JP2000344528A - Iron oxide particle and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce iron oxide particle excellent in fluidity, dispersibility, handleability, charge stability against environmental change and moisture absorption stability and capable of adjusting electric resistance corresponding to the application. SOLUTION: The iron oxide particle is covered with a multiple iron oxide of Si and Fe as an under layer and an Al component as an upper layer. The content of Si component in the multiple iron oxide of the under layer is desirably 0.05-2 wt.% per the whole iron oxide particle expressed in terms of Si and Fe and the content of the Al component of the upper layer is desirably 0.01-1 wt.% per the whole iron oxide particle expressed in terms of Al.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an iron oxide particle and a method for producing the same. More specifically, the present invention relates to a lower layer having a particle surface coated with a composite iron oxide of Si and Fe and an upper layer having the composite iron oxide coated with an Al component. By providing these materials, various properties such as fluidity, dispersibility, handling, and charging stability against environmental changes are improved in a well-balanced manner. Particularly, material powders for magnetic toner for electrostatic copying and carrier powders for developing electrostatic latent images. The present invention relates to iron oxide particles used for applications such as black pigment powder for paints and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Recently,
As a material for magnetic toner for electronic copiers and printers,
Magnetite particles by aqueous solution reaction are widely used. Various general development characteristics are required for magnetic toners, but in recent years, with the development of electrophotographic technology, in particular, copiers and printers using digital technology have rapidly developed, and the required characteristics have become more sophisticated. Have been.

That is, in addition to conventional characters, output of graphics, photographs, and the like is also required. Some copiers and printers have a capacity of 1200 dots or more per inch, and a latent image on a photoreceptor is required. Is becoming more elaborate. For this reason, there is a strong demand for high reproducibility of fine lines in development and for use in various environments without any problem.

[0004] For example, Japanese Patent Application Laid-Open No. 5-71801 discloses a magnetic toner. According to this, it is said that a magnetic powder having high resistance and good fluidity is desired. JP-A-8-101529 discloses a magnetic toner, and it is said that a magnetic powder having good fluidity and low resistance is desired.
This is to prevent the toner from being excessively charged under low humidity. Further, in order to prevent fogging under low humidity, a material having a high residual magnetization and a high coercive force is used. Japanese Patent Application Laid-Open No. 7-239571 also points out that there is a problem in the environment resistance of magnetic powder, particularly under high temperature and high humidity, as described above. Further, Japanese Unexamined Patent Application Publication No.
No. 60 discloses that the magnetic toner needs to have high resistance and low moisture absorption in order to satisfy it in various environments.

Japanese Patent Application Laid-Open No. 8-76519 discloses a resin-coated carrier in which a resin-coated carrier is produced using a magnetite particle of about 90% by weight of the total amount in a kneader. Have been. That is, it can be seen that magnetic powder that is very easily dispersed is required.

[0006] Further, there are negatively chargeable and positively chargeable toners, and there are also negatively chargeable or positively chargeable carriers using magnetite. And
Blackness and dispersibility as a black pigment are required, and handleability as a powder is required.

In other words, in order to satisfy these requirements, not only the properties normally required for magnetic powders, but also fluidity,
It is necessary to provide a magnetic powder excellent in dispersibility, handling properties, environmental resistance, and blackness. For example, the following proposal has been made.

[0008] JP-A-5-286723 discloses that Si
And / or magnetite particles containing an oxide of Al are disclosed. Thereby, blackness, dispersibility,
Although magnetite particles with excellent heat resistance can be obtained,
i and / or a mixture of Al is only co-precipitated (S
i), environmental resistance is inferior due to the hygroscopicity of Si,
Fluidity, resistance control, and handling properties are also insufficient.

Japanese Patent Application Laid-Open (JP-A) No. 7-110598 discloses magnetite particles in which Si is contained inside the particles and the surface of which is treated with a coprecipitate of silica or alumina. Thereby, magnetite particles having excellent charge stability can be obtained at the time of repeated measurement, but the charge stability, fluidity, handleability, and blackness with respect to environmental changes are insufficient.

Japanese Patent Application Laid-Open No. 7-240306 discloses magnetite particles in which Si is contained inside the particles, the surface of the particles is treated with a coprecipitate of silica or alumina, and nonmagnetic particles are further fixed. ing. As a result, although the charge stability is excellent, the fluidity and the initial dispersibility are excellent at the time of repeated measurement, it is costly to fix the non-magnetic particles, and the possibility of peeling, uniform and complete surface treatment Is difficult, and the charging stability against environmental changes is insufficient, resulting in a decrease in blackness.

The term "charge stability against environmental changes" as used herein refers to the degree of change in the amount of charge when exposed to a predetermined environment, and the rate of change in the amount of charge in any use environment from low to low humidity to high to high humidity. The lower the value, the better the charging stability.

Japanese Patent Application Laid-Open No. Hei 8-133745 discloses that Zn _x Fe _{2 + y} O _z is formed in a lower layer, and Si, Al, T is formed in an upper layer.
i) discloses magnetite particles surface-treated with the coprecipitate. This makes it excellent in heat resistance and coloring power,
Although the charge amount is controlled, fluidity, handling,
Insufficient charging stability against environmental changes and resistance control.

Japanese Patent Application Laid-Open No. Hei 10-182163 discloses that spinel-like magnetite particles containing silicon have an Si surface.
And the like are disclosed. As a result, magnetite particles with excellent fluidity are obtained without falling off from the toner particles. Insufficient charge stability against change.

That is, in the prior art, iron oxide particles which are excellent in fluidity, dispersibility, handling properties and environmental resistance as well as properties normally required for magnetic powder, and whose resistance can be adjusted depending on the application, In particular, magnetite particles have not yet been provided.

Accordingly, an object of the present invention is to provide fluidity, dispersibility, handleability, charge stability against environmental changes,
An object of the present invention is to provide iron oxide particles which are excellent in moisture absorption stability and the like and whose resistance can be adjusted according to the use, and a method for producing the same.

[0016]

As a result of the study, the present inventors have found that the surface of iron oxide particles is coated with a composite iron oxide of Si and Fe, and further coated with an Al component. It was found that the purpose could be achieved.

The present invention has been made on the basis of the above-mentioned findings, and has an iron oxide particle characterized in that a lower layer is coated with a composite iron oxide of Si and Fe and an upper layer is coated with an Al component. To provide.

Further, as a preferred method for producing the iron oxide particles of the present invention, the present invention provides a method of preparing a slurry containing iron oxide particles produced by a wet method, comprising adding an aqueous solution of a water-soluble silicate, a ferrous salt and an alkali. Add and mix, pH 5-10, temperature 60-
Oxidized at 98 ° C., the iron oxide particles are coated with a composite iron oxide of Si and Fe, a water-soluble aluminum salt is further added, and the composite iron oxide is coated with an Al component at pH 5 to 9. And a method for producing iron oxide particles.

[0019]

Embodiments of the present invention will be described below. The iron oxide particles referred to in the present invention are preferably those mainly composed of magnetite, and include those in which iron oxide particles mainly composed of magnetite containing various effective elements such as silicon and aluminum are included. Is done. In the following description, magnetite particles, which are typical iron oxide particles, will be described. Further, when referring to iron oxide particles or magnetite particles, it means either individual particles or an aggregate thereof depending on the content.

The magnetite particles of the present invention have a surface coated with a composite iron oxide of Si and Fe (lower layer). Here, the composite iron oxide of Si and Fe refers to an iron oxide that incorporates or binds silicon by oxidizing an iron component in the presence of a silicon component. Regarding the coating of the composite iron oxide, either a dense layer or a layer formed by fixing or adhering a large amount of fine particles may be used. In the present invention, the composite iron oxide (lower layer) is coated with an Al component (upper layer).

A magnetite core particle whose surface has been subjected to only a neutralization treatment of Si, for example, as described in JP-A-5-2867.
In those manufactured by the method represented by No. 23 gazette,
The Si component itself has a hygroscopic property, and various characteristics such as fluidity, resistance, and handling cannot be obtained. In the case where the surface of the magnetite core particles is coated only with the composite iron oxide of Si and Fe, the change in the charge amount with respect to the environmental change is large.

The content of the Si component in the composite iron oxide is preferably 0.05 to 2% by weight, more preferably 0.5 to 2% by weight in terms of Si, based on the whole magnetite particles. If the Si content is less than 0.05% by weight, the intended effect is small, and if it exceeds 2% by weight, the magnetic properties normally required for magnetite particles are reduced. The magnetic properties (saturated magnetization at 10 kOe) are preferably 70 emu / g or more, and more preferably 75 emu / g or more. In addition, Si and Fe forming complex iron oxide on the particle surface
Is preferably Si: Fe = 1 to 100 to 10
0: 1, more preferably 5: 100 to 75:25.

The content of the Al component in the upper layer is preferably 0.00 in terms of Al relative to the entire iron oxide particles.
1 to 1% by weight, more preferably 0.05 to 0.1% by weight.
5% by weight. If the Al content is less than 0.01% by weight, the coating effect is poor. If it exceeds 1% by weight, the coating effect of the composite iron oxide of Si and Fe is lost, and the saturation magnetization is reduced due to the increase in the Al component. Or poor hygroscopicity. Regarding the coating of the Al component in the upper layer, either a dense formed layer or a formed layer formed by fixing or adhering a large amount of fine particles may be used.

The magnetite particles of the present invention have a cohesion degree of 4
It is desirably 0 or less. If the agglomeration degree exceeds 40, the handleability, the mixing property with the resin, and the supply stability to the toner manufacturing equipment are poor, and the fluidity of the toner itself may be affected.

The magnetite particles of the present invention have an adhesive force of 5
It is desirably not more than × 10 ⁻⁵ dyne / contact. If the adhesive force exceeds this, the adhesion between the powders is strong, and the handling of the powder during the production of the toner is easy, that is, the load on the transport equipment due to the adhesion of the powders, and the resin and the magnetic material during the production of the toner The mixing condition worsens,
Dispersibility is poor.

In the magnetite particles of the present invention, the Si component in the composite iron oxide is present in an amount of 0.5 to 2% by weight in terms of Si with respect to the entire magnetite particles, and has an electric resistance of 1 × 10
Desirably, it is ⁴ Ω · cm or more. Si component is 0.
When the content is less than 5% by weight, the resistance is less than 1 × 10 ⁴ Ω · cm, and it is necessary that the content of Si is 0.5% by weight or more for toners and carriers which are desired to have high resistance. If the Si content exceeds 2% by weight, the saturation magnetization decreases, which is not desirable.

The magnetite particles of the present invention have a temperature of 10.degree.
It is desirable that each moisture absorption and specific surface area after exposure under each environment of 0% RH, 35 ° C. and 85% RH satisfy the following formula (1). When a toner or a resin-coated carrier is manufactured, the presence of magnetic powder exposed on the surface of the toner particles is considered. That is, with respect to the change in moisture absorption under each environment, outside the range of the following equation (1), there is a possibility that the fluidity, chargeability, and stability of resistance of the toner and carrier may be adversely affected.

(ΔW _HH −ΔW _LL ) /A≦0.05 (1) ΔW _HH ; moisture absorption at 35 ° C., 85% RH (wt%) ΔW _LL ; 10 ° C., 20% RH Moisture absorption (wt%) A: specific surface area (m ² / g)

The magnetite particles of the present invention have a temperature of 10.degree.
Assuming that the charge amount (μC / g) after exposure for 4 hours in each environment of 0% RH and 20 ° C., 60% RH, 35 ° C., and 85% RH is B _LL , B _NN , and B _HH , respectively, It is desirable that both B _LL and B _HH satisfy the following expression (2). | B _NN × 0.8 | ≦ | B _LL and B _HH | ≦ | B _NN × 1.2 | (2)

The environment in which the machine and the toner are used varies depending on the mode of use, and it is required that the same performance be exhibited in any environment from low temperature and low humidity to high temperature and high humidity. In a one-component toner, the magnetic powder is 40 to 5 particles.
0% by weight, and about 90% by weight in a two-component carrier. If the change in the charge amount of the magnetic powder itself exceeds 20% of the change in the environment, the environmental stability of the toner may be impaired.

The magnetite particles of the present invention have a blackness (L) of 18.5 or less and a reflectance (60 °) of 8 by a color difference meter.
It is desirable that it be 5% or more. The higher the L value, the lower the darkness. The higher the reflectance, the better the dispersibility in the resin. Pigments that have good dispersibility in resin as a powder and have a high degree of blackness after being dispersed rather than agglomerating blackness are most suitable as pigments. With the recent trend toward smaller toner diameters, the magnetic material used must have a high degree of blackness in order to achieve high resolution black.

The magnetite particles of the present invention are firmly covered with a composite iron oxide of Si and Fe forming a lower layer, so that a compound such as Si does not exist alone,
By being present in the composite iron oxide and controlled by the particle surface layer, moisture absorption due to the presence of a compound such as Si alone on the surface can be suppressed, and the Si component present in the composite iron oxide can be moderately reduced. Stably retains moisture, improves handling properties such as fluidity in response to external environmental changes, and forms an upper layer A
It is presumed that the above object was achieved by suppressing the change in the amount of charge with respect to the environmental change by the appropriate moisturizing property of the l component.

Further, the presence of the composite iron oxide of Si and Fe which forms the lower layer hinders electric conduction, provides high resistance with a small amount of components, and suppresses magnetic coagulation on the surface.
It is presumed that effects such as reduced adhesion of individual powders and good dispersibility were obtained due to not being only the i component.

A preferred method for producing the magnetite particles of the present invention is to add a water-soluble silicate, a ferrous salt and an aqueous alkali solution to a core magnetite slurry formed by a wet method, and mix the mixture. By oxidizing at a temperature of 60 to 98 ° C., the surface of the core particles is coated with a composite iron oxide of Si and Fe (lower layer), and a water-soluble aluminum salt is further added. Is coated with an Al component.

The magnetite particles serving as the core used at this time have an octahedral, hexahedral, spherical or the like shape.
It is not limited at all. The water-soluble silicate may be any water-soluble silicate such as sodium silicate. Examples of the ferrous salt include ferrous sulfate and ferrous chloride. Sodium hydroxide as the alkali,
Sodium carbonate, potassium hydroxide and the like are used.

The pH of the solution at this time is 5-10. p
When H is less than 5, the reaction speed in the oxidation step is slow and not industrial, and when the pH exceeds 10, it is costly and not economical. The temperature of the solution is 60 to 98.
° C, and if the temperature is lower than 60 ° C, FeOOH and the like are mixed and problems such as color, saturation magnetization, and particle uniformity occur. Temperatures above 98 ° C are not industrial. As a method of oxidizing, an oxygen-containing gas may be ventilated, and air is preferably used economically. Further, a liquid oxidizing agent may be used.

The water-soluble aluminum salts include:
For example, aluminum sulfate, aluminum chloride, aluminum nitrate, sodium aluminate and the like can be mentioned. Al
The pH for coating the components is 5-9. When the pH is less than 5, the reaction speed is low and the reaction is not industrial, and when the pH is more than 9, it is costly and not economical.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments and the like.

Example 1 As shown in Table 1, Fe 1
50 liters of an aqueous solution of ferrous sulfate containing ²⁺ 2.0 mol / l and an aqueous solution of 3.60 mol / l sodium hydroxide 5
0 liter was mixed and stirred. The pH at this time was 6.5. While maintaining the slurry at 85 ° C., air was blown at 65 L / min to terminate the reaction (manufacture of magnetite core particles).

The slurry containing the magnetite core particles was added to a sodium silicate aqueous solution (Si1.1mo).
l / l) is 3 liters and the molar ratio of Si to Fe is 1: 2.
Ferrous sulfate aqueous solution (Fe ²⁺ 1.3 mol /
l) and 5 liters of an aqueous sodium hydroxide solution,
The pH was set to 9. While maintaining the slurry at 80 ° C., air was blown at 65 L / min to oxidize again to terminate the reaction (coating of composite iron oxide; lower layer).

After coating the composite iron oxide, 3 liters of an aqueous solution of aluminum sulfate (Al 0.6 mol / l) was mixed, and then the pH was adjusted to 6 to terminate the reaction (A).
1-component coating; upper layer).

The resulting particles were subjected to usual filtration, washing, drying and pulverizing steps to obtain magnetite particles. In addition, the content of Si and Al, the specific surface area, the magnetic properties (saturated magnetization), the amount of charge obtained under each environment, the cohesion, the adhesion, the blackness, the reflectance, and the electrical resistance were determined by the following methods. Was evaluated. Table 2 shows the results.

[Measurement Method] (1) Analysis of Si and Al Contents Samples were dissolved and measured by ICP. (2) Specific surface area Shimadzu-Micromeritics 2200 type BET meter
Measured. (3) Magnetic properties Using a vibration sample type magnetometer VSM-P7 manufactured by Toei Industry Co., Ltd.
The measurement was performed at a magnetic field of 10 KOe. (4) Moisture absorption rate Dry in advance at 105 ° C for 1 hour with a dryer (dry weight W
1) Allow 10 ° C, 20% RH and 35 ° C, 8
Each was exposed for 4 hours in an environment of 5% RH (weight W after moisture absorption W
2) The weight measurement of each is determined by the following equation (% by weight)
Was calculated (ΔW _LL10 ° C., 20% RH, ΔW_HH3
5 ° C, 85% RH). ΔW: moisture absorption (% by weight) = [(W2−W1) / W1] ×
100 The change in the moisture absorption rate per area is expressed by the following equation.
You. (ΔW_HH-ΔW_LL) / A (Specific surface area) (5) Charge amount The sample was placed in an environmental chamber at 10 ° C., 20% RH, 20 ° C., 6
Exposure for 4 hours under each environment of 0% RH, 35 ° C and 85% RH
And using the iron powder carrier to determine by blow-off method
Was. (6) Degree of aggregation “Powder T” manufactured by Hosokawa Micron
Ester TypePT-E ”(trade name)
The measurement was performed at a vibration time of 65 sec. Measure the measurement results
The degree of agglomeration was determined by substituting into the formula. (7) Adhesion force Using Shimadzu powder adhesion measurement device (EB-3300CH)
And place the sample on the entire edge of the cell container (measure the powder weight).
Fixed). After pressing up to 1cm from the cut surface in the cell,
It was measured with a measuring instrument and calculated by a predetermined formula. (8) Electric resistance Place 10 g of the sample in the holder, 600 kg / cm^Two Pressure
, And after forming into a 25mmφ tablet, attach the electrode
Ke, 150kg / cm^Two The measurement was carried out under a pressurized state. Measurement
Calculated from the thickness, cross-sectional area and resistance value of the sample used for
The electrical resistance value of the magnetite particles was determined. (9) Blackness and reflectivity Styrene acrylic resin (TB-1000F)
60 g of a solution dissolved in toluene (resin: toluene = 1: 2)
Contains 10g of sample and 90g of 1mm diameter glass beads
Put the bottle in a 140 ml bottle, cover it,
The mixture was mixed for 30 minutes in a shaker (manufactured by Toyo Seiki). This
This is coated on a glass plate using a 4 mil applicator.
Cloth, after drying, blackness by color difference meter, Murakami GLOS
Measure the reflectance at 60 degrees with S METER (GM-3M)
Specified.

Examples 2 to 8 Magnetite was prepared in the same manner as in Example 1 except that the reaction conditions for producing magnetite core particles and the conditions for coating the surface of the composite iron oxide layer (lower layer) and the Al component layer (upper layer) were changed. Particles were produced. Table 1 shows the production conditions for the magnetite particles. Table 2 shows the results of evaluating various properties and characteristics in the same manner as in Example 1.

Comparative Example 1 Composite iron oxide layer (lower layer), Al
Magnetite particles were produced in the same manner as in Example 1, except that the coating treatment of the component layer (upper layer) was not performed. Table 1 shows the production conditions for the magnetite particles. Table 2 shows the results of evaluating various properties and characteristics in the same manner as in Example 1.

Comparative Examples 2 and 3 Magnetite particles were produced in the same manner as in Examples 2 and 3, except that the composite iron oxide layer and the Al component layer were not coated. Table 1 shows the production conditions for the magnetite particles. Table 2 shows the results of evaluating various properties and characteristics in the same manner as in Example 1.

[Comparative Example 4] After producing magnetite core particles under the same conditions as in Example 1, the composite iron oxide layer (lower layer), Al
Instead of the coating treatment of the component layer (upper layer), only the Fe compound was coated to produce magnetite particles. Table 1 shows the production conditions for the magnetite particles. Table 2 shows the results of evaluating various properties and characteristics in the same manner as in Example 1.

Comparative Example 5 After producing magnetite core particles under the same conditions as in Example 1, a composite iron oxide layer (lower layer), Al
Instead of the coating treatment of the component layer (upper layer), an aqueous solution of sodium silicate and sodium hydroxide are added (pH 10 after addition) to the slurry after the magnetite core particles are formed, and the mixture is neutralized to pH 6 with sulfuric acid, and only the Si component is added. To produce magnetite particles. Table 1 shows the production conditions for the magnetite particles. Table 2 shows the results of evaluating various properties and characteristics in the same manner as in Example 1.

Comparative Example 6 After producing magnetite core particles under the same conditions as in Example 1, the composite iron oxide was not coated.
Coating was performed with only the Al component to produce magnetite particles. Table 1 shows the production conditions for the magnetite particles. Table 2 shows the results of evaluating various properties and characteristics in the same manner as in Example 1.

[0050]

[Table 1]

[0051]

[Table 2]

As is clear from Table 2, Examples 1 to 8
The magnetite particles are stable in chargeability and hygroscopicity in each environment without greatly deteriorating the saturation magnetization, and have excellent fluidity, handling, dispersibility and blackness,
And high electrical resistance. As can be seen from Example 8, Si
Those having a large amount have good dispersibility and blackness, and have high resistance but low saturation magnetization.

In Comparative Examples 1 to 3, the composite iron oxide layer (lower layer)
Since there is no Al component layer (upper layer), although the saturation magnetization is high, the change in charge amount and the change in moisture absorption under each environment are large, and the fluidity, handling, dispersibility, and blackness are poor. In Comparative Example 4, the Fe compound coating treatment was performed, and the same results as Comparative Examples 1 to 3 were obtained.

In Comparative Examples 5 and 6, which were subjected to Si neutralization treatment and Al neutralization treatment, respectively, and coated with a Si component or an Al component, the dispersibility was better than Comparative Examples 1 to 4. Despite the neutralization treatment, the blackness was low, and the charging stability and moisture absorption stability under each environment were poor. In addition, since the cohesion and adhesion are large, fluidity and handling are poor.
In addition, the electric resistance value was low with respect to the throughput of the Si component.

[0056]

As described above, the iron oxide particles of the present invention are obtained by coating the surface of iron oxide core particles with a composite iron oxide of Si and Fe (lower layer) and an Al component (upper layer).
In addition to the properties normally required for magnetic powders, fluidity, dispersibility, handling, charging stability against environmental changes,
Suitable for applications such as material powder for magnetic toner for electrostatic copying, material powder for carrier for electrostatic latent image development, and black pigment powder for paints since it has excellent moisture absorption stability and can adjust the electric resistance arbitrarily. It is.

Claims (9)

[Claims] 1. An iron oxide particle comprising a lower layer coated with a composite iron oxide of Si and Fe, and an upper layer coated with an Al component. 2. A method according to claim 1, wherein the content of the Si component in the lower composite iron oxide is 0.0
5 to 2% by weight, and the content of the Al component in the upper layer is
2. The iron oxide particles according to claim 1, wherein the content of the iron oxide particles is 0.01 to 1% by weight in terms of Al with respect to the whole iron oxide particles. 3. The method according to claim 1, wherein the degree of aggregation is 40 or less.
The described iron oxide particles. 4. An adhesive force of 5 × 10 ⁻⁵ dyne / cont.
The iron oxide particles according to claim 1, wherein the particle size is less than or equal to act. 5. The content of the Si component in the composite iron oxide is 0.5 to 2 in terms of Si with respect to the entire iron oxide particles.
The iron oxide particles according to any one of claims 1 to 4, wherein the iron oxide particles are 1% by weight and have an electric resistance of 1 × 10 ⁴ Ω · cm or more. 6. 10 ° C., 20% RH and 35 ° C., 85% R
Moisture absorption after exposure to 4Hr under each environment of H (wt%)
And ΔW _LL and ΔW _HH , respectively, and specific surface area (m ² / g)
The iron oxide particles according to any one of claims 1 to 5, wherein the following formula (1) is satisfied when A is defined as A. (ΔW _HH −ΔW _LL ) /A≦0.05 (1) 7. 10 ° C., 20% RH and 20 ° C., 60% R
Exposure to H for 4 hours in each environment of 35 ° C and 85% RH
Charge amount (μC / g)_LL, B _NN, B_HHWhen
7. The method according to claim 1, wherein the following formula (2) is satisfied.
Iron oxide particles according to any of the above. | B_NN× 0.8 | ≦ | B_LLAnd B_HH| ≦ | B_NN× 1.2 | ・ ・ ・ ・ (2) 8. The blackness (L) measured by a color difference meter is 18.5 or less, and the reflectance (60 degrees) is 85% or more.
The iron oxide particles according to any one of the above. 9. An aqueous solution of a water-soluble silicate, a ferrous salt and an alkali is added to a slurry containing iron oxide particles produced by a wet method, and the mixture is oxidized at a pH of 5 to 10 and a temperature of 60 to 98 ° C. Then, the iron oxide particles are coated with a composite iron oxide of Si and Fe, and a water-soluble aluminum salt is further added thereto to adjust the pH to 5%.
9. The method for producing iron oxide particles, wherein the composite iron oxide is coated with an Al component according to any one of (1) to (9).