JPH089486B2 - Cobalt-containing ferromagnetic iron oxide powder and method for producing the same - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a cobalt-containing ferromagnetic iron oxide powder which is useful as a recording material for a magnetic recording medium, and particularly has improved dispersibility in an organic binder, and a method for producing the same.

[Technical Background of the Invention and its Problems] Cobalt-containing ferromagnetic iron oxide has a high coercive force, and when it is used as a recording element of a magnetic recording medium, high density recording is possible and its sensitivity in a high frequency region is excellent. Because of its unique features, it is widely used in the field of magnetic recording such as video recording.

As a recent tendency, with the trend toward high-quality magnetic recording media such as video tapes, the cobalt-containing ferromagnetic iron oxide as a recording element is becoming finer particles. Magnetic powder is
When the particles are made into fine particles, it becomes difficult to uniformly disperse them in the binder when kneading with various organic binders to prepare a magnetic coating material, and there is a problem in that the effect of making the particles fine cannot be obtained in terms of the characteristics of the magnetic recording medium. are doing.

On the other hand, in recent manufacturing methods of magnetic recording media, it is excellent to give highly smooth surface to the surface of the magnetic recording medium by highly filling and highly orienting the magnetic powder with high coercive force and by uniformly dispersing the magnetic powder. This is important for obtaining the magnetic properties, but for that purpose, it is necessary to disperse the magnetic powder up to near the primary particles without destroying it when preparing the magnetic coating material. However, in the magnetic powder obtained by the conventional manufacturing method, primary particles easily aggregate to form an aggregate,
It was difficult to disperse the particles to near the primary particles when preparing the magnetic paint.

Various proposals have already been made to solve these problems. For example, a method of coating the surface of particles of magnetic powder with a binder and a surfactant that is well compatible with the preparation of a magnetic coating (Japanese Patent Publication No. 19120/1978, Japanese Patent Publication No. 37297/1979).
53-141196, JP-A-54-82354, JP-A-54-85397),
A method in which a surfactant is added as a dispersant during the preparation of the magnetic coating (JP-A-55-151068, JP-A-55-151069), or a method of loosening agglomerates by using a mechanical dispersing means during the preparation of the magnetic coating (special Kai 50-22297, JP 55-157216, JP 56-1
0903) has been tried.

However, even with these conventional methods, there is no effect due to the problem of the resin selectivity of the surfactant, strength reduction of the magnetic tape, bleeding, powder falling (a phenomenon in which magnetic powder peels off due to abrasion of the tape), etc. However, there is a drawback that re-aggregation occurs depending on the degree of mechanical dispersion. In particular, the finer the magnetic powder is, the more conspicuous these defects become, so that it becomes extremely difficult to improve the dispersibility.

In the process of producing a cobalt-containing ferromagnetic iron oxide by depositing a cobalt compound on the surface of magnetic iron oxide particles,
The dryer used in the drying step after depositing the cobalt compound is generally an airflow box type dryer, a fluid type dryer, a rotary type air dryer, etc. There was no suggestion devised.

[Object of the Invention] An object of the present invention is to solve the above problems of the prior art and to provide a cobalt-containing ferromagnetic iron oxide powder having excellent dispersibility in an organic binder and useful as a recording material, and a method for producing the same. It is in.

[Summary of the Invention] The present inventors have conducted various studies to achieve the above-mentioned object, and as a result, in a process for producing a cobalt-containing ferromagnetic iron oxide by depositing a cobalt compound on the particle surface of magnetic iron oxide powder. By using a spray dryer in the drying step after depositing the cobalt compound, a spherical powder having a diameter of 5 to 200 μm, which is an aggregate of fine particles of cobalt-containing magnetic iron oxide, is produced at the time of manufacturing the magnetic recording medium. In the present invention, the inventors have found that a magnetic powder having extremely excellent dispersibility that can be easily dispersed even in primary particles before aggregation is obtained, and the present invention has been completed. That is, the first of the present invention is an aggregate powder of cobalt-containing ferromagnetic iron oxide fine particles having a specific surface area of 30 m ² / g or more by the BET method and having a diameter of 5 to 200 μm,
Further, it is a cobalt-containing ferromagnetic iron oxide powder having a spherical shape with an angle of repose of 35 degrees or less measured by a Hosokawa powder tester using an injection method.

A second aspect of the present invention is to treat magnetic iron oxide in a water-based medium with a cobalt salt or a cobalt salt and another metal salt and an alkali to deposit a metal compound containing cobalt on the surface of the particles, and This magnetic iron oxide is filtered, washed with water,
Alternatively, in the method for producing a cobalt-containing ferromagnetic iron oxide powder by further heating and then drying, the drying is performed by spray drying, which is a method for producing a cobalt-containing ferromagnetic iron oxide powder.

Cobalt-containing ferromagnetic iron oxide having a specific surface area of 30 m ² / g or more by the BET method is produced by various methods, and the production method is not particularly limited, but usually maghemite (γ-Fe ₂ O ₃ ), Magnetite (Fe ₃ O ₄ ), and verterlide compounds (FeOx, 1.33 << × <1.5) in an aqueous medium, treated with cobalt salt or cobalt salt and other metal salts and alkali. The surface of the particles is coated with a metal compound containing cobalt, and then the magnetic iron oxide is filtered, washed with water, or further heat-treated.
The shape of this product is typically acicular crystals with an average particle size (major axis length) of 0.08 to 0.3 μm, an axial ratio of 3 to 15 and a specific surface area of 30 to 70 m ² / g, preferably 35 to The fine particles have a particle size of 70 m ² / g, but in addition to the acicular crystal shape, they may have various shapes such as spindle shape and rice grain shape and have a specific surface area within the above range. Examples of the other metal salts include salts of metals such as ferrous iron, manganese, zinc, vanadium, barium and magnesium.

The effect of the present invention is particularly remarkable in the magnetic iron oxide particles having the above specific surface area. The reason is that as the specific surface area increases, the cohesive force between particles increases,
It is considered that the agglomeration state of dried particles significantly progresses during drying. In the case of a magnetic iron oxide having a small specific surface area (less than 30 m ² / g), agglomeration at the time of drying does not pose a problem and the effect of the present invention is not recognized so much.

The magnetic powder of the present invention remains in the form of droplets and has a diameter of 5 which is an aggregate of cobalt-containing ferromagnetic iron oxide fine particles having a specific surface area of 30 m ² / g or more as a primary particle by the BET method.
It has a spherical shape of ~ 200 μm. This product has excellent fluidity, and the angle of repose often used as a measure of powder fluidity is 35 degrees or less (measured using a "Powder Tester" manufactured by Hosokawa Micron KK). It is much lower than that obtained using a hot air circulation type or fluidized type drying device. The angle of repose, which is a characteristic of the powder with respect to fluidity, is 35 degrees or less, and preferably 30 degrees or less. Magnetic powders having an angle of repose of more than 35 degrees have poor dispersibility and also have poor magnetic properties such as squareness and orientation of the magnetic recording medium. In addition, powders exceeding 35 degrees generally have a strong adhesion and cohesion property, and are therefore unfavorable because there are problems in powder handling such as easy adhesion and blockage in air transportation equipment, storage equipment and the like. Another degree of compaction, sometimes used as another property of the powder for flowability, appears to be 0.25 or less, preferably 0.20 or less.

As described above, the magnetic powder of the present invention having remarkably excellent fluidity has various advantages in handling as a powder.
In addition, since the powder is easily dispersed to near the primary particles during the preparation of the magnetic coating material, the dispersion time is not only greatly shortened, but when a magnetic recording medium such as a magnetic tape is prepared using this, the coating film Not only gloss, but also magnetic properties such as squareness ratio (Br / Bm) and orientation (OR) of magnetic recording media are improved.

The present invention is a method for producing spherical cobalt-containing ferromagnetic iron oxide powder by the following steps.

That is, according to the present invention, magnetic iron oxide is treated with a cobalt salt or a cobalt salt and another metal salt and an alkali in an aqueous medium to deposit a metal compound containing cobalt on the surface of the particle, and then the magnetic compound The iron oxide is filtered, washed with water,
Alternatively, in a method for producing a cobalt-containing ferromagnetic iron oxide powder by further heating and then drying, the drying is performed by spray drying.

A spray dryer is a device that atomizes liquid raw materials such as solutions, colloids, pastes, slurries, etc., and makes them dry in seconds by contacting them with a high-temperature air stream. Depending on the contact method between hot air and droplets, or the spray method. There are various types, but any type can be used in the present invention. A cross-sectional view of an example of the spray dryer is shown in FIG.

The slurry concentration of magnetic iron oxide supplied to the spray dryer is
There is no particular limitation because it can be atomized in a pressure nozzle type or a disc type depending on its properties, but usually 30 to 400 g /
, Preferably about 150 to 300 g /. If the magnetic iron oxide concentration is too high, pumping and atomization will be hindered, and droplets for forming spherical particles will be insufficient. Also, for the purpose of removing water by drying, if the concentration is too low, the efficiency becomes economically inferior.
The concentration usually used industrially is preferably 150 g / or more.

There are various appropriate conditions for the discharge pressure when atomizing, depending on the spray type, supply pump type, and capacity, but usually 2 to 30
Kg / cm ^2, desirably from 10~25Kg / cm ² approximately.

The hot air flow inlet temperature as a drying heat source is usually 150 ° C or higher, but the outlet temperature rises depending on the inlet temperature, and when the bag filter method is used as a method for collecting spray-dried products, heat resistance of the filter cloth The upper limit temperature is regulated in terms of sex. In the case of magnetic iron oxide, if the material temperature is heated to 200 ° C. or more during drying, the magnetic properties and dispersion properties are likely to deteriorate, so it is better to set the inlet temperature so that the outlet temperature does not exceed 200 ° C. Further, if the outlet temperature is too low, the moisture content after drying becomes too high, and the spherical particle state is destroyed when additional drying is performed, or the degree of influence from the additional drying equipment becomes large, and spray drying causes It is not preferable because the characteristics of improving the dispersibility are reduced and the drying efficiency is deteriorated. Therefore, the drying outlet temperature is usually 60 to 20.
The temperature is 0 ° C, preferably about 70 to 200 ° C.

A magnetic powder having a spherical particle diameter of 5 to 200 μm can be obtained by appropriately combining the respective ranges of the magnetic iron oxide concentration, the discharge pressure, and the drying outlet temperature according to the model and ability of the spray dryer.

The dry atmosphere need not be a non-oxidizing atmosphere and may be air. A non-oxidizing atmosphere may be required as a dry atmosphere for the cobalt-containing ferromagnetic iron oxide,
This is generally to prevent the Fe ⁺⁺ in the cobalt-containing ferromagnetic iron oxide from being oxidized. In the method of the present invention, the oxidation reaction of Fe ⁺⁺ becomes difficult to proceed, probably because it can be dried instantaneously. Therefore, in the method of the present invention, even in the case of magnetic iron oxide, which requires a non-oxidizing atmosphere in a usual dryer, it is possible to dry in air without adjusting the atmosphere to be non-oxidizing, which is advantageous in terms of cost. is there.

[Examples] Next, examples of the present invention will be described. A comparative example is also shown for reference.

The cobalt-containing ferromagnetic iron oxide (hereinafter abbreviated as magnetic iron oxide) used in Examples and Comparative Examples has an average particle size (major axis length) of 0.18.
μ, axial ratio 8, Hc380Oe, σs71emu / g, γ-Fe ₂ O ₃ having characteristics of specific surface area of 50 m ² / g is used as a nuclear crystal, and cobalt sulfate, ferrous sulfate aqueous solution and caustic soda aqueous solution are used. A magnetic iron oxide layer containing cobalt was formed on the nuclei, which was filtered and thoroughly washed with water. The contents of cobalt and ferrous iron are 3.5% and 3.7%, respectively, on a dry basis.

Example 1 A spray dryer (manufactured by Ashizawa Nilo Atomizer KK, AN-12.5CN / CR) in which an aqueous slurry having a magnetic iron oxide concentration of 195 g / is stable at an air hot air inlet temperature of 300 ° C and an outlet temperature of 100 ° C.
Mould), sprayed at a nozzle discharge pressure of 15 kg / cm ² and dried to give a water content of 0.2% by weight and spherical particles having an average diameter of about 85 μm.
Magnetic powder (A) was obtained.

Example 2 The same procedure as in Example 1 was carried out except that the hot air inlet temperature was 400 ° C and the outlet temperature was 150 ° C to obtain a magnetic powder (B) having a water content of 0.1% by weight and spherical particles having an average diameter of about 83 µm. .

Example 3 A magnetic powder having a water content of 5.0% by weight was obtained in the same manner as in Example 1 except that the hot air inlet temperature was 250 ° C and the outlet temperature was 75 ° C. Using an aeration flow box type dryer,
Magnetic powder (C) with spherical particles having an average diameter of approximately 80 μm, which was additionally dried in nitrogen gas at 120 ° C. until the water content became 0.1% by weight.
Got

Comparative Example 1 The same magnetic iron oxide slurry as that used in the example was dehydrated to form a cake, which was dried in a nitrogen gas at 120 ° C. in a ventilated box dryer to obtain a magnetic powder having a water content of 0.1% by weight ( D)
Got

Comparative Example 2 The same magnetic iron oxide cake as in Comparative Example 1 was dried in a nitrogen gas at 120 ° C. with a fluidized dryer to obtain a magnetic powder (E) having a water content of 0.1% by weight.

Comparative Example 3 The same magnetic iron oxide cake as in Comparative Example 1 was dried at 120 ° C. in nitrogen gas using a rotary air dryer to obtain a magnetic powder (F) having a water content of 0.1% by weight.

Comparative Example 4 The same magnetic iron oxide cake as in Comparative Example 1 was dried at 120 ° C. in the atmosphere with a ventilation box dryer to obtain a magnetic powder (G) having a water content of 0.1% by weight.

With respect to the samples (A) to (G), the coercive force (Hc) and the saturation magnetization (σs) were measured by the usual methods. Further, the Fe ⁺⁺ content (% by weight) was measured by the potassium dichromate titration method. In addition, the Hosokawa powder tester was used to measure the angle of repose, the loose apparent specific gravity and the solid apparent specific gravity. The results are shown in Table 1.

Method of measuring angle of repose The angle of repose is measured by a Hosokawa powder tester using the injection method. The detailed measurement conditions are as described in the instruction manual for the Hosokawa powder tester, and the main points are as follows. First, the standard sieve (opening 710
Gently put the powder on top of μ) and start the vibration of the standard sieve.
The sieving powder is discharged from the tip of the funnel through the funnel. It is deposited on a circular table (8.0 cm in diameter) installed 7.5 cm below it in advance. At this time, the tip of the funnel should be positioned in the center of the table. Stop the outflow of powder when the state of the accumulated powder becomes constant,
A protractor is attached to the powder's ridgeline, and the angle of the ridgeline to the horizontal plane is calculated and used as the angle of repose.

Loosening apparent specific gravity measurement method A standard sieve (opening 710μ) is vibrated to drop the sample through a chute, and a specified container (internal volume 100cc)
Allow about 20 to 30 seconds until the powder piles up, stand up the blade vertically, scrape the surface of the powder, weigh it with a fine balance, and loosen it with the powder weight ÷ 100 Give the multiplication specific gravity value.

How to measure the apparent apparent specific gravity value Add a cap to the specified container (internal volume 100cc), put it in the tapping holder, put the powder to the top of the cap, cover the cap and tap (1 time / sec) 18
After turning it 0 times, weigh it out with a blade in the same way as the loose apparent specific gravity value, and weigh the powder ÷ 100 to give a solid apparent specific gravity value.

Further, for each sample, a blend was prepared according to the following blending ratio and dispersed by a ball mill to produce a magnetic coating material.

(1) Cobalt-containing ferromagnetic iron oxide powder 100.0 parts by weight (2) Surfactant 3.8 〃 (3) PVC-vinyl acetate copolymer resin 8.0 〃 (4) Polyurethane resin 35.5 〃 (5) Methyl ethyl ketone 108.1 〃 (6) Toluene 108.1 〃 (7) Cyclohexanone 36.0 〃 Next, each magnetic paint was applied to a polyester film by a usual method, oriented and dried to prepare a magnetic tape having a magnetic coating film of about 9 μm thick. The coercive force (Hc) and squareness ratio (Br
/ Bm), orientation (OR), switching field distribution (SFD), and gloss of coating film (60 ° -60 ° Gloss). The results are shown in Table 1.

Further, FIG. 2 shows the results of examining the relationship between the ball mill dispersion time and the gloss of the magnetic tape coating for the magnetic powder (A) and the magnetic powder (D).

From the results shown in Table 1, the cobalt-containing ferromagnetic iron oxide powder obtained by the present invention is rich in fluidity,
From the values of gloss, squareness ratio and orientation of the coating film, which can also be an index of dispersibility in the resin binder, it can be seen that the dispersibility is excellent. In addition, even though hot air was used, the same Fe ⁺⁺ as that of the comparative example performed in an inert atmosphere was used.
It is the content rate, which indicates that the oxidation reaction of Fe ⁺⁺ is difficult to proceed.

From FIG. 2, it can be seen from the results of the present invention that the dispersion time required to obtain a coating film gloss value equivalent to that obtained by using an ordinary dryer is greatly shortened when the magnetic coating is manufactured. It can be seen that the magnetic powder has been improved to have the feature that the cost can be greatly reduced.

From the results of Example 3, it can be seen that additional drying may be performed when the moisture value of the dried discharged product is insufficient. Since this additional drying is an auxiliary means for removing the difference from the desired moisture content, that is, the so-called slight residual moisture content, it is not preferable if the moisture content of the dried discharged product is too high (about 10% or more). 5,
%, The characteristics of the magnetic powder brought about by the spray dryer are not lost even if a normal dryer is used.

An electron micrograph (30,000 times) showing the crystal structure of the cobalt-containing ferromagnetic iron oxide particles used in Examples and Comparative Examples, and a micrograph showing the particle structure of magnetic powder A and magnetic powder D (40
2) are sequentially attached as FIGS. 3 to 5. It is clear that the one obtained according to the present invention (magnetic powder A) is a spherical powder having a particle diameter of 5 to 200 μm.

[Advantages of the Invention] The present invention, having the above-mentioned configuration, has the following various advantageous effects.

In other words, since this magnetic powder is composed of cobalt-containing ferromagnetic iron oxide, which is the primary particle, aggregated with a relatively gentle cohesive force, it can be easily dispersed to near the primary particle during the preparation of the magnetic coating, and the dispersion time can be greatly extended. Shorten to. Therefore, it leads to cost reduction in industrial operation.

When a magnetic recording medium such as a magnetic tape is produced by using this magnetic powder, not only the gloss of the coating film but also the magnetic characteristics such as the squareness ratio and the orientation are significantly improved.

The cobalt-containing ferromagnetic iron oxide powder according to the present invention is rich in fluidity and has various advantages in handling as a powder. For example, it is possible to prevent blockage and crosslinking phenomenon during powder transportation.

In the conventional product, the dispersibility is slightly improved by performing a crushing treatment after drying, but this magnetic powder has excellent dispersibility even without crushing, and a crushing operation is unnecessary.

According to the method of the present invention, even in the case of magnetic iron oxide which requires a non-oxidizing atmosphere during drying, it is not necessary to adjust the atmosphere to be non-oxidizing, which is industrially advantageous in cost.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of a spray dryer, and FIG. 2 is a diagram showing the relationship between the dispersion time in a resin binder and the coating film gloss for the magnetic powders obtained in Example 1 and Comparative Example 1. FIG. 3 is an electron micrograph showing the crystal structure of the cobalt-containing ferromagnetic iron oxide particles used in the examples (30,000
4), FIG. 4 and FIG. 5 are micrographs showing the particle structures of the magnetic powder obtained in Example 1 and the magnetic powder obtained in Comparative Example 1, respectively, and the magnification is 40 times. In FIG. 2, curve 1 shows the case of the magnetic powder of Example 1, and curve 2 shows the case of the magnetic powder of Comparative Example 1.

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Claims (2)

[Claims] 1. An aggregate powder of cobalt-containing ferromagnetic iron oxide fine particles having a specific surface area of 30 m ² / g or more as measured by the BET method, the diameter of which is 5 to 200 μm, and the Hosokawa method using the injection method. A spherical cobalt-containing ferromagnetic iron oxide powder having a repose angle of 35 degrees or less measured by a powder tester. 2. A magnetic iron oxide is treated with a cobalt salt or a cobalt salt and another metal salt and an alkali in an aqueous medium to deposit a metal compound containing cobalt on the surface of the particles, and then the magnetic oxidation is carried out. In a method for producing a cobalt-containing ferromagnetic iron oxide powder by filtering iron, washing with water, or further heating and then drying, the drying is carried out by spray drying. Production method.