JP3360351B2 - Method for producing ferromagnetic composite particles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing ferromagnetic composite particles having an iron compound coating layer serving as a shell on fine particles serving as a core.

[0002]

2. Description of the Related Art Conventionally, a liquid crystal display (hereinafter referred to as "LC
D ". )), Spacer particles (hereinafter referred to as LC) are used to regulate the gap between the substrates and keep the thickness of the liquid crystal layer at an appropriate level.
The spacer particles for D are simply referred to as “spacer particles”. ) Is used. In general, such spacer particles should have sufficient strength and heat resistance to withstand the pressure and heating conditions in the LCD substrate assembly process, have a narrow particle size distribution that can uniformly maintain the substrate gap, It does not dissolve and has few impurities such as metal ions (preferably 10 ppm per particle)
It is required that the orientation of the liquid crystal is not disturbed. Also, more recently, from the viewpoint of improving the image quality, particularly by improving the contrast of the color liquid crystal panel,
Although colored spacer particles (hereinafter, colored spacer particles for LCDs are simply referred to as “colored spacer particles”) have been used, it is strongly required that these colored spacer particles have sufficient light-shielding properties. In addition, it is also required that constituent materials such as coloring components do not peel off or fall off during a liquid crystal panel manufacturing process such as liquid crystal injection or during use of the liquid crystal panel. In particular, the colored particles used for the colored spacer particles and the like are obtained by (a) a method of dyeing colorless to white particles, and (ii) suspension polymerization or emulsion polymerization of a monomer containing a colorant to obtain spherical particles. (C) kneading a polymer and a colorant, and then granulating and classifying. However, among the conventional methods for producing colored particles,
In the above method (a), the dye does not sufficiently penetrate into the inside of the particles, and not only a dye having a light color tone and insufficient light-shielding properties can be obtained, but also the dye is eluted or transferred into the liquid crystal component, and In the methods (b) and (c), there is no affinity for the constituents of the base particles, or the poor coloring agent is dispersed at a high concentration, so that the method is often brittle. Only particles can be obtained, and the constituent materials of the colored particles may not only peel off or fall off, but also it is difficult to sufficiently lower the concentration of impurities due to the coloring agent. Therefore, in the conventional method for producing colored particles, there is a problem that colored spacer particles satisfying the above conditions are not efficiently provided.
In recent years, in an immunoassay, particularly an enzyme immunoassay, a bound form in which an antigen and an antibody bound as a result of an antigen-antibody reaction are separated from a free form in which the antigen and the antibody are not bound (that is, B / F separation), a method using magnetic particles has been proposed. As a method for producing the magnetic particles, for example, a method is known in which a coating layer made of iron oxide-based ferrite is formed on core particles made of an organic polymer material by an oxidation reaction of a ferrous salt (for example, a method of producing the magnetic particles). See JP-A-3-115862). However, these magnetic particles have disadvantages such as the ferrite coating layer being easily peeled or falling off, and low stability under long-term storage.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a coating layer which is excellent in strength and heat resistance, hardly causes peeling or falling off of the coating layer, does not dissolve or migrate the coating layer components into various media, and is stable in storage. It is an object of the present invention to provide a method for producing ferromagnetic composite particles having excellent light-shielding properties and exhibiting stable light-shielding properties, ferromagnetism and the like at all times.

[0004]

According to the present invention, there are provided the following objects: (1) To adjust the pH of a reaction system to 1.4 to 4 in a dispersion in which fine particles to be cores are uniformly dispersed in an aqueous medium. While controlling, the hydrolyzable iron salt is hydrolyzed in a solution state to form an iron compound coating layer serving as a shell on the fine particles, and if necessary, further (2-1) oxidation treatment and / or reduction treatment. At least a portion of the process and / or <br/> into a ferromagnetic iron oxide (2-2) ferromagnetic oxides in the outer layer of iron compound-coated layer of iron compounds iron compound coating layer by a row Ukoto A method for producing ferromagnetic composite particles, characterized by performing a step of forming a coating layer containing iron.

Hereinafter, the present invention will be described in detail. This will clarify the objects, configurations and effects of the present invention. The material of the fine particles serving as the core of the composite particles in the present invention (hereinafter, referred to as “core fine particles”) is not particularly limited, and examples thereof include glass, silica, aluminum, titanium, iron, nickel, copper, and silver. Inorganic materials such as gold, stainless steel, iron oxide, ferrite, and carbon black; (co) polymers of olefins such as ethylene and propylene; (co) polymers of aromatic vinyl compounds such as styrene and divinylbenzene; vinyl acetate (Co) polymers of vinyl esters such as acrylonitrile; (co) polymers of vinyl cyanide compounds such as acrylonitrile; (co) polymers of (meth) acrylate esters such as methyl methacrylate; vinyl chloride, tetrafluoroethylene, etc. (Co) polymers of vinyl halide compounds; polyacetals; polycarbonates; polyesters; Resins; unsaturated polyester; polyarylates; polysulfides; polysulfones;
Polyamide (for example, nylon-6, nylon-12
Polyimide, polysiloxane, epoxy resin, phenol resin, urea resin, melamine resin, benzoguanamine resin, cellulose, starch, ionomer and the like. When the core fine particles are an organic polymer material, they may have a crosslinked structure. These core fine particles may be granulated and classified after kneading and mixing two or more types of materials in advance, or may be a mixture of two or more types of particles composed of different materials. The color tone of these core fine particles is not particularly limited.

[0006] The method for preparing the core fine particles is not particularly limited, but includes, for example, tumbling granulation, fluidized bed granulation,
Agitation granulation, crushing / crushing granulation, compression granulation, extrusion granulation, melt granulation, mixed granulation, spray cooling granulation, spray drying granulation, sedimentation
Physical granulation methods such as precipitation granulation, freeze-drying granulation, suspension aggregation granulation, and drop cooling granulation; chemical granulation methods such as emulsion polymerization, suspension polymerization, precipitation polymerization, etc. The method may be appropriately selected according to the above, granulated, and classified. When the core fine particles are commercially available, they can also be used.

In the present invention, an iron compound coating layer serving as a shell is formed on the core fine particles described above, and if necessary,
A step of forming a coating layer comprising an outer layer in the ferromagnetic iron oxide at least a portion is converted to ferromagnetic iron oxide process and / or iron compound-coated layer of Flip further iron compound iron compound coating layer By doing so, ferromagnetic composite particles are produced.

The treatment process for providing the iron compound coating layer on the core fine particles is basically performed in a uniform dispersion of the core fine particles in a hydrolyzable iron salt (hereinafter simply referred to as “iron salt”). Is hydrolyzed in a solution state. This hydrolysis proceeds easily at room temperature or under heating. As a result, a coating layer composed of an iron compound such as iron oxide or iron hydroxide is formed on the core fine particles, and has a core / shell composite structure.

The iron salts include, for example, FeCl ₃ , FeCl ₂
Chlorides such as; nitrates such as Fe (NO ₃ ) ₃ and Fe (NO ₃ ) ₂ ; Fe ₂ (SO ₄ )
₃ , sulfates such as FeSO _{4 and the} like. These iron salts can be used alone or in combination of two or more.

The concentration of the iron salt in the hydrolysis reaction is usually 0.1 mmol / liter of reaction medium or more, particularly preferably 1 mmol / liter of reaction medium or more.
The upper limit concentration of the iron salt is a limit at which the iron salt can be completely dissolved in the reaction medium at room temperature or under heating and a uniform hydrolysis reaction can occur, and varies depending on the type of the iron salt and the reaction medium.
Usually, it is less than 1000 mmol / l liter of reaction medium.

The concentration of the core fine particles in the hydrolysis reaction is usually 0.01 g / liter or more of the reaction medium.
Particularly preferably, the amount is 0.5 g / l liter or more of the reaction medium. The upper limit concentration of the core fine particles is a limit capable of maintaining a uniform dispersion state of the core fine particles and the obtained composite particles,
Usually, it is 300 g / liter or less of the reaction medium.

As a reaction medium in the hydrolysis reaction,
Water is the main component, and if necessary, an organic solvent such as an alcohol such as methanol, ethanol, n-propanol, isopropanol, n-butanol, and t-butanol is used in combination. A particularly preferred reaction medium is water alone or a mixed solvent of water and a saturated alcohol. When alcohols are used in combination with water, the amount of each alcohol used is appropriately adjusted in consideration of the miscibility with water, the solubility of each iron salt in the mixed solvent, and the like.

At the time of the hydrolysis reaction, it is important to maintain a uniform dispersion state of the core fine particles and the obtained composite particles in the iron salt solution. For example, when the dispersion state of the core fine particles is poor, and several to several hundreds of the fine particles are aggregated, they are directly coated with the iron compound, so that uniform composite particles cannot be obtained or obtained. Even if the resulting composite particles cannot associate and aggregate in the reaction medium and cannot be redispersed, uniform composite particles cannot be obtained.

In order to maintain a uniform dispersion state of the core fine particles and the composite particles, a polymer compound soluble in the reaction medium containing an iron salt, a surfactant and the like are added to the reaction medium as a dispersion stabilizer. Is preferred.

[0015] As such a dispersion stabilizer, such as carboxymethyl sodium salt of methyl cellulose, hydroxyethyl cellulose, sodium Polo acrylate, Po Li et <br/>Chiren'okishido, polyvinyl alcohol, high molecular compounds such as polyvinylpyrrolidone; stearate sodium,
Examples of the surfactant include sodium dodecylbenzenesulfonate, sodium lauryl sulfate, sodium diphenylether disulfonate, and sodium dialkyl succinate sulfonate. Preferred dispersion stabilizers are polyvinylpyrrolidone, sodium dodecylbenzenesulfonate and the like. These dispersion stabilizers can be used alone or in combination of two or more of the polymer compound and the surfactant, and the polymer compound and the surfactant can be used in combination.

The amount of the dispersion stabilizer used is 100
The amount is preferably at least 1 part by weight, more preferably 3 to 300 parts by weight, particularly preferably 5 to 250 parts by weight with respect to parts by weight.

There are basically two mechanisms for forming an iron compound coating layer on the core fine particles by the hydrolysis reaction of the iron salt in the present invention. One of the mechanisms is (i) a mechanism in which a hydrolyzed iron ion and / or a complex generated by hydrolysis are adsorbed on the surface of the core fine particles to form a coating layer. The other is that (ii) through the hydrolysis step, nuclei composed of extremely small fine particles of an iron compound are formed in the initial stage of the reaction, and these fine particles are adsorbed on the surface of the core fine particles by a hetero-aggregation mechanism. The mechanism is that the coating layer grows. In the latter mechanism, the thickness of the coating layer can be controlled by adjusting the number and the particle size of the iron compound fine particles adsorbed on the core fine particles.

Of the above two mechanisms, the mechanism via (ii) the hetero-aggregation mechanism becomes dominant when the core fine particles have a charge. For example, when the hydrolysis reaction of the iron salt occurs at a pH equal to or higher than the isoelectric point of the generated fine particles of the iron compound, the fine particles are efficiently adsorbed by imparting a positive charge to the core fine particles. In addition, the hydrolysis reaction of the iron salt has a pH below the isoelectric point of the iron compound fine particles.
In the case where the microparticles occur, the core microparticles are given a negative charge, whereby the microparticles are adsorbed efficiently.

In the case where the mechanism (ii) is adopted in the present invention, it is preferable to introduce a sulfonic acid group, a carboxyl group, or the like to the surface of the core fine particles so as to have a negative charge.
Thereby, the dispersibility of the core fine particles is also improved.

When the iron compound coating layer is formed, the growth of the coating layer does not occur smoothly even if the hydrolysis rate of the iron salt is too fast or too slow. As a result, gaps and irregularities are formed in the iron compound coating layer. Or the iron compound cannot grow on the coating layer,
May be released as small particles. Therefore, in the present invention, in order to adjust the rate of the hydrolysis reaction of the iron salt, the pH of the reaction system at the time of the hydrolysis is 1.4 to 4, preferably 1.6 to 3, and particularly preferably 1.8 to 2. .5.

The pH of the reaction system during the hydrolysis of the iron salt is adjusted to 1.4 to 4 by adding an acid and / or an alkali.
The acid used at this time can be, for example, an inorganic acid or an organic acid such as hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, and acetic acid.As the alkali, for example, potassium hydroxide, Examples include hydroxides such as sodium hydroxide and calcium hydroxide and ammonia. In the hydrolysis reaction of the iron salt in the present invention, hydrochloric acid and nitric acid are preferable as the acid, and sodium hydroxide and ammonia are preferable as the alkali. These acids and alkalis can be used alone or in combination of two or more, and if necessary, the pH is finely adjusted by using both the acid and the alkali in combination.

When these acids or alkalis are added to the reaction medium, the amount of the acid or alkali used is usually 1 mol / l liter of the reaction medium, preferably 0.5 mol / l liter of the reaction medium, especially Preferably 0.005-
0.2 mol / l of reaction medium.

The reaction medium containing the iron salt usually shows an acidity, and the pH is further lowered by the hydrolysis reaction. Therefore, (a) a method in which urea and / or formamide is added to a reaction medium, and iron salts and / or acids are sequentially or continuously added so as to offset a rise in pH due to their thermal decomposition, b) collectively adding iron salts, sequentially or continuously, a method of offsetting the pH decrease due to hydrolysis of iron salts by sequential or continuous addition of alkali, (c) by a combination of these methods, etc. It is preferred to maintain the pH during hydrolysis substantially constant. As the acid and alkali added sequentially or continuously as described above,
Examples thereof include those used for adjusting the pH of the reaction system described above.

The normality of the acid or alkali added sequentially or continuously is usually 0.01 to 10 normal, preferably 0.05 to 1 normal.

The amount of urea and / or formamide used is usually 1000 g / l liter of reaction medium, preferably 200 g / l liter of reaction medium, particularly preferably 10-100 g / l liter of reaction medium. is there.

The thickness of the iron compound coating layer formed by the hydrolysis reaction of the iron salt can be appropriately adjusted by controlling the amount of the iron salt used, the reaction time and the like.

The thickness and density of the iron compound coating layer can be increased by repeating the hydrolysis reaction of the iron salt using the composite particles having the iron compound coating layer as seed particles.

According to the type and composition of the iron salt to be used, for example, α-
Fe ₂ O ₃ (reddish brown), γ-Fe ₂ O ₃ (reddish brown to black), Fe ₃ O
₄ (black), FeO (black), Fe (OH) ₃ (reddish brown), Fe (O
H) ₂ (white to pale green), α-FeO (OH) (reddish brown), γ-Fe
Iron compounds such as iron oxide such as O (OH) (yellow) and iron hydroxide,
Alternatively, a mixture thereof is formed, and a uniform coating layer is formed on the surface of the core fine particles.

The iron compound coating layer thus formed may show magnetism depending on the type of the iron compound. For example, γ-Fe ₂ O ₃ and Fe ₃ O ₄ show ferromagnetism,
Composite particles containing γ-Fe ₂ O ₃ and / or Fe ₃ O ₄ in the iron compound coating layer can be used as ferromagnetic composite particles as they are.

The above-mentioned process of forming an iron compound coating layer on core fine particles by hydrolysis of an iron salt is highly efficient and easy to carry out with high efficiency in producing desired composite particles, and is very industrially efficient. be able to.

In the present invention, when the iron compound coating layer formed by the hydrolysis reaction of the iron salt does not contain a ferromagnetic iron oxide, a part or all of the iron compound in the iron compound coating layer may be ferromagnetic. Ferromagnetic composite particles can be obtained by performing the step of converting to iron oxide and / or the step of forming a coating layer containing ferromagnetic iron oxide on the outer layer of the iron compound coating layer. Further, these treatments may be performed when the iron compound coating layer partially contains ferromagnetic iron oxide.

In the step of converting an iron compound other than ferromagnetic iron oxide to ferromagnetic iron oxide and / or the step of forming a coating layer containing ferromagnetic iron oxide on the outer layer of the iron compound coating layer.
That the treatment method, for example, and (d) oxidizing or reducing process by heating the composite particles, (e) method of performing a combination of said oxidation treatment and reduction treatment, the dispersion of (f) the composite particles And a method of hydrolyzing a hydrolyzable ferrous salt (hereinafter simply referred to as "ferrous salt").

In these treatments, the treatment conditions vary depending on the type and composition of the iron compound in the iron compound coating layer. For example, when the iron compound coating layer is made of FeO or Fe (OH) ₂ , the composite particles are heated at, for example, 50 to 400 ° C. and gradually oxidized while controlling the oxygen partial pressure in the atmosphere. FeO or Fe (OH) ₂ can be converted to γ-Fe ₂ O ₃ via Fe ₃ O ₄ .

Further, the iron compound coating layer is made of ferric oxide (α-
Fe ₂ O ₃ or γ-Fe ₂ O ₃ ) or iron oxide hydroxide [α-FeO (O
H) or γ-FeO (OH)]], the composite particles are placed in a hydrogen atmosphere at an appropriate temperature, for example, 150 to 40 ° C.
By performing the reduction treatment by heating to 0 ° C., ferric dioxide and iron oxide hydroxide can be converted to Fe ₃ O ₄ .

When the iron compound coating layer is made of FeO or iron hydroxide [Fe (OH) ₃ or Fe (OH) ₂ ], these iron compounds are converted to α-Fe ₂ O _{3 as described above.} After the oxidation, α-Fe ₂ O ₃ can be reduced and converted to Fe ₃ O ₄ .

If desired, ferric oxide or iron oxide hydroxide is reduced as described above, converted to Fe ₃ O ₄ , and then oxidized by heating to 150 to 400 ° C. in an air atmosphere. , It can be converted back to γ-Fe ₂ O ₃ .

Further, the hydrolysis of the ferrous salt in the dispersion liquid of the composite particles is carried out in the state of the solution of the ferrous salt when the iron compound coating layer is formed on the core fine particles. The reaction can be carried out in the same manner as in the reaction, but in this case, it is not always necessary to control the pH.

The hydrolysis of the ferrous salt in the dispersion liquid of such composite particles is caused by the fact that the iron compound coating layer of the composite particles
-Fe ₂ O ₃ , Fe (OH) ₃ , is applied when containing a ferric compound such as iron oxide hydroxide, these ferric compounds are reduced simultaneously with the hydrolysis of the ferrous salt At the same time, the ferrous salt is oxidized at the same time as the hydrolysis, and is converted to Fe ₃ O ₄ .

Examples of the ferrous salt include FeCl ₂ , Fe
(NO ₃ ) ₂ , FeSO _{4 and the} like.

The thus obtained ferromagnetic composite particles have a core / shell composite structure in which a coating layer containing ferromagnetic iron oxide is formed on core fine particles. In the ferromagnetic iron oxide-containing coating layer, (g) contained in the ferromagnetic iron oxide-containing coating layer according to the reaction conditions for converting the iron compound to the ferromagnetic iron compound, the properties of the iron compound coating layer of the composite particles, and the like. (H) when all of the iron compounds contained in the coating layer are converted to ferromagnetic iron compounds, (i) when the iron compounds are partially converted to ferromagnetic iron compounds at random, When only the iron compound present in the outer layer portion of the iron compound coating layer of the particles is partially or entirely converted to a ferromagnetic iron compound, (j) the iron compound coating layer of the composite particle has a ferromagnetic iron compound-containing coating. If further covered by a layer, or (k)
These may be mixed.

The process for producing ferromagnetic composite particles from the composite particles as described above has high production efficiency of desired ferromagnetic composite particles, and can be industrially extremely efficient and easily carried out.

The average particle size of the ferromagnetic composite particles obtained by the present invention can be appropriately set by adjusting the average particle size of the core fine particles, the thickness of the iron compound coating layer, and the like. 0.04 to 50 μm, preferably 0.04
To 40 μm, particularly preferably 0.1 to 10 μm.
Note that these particles are usually obtained in a spherical shape or a substantially spherical shape, but the average particle size in a case where the particles are not necessarily symmetrical is assumed to be a short axis diameter.

The ratio of the particle diameter of the core fine particles to the particle diameter of the ferromagnetic composite particles (hereinafter referred to as the “inner diameter ratio”) is usually 0.3 to 0.99, preferably 0.5. To 0.99, particularly preferably 0.7 to 0.99. When the inner diameter ratio is out of the range of 0.3 to 0.99, the thickness of the iron compound coating layer is too thick or too thin, so that it may be difficult to efficiently produce particles having good characteristics.

The ferromagnetic composite particles obtained by the present invention include:
For example, even when dispersed in water and exposed to ultrasonic waves, the iron compound coating layer no longer peels or falls off from the core fine particles, and the iron compound coating layer components do not elute or migrate into various media, In addition, it has excellent storage stability, and can always exhibit stable light-shielding properties, magnetism, and the like.
In addition, since the intrinsic properties of the core fine particles are not impaired at all, the strength and heat resistance of the whole particles can be made as excellent as those of the core fine particles.

The ferromagnetic composite particles obtained by the present invention include:
It can be suitably used as an electronic material, a magnetic material, an optical material, a catalyst, a spacer particle, a column filler, a carrier for a developer, a carrier for a diagnostic agent, a masking material, and the like.

[0046]

Next, the present invention will be described more specifically with reference to examples. However, the present invention, unless it exceeds the gist,
The embodiment is not limited in any way. Reference Example 1 A mixture of 100 g of crosslinked divinylbenzene polymer particles having an average particle size of 5.0 μm and 800 ml of concentrated sulfuric acid was prepared.
After stirring at room temperature for 12 hours, the mixture was washed to obtain sulfonated crosslinked particles. The sulfonated crosslinked particles were prepared at a concentration of 15.7.
642 ml of an aqueous dispersion dispersed in a weight%, 800 ml of a 3% by weight aqueous solution of polyvinylpyrrolidone,
A mixture of 120 ml of normal hydrochloric acid and 1.8 liter of distilled water was sonicated for 2 minutes and then heated to 95 ° C. Next, a solution prepared by dissolving 1.08 g of ferric chloride hexahydrate and 24.00 g of urea in 80 ml of distilled water was added all at once, and one hour and 40 minutes later, urea 12
A solution of 0.00 g dissolved in 160 ml of distilled water was added all at once. One minute later, a solution prepared by dissolving 120.00 g of ferric chloride hexahydrate in 400 ml of distilled water was added over 4 hours and 30 minutes while controlling the pH of the reaction system to 2. Then, after heating for 1 hour, the reaction solution was added to 4 liters of ice-cold water to stop the reaction, and filtered. After the obtained particles were washed with water, they were dried under reduced pressure at 60 ° C. for 12 hours. Observation of these particles with a scanning electron microscope revealed that they were red-brown, spherical composite particles having an average particle size of 5.3 μm, an inner diameter ratio of 0.94, and a uniform coating layer. When the composite particles were analyzed by X-ray diffraction and thermogravimetric analysis, it was confirmed that the core was composed of crosslinked divinylbenzene polymer particles and the shell was composed of γ-FeO (OH). This composite particle is referred to as composite particle A.

Example 1 100 g of the composite particles A was heated from room temperature to 320 ° C. in a hydrogen atmosphere at a rate of 10 ° C./min.
C. for 1 hour and then cooled to room temperature at a rate of 10 ° C./min. Observation of the obtained particles with a scanning electron microscope showed that the average particle size was 5.2 μm and the inner diameter ratio was 0.9.
No. 6 were black, spherical composite particles having a uniform coating layer. When the composite particles were analyzed in the same manner as described above, it was confirmed that the core was ferromagnetic composite particles composed of crosslinked divinylbenzene polymer particles and the shell composed of magnetite (Fe ₃ O ₄ ).

Example 2 A mixture of 100 g of the composite particles A, 10 g of ferrous chloride tetrahydrate and 200 ml of a 3% by weight aqueous solution of polyvinylpyrrolidone was heated to 95 ° C. and stirred at 95 ° C. for 6 hours. Next, the reaction solution was added to 1 liter of ice-cold water to stop the reaction, followed by filtration. The obtained particles were washed with water and dried under reduced pressure at 60 ° C. for 12 hours. Observation of these particles with a scanning electron microscope showed that the average particle size was 5.3 μm,
It was a black, spherical composite particle having an inner diameter ratio of 0.94 and a uniform coating layer. Also, the composite particles were prepared in Example 1.
As a result, it was confirmed that the core was a ferromagnetic composite particle composed of crosslinked divinylbenzene polymer particles and the shell composed of magnetite (Fe ₃ O ₄ ). The ferromagnetic composite particles are referred to as composite particles B.

Example 3 100 g of the composite particles B was heated from room temperature to 250 ° C. in an air atmosphere at a rate of 10 ° C./min.
C. for 2 hours and then cooled to room temperature at a rate of 10 ° C./min. Observation of the obtained particles with a scanning electron microscope showed that the average particle size was 5.2 μm and the inner diameter ratio was 0.9.
6 was a brown, spherical composite particle having a uniform coating layer. When the composite particles were analyzed in the same manner as in Example 1, it was confirmed that the core was ferromagnetic composite particles composed of crosslinked divinylbenzene polymer particles and the shell composed of maghemite (γ-Fe ₂ O ₃ ).

[0050]

According to the present invention, the coating layer is excellent in strength and heat resistance, hardly causes peeling or falling off of the coating layer, and the iron compound coating layer component does not elute or migrate into various media and is stable in storage. Ferromagnetic composite particles having excellent light-shielding properties and constantly exhibiting stable light-shielding performance and ferromagnetism can be industrially extremely efficiently and easily produced.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-2-1307 (JP, A) JP-A-4-149237 (JP, A) European Patent Application Publication 462388 (EP, A1) (58) Field (Int.Cl. ⁷ , DB name) B01J 13/02

Claims (1)

(57) [Claims] (1) In a dispersion liquid in which fine particles to be cores are uniformly dispersed in an aqueous medium, the pH of the reaction system is controlled to 1.4 to 4 while the hydrolyzable iron salt is kept in a solution state. It is hydrolyzed to form an iron compound-coated layer as a shell on the microparticles, optionally further (2-1) oxidation and /
Engineering be converted to ferromagnetic iron oxide at least a portion of, or iron compounds iron compound coating layer by a reduction process line Ukoto
And / or (2-2) performing a step of forming a coating layer containing ferromagnetic iron oxide on the outer layer of the iron compound coating layer .
A method for producing ferromagnetic composite particles, comprising: