JPH06304470A - Preparation of composite particle - Google Patents

Abstract

PURPOSE:To prepare efficiently and easily a composite particle which exhibits excellent strength and heat resistance and has a coating layer being hardly released off or fallen down and wherein components of the coating layer are not dissolved out or bleed out into various media and which exhibits excellent storage stability and always exhibits stable light screening properties and ferromagnetic properties. CONSTITUTION:A method for preparing a composite particle passes through a process wherein a hydrolyzable iron salt is hydrolyzed under a solution condition at pH1.4-4 in an aqueous dispersion of fine particles to be a core to form a coating layer of an iron compd. to be a shell on the fine particle. When the iron compd. coating layer does not contain a ferromagnetic iron oxide, the composite particle is furthermore oxidatively and/or reductively treated to convert a part of the iron compd. in the iron compd. coating layer to a ferromagnetic iron oxide or a coating layer contg. a ferromagnetic iron oxide is formed on the outer layer of the iron compd. coating layer to make a ferromagnetic composite particle.

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 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 particles) are used to regulate the substrate gap and to keep the thickness of the liquid crystal layer appropriate.
The spacer particles for D are simply referred to as "spacer particles". ) Is used. Generally, such spacer particles 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 capable of uniformly maintaining the substrate gap, and have a liquid crystal component. It does not dissolve and contains few impurities such as metal ions (preferably 10 ppm per particle)
Below), it is required not to disturb the alignment of the liquid crystal. In addition, more recently, especially from the viewpoint of improving the image quality by improving the contrast of the color liquid crystal panel,
Colored spacer particles (hereinafter, the colored spacer particles for LCD are simply referred to as “colored spacer particles”) have come to be used, but it is strongly demanded that these colored spacer particles have sufficient light shielding properties. In addition, it is also necessary 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 (a) a method of dyeing colorless to white particles, and (b) a spherical particle obtained by suspension polymerization or emulsion polymerization of a monomer containing a colorant. , A method of classifying, a method of granulating and classifying after kneading the (c) polymer and a colorant, and the like. However, among the conventional methods for producing colored particles,
According to the method (a), not only the dye does not sufficiently penetrate into the inside of the particles, but the color tone is thin and the light-shielding property is insufficient. However, the methods (b) and (c) are often fragile because they have no affinity for the constituents of the base particles or a poor colorant is dispersed in a high concentration. Only particles can be obtained, and the constituent material of the colored particles may be peeled off or fall off, and it is difficult to sufficiently reduce the concentration of impurities due to the colorant. Therefore, the conventional method for producing colored particles has a problem that colored spacer particles satisfying the above conditions (1) to (3) are not efficiently provided.
In recent years, in immunoassays, particularly enzyme immunoassays, a bound form in which an antigen and an antibody are bound as a result of an antigen-antibody reaction and a free form in which the antigen and the antibody are not bound are separated (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 a core particle made of an organic polymer material by an oxidation reaction of a ferrous salt (for example, a special method). See Kaihei 3-1155862). However, these magnetic particles have drawbacks such that the ferrite coating layer is easily peeled off or fallen off, and the stability under long-term storage is low.

[0003]

The object of the present invention is excellent in strength and heat resistance, the coating layer does not easily peel or fall off, the components of the coating layer do not elute or migrate into various media, and the storage stability is stable. An object of the present invention is to provide a method for producing composite particles having excellent properties and always exhibiting stable light-shielding properties and ferromagnetism.

[0004]

According to the present invention, the above-mentioned object is to control the pH of the reaction system to 1.4 to 4 in a dispersion liquid in which core fine particles are uniformly dispersed in an aqueous medium. , A hydrolyzable iron salt is hydrolyzed in a solution state to form an iron compound coating layer serving as a shell on the fine particles, or if necessary, further subjected to an oxidation treatment and / or a reduction treatment to obtain the iron compound. At least a part of the iron compound in the coating layer is converted into ferromagnetic iron oxide, and / or a coating layer containing ferromagnetic iron oxide is formed on the outer layer of the iron compound coating layer. The manufacturing method of.

The present invention will be described in detail below. This will clarify the objects, configurations and effects of the present invention. The material of the fine particles (hereinafter, referred to as “core fine particles”) to be the core of the composite particles in the present invention is not particularly limited, but for example, glass, silica, aluminum, titanium, iron, nickel, copper, 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) acrylic acid esters such as methyl methacrylate; vinyl chloride, tetrafluoroethylene, etc. (Co) polymer of halogenated vinyl compound; polyacetal; polycarbonate; polyester; alk Resins; unsaturated polyester; polyarylates; polysulfides; polysulfones;
Polyamide (eg nylon-6, nylon-12
Etc.); 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 particles obtained by kneading and mixing two or more kinds of materials in advance and then granulating and classifying the particles, or may be a mixture of two or more kinds of particles made of different materials. The color tone of these core particles is not particularly limited.

The method for preparing the core fine particles is not particularly limited, but for example, rolling granulation, fluidized bed granulation,
Agitation granulation, crushing / pulverization granulation, compression granulation, extrusion granulation, melt granulation, mixed granulation, spray cooling granulation, spray drying granulation, precipitation
Physical granulation methods such as precipitation granulation, freeze-drying granulation, suspension aggregation granulation, drop cooling granulation; chemical granulation methods such as emulsion polymerization, suspension polymerization, precipitation polymerization, etc. A method of appropriately selecting, granulating and classifying can be mentioned. In addition, when the core fine particles are commercially available, they can be used.

In the present invention, first, an iron compound coating layer serving as a shell is formed on the above core fine particles, or at least a part of the iron compound in the iron compound coating layer is converted into ferromagnetic iron oxide. Composite particles are produced by converting and / or forming a coating layer containing ferromagnetic iron oxide on the outer layer of the iron compound coating layer.

The treatment process for providing the iron compound coating layer on the core fine particles is basically a hydrolyzable iron salt (hereinafter simply referred to as "iron salt") in a uniform dispersion liquid of the core fine particles. Is hydrolyzed in solution. This hydrolysis easily proceeds at room temperature or under heating. As a result, a coating layer made 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.

Examples of the iron salt include FeCl ₃ and FeCl ₂
Such as chlorides; 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 can be mentioned. These iron salts may be used alone or in admixture 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 it varies depending on the type of the iron salt or the reaction medium.
Usually, it is 1000 mmol / liter or less of the reaction medium.

The concentration of the core fine particles in the hydrolysis reaction is usually 0.01 g / liter of the reaction medium or more,
It is particularly preferably 0.5 g / liter of reaction medium or more. The upper limit concentration of the core fine particles is a limit that can maintain a uniform dispersion state of the core fine particles and the resulting composite particles,
Usually, it is 300 g / liter of reaction medium or less.

The reaction medium in the hydrolysis reaction is
Water is the main component, and if necessary, an organic solvent such as alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, t-butanol, etc. is used together. 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 miscibility with water, solubility of each iron salt in the mixed solvent, and the like.

In the hydrolysis reaction, it is important to maintain the dispersion state of the core fine particles and the resulting 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 agglomerated, they are coated with the iron compound as they are, and uniform composite particles cannot be obtained. Even if the composite particles are associated with each other, aggregated in the reaction medium, and cannot be redispersed, uniform composite particles cannot be obtained.

In order to maintain the uniformly dispersed 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. It is preferable.

Examples of such dispersion stabilizers include polymeric compounds such as sodium salt of carboxymethyl cellulose, hydroxyethyl cellulose, sodium poloacrylate, poloethylene oxide, polyvinyl alcohol and polyvinylpyrrolidone; sodium stearate,
Surfactants such as sodium dodecylbenzene sulfonate, sodium lauryl sulfate, sodium diphenyl ether disulfonate, sodium succinate dialkyl ester sulfonate can be mentioned. Preferred dispersion stabilizers are polyvinylpyrrolidone, sodium dodecylbenzene sulfonate and the like. These dispersion stabilizers can be used either individually or as a mixture of two or more of the polymer compound and the surfactant, or the polymer compound and the surfactant can be used in combination.

The amount of the dispersion stabilizer used is 100
It is preferably 1 part by weight or more, more preferably 3 to 300 parts by weight, and particularly preferably 5 to 250 parts by weight, based on parts by weight.

There are basically two mechanisms by which the iron compound coating layer is formed on the core fine particles by the hydrolysis reaction of the iron salt in the present invention. One of them is (i) a mechanism in which a hydrolyzed iron ion and / or a complex produced by hydrolysis is adsorbed on the surface of the core fine particles to form a coating layer. The other one is that (ii) after the hydrolysis step, nuclei consisting of very small particles of iron compounds are formed in the initial stage of the reaction, and these fine particles are adsorbed on the surface of the core fine particles by the heteroaggregation mechanism. The mechanism is that the coating layer grows. With this 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.

Among the above two mechanisms, the mechanism (ii) that goes through the hetero aggregation mechanism becomes dominant when the core fine particles have an electric 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 produced iron compound fine particles, the fine particles are efficiently adsorbed by giving the core fine particles a positive charge. 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 of 2, the fine particles are efficiently adsorbed by giving the core fine particles a negative charge.

When the mechanism (ii) is adopted in the present invention, it is preferable to introduce a sulfonic acid group, a carboxyl group or the like on the surface of the core fine particles to give a negative charge,
This also improves the dispersibility of the core fine particles.

When the iron compound coating layer is formed, if the hydrolysis rate of the iron salt is too fast or too slow, the growth of the coating layer does not occur smoothly, and as a result, the iron compound coating layer has gaps and irregularities. Or even the iron compound could not grow in the coating layer,
It may be released as small particles. Therefore, in the present invention, in order to control the rate of the hydrolysis reaction of the iron salt, the pH of the reaction system during hydrolysis is 1.4 to 4, preferably 1.6 to 3, and particularly preferably 1.8 to 2. Control to .5.

The pH of the reaction system during the hydrolysis of the iron salt is 1.4 to 4 by adding an acid and / or an alkali.
The acid used in this case may be an inorganic acid or an organic acid such as hydrochloric acid, sulfuric acid, nitric acid, oxalic acid or acetic acid, and the alkali may be potassium hydroxide, for example. Mention may be made of hydroxides such as sodium hydroxide, 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 admixture of two or more, and if necessary, both the acid and the alkali are used in combination to finely adjust the pH.

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

The reaction medium containing the iron salt is usually acidic, and the pH is further lowered by the hydrolysis reaction. Therefore, (a) a method in which urea and / or formamide are added to the reaction medium in advance, and an iron salt and / or an acid is added sequentially or continuously so as to offset the increase in pH due to their thermal decomposition, b) a method in which iron salts are added all at once, sequentially or continuously, and the pH decrease due to hydrolysis of the iron salts is offset by sequential addition or continuous addition of alkali, (c) by a combination of these methods, It is preferred to maintain the pH during hydrolysis substantially constant. Thus, as the acid and the alkali that are sequentially added or continuously added,
The thing used for pH adjustment of the reaction system mentioned above can be mentioned.

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

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

Depending on the kind and composition of the iron salt 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 light green), α-FeO (OH) (reddish brown), γ-Fe
Iron oxides such as O (OH) (yellow), iron compounds such as iron hydroxide,
Alternatively, a mixture of these is produced to form a uniform coating layer on the surface of the core fine particles.

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

The above-mentioned process of forming the iron compound coating layer on the core fine particles by the hydrolysis of the iron salt is highly efficient in producing the desired composite particles, is safe, and is industrially extremely efficient and easy to carry out. 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 ferromagnetic iron oxide, part or all of the iron compound in the iron compound coating layer is ferromagnetic. Ferromagnetic composite particles can be obtained by converting to iron oxide and / or by forming a coating layer containing ferromagnetic iron oxide on the outer layer of the iron compound coating layer. Further, these treatments may be carried out when the iron compound coating layer partially contains ferromagnetic iron oxide.

In order to convert iron compounds other than ferromagnetic iron oxide into ferromagnetic iron oxide and / or to form a coating layer containing ferromagnetic iron oxide on the outer layer of the iron compound coating layer,
The composite particles are subjected to an oxidation treatment and / or a reduction treatment. Examples of such a treatment method include (d) a method of heating the composite particles to perform an oxidation treatment or a reduction treatment, and (e) the oxidation treatment and the reduction treatment. Examples of the method include a combination method, (f) a method of hydrolyzing a hydrolyzable ferrous iron salt (hereinafter, simply referred to as “ferrous iron salt”) in a dispersion liquid of composite particles, and the like.

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 to, 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 .

In addition, the iron compound coating layer is ferric dioxide (α-
Fe ₂ O ₃ or γ-Fe ₂ O ₃ ) and 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, 150-40.
By heating to 0 ° C. and performing reduction treatment, ferric oxide 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 mixed with α-Fe ₂ O _{3 as described above.} After oxidation, it can be converted by reduction of α-Fe ₂ O ₃ to Fe ₃ O _4.

If desired, ferric dioxide or iron oxide hydroxide may be reduced as described above to be converted into Fe ₃ O ₄ and then heated in an air atmosphere, for example, at 150 to 400 ° C. for oxidation. Can be converted again into γ-Fe ₂ O ₃ .

Further, the hydrolysis of the ferrous salt in the dispersion liquid of the composite particles is carried out by hydrolyzing the iron salt when the iron compound coating layer is formed on the core fine particles in the solution state of the ferrous salt. 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 carried out by the iron compound coating layer of the composite particles being α
-Applicable when containing ferric iron compounds such as Fe ₂ O ₃ , Fe (OH) ₃ and iron oxide hydroxide, and these ferric iron compounds are reduced simultaneously with hydrolysis of ferrous salt At the same time, the ferrous salt is oxidized at the same time as being hydrolyzed and converted into Fe ₃ O ₄ .

Examples of the ferrous salt include FeCl ₂ and Fe.
(NO ₃ ) ₂ and FeSO ₄ may be mentioned.

The ferromagnetic composite particles thus obtained have a core / shell composite structure in which a coating layer containing ferromagnetic iron oxide is formed on the core fine particles. In this ferromagnetic iron oxide-containing coating layer, depending on the reaction conditions for converting the above-mentioned iron compound to a ferromagnetic iron compound, the properties of the iron compound coating layer of the composite particles, etc., (g) it is contained in the coating layer. When a part of the iron compound contained in the coating layer is randomly converted to a ferromagnetic iron compound, (h) When all of the iron compound contained in the coating layer is converted to a ferromagnetic iron compound, (i) the composite When only a part or all of the iron compound existing in the outer layer portion of the iron compound coating layer of the particles is converted into a ferromagnetic iron compound, (j) the iron compound coating layer of the composite particle is a ferromagnetic iron compound-containing coating. If further covered by layers, or (k)
These may be mixed.

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

The average particle size of the 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, etc.
Usually, 0.04 to 50 μm, preferably 0.04 to 40
μm, particularly preferably 0.1 to 10 μm. In addition,
These particles are usually obtained in a spherical shape or a substantially spherical shape, but the average particle diameter when the shape is not necessarily symmetrical is taken to be the minor axis diameter.

The ratio of the particle size of the core particles to the particle size of the composite particles (hereinafter referred to as "inner diameter ratio") is
Usually, it is 0.3 to 0.99, preferably 0.5 to
0.99, particularly preferably 0.7 to 0.99. If the inner diameter ratio is out of the range of 0.3 to 0.99, the iron compound coating layer may be too thick or too thin, so that it may be difficult to efficiently produce particles having good properties.

The composite particles obtained according to the present invention, when dispersed in water, for example, and subjected to ultrasonic waves, the iron compound coating layer is no longer peeled or dropped from the core fine particles, and the iron compound coating layer component is There is no risk of elution or migration into various media, storage stability is excellent, and stable light-shielding properties, magnetism, etc. can always be exhibited. Moreover, since the original characteristics of the core fine particles are not damaged at all,
The strength and heat resistance of the particles as a whole can be made as excellent as those of the core particles.

The composite particles obtained by the present invention can be suitably used as electronic materials, magnetic materials, optical materials, catalysts, spacer particles, column fillers, carriers for developers, carriers for diagnostic agents, concealing materials and the like. .

[0046]

EXAMPLES Next, the present invention will be described more specifically by way of examples. However, the present invention does not exceed the gist of the invention.
There is no limitation to these examples. 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,
The mixture was stirred at room temperature for 12 hours and then washed to obtain sulfonated crosslinked particles. The concentration of the sulfonated crosslinked particles was 15.7.
Aqueous dispersion 642 ml, dispersed in 3% by weight, 3% by weight polyvinylpyrrolidone aqueous solution 800 ml, 1
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. Then, 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 at once, and after 1 hour and 40 minutes, urea 12
A solution of 0.00 g dissolved in 160 ml of distilled water was added at once. One minute later, a solution of 120.00 g of ferric chloride hexahydrate dissolved 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 the mixture was filtered. The obtained particles were washed with water and dried under reduced pressure at 60 ° C. for 12 hours. When the particles were observed with a scanning electron microscope, they were red-brown, spherical composite particles having an average particle diameter of 5.3 μm, an inner diameter ratio of 0.94, and a uniform coating layer. Further, when the composite particles were analyzed by X-ray diffraction and thermogravimetric analysis, it was confirmed that the core consisted of crosslinked divinylbenzene polymer particles and the shell consisted of γ-FeO (OH) 2. This composite particle is referred to as composite particle A.

Example 2 100 g of composite particles A was heated from room temperature to 320 ° C. at a heating rate of 10 ° C./min in a hydrogen atmosphere, and then 320
The temperature was kept at 1 ° C for 1 hour, and then the temperature was lowered to room temperature at a temperature decreasing rate of 10 ° C / min. Observation of the obtained particles with a scanning electron microscope revealed that the average particle size was 5.2 μm and the inner diameter ratio was 0.9.
In No. 6, it was a black, spherical composite particle having a uniform coating layer. When the composite particles were analyzed in the same manner as above, it was confirmed that the core was a crosslinked divinylbenzene polymer particle and the shell was a ferromagnetic composite particle composed of magnetite (Fe ₃ O ₄ ).

Example 3 A mixture of 100 g of composite particles A, 10 g of ferrous chloride tetrahydrate and 200 ml of a 3% by weight aqueous polyvinylpyrrolidone solution was heated to 95 ° C. and stirred at 95 ° C. for 6 hours. Then, 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 the particles with a scanning electron microscope revealed that the average particle diameter was 5.3 μm.
It was a black, spherical composite particle having an inner diameter ratio of 0.94 and a uniform coating layer. In addition, this composite particle was used in Example 1.
When analyzed in the same manner as in 1., it was confirmed that the core was a crosslinked divinylbenzene polymer particle and the shell was a ferromagnetic composite particle composed of magnetite (Fe ₃ O ₄ ). This ferromagnetic composite particle is referred to as composite particle B.

Example 4 100 g of composite particles B was heated from room temperature to 250 ° C. at a heating rate of 10 ° C./min in an air atmosphere, and then 250
The temperature was maintained at 0 ° C for 2 hours, and then the temperature was cooled to room temperature at a temperature decreasing rate of 10 ° C / min. Observation of the obtained particles with a scanning electron microscope revealed that the average particle size was 5.2 μm and the inner diameter ratio was 0.9.
In No. 6, it 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 a crosslinked divinylbenzene polymer particle and the shell was a ferromagnetic composite particle composed of maghemite (γ-Fe ₂ O ₃ ).

[0050]

EFFECTS OF THE INVENTION According to the present invention, the strength and heat resistance are excellent, the coating layer does not easily peel or fall off, the iron compound coating layer component does not elute or migrate into various media, and the storage stability is stable. It is possible to industrially produce extremely efficiently and easily composite particles having excellent properties and exhibiting constantly stable light-shielding performance and ferromagnetism.

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Claims (1)

[Claims] 1. A hydrolyzable iron salt is hydrolyzed in a solution state while controlling the pH of the reaction system to 1.4 to 4 in a dispersion liquid in which fine particles to be a core are uniformly dispersed in an aqueous medium. Forming an iron compound coating layer serving as a shell on the fine particles, or if necessary further oxidizing treatment and / or
Alternatively, a reduction treatment is performed to convert at least a part of the iron compound in the iron compound coating layer into ferromagnetic iron oxide, and / or a coating layer containing ferromagnetic iron oxide is provided as an outer layer of the iron compound coating layer. A method for producing composite particles, which comprises forming the composite particles.