Patent Application
20070254157
The present invention is described in detail below. Meanwhile, in the present invention, since it is sometimes required that properties of the black magnetic iron oxide particles are defined as those of a molded product comprising the black magnetic iron oxide particles, the molded product comprising the black magnetic iron oxide particles are hereinafter occasionally referred to as a “molded product”.
The particle shape of the black magnetic iron oxide particles according to the present invention is not particularly limited. The black magnetic iron oxide particles may have a hexahedral shape, an octahedral shape, a polyhedral shape, a granular shape, a spherical shape, etc.
The black magnetic iron oxide particles of the present invention comprise core particles and a surface layer formed on the respective core particles. The “surface layer” means a portion of the respective magnetic iron oxide particles except for an Fe-containing portion which extends from a center of each particle toward the surface thereof. Also, the “core particles” mean an inside portion of the respective black magnetic iron oxide particles except for the surface layer.
The surface layer of the respective black magnetic iron oxide particles according to the present invention is a layer formed by uniformly dispersing a compound of at least one alkali earth element selected from the group consisting of Mg, Ca, Sr and Ba and an aluminum element over the surface of the respective core particles.
In the black magnetic iron oxide particles of the present invention, the content of the aluminum element present in the surface layer of the respective black magnetic iron oxide particles is not less than 1000 ppm and not more than 20000 ppm on the basis of a whole weight of the black magnetic iron oxide particles. When the content of the aluminum element present in the surface layer is less than 1000 ppm, the resultant black magnetic iron oxide particles tend to exhibit not only a low electric resistance value but also a low breakdown voltage. When the content of the aluminum element present in the surface layer is more than 20000 ppm, the resultant black magnetic iron oxide particles tend to not only exhibit a high moisture absorption but also be lowered in both electric resistance value and breakdown voltage. The content of the aluminum element present in the surface layer is preferably 1000 to 18000 ppm and more preferably 1000 to 15000 ppm. Meanwhile, in the present invention, all of the “ppm” values represent “ppm by weight”.
In the black magnetic iron oxide particles of the present invention, the content of the at least one alkali earth metal element selected from the group consisting of Mg, Ca, Sr and Ba which is present in the surface layer of the respective black magnetic iron oxide particles is not less than 100 ppm and not more than 1000 ppm on the basis of a whole weight of the black magnetic iron oxide particles. When the content of the at least one alkali earth metal element present in the surface layer is less than 100 ppm, the resultant black magnetic iron oxide particles may fail to exhibit a breakdown voltage of not less than 400 V/cm. When the content of the at least one alkali earth metal element present in the surface layer is more than 1000 ppm, the surface of the resultant black magnetic iron oxide particles tends to show a moisture absorption, resulting in not only low electric resistance value and low breakdown voltage of the obtained particles but also strong interaction with a resin component in a toner composition and, therefore, deterioration in mixing property and dispersibility therein. The content of the at least one alkali earth metal element present in the surface layer of the respective black magnetic iron oxide particles is preferably 120 to 990 ppm and more preferably 130 to 980 ppm.
The ratio [A/B] of the content [A (ppm)] of the aluminum element present in the surface layer of the respective black magnetic iron oxide particles to the content [B (ppm)] of the at least one alkali earth metal element selected from the group consisting of Mg, Ca, Sr and Ba which is in the surface layer, is not less than 1.0 and not more than 100. When the ratio [A/B] of the content [A (ppm)] of the aluminum element to the content [B (ppm)] of the at least one alkali earth metal element selected from the group consisting of Mg, Ca, Sr and Ba is less than 1.0, the resultant black magnetic iron oxide particles exhibit a relatively high electric resistance value in a low voltage range, but tends to have a low electric resistance value in a high voltage range. When the ratio [A/B] is more than 100, the resultant black magnetic iron oxide particles tend to have a high moisture absorption, resulting in low electric resistance value in a high voltage range. The ratio [A/B] of the content [A (ppm)] of the aluminum element to the content [B (ppm)] of the at least one alkali earth metal element selected from the group consisting of Mg, Ca, Sr and Ba is preferably 1.0 to 90 and more preferably 1.0 to 80.
The molded product comprising the black magnetic iron oxide particles of the present invention has a breakdown voltage of not less than 400 V/cm. When the breakdown voltage of the molded product is less than 400 V/cm, the resultant black magnetic iron oxide particles may fail to exhibit a high electric resistance value in a high voltage range. The breakdown voltage of the molded product comprising the black magnetic iron oxide particles is preferably not less than 500 V/cm and more preferably not less than 600 V/cm. As to the upper limit of the breakdown voltage of the molded product comprising the black magnetic iron oxide particles, since a maximum voltage applied by a measuring apparatus used in the present invention was 1000 V, the upper limit of the breakdown voltage measured was substantially about 1.8 kV/cm.
In the black magnetic iron oxide particles of the present invention, an amount of Al eluted out therefrom (amount of weak acid-soluble Al) is preferably not more than 500 ppm when measured by mixing the black magnetic iron oxide particles in a 0.002 N HCl aqueous solution at 40° C. under stirring for 10 min. When the amount of weak acid-soluble Al contained in the black magnetic iron oxide particles is more than 500 ppm, the resultant black magnetic iron oxide particles tend to be lowered in electric resistance value and breakdown voltage. The reason therefor is that since the surface layer formed on the respective core particles which is made of the alkali earth metal element and the Al element becomes uneven under the above condition, the Al element contained in the surface layer tends to be readily eluted out from the surface layer when exposed to a weak acid aqueous solution. Further, owing to the uneven surface treatment, a thinner surface layer portion or a surface-untreated portion tends to exist on the core particles, and an electric current tends to leak through such a portion, resulting in low electric resistance value and low breakdown voltage of the obtained black magnetic iron oxide particles. The amount of the weak acid-soluble Al contained in the surface layer is more preferably not more than 450 ppm and still more preferably not more than 400 ppm.
The molded product having a density of 2.7 g/cm3 which is formed from the black magnetic iron oxide particles of the present invention exhibits an electric resistance of preferably not less than 1×106 Ωcm and more preferably not less than 1×107 Ωcm when applying a D.C. voltage of 500 V thereto. Incidentally, the above density of 2.7 g/cm3 is a substantial value in the practical measurement, it may be understand that the molded product having a density of within 2.5 to 2.8 g/cm3 which is formed from the black magnetic iron oxide particles of the present invention exhibits an electric resistance of preferably not less than 1×106 Ωcm and more preferably not less than 1×107 Ωcm when applying a D.C. voltage of 500 V thereto. If the density is extremely out of the range of 2.5 to 2.8 g/cm3, the electric resistance is changed and it may be difficult to compare respective measured values.
The black magnetic iron oxide particles of the present invention preferably have an average particle diameter of 0.10 to 0.30 μm. When the average particle diameter of the black magnetic iron oxide particles is less than 0.10 μm, it may be difficult to well disperse the obtained black pigment in toner particles when used in a toner. When the average particle diameter of the black magnetic iron oxide particles is more than 0.30 μm, the number of magnetic particles contained in the toner particles tends to be comparatively reduced, resulting in poor tinting strength.
The black magnetic iron oxide particles of the present invention preferably have a BET specific surface area of 4 to 20 m2/g. When the BET specific surface area of the black magnetic iron oxide particles is less than 4 m2/g, the BET specific surface area is smaller than the theoretical calculated value so that there is a possibility that particles are coagulated each other and these are present as coarse particles having larger size than each particle size. Therefore, this is not preferable in view of dispersibility thereof. When the BET specific surface area of the black magnetic iron oxide particles is more than 20 m2/g, the resultant black magnetic iron oxide particles tend to exhibit a high moisture absorption, resulting in deterioration of image density.
In the black magnetic iron oxide particles of the present invention, a product a of the BET specific surface area and the average particle diameter thereof which is represented by the formula is preferably 1.2 to 2.0 and more preferably 1.3 to 1.9.
α=(BET specific surface area (m2/g))×(average particle diameter (μm)),
When the product α is more than 2.0, the BET specific surface area value of the black magnetic iron oxide particles becomes increased if the average particle diameters thereof are the same, so that the obtained black magnetic iron oxide particles tend to exhibit a high moisture absorption, resulting in low electric resistance value and low breakdown voltage. The large BET specific surface area value of the black magnetic iron oxide particles having a product a of more than 2.0 tends to be caused by such a condition that the compound of the alkali earth metal element and the Al element to be present on the surface of the respective core particles fails to be uniformly surface-treated thereon, or tends to be deposited on portions other than the surface of the respective core particles, i.e., separately from the magnetic particles. When the product α is 1.2, the black magnetic iron oxide particles have a minimum BET specific surface area value assuming that the particles have a spherical shape. This means that the most uniform and smoothest surface layer is formed on the respective core particles. In the above specified range of the average particle diameter, it is theoretically impossible to obtain the particles having a BET specific surface area value lower than the minimum value.
Meanwhile, in the core particles of the black magnetic iron oxide particles according to the present invention, in order to improve various properties as required upon use, various elements may be contained in a whole portion or specific portions within the core particles. However, if Al is contained in the core particles, for the same reason as described above, the amount of Al eluted out from the black magnetic iron oxide particles finally produced after the surface treatment, namely, the amount of the weak acid-soluble Al contained therein, is preferably not more than 500 ppm on the basis of the weight of the black magnetic iron oxide particles.
Next, the process for producing the black magnetic iron oxide particles according to the present invention is described.
The black magnetic iron oxide particles of the present invention may be produced by forming the core particles made of magnetite by an ordinary method, adding an aluminum salt and an alkali earth metal salt to a slurry containing the core particles, and then controlling a pH value of the resultant slurry under heating.
As described above, the core particles used for obtaining the black magnetic iron oxide particles of the present invention may be selected from those particles having various shapes and particle diameters from the standpoints of magnetic properties, dispersibility, etc., which are required as a black magnetic pigment, and may be produced by various methods. In order to effectively achieve the objects of the present invention, from the standpoint of uniformly performing the subsequent surface treatment, the slurry containing the core particles preferably include none of substances which tend to prohibit the surface treatment, such as, for example, unreacted fine iron hydroxide particles. Also, from the same standpoint, the core particles more preferably have a smooth surface.
As described above, the slurry containing the core particles can be obtained by various methods. For example, by controlling the pH value of a ferrous (Fe2+) aqueous solution during an oxidation reaction thereto to a predetermined suitable value, there can be obtained the core particles having an octahedral shape, a polyhedral shape, a spherical shape or an irregular shape. In addition, by suitably adjusting conditions for particle growth during the oxidation reaction, there can be obtained the core particles having a desired particle diameter. Further, the core particles having a well-controlled surface smoothness can be produced by suitably controlling the conditions for particle growth at an end stage of the oxidation reaction or by adding a silicon component or an aluminum component, or compounds which tend to form a spinel ferrite structure such as zinc, magnesium and calcium, to the slurry, as generally known in the art.
As to the ferrous (Fe2+) aqueous solution, there may be used, for example, aqueous solutions of ordinary ion compounds such as ferrous sulfate and ferrous chloride. In addition, as the alkali solution which is used for obtaining the iron hydroxide or serves as a pH modifier, there may be used aqueous solutions of sodium hydroxide, sodium carbonate, etc. The respective raw materials may be appropriately selected in view of economy or reaction efficiency.
For example, the spherical core particles may be produced by the following method.
A ferrous sulfate aqueous solution is reacted with an aqueous solution containing sodium hydroxide in an amount of 0.95 equivalent on the basis of Fe2+ contained in the ferrous sulfate aqueous solution to obtain a ferrous salt aqueous reaction solution containing a ferrous hydroxide colloid. Then, the thus obtained ferrous salt aqueous reaction solution containing the ferrous hydroxide colloid is subjected to oxidation reaction at 90° C. by passing an oxygen-containing gas therethrough, thereby producing magnetite particles. Next, the slurry containing the thus obtained magnetite particles is mixed with a sodium hydroxide aqueous solution in an amount of 1 equivalent or more on the basis of residual Fe2+ contained in the slurry, and the resultant slurry is successively subjected to oxidation reaction to obtain the core particles.
In the below mentioned surface treatment, when Ca, Sr and Ba are used as the alkali earth metal elements, the slurry containing the core particles preferably contains a less amount of a component which tends to form a hardly water-soluble salt, such as a sulfate group (SO42−).
The slurry containing the core particles preferably has a pH value of not less than 10 and preferably not less than 11. When the pH value of the slurry is less than 10, when adding the Al component to the slurry containing the core particles, aluminum hydroxide tends to be produced simultaneously with the addition of the Al component, so that it may be difficult to form a uniform layer on the surface of the respective core particles.
The slurry containing the core particles is preferably maintained at a temperature of 60 to 95° C. When the temperature of the slurry is less than 60° C., it may be difficult to form a uniform layer comprising the Al component and the alkali earth metal component on the surface of the respective core particles. The upper limit of the temperature of the slurry containing the core particles is not particularly limited. However, since the slurry is in the form of an aqueous slurry, the upper limit of the temperature of the slurry containing the core particles is about 95° C. in view of a good productivity and low costs.
The amount of the Al compound added to the slurry containing the core particles may be controlled depending upon the amount of the core particles contained in the slurry such that the Al content in the finally obtained surface-treated particles is not less than 1000 ppm and not more than 20000 ppm. When the amount of the Al compound added to the slurry containing the core particles is less than 1000 ppm as the Al content in the surface-treated particles, the resultant black magnetic iron oxide particles may fail to exhibit a high electric resistance value as aimed by the present invention. When the amount of the Al compound added to the slurry containing the core particles is more than 20000 ppm as the Al content in the surface-treated particles, the resultant black magnetic iron oxide particles tend to have a high moisture absorption, resulting in low electric resistance value thereof.
The amount of the alkali earth metal compound added to the slurry containing the core particles may also be controlled depending upon the amount of the core particles contained in the slurry such that the alkali earth metal element content in the finally obtained surface-treated particles is not less than 100 ppm and not more than 2000 ppm. When the amount of the alkali earth metal compound added to the slurry containing the core particles is less than 100 ppm as the alkali earth metal content in the surface-treated particles, the resultant black magnetic iron oxide particles may fail to exhibit a high electric resistance value as aimed by the present invention. When the amount of the alkali earth metal compound added to the slurry containing the core particles is more than 2000 ppm as the alkali earth metal content in the surface-treated particles, the resultant black magnetic iron oxide particles tend to have a high moisture absorption, resulting in low electric resistance value thereof.
The order of addition of an aqueous solution containing the Al component and an aqueous solution containing the alkali earth metal component to the slurry containing the core particles is not particularly limited. These aqueous solutions may be added to the slurry separately from each other, or both the aqueous solutions may be previously mixed with each other to form a mixed aqueous solution, followed by adding the mixed aqueous solution to the slurry.
The pH value of the slurry obtained after adding the Al compound and the alkali earth metal salt to the slurry containing the core particles is preferably controlled to 4 to 10 and more preferably 5 to 8. Upon controlling the pH value of the slurry, the slurry and the mixed solution are preferably intimately stirred with each other. In the pH control procedure, the pH value of the slurry of the core particles containing the aqueous solutions of the Al compound and the alkali earth metal salt which slurry has a pH value of not less than 10, is preferably gradually reduced to the aimed value. More specifically, an acid aqueous solution is added to the slurry to once reduce the pH value thereof to 8 to 10, and the resultant slurry is uniformly mixed for not less than 5 min. Then, an acid aqueous solution is added again to the slurry to gradually reduce the pH value and allow the pH value to finally reach 6.5 to 7.5. When the finally obtained slurry has a pH value of less than 4, it may be difficult to form a uniform Al compound containing layer on the surface of the respective core particles. When the finally obtained slurry has a pH value of more than 10, it may be difficult to produce the Al compound.
After adding the Al compound and the alkali earth metal salt to the slurry, the temperature of the slurry is preferably controlled to 60 to 95° C. When the temperature of the slurry is less than 60° C., it may be difficult to form a uniform layer comprising the Al component and the alkali earth metal component on the surface of the respective core particles. The upper limit of the temperature of the slurry is not particularly limited. Since the slurry is in the form of aqueous slurry, the upper limit of the temperature of the slurry is about 95° C. in view of a good productivity and low costs.
After the reaction, the resultant particles may be subjected to water-washing and drying by ordinary methods.
In the black magnetic iron oxide particles of the present invention, it is important that the Al compound is present together with the alkali earth metal component on an outside of the core particles (as the surface layer). In addition, the Al component present on the surface of the core particles may be derived from residual Al remaining in the reaction system after charging a whole Al component at one time during the oxidation reaction for producing the black magnetic iron oxide particles and incorporating a part of the Al component into the core particles. However, even in this case, it is essential that the Al component is present together with the alkali earth metal component on the surface of the core particles. In fact, it was confirmed that the particles obtained by adding an Al component to magnetite particles produced in order to surface-treat the magnetite particles with the Al component as already reported by the present inventors (Japanese Patent No. 3259744) or the particles obtained by surface-treating the Al-doped magnetite particles with the Al component solely as described in the below-mentioned Comparative Examples, have failed to be improved in electric properties. The reason therefor is considered by the present inventors as follows, though it is not clearly determined. That is, it is suggested that the Al component and the alkali earth metal component which are present on the surface of the core particles are reacted with each other to produce a compound thereof, thereby forming an insulating layer made of a hydroxide phase or an oxide hydroxide phase in the form of a uniform film covering the surface of the core particles without any clearances or voids.
The black magnetic iron oxide particles of the present invention can achieve a high image density, in particular, when used as a pigment for a toner, and can exhibit a high electric resistance value in a high voltage range and are, therefore, suitably used especially in the applications having the purpose of attaining a high image density under high-temperature and high-humidity conditions.
The present invention is described in more detail by the following Production Examples, Examples and Comparative Examples. However, the following Examples are only illustrative and not intended to limit the scope of the present invention thereto. The methods for measuring and evaluating various properties as well as the methods for producing samples for the evaluation are as follows.
The average particle diameter of the black magnetic iron oxide particles is expressed by an average value of Martin diameters of 800 or more particles observed in a field of view photographed by a transmission electron microscope “JEM-1200EX” manufactured by JEOL, Ltd., which were measured by processing the TEM image using an image processing system.
The shape of the black magnetic iron oxide particles was determined from micrographs obtained by observing particles using the transmission electron microscope and a scanning electron microscope “S-4800” manufactured by Hitachi High-Technologies Corporation.
The BET specific surface area value of the black magnetic iron oxide particles was measured by a BET method using “Mono Sorb MS-II” manufactured by Yuasa Ionics Co., Ltd.
The amounts of alkali earth metal elements (Mg, Ca, Sr and Ba) and Al element contained in the black magnetic iron oxide particles were measured by a “Fluorescent X-ray Analyzer RIX-2100” manufactured by Rigaku Denki Kogyo Co., Ltd., and expressed by the values (calculated as Mg, Ca, Sr and Ba and Al) on the basis of the weight of the black magnetic iron oxide particles.
The amount of weak acid-soluble Al contained in the black magnetic iron oxide particles was quantitatively determined by mixing particles to be measured in a 0.002 N HCl aqueous solution at 40° C. under stirring for 10 min, subjecting the resultant mixture to filtration to separate a filtrate therefrom, and then measuring an amount of Al element contained in the filtrate using an inductively coupled plasma atomic emission spectrometer (ICP) “SPS-4000 Model” manufactured by Seiko Instruments Inc.
More specifically, 8 g of the black magnetic iron oxide particles were added to water and sufficiently stirred therewith to obtain a slurry. While allowing the obtained slurry to stand at 40° C., a hydrochloric acid aqueous solution was added thereto to prepare a slurry having a HCl concentration of 0.002 N and a total volume of 800 mL. After stirring for 10 min, the slurry was sampled and passed through a membrane filter having a pore size of 0.1 μm to separate a filtrate therefrom. The obtained filtrate was subjected ICP spectroscopic analysis to quantitatively determine the amount of Al contained in the particles. The 10 min-stirring of the slurry was initiated at the time (0 min) at which the HCl solution was added to the slurry. Also, the quantitative determination of Al was stabilized with a good reproducibility by intimately deaggregating the water slurry before adding the HCl solution thereto using a homogenizer, etc.
The electric resistance value of the black magnetic iron oxide particles upon applying a voltage of 15 V thereto was determined by the following method. That is, 0.5 g of the particles to be measured were weighed, and pressure-molded for 10 seconds using a KBr tableting machine manufactured by Shimadzu Corporation, under a gauge pressure of 14 MPa as a read value on a hand press “SSP-10 Model” manufactured by Shimadzu Corporation, to obtain a molded product having a density of about 2.7 g/cm3. If the other molding apparatus was used for the above procedure, the molding conditions thereof were appropriately controlled so as to obtain a molded product having a density of around 2.7 g/cm3. Next, the thus pressure-molded product as a sample was set between stainless steel electrodes. In this case, the space between the electrodes was completely isolated from an outside by a fluororesin holder. Then, a voltage of 15 V was applied to the sample set between the electrodes using a Wheatstone bridge “Type 2768 Model” manufactured by Yokogawa Denki Co., Ltd., to measure an electric resistance R thereof. When the electric resistance value of the sample was too high and therefore unmeasurable by the Wheatstone bridge, a D.C. voltage of 15 V or a constant voltage of 500 V was applied to the same pressure-molded product using a “High Resistance Meter 4339B” manufactured by Hewlett Packard Inc., to measure the electric resistance R (Ω) as well as an electrode area A (cm2) and a thickness (t) of the sample, and calculate a volume resistivity X (Ωcm) thereof according to the following formula:
X=R/(A/t).
The thus calculated volume resistivity X (Ωcm) was determined as an electric resistance value of the sample.
Using the same electric resistance measuring apparatus as used above, a voltage of 10 V was applied to a molded product formed from 2.0 g of the particles to be measured, to measure an electric resistance value thereof. Thereafter, the voltage applied to the molded product was increased at intervals of 10 V to measure an electric current flowed through the sample at each voltage. The voltage-applying time was within 20 sec for each voltage. When the applied voltage was increased and reached a certain value, the sample suffered from dielectric breakdown so that the electric current flowed therethrough became remarkably high and unmeasurable. The voltage E (V) at which the electric current flowed through the sample was unmeasurable was measured, and the measured value was divided by the thickness of the sample to determine a breakdown voltage (V/cm) of the sample.
23.75 L of a ferrous sulfate aqueous solution containing 1.6 mol/L of Fe2+ was added to a reactor previously filled with 26.25 L of an aqueous solution containing 2.75 mol/L of sodium hydroxide (corresponding to 0.95 equivalent based on Fe2+). While maintaining the obtained mixed solution at a pH value of 6.5 to 7.5 and a temperature of 90° C., air was passed through the solution at a flow rate of 80 L/min to conduct an oxidation reaction thereof (first stage reaction). At the time at which the oxidation-reduction potential was raised (0.05 equivalent of Fe2+ still remained in the solution), 1.25 L of a ferrous sulfate aqueous solution containing 1.6 mol/L of Fe2+ was added to the reaction solution, and then a sodium hydroxide aqueous solution was further added to the reactor to oxidize residual Fe2+ (3.9 mol) still remaining therein. Thus, after adjusting the pH value of the slurry in the reactor to 10 to 12, the slurry was successively subjected to oxidation reaction (second stage reaction) to complete the oxidation reaction of the slurry, thereby obtaining a slurry containing the core particles.
The thus obtained core particles were sampled, and subjected to filtration, washing with water and then drying by ordinary methods. As a result of observing a micrograph of the thus obtained core particles, it was confirmed that the core particles had a spherical shape.
The slurry containing the thus obtained core particles and having a pH value of not less than 10 was mixed with 0.5 L of an aqueous solution containing 2.30 mol/L of aluminum sulfate (corresponding to 10340 ppm based on the total amount of the particles). The resultant mixture was stirred at a temperature of 75 to 85° C. and then mixed with 0.05 L of an aqueous solution containing 1.23 mol/L of magnesium sulfate (corresponding to 498 ppm based on the total amount of the particles). The obtained mixture was uniformly mixed for at least 10 min, and then mixed with an acid aqueous solution to once adjust the pH value thereof to 8 to 10, followed by uniformly mixing the mixture for not less than 5 min. The thus obtained mixture was mixed again with an acid aqueous solution to gradually reduce the pH value thereof and finally reduce the pH value to 6.5 to 7.5. The thus obtained slurry was subjected to water-washing, filtration and then drying, thereby obtaining black magnetic iron oxide particles made of magnetite.
The same procedure as defined in Example 1 was conducted except that the conditions for production of the black magnetic particles were changed variously, thereby obtaining black magnetic iron oxide particles.
Production conditions of the core particles are shown in Table 1, and various properties of the obtained core particles are shown in Table 2. Further, production conditions of the black magnetic iron oxide particles are shown in Table 3, and various properties of the obtained black magnetic iron oxide particles are shown in Table 4.
Meanwhile, as shown in Table 2, the electric resistance values of all of the core particles 1 to 6 already exceeded the measurement limit when the voltage applied thereto reached 500 V. Therefore, the electric resistance values were unmeasurable.
Also, the Al amounts being present on the surface of the core particles obtained in Examples 8 and 9 and Comparative Example 2 as shown in Table 4 are values calculated by subtracting the Al amounts contained in the core particles from the Al amounts contained in the finally obtained surface-treated products.
As shown in Table 4, as to the black magnetic iron oxide particles obtained in Comparative Examples 1 to 10, the electric resistance values of these particles upon applying a voltage of 500 V thereto were unmeasurable.
The black magnetic iron oxide particles having a high electric resistance value in a high voltage range according to the present invention can be suitably used as a pigment in various applications. The black magnetic iron oxide particles can be suitably used, in particular, for a toner, since the obtained toner can exhibit a high image density even under high-temperature and high-humidity conditions.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-126173 | Apr 2006 | JP | national |
