JPH07257930A - Spherical magnetite grain and production thereof - Google Patents

Abstract

PURPOSE:To easily obtain a spherical magnetite particle excellent in dispersibility and change of color tone with time by coating the surface of a magnetite particle having a specified diameter. CONSTITUTION:An aq. mixed soln. contg. a 1:1.2 to 1:1 ratio of the ferrous ion and ferric ion is added with <=1.3 equiovalents of alkali to control the soln, to pH5-12, and the soln. is heated to 80-100 deg.C to form magnetite fine particles having 0.01-0.05mum average diameter. A straight-chain fatty acid having a 12-22C fatty acid part or fatty acid salt is added to a suspension of the magnetite fine particle and agitated to obtain a spherical magnetite particle coated with the fatty acid or its salt and having 0.01-0.05mum average diameter. After fatty acid treatment, 0.025-0.15mol/l soluble silicate is added, as required, agitated for 0.1hr at 50-120 deg.C to obtain a spherical magnetite particle coated with a silicon oxide layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to spherical magnetite particles and a method for producing the same, and more particularly to spherical magnetite particles excellent in dispersibility and stability of color tone over time and a method for producing the same. The spherical magnetite particles obtained by the present invention are particularly useful as a black pigment, especially a black pigment for cosmetics.

[0002]

2. Description of the Related Art Conventionally, carbon black and magnetite (ideal composition is Fe ^II Fe ^III ₂ O ₄ ) are generally used as black pigments. Of these, carbon black has a safety problem because it has a risk of being mixed with a carcinogen such as benzpyrene during production. For this reason, magnetite is widely used in applications such as cosmetics in which pigment particles come into direct contact with the human body.

As a method for industrially producing magnetite, a method of reducing goethite (α-FeOOH) and a method of oxidizing ferrous hydroxide are known. However, the magnetite obtained by these methods has an average particle size of 0.1
It is larger than μm. Therefore, when it is dispersed in an aqueous system, the sedimentation rate is high and the stability of the dispersion is poor.
Further, since the particle size is large, visible light is scattered and a deep black color cannot be obtained. Therefore, there is a problem that it is not preferable as a black pigment to be added to cosmetics, particularly mascara and eyeliner used as an aqueous dispersion.

In order to solve these problems, conventionally, studies have been made from two directions of reducing the particle size of magnetite particles produced and improving the surface characteristics. As a method for obtaining magnetite with a fine particle diameter, the pH of an aqueous solution containing ferrous ions and ferric ions in a molar mixing ratio of 1: 1
It is known to increase the pH to 11 or more by adding alkali to generate magnetite fine particles. According to this method, particles having a particle size of 0.01 μm or less can be obtained, but since the particle size and the shape are non-uniform, the secondary agglomeration is strong, and it is difficult to maintain a good dispersion state. Further, magnetite tends to change to Fe ₂ O ₃ in the air to change its color to blackish brown or brown, but when the surface area is large, such a change with time becomes remarkable.

A method for producing magnetite particles having a particle size of 0.01 to 0.1 μm, which is slightly larger than that obtained by the above method, has also been proposed. For example, in JP-A-4-238819, an inert gas is blown into a ferrous ion-containing aqueous solution to generate ferrous hydroxide by adding an alkali while reducing the dissolved oxygen concentration in the aqueous solution. Dispersion 6
A method for growing magnetite crystallites by heating to 0 to 100 ° C. is disclosed. However, this method has complicated steps and poor productivity. Further, there remains a problem in stability with respect to changes with time.

As a method of improving the surface characteristics, a method of coating the particle surface with a fatty acid is known. For example, JP-A-54-139544, JP-A-56-64348, and JP-A-
56-128957, JP-A-58-14773, JP-A-61-5
No. 3654 discloses a method of coating the surface of magnetite particles produced by various methods with a fatty acid or the like. However, the fatty acid coating alone does not provide a sufficient effect, and various improvements have been proposed at present. For example, in JP-A-60-69011, 3 to 35% of a pigment is added and suspended in water, and
0.5 to 10% of a fatty acid-soluble salt is added dropwise and uniformly mixed, and then a 1 to 30% aqueous solution of a soluble salt selected from Al, Mg, Ca, Zn, Zr and Ti is added dropwise to produce A method for orienting and adsorbing a metal salt on the surface of a pigment is described. Further, JP-A-63-17222 discloses a method in which a water-soluble silicate is added at the time of oxidation of ferrous hydroxide to produce magnetite particles containing Si, and then fatty acid coating is performed. Have been described. Further, Japanese Patent Application Laid-Open No. 4-144924 describes a method of adding Mn salt aqueous solution in place of fatty acid to perform Mn coating, and heating and firing this to obtain Mn solid solution hematite. However, in these methods, the steps are complicated because the magnetite particles obtained by the complicated coprecipitation reaction are subjected to more complicated surface coating and the like. Moreover, careful reaction control is necessary to obtain a uniform product.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide spherical magnetite particles and a method for manufacturing the same, which solve the above problems in the conventional manufacturing method. It is possible to easily produce magnetite particles having excellent dispersibility and stability with respect to changes in color tone with time without complicated steps.

[0008]

[Means for Solving the Problems] In the method for producing magnetite by adding an alkali to a mixed solution of ferrous and ferric ions, the present inventors have found that pH, temperature conditions and concentration at the time of producing magnetite. By specifying the reaction time, it is possible to generate magnetite with a particle size of 0.01 to 0.05 μm, and if fatty acid or its salt is added to the liquid in which particles of such particle size are suspended, the shape of The present invention has been accomplished by finding that particles that are regular and have excellent dispersibility can be obtained, and that such particles exhibit excellent dispersion stability over time.

That is, the present invention provides a magnetite particle having the following constitution and a method for producing the same. (1) Spherical magnetite particles having an average particle size of 0.01 to 0.05 μm, the particle surface of which is coated with a fatty acid or a salt thereof. (2) The spherical magnetite particles according to (1), wherein the particle surface is coated with a fatty acid or a salt thereof, and further coated with a silicon oxide layer. (3) In a mixed aqueous solution of ferrous and ferric ions
1.3 average equivalent or less of alkali was added to produce an average particle size of 0.
An average particle size of 0.01-0.05 μm coated with a fatty acid or its salt by adding a fatty acid or fatty acid salt to a liquid in which magnetite fine particles of 01-0.05 μm are suspended and stirring.
To produce spherical magnetite particles. (4) The content ratio of ferrous ion to ferric ion is 1:
The method according to (3) above, which uses a mixed aqueous solution of 1.2 to 1: 2. (5) The method according to the above (3) or (4), wherein after adding the alkali, the liquid temperature is heated to 80 to 100 ° C. to generate magnetite fine particles. (6) The fatty acid portion of the fatty acid or fatty acid salt is 12 to 2
(3), which is a straight-chain fatty acid containing two carbons,
The method according to any one of (5). (7) A soluble silicate aqueous solution is added to a suspension containing spherical magnetite particles coated with a fatty acid or a salt thereof to further form a silicon oxide layer.
The method according to any one of (6).

The details of each constituent element of the present invention will be described below. The method of the present invention uses a mixed aqueous solution of ferrous ions and ferric ions. Examples of compounds that generate ferrous ions include halides such as ferrous chloride, ferrous sulfate, ferrous nitrate, and ferrous perchlorate. In addition, examples of the compound that produces ferric ion include ferric salts corresponding to these. In industrial production, ferrous chloride, ferrous sulfate, ferric chloride and ferric sulfate, which are inexpensive, are preferable.

The concentration of the ferrous ion-containing aqueous solution is not particularly limited, but a concentration of about 0.5 to 1.5 mol / liter is preferable. If it is less than 0.5 mol / liter, the production efficiency is poor and 1.5 mol / liter
Even if it exceeds the liter, the rate of improvement in productivity is small and the economy is rather reduced. The same applies to the type of ferric ion-containing compound and its concentration in the liquid. Generally, it is preferable to use a ferric salt corresponding to the ferrous salt. Since the ideal composition of magnetite is Fe ^II Fe ^III ₂ O ₄ ,
The content ratio of ferrous ions to ferric ions in the aqueous solution is 1: 2 in principle, but the actual composition has a range, so if it is in the range of 1: 1.2 to 1: 2. good.

In the present invention, magnetite fine particles are produced by adding an alkali to a mixed aqueous solution of ferrous and ferric ions. It is generally known that addition of an alkali to ferrous ions causes precipitation of hydroxide, but addition of an alkali to a mixed aqueous solution of ferrous salt and ferric salt causes precipitation of magnetite. The alkali used is preferably alkali hydroxide. Specific examples thereof include alkali metal hydroxides such as sodium hydroxide and potassium hydroxide, and alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide. Alkaline substances such as ammonia can also be used. In addition,
The reaction when sodium hydroxide is added to the mixed liquid of ferrous and ferric ions is considered to follow the following formula.

## STR1 ## FeCl ₂ + 2FeCl ₃ + 8NaOH → Fe ₃ O ₄ + 8NaCl + 4H ₂ O The amount of alkali hydroxide is 1.3 equivalents or less with respect to all iron ions. At 1.3 equivalents or more, acicular goethite occurs. The alkali hydroxide is preferably 1.0 equivalent or more. If it is less than 1.0 equivalent, the production efficiency of magnetite is poor.

In the present invention, the pH after addition of alkali
Is required to be 5 to 12, preferably pH 5
-11, and more preferably 7-9. By maintaining the pH after addition of the alkali within the above range and heating and stirring as described below, spherical magnetite particles having an average particle diameter of 0.01 to 0.05 μm can be obtained. Magnetite does not occur on the acidic side of pH 5. Further, when the pH is 12 or more, the generated magnetite becomes plate-like and secondary aggregation easily occurs. Furthermore, as described below, since it is necessary that iron ions are present on the surface of magnetite when the fatty acid is added, it is preferable that the alkali is not excessive. For these reasons, the upper limit of pH is approximately 12, preferably 11, and more preferably 9. Alkali hydroxide is usually added as an aqueous solution so that the reaction is smooth and the solution pH is uniform. The concentration at this time is preferably 1 to 4 mol /
It is about 1 liter, more preferably about 1.5 to 2.5 mol / liter, most preferably about 2 to 2.1 mol / liter. After adding the alkali, the liquid is heated and stirred to allow the reaction to proceed. Liquid temperature 80
It is preferably in the range of -100 ° C. When the liquid temperature is low, the oxidation rate is low and the particle size of the generated magnetite is large. However, even if the liquid temperature is 100 ° C or higher, the oxidation rate is not so different, and from the economical viewpoint, 100 ° C or lower is preferable.

When magnetite particles are generated and grow and fine particles having an average particle size of 0.01 to 0.05 μm are suspended in the liquid, a fatty acid or a fatty acid salt is added thereto. Various methods can be used to confirm whether or not the average particle size has reached the above range, but this value is reached within about 1 hour under the above-mentioned reaction conditions. The reaction time can be shortened or extended by adjusting the reaction conditions. The fatty acid portion of the fatty acid or fatty acid salt is preferably a fatty acid having 12 to 22 carbon atoms. Saturated fatty acids are more preferable from the viewpoint of stability, but unsaturated fatty acids can also be used. Examples of such fatty acids include saturated fatty acids such as lauric acid, myristic acid, stearic acid, palmitic acid and behenic acid, and unsaturated fatty acids such as oleic acid. Examples of metal ions contained in the fatty acid salt include Na and Ca. An iron salt may be used as long as the effect of the present invention is not impaired. Sodium salt is preferable from the viewpoint of solubility and easy availability.

The amount of fatty acid added is preferably at least the amount required to form a monolayer on the surface of magnetite particles. The BET value of the magnetite obtained by the present invention is about ²⁰ to 100 m ² / g, and 1 g of magnetite is used.
It may be about 4.3 × 10 ⁻⁴ mol or more per unit. If the amount is less than this amount, the L value (blackness) is lowered and the effect of the present invention is not exhibited. Further, addition of more fatty acid does not change the effect and may adversely affect the quality of the product. Therefore, the amount is preferably twice or less than the amount required for forming a monomolecular film.

The fatty acid is added by adding an aqueous solution in which a fatty acid is suspended or an aqueous solution in which the above fatty acid salt is dissolved to the liquid in which magnetite is produced and suspended. A preferable concentration is about 0.025 to 0.15 mol / liter. 0.025mol /
If it is less than liter, the production efficiency is poor. If it exceeds 0.15 mol / liter, it becomes impossible to dissolve fatty acids and fatty acid salts. After the fatty acid addition, the suspension is continued, or 50-100
The reaction is allowed to proceed by heating and stirring in the temperature range of ℃ for 0.5 to 3 hours. By such a treatment, the fatty acid reacts with iron ions existing near the surface of the magnetite particles to form an iron salt, and the hydrophobic group is adsorbed on the particle surface with the hydrophobic group facing outward. The presence of the fatty acid or its salt on the surface of the particles interrupts the contact with the air and prevents the change in color due to oxidation. In addition, secondary aggregation of magnetite particles is prevented.

After the treatment with the fatty acid described above, a soluble silicate is added to form a silicic acid film on the surface of the particles coated with the fatty acid or its salt, thereby alleviating the hydrophobicity due to the fatty acid treatment, and glossing. It is possible to obtain a black pigment having a matte feeling due to erasing. Specifically, from 0.025
After adding silicate at a concentration of about 0.15 mol / liter, 5
Continue stirring at 0 to 120 ° C. for about 0.5 to 1 hour. When the temperature is low, the reaction proceeds slowly. On the other hand, if the temperature exceeds 120 ° C, gelation of silicate becomes a problem. By this processing,
The temporal stability is further improved. Examples of water-soluble silicates include sodium and potassium silicates. The addition amount is 0.2 to 0.5 atom% in terms of Si with respect to Fe. If it is less than 0.2 atomic%, no effect of relaxing hydrophobicity is observed. On the other hand, if it exceeds 0.5 atom%, the productivity is lowered and the blackness is lowered.

The magnetite fine particles obtained by the present invention are filtered and washed with water after being subjected to the above treatment, and the temperature is 40 to 130 ° C.
Can be dried for 24 to 48 hours to obtain powder particles. Alternatively, the slurry may be washed with water by decantation, and the water content may be adjusted as necessary before use.

The magnetite produced according to the present invention has an L value of about 4 to 12 as a dry powder. Further, since the particle size is smaller than the wavelength of visible light, there is a sense of transparency. Therefore, it is useful as a black pigment, especially as a black pigment for cosmetics which gives a deep black color. Further, since the particle size is small, it can be used as a magnetic toner for an electrostatic copying machine. In addition, paints, inks, coloring agents for synthetic resins, etc.
It can be used as a pigment in various fields. In addition, when using it for these uses, a component known in the said technical field is added and it is used as a composition. For example, when used as a cosmetic pigment, 0.01% of a preservative such as paraben is used.
˜0.3% is added, and the slurry is dispersed in water or the water in the slurry is replaced with a non-aqueous solvent. Examples of such non-aqueous solvents include hydrocarbon oils such as isoparaffin and silicone oils such as octamethylcyclotetrasiloxane and methylcyclopentanesiloxane.

[0020]

EXAMPLES The present invention will be described more specifically below with reference to examples and comparative examples. The average particle size of the obtained particles,
The following method and conditions were used for measuring the color tone and taking a TEM (transmission electron microscope) photograph. (1) Average particle diameter: determined by the BET method. (2) Measurement of color tone: The color tone of magnetite is measured by using a color computer MS-7-IS-2B (manufactured by Suga Test Instruments Co., Ltd.) for L value, a value and b value of powder, dispersion liquid and dispersion liquid coated paper. Was measured and evaluated. (i) Powder: Magnetite particles were dried at 45 ° C. or lower for 48 hours or more and ground in a mortar. About 5 g of this powder was put into a pellet cell and L value, a
Values and b values were measured. The measurement was performed twice in the static state and the tap-filled state, and the arithmetic average value was used as the measured value. (ii) Dispersion liquid: Magnetite particles, ion-exchanged water and a dispersant were put in a sample bottle and subjected to ultrasonic waves for about 20 minutes while stirring with a spatula. Magnetite solid content is 10%
And Two types of samples were prepared for the purpose of comparison with the case where the dispersant was added later, that is, Dispersion 1 containing 3% of the dispersant (C ₁₇ H ₃₃ COONa) and Dispersion 2 containing no dispersant. . The total amount of dispersion was 50 g each. 20 ml of this dispersion was put in a pellet cell and the L value, a value and b value were measured by a color computer. (iii) Dispersion coated paper: Dispersion 1 of (ii) 0 ml, 20 μm
It was applied to a white paper (quality paper) with a thickness of m. This is 24-4
After drying for 8 hours, L value, a value and b value were measured by a color computer. At the time of measurement, a white paper was placed on the dispersion-coated paper to prevent light transmission. The measurement was performed at two points and the arithmetic mean value was used as the measurement value. (3) TEM photography conditions (i) Sample preparation method: An extremely small amount of magnetite particles and an extremely small amount of linseed oil were placed in a mortar and stirred with a pestle in a certain direction for 10 minutes or more to disperse the magnetite particles. By this operation, the magnetite attached to the tip of the pestle is dispersed to the state of primary particles. Take this on a collodion membrane-attached mesh and wash with toluene to remove linseed oil.
It was used as an EM observation sample. (ii) TEM shooting conditions: Shooting at an acceleration voltage of 200 kV (Hitachi
H-800 transmission electron microscope).

Example 1 1 liter of an aqueous ferrous chloride solution containing 1.0 mol / liter of Fe ^{2+ was} added to Fe ^3+.
The mixed solution added to 1 liter of ferric chloride aqueous solution containing 2.0 mol / liter was added to 4 liter of 2 mol / liter NaOH aqueous solution to adjust the pH to 1
Stirring was continued at 0.43 at a temperature of 90 ° C. for about 1 hour to produce magnetite particles. Then, a 0.1 mol / liter aqueous solution of sodium oleate 2 was added to the suspension containing magnetite.
1 liter was added, and the mixture was stirred at a temperature of 90 ° C. The produced particles were washed with water by a conventional method, filtered off, and dried at 45 ° C. for 118 hours to obtain a black powder (water content 0.62%). BET of the obtained powder
The value was 59.1 m ² / g, and the average particle size calculated from this was 19.5 nm. The color tone of the obtained powder and the powder prepared as a coating liquid according to the conditions shown in Table 1 were measured by the above-described colorimetric method. The results are also shown in Table 1. A TEM photograph of this powder is shown in FIG.

[0022]

[Table 1]

Comparative Example 1 Magnetite particles were produced under substantially the same conditions as in Example 1 except that the fatty acid treatment was not carried out as follows. Fe ²⁺ 1.
1 liter of ferrous chloride aqueous solution containing 0 mol / liter is Fe ³⁺ 2.0 mol
The mixed solution added to 1 liter of an aqueous ferric chloride solution containing 1 / liter was added to 4 liters of an 8 mol / liter NaOH aqueous solution, and the pH was 7.76.
The stirring was continued at a temperature of 90 ° C. for about 1 hour to generate magnetite particles. The produced particles were washed with water by a conventional method, filtered off, and dried at 40 ° C. for 48 hours to obtain a black powder. The BET value of the obtained powder was 75.5 m ² / g, and the average particle size calculated from this was 16.1 nm. The color tone of the obtained powder and that obtained as a coating liquid according to the conditions of Table 2 were measured as described above. The results are also shown in Table 2.
A TEM photograph of this powder is shown in FIG.

[0024]

[Table 2]

Example 2 1 liter of an aqueous ferrous chloride solution containing 1.2 mol / liter of Fe ^{2+ was} added to Fe ^3+.
The mixed solution added to 1 liter of ferric chloride aqueous solution containing 2.0 mol / liter was added to 4 liter of 2.4 mol / liter NaOH aqueous solution to adjust pH.
Stirring was continued for about 1 hour at 9.24 at a temperature of 90 ° C. Then, 2 liters of 0.1 mol / liter sodium oleate aqueous solution was added to the above magnetite-containing suspension, and the temperature was adjusted to 29
Stirred at ° C. The produced particles are washed with water by a conventional method, filtered off, and dried at 45 ° C. for 55 hours to obtain a black powder (water content: 0.20
%) Was obtained. The BET value of the obtained powder was 36.2 m ² / g, and the average particle size calculated from this was 31.8 nm. The color tone of the obtained powder and that obtained as a coating liquid according to the conditions in Table 3 were measured by the above-described colorimetric method.
The results are also shown in Table 3. Further, a TEM photograph of this powder is shown in FIG.

[0026]

[Table 3]

Comparative Example 2 Magnetite particles were produced under substantially the same conditions as in Example 2 except that the fatty acid treatment was not carried out as follows. Fe ²⁺ 1.
1 liter of ferrous chloride aqueous solution containing 0 mol / liter is Fe ³⁺ 2.0 mol
The mixed solution added to 1 liter of ferric chloride aqueous solution containing 1 / liter was added to 4 liters of 2.1 mol / liter NaOH aqueous solution to adjust the pH to 10.38.
Then, stirring was continued at a temperature of 90 ° C. for about 1 hour to generate magnetite particles. The produced particles were washed with water by a conventional method, filtered off, and dried at 40 ° C. for 48 hours to obtain a black powder. The BET value of the obtained powder was 73.1 m ² / g, and the average particle size calculated from this was 16.6 nm. The color tone of the obtained powder and that obtained as a coating liquid according to the conditions of Table 2 were measured as described above. The results are also shown in Table 4. In addition, a TEM photograph of this powder is shown in FIG.

[0028]

[Table 4]

Example 3 1 liter of an aqueous ferrous chloride solution containing 1.0 mol / liter of Fe ^{2+ was} added to Fe ^3+.
The mixed solution added to 1 liter of ferric chloride aqueous solution containing 2.0 mol / liter was added to 4 liter of 2 mol / liter NaOH aqueous solution to adjust the pH to 1
The mixture was stirred at 1.67 and a temperature of 90 ° C. for 1 hour. Subsequently, 2 liters of a 0.1 mol / liter sodium oleate aqueous solution was added to the magnetite-containing suspension at a temperature of 90 ° C.
After stirring for 30 minutes at room temperature, 2 liters of an aqueous solution of sodium disilicate hydrate (SiO ₂ : 52.75%) was added so as to contain 0.3 mol in terms of Si, and the mixture was stirred at a temperature of 90 ° C. The produced particles were washed with water by a conventional method, filtered off, and dried at 45 ° C. for 163 hours to obtain a black powder (water content 0.70%). The BET value of the obtained powder was 74.5 m ² / g, and the average particle size calculated from this was 15.5 nm. The color tone of the obtained powder and that obtained as a coating liquid according to the conditions in Table 5 were measured by the above-described colorimetric method. The results are also shown in Table 5. Further, a TEM photograph of this powder is shown in FIG.

[0030]

[Table 5]

Example 4 1 liter of an aqueous ferrous chloride solution containing 1.0 mol / liter of Fe ^{2+ was} added to Fe ^3+.
The mixed solution added to 1 liter of ferric chloride aqueous solution containing 2.0 mol / liter was added to 4 liter of 2 mol / liter NaOH aqueous solution to adjust pH.
Stirring was continued for about 1 hour at 8.68 and a temperature of 90 ° C. to generate magnetite particles. Then, 2 liters of a 0.1 mol / liter sodium stearate aqueous solution was added to the suspension containing magnetite, and the mixture was stirred at a temperature of 90 ° C. The produced particles were washed with water by a conventional method, filtered off, and dried at 45 ° C. for 79 hours to obtain a black powder (water content 0.24%). BE of the obtained powder
The T value was 39.5 m ² / g, and the average particle size calculated from this was 29.2 nm. The powder obtained and this are shown in Table 6.
The color tone of the coating solution was measured according to the above conditions by the above colorimetric method. The results are also shown in Table 6.

[0032]

[Table 6]

The T of the particles obtained in each of the above Examples and Comparative Examples
As shown in the EM photograph, it can be seen that the particles of Comparative Example are formed by irregularly agglomerating fine particles.
That is, the primary particles constituting each secondary particle can be easily identified, and the size and shape of each primary particle are also extremely non-uniform. On the other hand, the particles of the present invention are primary particles having high sphericity and a uniform size.

Test Example (Aging Change Measurement Test) The magnetite, ion-exchanged water and dispersant obtained in the examples and comparative examples were put in a sample bottle and subjected to ultrasonic waves for about 20 minutes while stirring with a spatula to solidify the magnetite. Dispersant (C ₁₇ H ₃₃ COONa) 3% with a concentration of 10%
% To prepare an aqueous dispersion. Total amount of dispersion is 50g
And 20 ml of this dispersion is put in a pellet cell,
Changes in L value, a value and b value were measured over about 150 hours. From the results shown in FIG. 6, it can be seen that the dispersion liquid using the magnetite particles of the present invention shows good stability over a long period of time.

[0035]

The magnetite particles of the present invention have a particle size of 0.
Since the particle size is 01 μm to 0.05 μm, which is smaller than that of conventional magnetite particles, the dispersibility is good, and since the wavelength is shorter than the wavelength of visible light, the magnetite particles of the present invention have a fatty acid coating. It is resistant to changes with time, such as oxidation, and there is no change in color tone with time. Therefore, it is particularly useful as a black pigment. Further, the magnetite of the present invention can be manufactured relatively easily, and does not require a complicated process as in the past.

[Brief description of drawings]

FIG. 1 shows T showing the particle structure of magnetite particles of the present invention.
EM photograph.

FIG. 2 is a TEM photograph showing a particle structure of magnetite particles not subjected to a fatty acid coating treatment.

FIG. 3 shows the particle structure of magnetite particles of the present invention T
EM photograph.

FIG. 4 is a TEM photograph showing a particle structure of magnetite particles not subjected to a fatty acid coating treatment.

FIG. 5: T showing the particle structure of magnetite particles of the present invention
EM photograph.

FIG. 6 is a graph showing changes in L value with time for dispersions of magnetite particles of the present invention and comparative examples.

Claims (7)

[Claims] 1. Spherical magnetite particles having an average particle size of 0.01 to 0.05 μm, the surface of which is coated with a fatty acid or a salt thereof. 2. The spherical magnetite particles according to claim 1, wherein the particle surface is coated with a fatty acid or a salt thereof, and further coated with a silicon oxide layer. 3. An average particle diameter of 0.01 is obtained by adding an alkali of 1.3 equivalents or less to a mixed aqueous solution of ferrous and ferric ions.
~ 0.05μm magnetite fine particles are generated, and the fatty acid or fatty acid salt is added to the liquid in which the generated magnetite fine particles are suspended and stirred to form a spherical shape with an average particle size of 0.01 to 0.05μm Method for producing magnetite particles. 4. A mixed aqueous solution in which the content ratio of ferrous ions to ferric ions is 1: 1.2 to 1: 2 is used.
The method described in. 5. The method according to claim 3, wherein after adding the alkali, the liquid temperature is heated to 80 to 100 ° C. to generate magnetite fine particles. 6. The fatty acid portion of the fatty acid or fatty acid salt is 1
A linear fatty acid containing 2 to 22 carbons.
5. The method according to any one of 5 above. 7. A solution of a soluble silicate solution is added to a suspension containing spherical magnetite particles coated with a fatty acid or a salt thereof to further form a silicon oxide layer.
7. The method according to any one of 6 to 6.