JPH0822746B2 - Ferromagnetic fine powder and its manufacturing method - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a ferromagnetic fine powder and a method for producing the same, and more specifically, a cobalt-modified iron oxide fine powder useful as a magnetic recording material for a magnetic recording medium. And the method of manufacturing it.

BACKGROUND OF THE INVENTION In recent years, demands for higher density and higher SN ratio for magnetic recording media have become stronger and stronger, and various improvements have been made in order to meet these demands. Since those characteristics are influenced by the characteristics of the particulate magnetic material exhibiting the magnetic recording function, it is necessary to improve the magnetic material. For the purpose of improving the characteristics of magnetic materials, many studies have been conducted to make them finer particles than ever before, and the magnetic recording media that have achieved a considerably high SN ratio and high density have been achieved by making them finer particles. Already obtained.

As the magnetic material used for the magnetic recording medium is made finer as described above, noise components are particularly reduced, and thereby a high SN ratio has been achieved. However, as the magnetic material becomes finer, many problems arise.

Generally, a magnetic recording medium is prepared by uniformly dispersing a fine particle magnetic material (ferromagnetic fine powder) in a binder solution (solution consisting of a binder and a solvent) to prepare a magnetic coating solution, which is then applied to a support such as a plastic sheet. It is produced by a method in which the solvent is applied and then the solvent is removed by means such as evaporation.
When the magnetic material is made finer, that is, when the magnetic material used is finer than ever before, there is a marked tendency to cause troubles in the preparation, storage and coating operation of this magnetic coating liquid. Becomes

For example, there is a large increase in viscosity of a magnetic coating solution containing such a finely divided magnetic material, it becomes difficult to disperse the particles into primary particles, and there is a strong tendency to once aggregate into primary particles and then aggregate again. It will be. The particulate magnetic material is composed of primary particles and secondary particles formed by aggregating a plurality of primary particles. When preparing a coating liquid from a conventional particulate magnetic material, the secondary particles A considerable part of them was disaggregated during the preparation of the coating solution and dispersed as primary particles, and the coating was performed in this state. However, as the magnetic material becomes finer, it becomes more difficult to disperse the secondary particles in the primary particles, and even when the magnetic particles are dispersed, there is a tendency for the secondary particles to aggregate again in the coating liquid to form secondary particles. On the other hand, the increase in the viscosity of the coating solution is not desirable because it hinders the preparation of a uniform coating solution and also prevents the formation of a uniform coating layer and the formation of a coating layer having a uniform thickness even when applied to a support.

In order to solve these problems, improvement means such as improving the performance of a dispersion device, using a dispersion aid, and surface-treating a magnetic material have been investigated.

By using these means, it is possible to use magnetic materials that have been made finer, and the high SN of magnetic recording media can be used.
Higher ratios and higher densities have been achieved.

However, the particulate magnetic material that can be used by utilizing the above-mentioned improving means is 30 m ² / g to 35
It has a specific surface area of about m ² / g.

On the other hand, today, there is a further demand for higher SN ratio and higher density, and it is necessary to put into practical use a particulate magnetic material having a specific surface area of 40 m ² / g to 50 m ² / g. Typical magnetic materials used for magnetic recording are ferromagnetic substances such as γ-iron oxide (γ-Fe ₂ O ₃ ), cobalt-modified iron oxide, and iron atom-containing alloys. It is possible to produce a fine particle magnetic material having a high specific surface area as described above from such a ferromagnetic material by utilizing the fine particle technology known so far. Since the problem associated with the increase in the specific surface area becomes even greater, it has not been put to practical use. In particular, the fact that the viscosity of the dispersion liquid (magnetic coating liquid) becomes high is a serious obstacle, and it is fatal that sufficient dispersion is difficult with an ordinary dispersion device.

It is not impossible to uniformly disperse such a ferromagnetic fine powder having a high specific surface area in a binder solution and reduce the viscosity even by the conventional technique.

For example, there is a method of using more dispersion aid. However, in this method, the mechanical strength of the magnetic layer formed by coating and drying the magnetic coating liquid on the support is lowered, and blooming occurs. As a result, there is a problem that it is difficult to obtain a magnetic recording medium having sufficient durability and aging characteristics.

There is also a method of using a large amount of a binder, and this method can avoid a decrease in mechanical strength of the magnetic layer,
This time, the saturation magnetic flux (Bm) decreases and high output cannot be obtained, so the improvement of the SN ratio becomes insufficient.

There is also a method of increasing the amount of solvent used in the binder solution to reduce the viscosity. However, in this case, the disadvantage that the time required for dispersion increases increases.

[Object and Summary of the Invention] The present invention solves the above problems and has a specific surface area of 36
The present invention makes it possible to put the cobalt-modified iron oxide ferromagnetic fine powder exceeding m ² / g into practical use as a magnetic material for magnetic recording.

That is, an object of the present invention is to provide a magnetic material for magnetic recording comprising a cobalt-modified ferromagnetic fine powder having a high specific surface area, wherein the viscosity of a magnetic coating solution obtained by dispersing the powder is Provided is a ferromagnetic fine powder capable of uniformly dispersing the fine powder and achieving a low level that does not impede the uniformity of the coating layer and the uniformity of the coating layer thickness in the coating operation on the support. To do.

Another object of the present invention is to provide a magnetic material for magnetic recording comprising cobalt-modified ferromagnetic fine powder having a high specific surface area and exhibiting a high degree of dispersibility.

Another object of the present invention is to provide a cobalt-modified ferromagnetic fine powder having a sharp coercive force distribution.

Another object of the present invention is to provide a method for efficiently producing a ferromagnetic fine powder composed of the above-mentioned excellent cobalt-modified magnetic fine powder.

The present invention has a specific surface area of 36m ² / g or more (preferably 150 meters ² /
g or less, and more preferably 100 m ² / g or less), which is prepared so that the residual amount of 350-mesh water sieve is 2% or less. In powder.

Here, the 350 mesh water sieve residual ratio means that a certain amount of a ferromagnetic material is taken up on a 350 mesh sieve, and this is immersed in water or thoroughly washed under running water, and then the coarse particles remaining on the sieve are left. Say the rate.

The cobalt-modified ferromagnetic fine powder of the present invention contains secondary particles, and a dispersion liquid of the cobalt-modified ferromagnetic iron oxide fine powder having a specific surface area of 36 m ² / g or more is subjected to a classification operation to give a 350 mesh water sieving residual ratio of 2 % Or less of the cobalt-modified ferromagnetic iron oxide fine powder can be obtained.

The cobalt-modified ferromagnetic fine powder of the present invention also contains secondary particles, and a dispersion liquid of the ferromagnetic iron oxide fine powder having a specific surface area of 36 m ² / g or more is subjected to a classification operation, and the 350 mesh water sieve residual ratio is 2%.
It can also be obtained by a method comprising recovering the following ferromagnetic iron oxide fine powder and then modifying the fine powder with cobalt.

[Detailed Description of the Invention] The cobalt-modified ferromagnetic fine powder of the present invention will be described in more detail below.

Cobalt-modified iron oxide magnetic fine powder having a specific surface area of 36 m ² / g or more and a 350 mesh water sieving residue ratio of 2% or less is a fine particulate ferromagnetic iron oxide (specific surface area of about 36 m ² / g or more) E.g. Fe
It can be prepared from ₂ O ₃ , Fe ₃ O ₄ , FeOx (4/3 <x <3/2, etc.). Then, this fine particle-shaped ferromagnetic iron oxide having a specific surface area of about 36 m ² / g or more can be produced by firing finely particulate hydrated iron oxide having a specific surface area of about 60 m ² / g or more. Since the preparation and calcination of the particulate hydrated iron oxide having a specific surface area of about 60 m ² / g or more can be easily carried out by using a known method, a detailed description on these points will be given. Omit it.

As a typical example of the method for preparing a cobalt-modified iron oxide magnetic substance fine powder having a specific particle size distribution from a ferromagnetic iron oxide fine powder having a specific surface area of about 36 m ² / g or more, a particle size distribution after carrying out cobalt modification in advance is shown. And the cobalt modification after adjusting the particle size distribution.

The particle size distribution of the ferromagnetic iron oxide fine powder or its cobalt modified product is adjusted in the dispersion state. That is, finely divided iron oxide is dispersed in a solvent that is inert to iron oxide, and then ferromagnetic fine powder having a controlled particle size distribution is recovered from the dispersion, and then the ferromagnetic fine powder is modified with cobalt. Alternatively, a method may be used in which the ferromagnetic fine powder whose particle size has not been adjusted is cobalt-modified in the dispersion liquid and then the particle size of the cobalt-modified product is adjusted.

Examples of the above-mentioned solvent inert to iron oxide include:
Water, alcohols (eg, methanol, ethanol, propanol, butanol), ketones (eg, acetone, methyl ethyl ketone, methyl isobutyl ketone), esters (eg, ethyl acetate, butyl acetate), paraffins (eg, pentane, hexane) , Heptane, 2-ethylhexane), alicyclic compounds (eg, cyclohexane), aromatic compounds (eg, benzene, toluene, xylene) and the like. These solvents can be used alone or as a mixture.

A method of modifying iron oxide with cobalt is already known,
The cobalt modification in the present invention can also be carried out by a method arbitrarily selected from those methods. An example of a typical method thereof is a method of treating γ-iron oxide particles with a cobalt salt (eg, cobalt sulfate, cobalt chloride, cobalt nitrate) in an alkaline aqueous solution. Further, if desired, cobalt modification may be carried out using one or more salts of metals such as ferrous iron, ferric iron, manganese, zinc, nickel and chromium.

Regarding the cobalt-modified fine particle iron oxide, the one whose particle size distribution has been adjusted is as it is, and the one whose particle size distribution has not been adjusted is adjusted, then separated from the solvent and dried to obtain the ferromagnetic fine particles of the present invention. It becomes powder.

The particle size distribution of the fine powder in the present invention is adjusted by removing a certain amount or more of coarse particles (agglomerates of primary particles in the form of relatively large lumps) in the dispersion state. As a specific method thereof, (1) a method of accelerating the separation of secondary particles in the raw material by vigorously stirring the dispersion liquid to reduce coarse particles.

(2) A method of removing coarse particles by allowing the dispersion liquid to stand to precipitate coarse particles. At this time, the concentration of the dispersion liquid is adjusted if necessary.

(3) A method of filtering coarse particles by filtering the dispersion liquid using an appropriate filtering device. At this time, the concentration of the dispersion liquid is adjusted if necessary. As a filtering device, for example, 350
A mesh strainer is used.

These methods can be used alone or in combination. However, as an industrial method for reducing coarse particles, a method utilizing the filtration operation of the above (3) is advantageous.

The ferromagnetic fine powder whose particle size distribution has been adjusted as described above is separated from the liquid and then dried. The separation operation from the liquid and the drying operation may be performed in one step.

Ferromagnetic fine powders that have undergone the drying process are generally obtained in the form of agglomerates, and therefore, after that, they may be subjected to a light grinding process for the purpose of facilitating their handling. Examples of the crushing device used for such a purpose include a jaw crusher and a roll crusher.

The ferromagnetic fine powder obtained through the above steps is a ferromagnetic fine powder composed of cobalt-modified iron oxide particles, having a specific surface area of 36 m ² / g or more, and a 350 mesh water sieving residual rate of 2% or less, Ferromagnetic fine powder according to this regulation has excellent dispersibility in a binder solution and also suppresses the formation of secondary particles, so that the magnetic coating liquid produced tends to be homogeneous and have a low viscosity. is there. Therefore, a magnetic recording medium manufactured using this magnetic coating liquid has a magnetic layer that is homogeneous and has a high surface smoothness. As a result, the distribution of coercive force (Hc) becomes sharp, and the transfer effect is improved.

That is, according to the research by the present inventor, among the conventionally known cobalt-modified ferromagnetic fine powder, 20 m ² / g to 30
At a viscosity of up to m ² / g, the viscosity of the magnetic coating liquid containing them may increase and the workability may deteriorate, and in a magnetic recording medium, it may easily deteriorate the magnetic recording characteristics. Grain contains only about 0.1%. Therefore, as long as a fine powder having a specific surface area of this level is used, no particular trouble occurs in the actual magnetic coating liquid adjusting step and coating step, and also in magnetic recording media, deterioration of characteristics is not a problem. There wasn't. However, when the specific surface area of the cobalt-modified ferromagnetic fine powder is about 35 m ² / g, the content ratio (ratio by weight) of coarse particles is about 2%, and further, when the specific surface area is 40 m ² / g, Content exceeds about 2% 3
It turned out that it is not uncommon for it to be ~ 4%.

Even if the cobalt-modified ferromagnetic fine powder containing a considerable amount of such coarse particles is dispersed, the fine particles within the coarse particles are strongly bonded, making it difficult to disperse uniformly. When the magnetic layer is formed by using it, various characteristics of the magnetic recording medium are naturally deteriorated. Further, according to the research conducted by the present inventor, as schematically shown in the attached FIG. 1, the magnetic coating liquid (dispersion liquid) A containing a relatively large amount of coarse particles has a viscosity in the early stage of dispersion. Is low,
It was found that the viscosity increased rapidly as the dispersion proceeded. On the other hand, when the magnetic coating liquid B is prepared using the cobalt-modified ferromagnetic fine powder having the adjusted particle size distribution provided by the present invention, the viscosity at the initial stage of dispersion tends to be relatively high. Shows almost no change even after a lapse of time, and after the dispersion operation has been carried out for a certain period of time, the latter magnetic coating liquid B is clearer than the former magnetic coating liquid A containing a relatively large amount of coarse particles. It has a low viscosity. Further, regarding the surface property (surface smoothness) of the magnetic layer of the magnetic recording medium prepared by using these magnetic coating solutions, as schematically shown in FIG. The magnetic coating liquid B using the magnetic fine powder gives a magnetic layer exhibiting a good surface property, whereas the magnetic coating liquid A containing many coarse particles gives an insufficient surface property.

Next, the present invention will be described in more detail with reference to Examples and Comparative Examples.

[Example 1] Water 8 was put into a 20 stainless beaker, and the blade diameter was 150 m.
γ-Fe ₂ O ₃ with a specific surface area of 40 m ² / g while stirring with a m
2 kg was added and dispersed for 30 minutes. Slurry (total amount 10kg)
Had a very high viscosity, and no supernatant could be obtained even if it was allowed to stand.

When this slurry was diluted with water to make the total 50, it became a liquid with high fluidity. After 5 minutes from the stirring, the liquid substance was drawn into another beaker with a siphon. At this time, the suction port was placed near the liquid level so as not to suck out the coarse powdered material near the bottom, and the suction port was sequentially lowered as the liquid level lowered. Finally, the beaker from which the liquid was extracted left behind a slurry containing coarse undispersed particles. The amount of undispersed solids in the above classification operation is 57 g
Met. Therefore, 1 gamma-Fe ₂ O ₃ is contained in the extracted liquid material.
943g will be included.

The extracted liquid material is then allowed to stand still to form a supernatant liquid, and the total amount of the slurry is reduced to 10 by removing the supernatant liquid.
As kg, transferred to 20 reaction vessels. To this was added 300 g of cobalt sulfate heptahydrate, and the resulting slurry was vigorously stirred while blowing nitrogen gas at 1 / minute. Then 30
After a minute, an aqueous sodium hydroxide solution (NaOH 1.0 kg / 2 water) was added. Then, while further blowing nitrogen gas, the slurry was heated to 95 ° C. and kept at that temperature for 5 hours. Then
The slurry was washed with water to remove alkali and the like, dehydrated, and dried at 70 ° C. for 24 hours. The lumps produced by drying were crushed to a diameter of 2 mm or less to obtain fine particles of cobalt-modified iron oxide (magnetic powder). This was designated as Sample 1.

Comparative Example 1 Under the same conditions as in Example 1, 40 m ² / g of γ-Fe ₂ O ₃ was dispersed to obtain a total amount of 10 kg of slurry.

285 g of this slurry was taken, and 285 g of water was added thereto. Then, using this diluted slurry, the cobalt modification reaction of γ-Fe ₂ O ₃ was performed by the method described in Example 1.

After the reaction was completed, the reaction solution was divided into two, and one was washed with water in the same manner.
It was dehydrated and then dried. The lumps produced by drying were crushed to a diameter of 2 mm or less to obtain cobalt-modified iron oxide fine powder. This was designated as Comparative Sample 1.

[Example 2] The other one of the reaction solutions divided into two in Comparative Example 1 was passed through a 350-mesh sieve (diameter 300 mm). Since the sieve was apt to be clogged, the reaction solution was not passed at once, and the classification treatment was carried out while replenishing the sieve with water little by little. Similarly, the slurry that passed through the sieve was filtered, washed with water, dehydrated, and dried.
The lumps produced by drying were crushed to a diameter of 2 mm or less to obtain cobalt-modified iron oxide fine powder. This was designated as Sample 2.

[Example 3] A cobalt-modified iron oxide fine powder was obtained in the same manner as in Example 1 except that γ-Fe ₂ O ₃ having a specific surface area of 60 m ² / g was used as iron oxide, and this was used as sample 3. .

[Comparative Example 2] A cobalt-modified iron oxide fine powder was obtained in the same manner as in Example 3 except that the classification operation was not performed, and this was designated as Comparative Sample 2.

Example 4 40 m ² / g of γ-Fe ₂ O ₃ was dispersed under the same conditions as in Example 1 to obtain a total amount of 10 kg of slurry.

5 kg of glass beads (diameter 1 mm) was added to this slurry, and the mixture was further stirred for 60 minutes. The obtained slurry was passed through a 16-mesh sieve to separate glass beads, and then the filtrate (slurry) was similarly modified with cobalt. No particular coarse particles were found in the cobalt-modified slurry, and thus this was used as Sample 4 as it was.

[Example 5] In Example 1, instead of modification with 300 g of cobalt sulfate heptahydrate, 200 g of cobalt sulfate heptahydrate and ferrous sulfate heptahydrate were used.
The same treatment was carried out except that the modification was carried out with 500 g to obtain cobalt-modified iron oxide fine powder, which was designated as Sample 5.

[Evaluation of Cobalt-Modified Iron Oxide Fine Powder] The samples obtained in the above Examples and Comparative Examples were evaluated by the following methods.

1) Specific surface area: measured by the BET method.

Unit: m ² / g 2) 350 mesh water sieve residual ratio: Take 100 g of sample into a 350 mesh water sieve (diameter 300 mm), immerse this in a 0.1% hexametaphosphoric acid aqueous solution, and vibrate the sieve sufficiently in that state. After filtering the sample as much as possible, further exposing the sieve to running water and performing this classification operation until no particles are found in the filtered portion, weigh the coarse particles remaining on the sieve, and remove the residual coarseness of the sample used in the test. The weight ratio of the particles was determined, and this was taken as the 350 mesh water sieving residue ratio (described as the water sieving residue ratio in Tables 1 to 3).

3) Viscosity of Magnetic Coating Liquid and Properties as Magnetic Recording Material A sample (magnetic powder) was blended according to the following formulation, and kneaded for 2 hours with a batch kneader. Methyl ethyl ketone and butyl acetate were added to the obtained kneaded product to dilute it, and the diluted slurry was subjected to a dispersion treatment for 4 hours in a sand mill to obtain a magnetic coating liquid.

The viscosity of the obtained magnetic coating liquid was measured with a B-type viscometer. The viscosity is shown in Tables 1-3.

The above magnetic coating liquid was applied to a polyethylene terephthalate support (thickness: 20 μm) using a doctor blade with a gap of 50 μm.
m), the magnetic powder is oriented by using a magnetic field of 3000 gauss and the solvent is evaporated to form a magnetic layer.
A magnetic recording medium sample was prepared.

The gloss and various magnetic properties [coercive force (Hc), squareness ratio (SQ), SFD] of this magnetic recording medium sample were measured. Gloss was evaluated according to JIS-Z-8471 by measuring the intensity of reflected light when white light was incident on the surface of the magnetic layer at an angle of 45 degrees. Measured glass surface gloss of 10
It is a relative value with 0%.

The magnetic characteristics were measured using a VSM (vibrating sample magnetometer).

Reference Example 1 γ-Fe ₂ O ₃ having a different specific surface area was prepared from goethite (γ-FeOOH) having a different particle size as a raw material, and treated in the same manner as in Comparative Example 1 to produce a cobalt-modified iron oxide fine powder. . Table 4 shows the specific surface area and the 350-mesh water-sieve residual rate (measured by the above method) of the obtained cobalt-modified iron oxide fine powder.

From the above results, the cobalt-modified iron oxide fine powder having a small specific surface area produces a small amount of coarse particles even if no particular classification operation is performed, but the generation of coarse particles is remarkable when the specific surface area exceeds about 36 m ² / g. It can be seen that it is.

[Reference Example 2] The sample 2 (specific surface area 39.8 m ² / g) and the comparative sample 1 (specific surface area 39.5 m ² / g) were mixed in various amounts to obtain various 350
A cobalt-modified iron oxide fine powder having a mesh water sieving ratio was prepared. Using this cobalt-modified iron oxide fine powder, a magnetic coating solution similar to that described above was prepared, and this magnetic coating solution was similarly coated on the surface of polyethylene terephthalate to perform orientation treatment to form a magnetic layer. The gloss of this magnetic layer was measured by the same method. 35 of cobalt-modified iron oxide fine powder used
Table 5 shows the residual ratio of 0-mesh water sieve, the viscosity of the magnetic coating solution prepared by using the magnetic powder, and the gloss of the magnetic layer. The unit of each measurement item in Table 5 is the same as described above.

From the results in Table 5, it can be seen that the viscosity increases at an accelerated rate when the residual ratio of 350 mesh water sieve exceeds 2.0%. It is also clear that the gloss decreases in inverse proportion to the increase of the 350 mesh water screen residual rate.

[Brief description of drawings]

FIG. 1 shows the dispersion time in a magnetic coating liquid (dispersion liquid) A containing a relatively large amount of coarse particles and a magnetic coating liquid B prepared by using magnetic powder having a controlled particle size distribution provided by the present invention. 3 is a graph schematically showing the relationship between the viscosity and dispersion time of the coating liquid and the surface property (surface smoothness of the magnetic layer obtained using the magnetic coating liquid).

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Claims (6)

[Claims] 1. A ferromagnetic fine particle characterized by comprising cobalt-modified ferromagnetic iron oxide having a specific surface area of 36 m ² / g or more and having a 350 mesh water sieving residue ratio of 2% or less. Powder. 2. The ferromagnetic fine powder according to claim 1, which is adjusted so that the residual ratio of 350 mesh water sieve is 1.5% or less. 3. A dispersion of cobalt-modified ferromagnetic iron oxide fine powder containing secondary particles and having a specific surface area of 36 m ² / g or more is subjected to a classification operation, and a 350-mesh water-sieve residual ratio of 2% or less is used. A method for producing ferromagnetic fine powder, which comprises recovering magnetic iron oxide fine powder. 4. The method for producing a ferromagnetic fine powder according to claim 3, wherein the cobalt-modified ferromagnetic iron oxide fine powder having a 350-mesh residual water content of 1.5% or less is recovered. 5. A dispersion of ferromagnetic iron oxide fine powder containing secondary particles and having a specific surface area of 36 m ² / g or more is subjected to a classification operation, and a 350 mesh water sieving residual ratio is 2% or less. A method for producing a ferromagnetic fine powder, which comprises recovering the powder and then modifying the fine powder with cobalt. 6. The method for producing a ferromagnetic fine powder according to claim 5, wherein the ferromagnetic iron oxide fine powder having a 350-mesh residual water content of 1.5% or less is recovered.