JP4779141B2 - Carrier core material for electrophotographic development, method for producing the same, and magnetic carrier - Google Patents

Description

  The present invention relates to a carrier core material for dry development for providing high-quality electrophotography, a production method thereof, and a carrier using the carrier core material.

  The electrophotographic dry development method is a method in which a powder toner as a developer is attached to an electrostatic latent image on a photosensitive member, and the attached toner is transferred to a predetermined paper or the like for development. This method is roughly divided into a method using a one-component developer containing only toner and a method using a two-component developer containing toner and a magnetic carrier. In recent years, a two-component development method has become mainstream, in which toner charge control is easy, stable image quality can be obtained, and high-speed development is possible.

  Electrophotographic developing machines tend to be full color, high image quality, and high speed. To meet these demands, polymerized toner with small particle size has been developed, and the particle size of magnetic carrier has been reduced accordingly. Is progressing. On the other hand, with the widespread use of personal computers, the so-called MFP (multifunction printer) market has expanded in electrophotographic developing machines, and not only the document output capability but also running costs have been rigorously evaluated.

  The running cost of an electrophotographic developing machine greatly depends on the cost of consumables such as toner and magnetic carrier. Many of the magnetic carriers currently used use spherical soft ferrite as a core material, and the surface of the core material is coated with a resin. However, as the number of times of printing progresses, the phenomenon that the carrier surface is contaminated with toner or external additives, that is, spent occurs, and as a result, the charge of the toner decreases. Therefore, in many electrophotographic developing machines, when the counted number of printed documents reaches a certain value, the magnetic carrier is exchanged together with the toner.

Therefore, in order to alleviate such problems, a ferrite carrier using a light metal is used instead of the iron powder carrier (Patent Document 1), and the true specific gravity of the carrier such as a magnetic material dispersion type carrier is reduced (patent) Document 2) has been proposed. However, the damage to the toner has not yet been reduced to a satisfactory level.
On the other hand, a technique for reducing the specific gravity of a carrier by making a non-magnetic oxide phase present in the carrier core material is also known (Patent Document 3).

Japanese Patent Laid-Open No. 10-104884 Japanese Patent No. 2738734 Japanese Unexamined Patent Publication No. Sho 63-184664

  In order to extend the exchange life of the magnetic carrier, it is important to reduce the stress applied to the toner by the carrier core material. If the specific gravity of the carrier core material is reduced, the stress applied to the toner during the stirring and mixing of the electrophotographic developer in the electrophotographic developing machine can be reduced. In this regard, it is considered that the technique of reducing the specific gravity of carriers by making the nonmagnetic oxide phase disclosed in Patent Document 3 exist is effective.

  However, according to the study by the present inventors, for example, a magnetic carrier is manufactured by a method described in Patent Document 3, and an electrophotographic developer using the magnetic carrier is applied to the MFP (multifunction printer) or the like. In this case, it has been found that the effect of extending the exchange life of the magnetic carrier cannot be obtained. In other words, if the specific gravity of the carrier core material is simply reduced, a sufficiently satisfactory improvement effect cannot be obtained in recent high-performance electrophotographic developing machines.

  The inventors have made various investigations on the cause of this, and as a result of the addition of the non-magnetic substance, the sintering of the magnetic part is hindered, which increases the residual magnetization of the carrier core material, resulting in poor developer fluidity in the developing machine. I understood it. As a result, it was considered that the agitation resistance in the developing machine was increased, so that spent was generated and the exchange life of the magnetic carrier was not extended.

  In view of such problems, the present invention provides a carrier core material and a magnetic carrier capable of high-speed development with stable high image quality and improved magnetic carrier replacement life even when applied to a high-performance electrophotographic developing machine such as an MFP. Is to provide.

  As a result of detailed studies, the inventors have found that the above object can be realized by a carrier core material having a small apparent density, a small residual magnetization, and a high electric resistance when a high voltage is applied. And in order to obtain such a carrier core material, the knowledge that it was important to grind | pulverize a nonmagnetic component very finely at the time of a raw material, and to give an oxidation process after baking was acquired.

That is, in the present invention, MO · Fe ₂ O ₃ , the M component has a magnetic part having a composition represented by one or more divalent metal elements, and a nonmagnetic part containing SiO ₂ , and the apparent density There 2.0 g / cm ³ or less, residual magnetization 3A · m ² / kg or less, an electrophotographic developer carrier core material for electrical resistance made of the powder is 10 ⁵ Ω · cm or more is provided at an applied voltage of 250V The The SiO ₂ constituting the nonmagnetic portion is derived from a raw material made of a nonmagnetic oxide refined to an average particle size of 2 μm or less. The nonmagnetic portion may contain Fe ₂ O _{3 in} addition to SiO ₂ . The Si content in the powder is desirably 4 to 20% by mass.

Examples of the elements constituting the M component include divalent metal elements such as Mn, Mg, and Fe. Fe in the raw material constitutes “Fe ₂ O ₃ ” part of “MO · Fe ₂ O ₃ ” and part of “MO” as a divalent M element. Further, a nonmagnetic part is formed as Fe ₂ O ₃ . When M component consists of 1 or more types of Mn, Mg, and Fe, it is desirable that the compounding ratio of Mn, Mg, and Fe contained in the powder satisfies the following formula (1).
50 ≦ Fe ₂ O ₃ / (MnO + MgO + Fe ₂ O ₃ ) × 100 ≦ 100 (1)

However, the values obtained by converting the compounding ratios of Mn, Mg, and Fe into the molar ratios of MnO, MgO, and Fe ₂ O ₃ are substituted for MnO, MgO, and Fe ₂ O _{3 in} the above formula.
For example, when the molar ratio of the metal elements contained in the powder is Mn: Mg: Fe = 1: 1: 6, MnO = 1, MgO = 1, Fe ₂ O _{3 in the} formula (1) = 6/2 = 3 is substituted, and the value of Fe ₂ O ₃ / (MnO + MgO + Fe ₂ O ₃ ) × 100 is 3 / (1 + 1 + 3) × 100 = 60. Further, in the case of Example 3 in Table 1 described later, when MnO = 43, MgO = 0, and Fe ₂ O ₃ = 57 are substituted into the formula (1), Fe ₂ O ₃ / (MnO + MgO + Fe ₂ O ₃ ) × The value of 100 is 57 / (43 + 0 + 57) × 100 = 57.

  By covering this carrier core material with resin, a magnetic carrier for electrophotographic development is constructed.

As a method for producing such a carrier core material, a step of obtaining a slurry by mixing SiO ₂ powder refined to an average particle size of 2 μm or less with other raw materials, and spray-drying the slurry to obtain a granulated product A step of firing the granulated product at 1150 to 1350 ° C. to obtain a fired product having a composite structure having a magnetic part and a non-magnetic part, and forming the high resistance layer by heating the fired product in an oxidizing atmosphere There is provided a method for producing a carrier core material for an electrophotographic developer having a step (referred to as “high resistance treatment step”). The blending of raw materials to be mixed in the step of obtaining the slurry is preferably in the range of 10 to 70 parts by mass of the nonmagnetic part raw material with respect to 100 parts by mass of the total magnetic part raw material.
A step of adjusting the particle size is appropriately inserted before firing and before the resistance increasing treatment step.
Here, as the “average particle diameter”, D ₅₀ (median diameter) obtained by a laser diffraction particle size distribution analyzer can be adopted.

  In addition, the “carrier core material” in the present specification means a stage before resin coating, that is, a stage after the high resistance treatment process is finished. Therefore, the characteristics of the carrier core material (the apparent density, residual magnetization, and electrical resistance) are measured using the powder at this stage.

  According to the present invention, a carrier core material having a small specific gravity (apparent density) and a small residual magnetization is provided by using a refined nonmagnetic oxide powder as a raw material. When this carrier core material is used, the problem that the stirring resistance increases in the developing machine, which is seen with the conventional carrier core material having a non-magnetic part, is solved, and good image quality is obtained in a high-performance electrophotographic developing machine such as an MFP. It is expected that the properties can be realized and the exchange life of the magnetic carrier can be remarkably improved.

  As described above, in the conventional carrier core material that is reduced in weight by mixing the magnetic part and the nonmagnetic part, the nonmagnetic oxide added as a raw material becomes an obstacle, and the magnetic part is sintered during firing. As a result, the residual magnetization increases and the stirring resistance in the developing machine increases. When the high resistance treatment (described later) is performed, the increase in residual magnetization is further increased, and it has been particularly difficult to achieve both reduction in residual magnetization and high resistance.

The inventors have conducted detailed studies on a nonmagnetic oxide addition method that does not inhibit the sintering of the magnetic part. As a result, it has been found that it is extremely effective to sufficiently refine the nonmagnetic oxide (SiO ₂ ) added as a raw material in advance. When a magnetic part such as soft ferrite is produced by firing, in order to obtain a granulated product for firing, usually a process of wet-grinding a mixture of various raw material powders using a binder or the like in a step prior to firing. Is done. The above-described sintering inhibition due to the nonmagnetic oxide cannot be sufficiently eliminated only by the refinement by wet pulverization. In order to obtain the carrier core material of the present invention, it is important to sufficiently refine the nonmagnetic oxide at the stage of “raw material powder” before wet pulverization.

The reason why the refinement of the nonmagnetic oxide at the raw material powder stage is extremely advantageous in reducing the inhibition of sintering of the magnetic part has not been fully elucidated at this time. The non-magnetic oxide refined in stages has the property that it is difficult to inhibit the diffusion of Fe, Mn, Mg, etc. during firing, and this is thought to promote the sintering of the magnetic part.
Hereinafter, matters for specifying the present invention will be described.

[Magnetic part]
The magnetic part in the carrier core material of the present invention is the one having a ferrite structure having a composition represented by MO.Fe ₂ O ₃ . The M component is a divalent metal element, which is an alkaline earth metal or transition metal, and examples thereof include Mn, Mg, and Fe. The M component can be composed of only Fe, but the controllable range of magnetic characteristics can be expanded by using a mixture of two or more types. Inclusion of Mg, which is a light element, is also effective in reducing the apparent density of the carrier core material.

[Nonmagnetic part]
The nonmagnetic part preferably employs a nonmagnetic oxide having a true specific gravity of 3.5 g / cm ³ or less from the viewpoint of reducing the weight of the magnetic carrier. As such a preferred example, SiO ₂ is used in the present invention. The content of the non-magnetic part needs to be sufficient to ensure that the apparent density is in the appropriate range described later. Specifically, the apparent density is within the composition range of the magnetic part raw material and the non-magnetic part raw material described later. Density and electrical resistance can be controlled within appropriate ranges. On the other hand, in order to control the electrical resistance, Fe ²⁺ may be changed to Fe ³⁺ and Fe ₂ O ₃ may be included as a part of the non-magnetic portion by a high resistance treatment described later. The electric resistance can be controlled within an appropriate range by controlling the proportion of the Fe ₂ O ₃ phase. The nonmagnetic portion may contain impurity crystals in addition to SiO ₂ and Fe ₂ O ₃ . However, it is desirable that the amount of SiO ₂ + Fe ₂ O ₃ occupying in the nonmagnetic part is secured by 50% by mass or more.

[Apparent density]
As a result of investigations by the inventors, it has been found that it is important to reduce the apparent density of the carrier core material to 2.0 g / cm ³ or less in order to sufficiently reduce the damage to the toner by the magnetic carrier. It is more preferably 1.7 g / cm ³ or less, and even more preferably 1.5 g / cm ³ or less. The apparent density of the carrier core material can be controlled by the kind of oxide constituting the nonmagnetic part, the content of SiO ₂ in the nonmagnetic part, the kind of M component element used in the magnetic part, and the like.

[Residual magnetization]
As the residual magnetization of the magnetic carrier increases, the stirring resistance of the developer in the developing machine increases as described above, resulting in a decrease in spent resistance and, in turn, a decrease in the lifetime of the magnetic carrier. As a result of various studies, it has been found that in order to improve the life of the magnetic carrier, it is necessary to suppress the residual magnetization to 3 A · m ² / kg or less as the carrier core material. More preferably, it is 2.5 A · m ² / kg or less. The residual magnetization of the carrier core material can be controlled by using a refined raw material powder of SiO ₂ .

[Electric resistance when high voltage is applied]
In order to stably realize high-quality image characteristics in electrophotographic development, the magnetic carrier is required to have a high electric resistance. According to the detailed study by the inventors, by using a carrier core material having an electric resistance of 10 ⁵ Ω · cm or more at an applied voltage of 250 V, high-quality image characteristics can be stably obtained even in a high-performance MFP. Therefore, breakdown of carriers can be prevented even when a high voltage is applied. The high electrical resistance of the carrier core material can be imparted by oxidation treatment after firing (high resistance treatment described later).

The electrical resistance at an applied voltage of 250 V can be obtained as follows. Two brass plates having a thickness of 2 mm and having an electropolished surface as electrodes are placed on an insulating plate (for example, an acrylic plate coated with Teflon (registered trademark)) placed horizontally so that the distance between the electrodes is 2 mm. To place. The normal direction of the two electrode plates is set to be the horizontal direction. After inserting 200 ± 1 mg of the powder to be measured into the gap between the two electrode plates, a magnet having a cross-sectional area of 240 mm ² is arranged behind each electrode plate to form a bridge of the powder to be measured between the electrodes. . In this state, a DC voltage of 250 V is applied between the electrodes, and the current value flowing through the powder to be measured is measured by the four-terminal method. From the current value, the distance between the electrodes of 2 mm, and the cross-sectional area of 240 mm ² , the electric resistance of the powder to be measured (the dimension corresponding to the volume resistance) is calculated. Various magnets can be used as long as the powder can form a bridge. In the examples described later, a permanent magnet (ferrite magnet) having a surface magnetic flux density of 1000 gauss or more is used.

[Particle diameter of carrier core material]
The carrier core material of the present invention has an average particle diameter D50 of about 20 to 70 [mu] m by a laser diffraction particle size distribution measuring apparatus (Microtrack, Model 9320-X100 manufactured by Nikkiso Co., Ltd.), and more preferably 25 to _{50 [} mu] m. Suitable target.

Such a carrier core material of the present invention can be obtained, for example, by the following production method.
[Raw material]
To produce a spinel type soft ferrite having a composition represented by the MO · Fe ₂ O ₃ as a magnetic unit, as the Fe source Fe ₂ O ₃ can be preferably used. As the M component supply source other than Fe, for example, MnCO ₃ , Mn ₃ O ₄ or the like can be suitably used in the case of Mn, and MgCO ₃ , Mg (OH) ₂ , MgO or the like can be suitably used in the case of Mg. These raw materials for the magnetic part are referred to herein as “magnetic part raw materials”. Each magnetic part raw material is weighed so that the mixing ratio of Fe and M component metal elements becomes a target value after firing.

Each magnetic part raw material is desirably refined to an average particle diameter D ₅₀ of 5 μm or less, for example, in the range of 0.1 to 2.0 μm, in a stage comprising dry particles that have not yet been granulated. A powder raw material that has been previously adjusted to the above particle size is prepared, or the particle size is adjusted by grinding with a dry ball mill or the like. The particle size of each magnetic part material may be adjusted so as to correspond to the above particle size by pulverizing after mixing with the nonmagnetic oxide material.

[SiO ₂ raw material]
It is necessary to use SiO ₂ (made of particles that have not been granulated yet) that has been refined so that the average particle diameter D ₅₀ is 2 μm or less as the raw material for the nonmagnetic part. . It is more preferable to use one having a D ₅₀ of 1 μm or less, for example, in the range of 0.1 to 1.0 μm. If the particle size at this stage is too large, sintering of the magnetic part is hindered during firing, which is not preferable. Moreover, since the uniformity of the composition inside a particle | grain falls and a carrier may fly, it is unpreferable. Therefore, as the nonmagnetic part raw material, a powder raw material in which particles are refined as described above is prepared in advance, or a material sufficiently refined in the above range by pulverizing with a dry ball mill or the like is used. . The raw material powder of SiO ₂ is preferably weighed so that 4 to 20% by weight of Si is contained in the powder of the carrier core material.

[Slurry]
After weighing the magnetic part raw material and the nonmagnetic part raw material, they are slurried by mixing and stirring them in a medium solution. Prior to slurrying, the raw material mixture may be subjected to a dry pulverization treatment as necessary. The mixing ratio of the raw material powder and the medium liquid is preferably such that the slurry has a solid content concentration of 50 to 90% by mass. A medium solution prepared by adding a binder, a dispersant and the like to water is prepared. For example, polyvinyl alcohol can be suitably used as the binder, and the concentration in the medium liquid may be about 0.5 to 2% by mass. As the dispersant, for example, an ammonium polycarboxylate-based one can be preferably used, and the concentration in the medium liquid may be about 0.5 to 2% by mass. In addition, phosphorus, boric acid, or the like can be added as a lubricant or a sintering accelerator. It is preferable to further wet-grind the slurry obtained by mixing and stirring.

[Granulation]
Granulation can be suitably carried out by introducing the slurry into a spray dryer. The atmospheric temperature during spray drying may be about 100 to 300 ° C. Thereby, a granulated powder having a particle diameter of about 10 to 200 μm can be obtained. It is desirable to adjust the particle size of the obtained granulated powder by taking into consideration the final particle size of the product and removing particles and fine particles that are too large using a vibration sieve or the like.

[Baking]
Next, the granulated powder is put into a furnace heated to 1150 to 1350 ° C. and fired by a general method for synthesizing soft ferrite, thereby generating ferrite. It is preferable to advance the sintering of the magnetic part at this stage to reduce the internal vacancies and the surface vacancies. If there are a large number of internal vacancies and surface vacancies, the residual magnetization increases when the resistance is increased thereafter, which is not preferable. If the firing temperature is lower than 1150 ° C., sintering may not proceed sufficiently, and it will be difficult to stably reduce the residual magnetization. It is particularly preferable to fire at about 1170 to 1300 ° C. By this firing, a fired product having a composite structure having a ferrite magnetic part and a non-magnetic part is obtained.

  It is desirable to adjust the particle size of the obtained fired product at this stage. For example, the baked product is coarsely pulverized with a hammer mill or the like, then subjected to primary classification with an airflow classifier, and further subjected to a process of aligning the particle size with a vibration sieve or an ultrasonic sieve to obtain a baked product with an adjusted particle size. be able to. Further, it is desirable to remove the non-magnetic particles by applying a magnetic separator.

[High resistance treatment]
By heating the fired product in an oxidizing atmosphere, a high resistance layer is formed and the resistance is increased. The heating atmosphere may be air or a mixed atmosphere of oxygen and nitrogen. The heating temperature may be 200 to 800 ° C., preferably 250 to 600 ° C., and the treatment time may be about 30 min to 5 h.
As described above, it is possible to obtain the carrier core material of the present invention that simultaneously realizes a reduction in apparent density, a reduction in residual magnetization, and an increase in resistance.

[Manufacture of magnetic carrier]
Resin coating is applied to the high-resistance carrier core material. As the coating resin, a silicone resin is preferable. The coating resin is dissolved in a solvent (toluene or the like) at about 20 to 40% by mass to prepare a resin solution. In the coating operation, the mixing ratio of the resin solution and the carrier core material is mixed in a container so that the mixing ratio of the carrier core material: resin solution = 10: 1 to 5: 1 is 150 to 250 ° C. It can be carried out by heating and stirring at. What is necessary is just to adjust a coating amount so that 0.1-10 mass parts of resin may adhere with respect to 100 mass parts of carrier core materials in the state before solvent drying. The resin coating amount can be controlled by the concentration of the resin solution and the mixing ratio of the resin solution and the carrier core material. After the coating, a heat treatment is further performed to cure the resin coating layer, thereby obtaining a magnetic carrier.

[Manufacture of electrophotographic developer]
An electrophotographic developer can be obtained by mixing the obtained magnetic carrier with a toner having an appropriate particle size.

Example 1
Fe ₂ O ₃ powder finely pulverized to an average particle diameter D ₅₀ of about 1 μm was prepared. Further, a fine SiO ₂ powder having an average particle diameter D ₅₀ of about 1 μm was prepared by pulverizing SiO ₂ powder having an average particle diameter D ₅₀ of about 4 μm with a dry ball mill. These raw material powders were weighed to obtain a blending ratio of SiO ₂ = 20 parts by mass with respect to Fe ₂ O ₃ = 100 parts by mass. On the other hand, in water, 1.5% by mass of an ammonium polycarboxylate dispersant as a dispersant, 0.05% by mass of “SN Wet 980” manufactured by San Nopco Co., Ltd. as a wetting agent, and 0.02% of polyvinyl alcohol as a binder. A liquid (medium liquid) added by mass% was prepared. The above-mentioned weighed raw material powder was put into this medium solution and stirred to obtain a slurry having a concentration of 75% by mass of these charged substances. This slurry was wet pulverized with a wet ball mill, stirred for a while, and then sprayed into hot air at about 180 ° C. with a spray dryer to obtain a dry granulated product having a particle size of 10 to 200 μm.

  A granulated product obtained by separating coarse particles and fine particles from the granulated product using a sieve mesh having a mesh size of 61 μm and 25 μm was baked at 1170 ° C. in a nitrogen atmosphere for 5 hours to be ferritized. The ferritized fired product was pulverized with a hammer mill, fine powder was removed using an air classifier, and the particle size was adjusted with a vibrating screen having a mesh size of 54 μm.

The fired product whose particle size was adjusted was held at 400 ° C. in the atmosphere for 3 hours to give a high resistance treatment, thereby obtaining a carrier core material.
As a result of X-ray diffraction, this powder was confirmed to have an MO · Fe ₂ O ₃ type magnetic part and a composite structure containing SiO ₂ and Fe ₂ O ₃ (the same applies to the following examples). .

With respect to this carrier core material, the apparent density, residual magnetization, and electric resistance when 250 V was applied were measured as follows.
[Apparent density]
This was performed according to JIS Z2504: 2000.

[Residual magnetization]
It measured with the vibration sample type magnetometer (VSM) (made by Toei Industry Co., Ltd.) only for room temperature.

[Electric resistance when 250V is applied]
A sample of 200 ± 1 mg was collected from a sample of this carrier core material left in a measurement environment temperature of 20 ± 2 ° C. and a humidity of 60 ± 5% RH for one day. And the electrical resistance was measured. For the measurement, an insulation resistance meter (SM-8220, manufactured by Toa DKK Co., Ltd.) was used, and the value indicated 1 min after applying a predetermined voltage (250 V) was read as the value of the static resistance. If the calculated electrical resistance (expressed in the same dimension as the volume resistance) is 10 ⁵ Ω · cm or more, it is evaluated that a sufficiently high resistance can be realized. When this value was less than 10 ⁴ Ω · cm, it was evaluated as breakdown, and it was expressed as BD in Table 1 described later.

  Next, a silicone resin (manufactured by Shin-Etsu Chemical Co., Ltd., KR251) was dissolved in toluene to prepare a coating resin solution. The carrier core material and the resin solution are introduced into the stirrer at a mass ratio of carrier core material: resin solution = 9: 1, and heated in the range of 150 to 250 ° C. while immersing the carrier core material in the resin solution for 3 hours. Stir. Thereby, the resin was coated at a ratio of 1.0 part by mass with respect to 100 parts by mass of the carrier core material. The resin-coated carrier core material was heated at 250 ° C. for 5 hours with a hot-air circulating heating device to cure the resin coating layer, whereby the magnetic carrier according to Example 1 was obtained.

This magnetic carrier was mixed with a commercially available toner having a particle size of about 1 μm to produce an electrophotographic developer, and the spent resistance and image characteristics were evaluated.
The spent resistance was evaluated by the following method. The above-mentioned resin-coated carrier and commercially available toner are mixed so that the toner concentration becomes 10%, and a developer is prepared. The developer is left for one day in a measurement environment temperature of 20 ± 2 ° C. and humidity of 60 ± 5% RH. did. 100 g of the obtained developer was put into a plastic bottle (100 cc) with an inner lid, set on a shaking machine (red devil), and stirred. The carrier after receiving this stirring operation was observed with a scanning electron microscope (SEM). ◎ (very good) when toner spent is not observed, ○ (good) when toner spent is slight and within acceptable range (usable), △ (slightly poor) when toner spent is observed and outside acceptable range Those that could not be used due to a large amount of toner spent were evaluated as x (defect), and those rated as o or better were determined to be acceptable.

As for image characteristics, the 40-sheet machine adopting the digital reversal development system using this electrophotographic developer is used as an evaluation machine, and the initial image is evaluated for image density, fog density, carrier skip, fine line reproducibility, and image quality. did. Of these, “image quality” indicates the overall evaluation. The evaluation criteria are as follows: ◎ is a very good level, ◯ is a good level, Δ is a usable level, and x is a non-usable level. Here, ◯ evaluation is the same level as a high-performance electrophotographic developer that is currently in practical use, and ◯ evaluation or more was determined to be acceptable.
These results are shown in Table 1. The Si content in the table is an analysis result (the same applies in the following examples).

Example 2
The experiment was performed under the same conditions as in Example 1 except that the mixture was weighed at a blending ratio of SiO ₂ = 60 parts by mass with respect to Fe ₂ O ₃ = 100 parts by mass and the firing temperature was 1290 ° C. The raw material powder used is the same as in Example 1.

Example 3
As a raw material, Mn ₃ O ₄ finely pulverized to an average particle diameter D ₅₀ of about 1 μm is added, and the MO / Fe ₂ O ₃ ferrite composition has a Mn / Fe molar ratio of MnO / Fe ₂ O _3. The experiment was performed under the same conditions as in Example 1 except that the weight was converted so that MnO: Fe ₂ O ₃ = 43: 57. In this case, the blending ratio was SiO ₂ = 20 parts by mass with respect to (Fe ₂ O ₃ + Mn ₃ O ₄ ) = 100 parts by mass. The same raw material powder as in Example 1 was used except for Mn ₃ O ₄ .

Example 4
The experiment was performed under the same conditions as in Example 3, except that the weight ratio was (Fe ₂ O ₃ + Mn ₃ O ₄ ) = 100 parts by mass with a mixing ratio of SiO ₂ = 40 parts by mass. The raw material powder used is the same as in Example 3.

Example 5
As a raw material, MgCO ₃ finely pulverized to an average particle diameter D ₅₀ of about 1 μm is added. In the MO · Fe ₂ O ₃ ferrite composition, the compounding ratio of Mg and Fe is converted to a molar ratio of MgO and Fe ₂ O _3. Then, the experiment was performed under the same conditions as in Example 1 except for weighing so that MgO: Fe ₂ O ₃ = 20: 80. In this case, the blending ratio was SiO ₂ = 20 parts by mass with respect to (Fe ₂ O ₃ + MgCO ₃ ) = 100 parts by mass. The same raw material powder as in Example 1 was used except for MgCO ₃ .

<< Comparative Example 1 >>
The experiment was performed under the same conditions as in Example 1 except that the mixture was weighed at a compounding ratio of SiO ₂ = 10 parts by mass with respect to Fe ₂ O ₃ = 100 parts by mass. The raw material powder used is the same as in Example 1.

<< Comparative Example 2 >>
The experiment was performed under the same conditions as in Example 1 except that the high resistance treatment step was omitted.

<< Comparative Example 3 >>
The experiment was performed under the same conditions as in Example 1 except that the mixture was weighed at a compounding ratio of SiO ₂ = 40 parts by mass with respect to Fe ₂ O ₃ = 100 parts by mass and the firing temperature was 1050 ° C. The raw material powder used is the same as in Example 1.

<< Comparative Example 4 >>
A SiO ₂ raw material powder having an average particle diameter D ₅₀ of about 4 μm was used, and weighed at a blending ratio of SiO ₂ = 20 parts by mass with respect to Fe ₂ O ₃ = 100 parts by mass, and the firing temperature was 1170 ° C. The experiment was performed under the same conditions as in Example 1. The raw material powder used is the same as in Example 1.

As can be seen from Table 1, the carrier core material of each example using an appropriate amount of a very fine non-magnetic part material has an apparent density of 2.0 g / cm ³ or less, a residual magnetization of 3 A · m ² / kg or less, The electric resistance at an applied voltage of 250 V has a characteristic of 10 ⁵ Ω · cm or more. In the electrophotographic developer using this, no spent was generated and a high-quality image could be obtained.

On the other hand, in Comparative Example 1, the apparent density of the carrier core material increased because the amount of nonmagnetic oxide added was small. For this reason, the stirring torque in the developing machine cannot be reduced, and the electrophotographic developer using the developing machine generates a spent and inferior image characteristics. In Comparative Example 2, since the high resistance treatment was not performed, the image quality characteristics of the electrophotographic developer were inferior to those of the above examples. In Comparative Example 3, the remanent magnetization was increased because the firing temperature was too low. For this reason, although the apparent density and electrical resistance were good, the fluidity was poor when used as a developer, and the stirring torque could not be reduced satisfactorily. Further, the mixing property with the toner was poor, and the image quality characteristics were inferior to those of the above examples. In Comparative Example 4, since the average particle diameter D ₅₀ of the used nonmagnetic part raw material was larger than 2 μm, it is considered that this nonmagnetic part raw material became a factor inhibiting the sintering of the magnetic part. The remanent magnetization of the core material increased. As a result, the image characteristics were considerably worse than those of Example 1.

Claims (6)

MO · Fe ₂ O ₃ , the M component has a magnetic part having a composition represented by a divalent metal element, and a nonmagnetic part containing SiO ₂ , and an apparent density of 2.0 g / cm ³ or less, A carrier core material for an electrophotographic developer comprising a powder having a remanent magnetization of 3 A · m ² / kg or less and an electric resistance of 10 ⁵ Ω · cm or more at an applied voltage of 250 V. 2. The electrophotography according to claim 1, wherein the M component is composed of one or more of Mn, Mg, and Fe, and a mixing ratio of Mn, Mg, and Fe contained in the powder satisfies the following formula (1). Carrier core material for developer.
50 ≦ Fe ₂ O ₃ / (MnO + MgO + Fe ₂ O ₃ ) × 100 ≦ 100 (1)
However, the values obtained by converting the compounding ratios of Mn, Mg, and Fe into the molar ratios of MnO, MgO, and Fe ₂ O ₃ are substituted for MnO, MgO, and Fe ₂ O _{3 in} the above formula. 3. The carrier core material for an electrophotographic developer according to claim 1, wherein the SiO ₂ in the nonmagnetic part is derived from a SiO ₂ raw material powder refined to an average particle diameter of 2 μm or less.   The carrier core material for an electrophotographic developer according to any one of claims 1 to 3, wherein the Si content in the powder is 4 to 20% by mass. A step of obtaining a slurry by mixing SiO ₂ powder refined to an average particle size of 2 μm or less with other raw materials, a step of obtaining a granulated product by spray drying the slurry, and a step of 1150 to 1350 ° C. A step of obtaining a fired product having a composite structure having a magnetic part and a non-magnetic part by firing at a step of forming a high resistance layer by heating the fired product in an oxidizing atmosphere (referred to as “high resistance treatment step”) The manufacturing method of the carrier core material for electrophotographic developers in any one of Claims 1-4 which have these.   A magnetic carrier for electrophotographic development, wherein the carrier core material according to claim 1 is coated with a resin.