JP7548849B2 - Carrier core material, and electrophotographic development carrier and electrophotographic developer using the same - Google Patents

Description

The present invention relates to a carrier core material and a carrier for electrophotographic development and a developer for electrophotography that use the carrier core material.

When a resin-coated carrier, which is made of ferrite particles and has its surface coated with resin, is mixed with toner to form a two-component developer, a problem called "development memory" can occur, in which the image density decreases due to the influence of the image from the previous rotation of the developing roller. This development memory is thought to be caused by the high electrical resistance of the resin-coated carrier, and one of the countermeasures considered is to make the surface of the carrier core uneven, exposing part of the carrier core on the surface of the resin-coated carrier, thereby lowering the electrical resistance of the resin-coated carrier.

However, if the electrical resistance of the resin-coated carrier becomes low, there is a risk of "carrier adhesion" occurring, in which charge is injected into the resin-coated carrier in the development area, causing the resin-coated carrier to move to the photoconductor drum.

In addition, one possible way to make the surface of the ferrite particles that make up the carrier core material uneven is to lower the firing temperature in the manufacturing process to weaken the sintering of the ferrite particles. However, lowering the firing temperature increases the pore volume within the ferrite particles, decreasing the magnetization per particle volume. As a result, the magnetic adhesion of the carrier to the developing roller weakens, and there is a risk of "carrier scattering" occurring, in which the carrier is scattered from the developing roller due to the centrifugal force of the rotating developing roller.

As a carrier core material that can be used stably for a long period of time while suppressing the occurrence of development memory, carrier adhesion, and carrier scattering, for example, Patent Document 1 proposes a carrier core material with a specific core composition, in which the average of the average length RSm of the elements of the particle group that constitutes the carrier core material is within a specific range, and the average of the skewness Rsk of the particle group that constitutes the carrier core material is within a specific range.

Patent Document 2 proposes a carrier core material having a specific composition, containing a specific amount of Sn and having a specific range of saturation magnetization σs, and Patent Document 3 proposes a carrier core material containing ferrite particles containing a crystalline phase component made of perovskite crystals represented by _RZrO3 (R: alkaline earth metal element).

JP 2014-197133 A JP 2020-144310 A Patent No. 6757872

The object of the present invention is to provide a carrier core material that can further suppress the occurrence of development memory, carrier adhesion, and carrier scattering.

Another object of the present invention is to provide a carrier for electrophotography and a developer for electrophotography that can stably form images of good quality even after long-term use.

A carrier core material according to one embodiment of the present invention that achieves the above object is a carrier core material that contains as its main component a material represented by the composition formula _{MnXFe3 - XO4} (where 0<X<1) and also contains Sr and Zr, and is characterized in that the Sr content is from 0.01 mol% to 0.50 mol%, the Zr content is from 0.10 mol% to 0.50 mol%, the saturation magnetization _σs is from 77 ^{Am2 /} kg to 87 Am2 ^/ kg, the remanent magnetization _σr is from 0.2 ^Am2 /kg to 2.0 ^Am2 /kg, and the magnetization _σ1k when a magnetic field of 79.58× ¹⁰³ A/m (1000 Oersted) is applied is from 63 Am2/kg to 68 ^Am2 /kg.

In the carrier core material having the above configuration, it is preferable that the maximum height Rz is 1.60 μm or more and the average length RSm is 7.0 μm or less.

In the carrier core material having the above structure, the pore volume is preferably 0.015 cm ³ /g or less.

In the carrier core material having the above-mentioned structure, the true density is preferably 4.800 g/cm ³ or more and 5.000 g/cm ³ or less.

In the carrier core material having the above-mentioned configuration, it is preferable that the particle volume magnetization calculated from the following formula (1) is 0.284 Am ² /cm ³ or more and 0.320 Am ² /cm ³ or less.
Particle volume magnetization = magnetization σ _1k (Am ² /kg) x true density (g/cm ³ ) x 10 ^-3 x (100 - excluded volume (%)) / 100... (1)
(wherein, magnetization σ _1k : magnetization when a magnetic field of 79.58×10 ³ A/m (1000 Oersted) is applied, and excluded volume (%)=(1−1/(1+pore volume×true density))×100)

The present invention also provides a carrier for electrophotographic development, characterized in that the surface of the carrier core material described above is coated with a resin.

The present invention also provides an electrophotographic developer that contains the electrophotographic development carrier and toner described above.

In this specification, the saturation magnetization _σs , residual magnetization _σr , magnetization _σ1k , maximum height Rz, average length RSm, pore volume, and true density are values measured by the measurement methods and conditions described in the Examples below. In addition, in this specification, the terms "ferrite particles,""carrier core material,""electrophotographic development carrier," and "toner" each mean an aggregate (powdery material) of individual particles.

The carrier core material of the present invention suppresses the occurrence of development memory, carrier adhesion, and carrier scattering.

In addition, the electrophotographic development carrier and electrophotographic developer according to the present invention can stably form images of good quality even after long-term use.

Carrier core material of Example 1 and SEM photograph of cross section Carrier core material and cross-section SEM photograph of Comparative Example 2 Carrier core material of Comparative Example 8 and SEM photograph of cross section FIG. 1 is a schematic diagram showing an example of a developing device using a carrier according to the present invention;

One of the major features of the carrier core material according to the present invention is that the main component of the carrier core material is a material represented by Mn _x Fe _3-x O ₄ (where 0<x<1) and contains a predetermined amount of Sr (strontium) and Zr (zirconium). By containing a predetermined amount of Sr and Zr in the carrier core material, a high-melting point complex oxide containing Sr and Zr components is generated at the grain boundaries in the firing process when manufacturing a carrier core material made of ferrite particles. Usually, when the firing temperature is high, excessive crystals are generated by excessive sintering. When the crystals constituting the particles of the carrier core material become large, the resin coating layer tends to become non-uniform when the particle surface of the carrier core material is coated with a resin. On the other hand, when the high-melting point complex oxide is generated at the grain boundaries, the growth of the crystals is suppressed by the high-melting point complex oxide, and excessive crystals are difficult to generate even when firing is performed at a high temperature. As a result, the resin coating layer on the particle surface of the carrier core material tends to become uniform.

The Sr content in the carrier core material is in the range of 0.01 mol% or more and 0.50 mol% or less. The Zr content is in the range of 0.10 mol% or more and 0.50 mol% or less. By having the Sr and Zr contents in this range, a high melting point composite oxide containing Sr and Zr components is effectively generated at the crystal grain boundaries. A more preferable Sr content is in the range of 0.1 mol% or more and 0.30 mol% or less. A more preferable Zr content is in the range of 0.15 mol% or more and 0.45 mol% or less.

Another major feature of the carrier core material according to the present invention is that the residual magnetization _σr is 0.2 ^Am2 /kg or more and 2.0 ^Am2 /kg or less. When the residual magnetization _σr of the carrier core material is in this range, carrier scattering is suppressed. A preferred upper limit value of the residual magnetization _σr of the carrier core material is 1.1 ^Am2 /kg. On the other hand, a preferred lower limit value of the residual magnetization _σr of the carrier core material is 0.7 ^Am2 /kg. The residual magnetization _σr of the carrier core material can be adjusted, for example, by the firing temperature, firing time, oxygen concentration during firing, etc.

Another major feature of the carrier core material according to the present invention is that the saturation magnetization _σs is in the range of 77 ^Am2 /kg or more and 87 ^Am2 /kg or less. When the saturation magnetization _σs of the carrier core material is in this range, the occurrence of carrier scattering is suppressed. The preferred lower limit of the saturation magnetization σs of the carrier core material is 80 ^Am2 /kg. On the other hand, the preferred upper limit of the saturation magnetization _σs of the carrier core material is 85 ^Am2 /kg.
The saturation magnetization σ _s of the carrier core material can be adjusted by, for example, the blending amount of the Fe component raw material.

The maximum height Rz of the carrier core material according to the present invention is preferably 1.60 μm or more, and the average length RSm is preferably 7.0 μm or less. By forming unevenness of a predetermined height or more at a predetermined interval or less on the particle surface of the carrier core material, when the particle surface is coated with resin, the coating resin can be applied uniformly, and the coating resin is less likely to peel off even after long-term use. Even if a part of the coating resin peels off, the coating resin remaining in the recesses suppresses the decrease in the ability to impart charge to the toner. Furthermore, cracking and chipping of the particles are also suppressed. A more preferable lower limit of the maximum height Rz of the carrier core material is 1.80 μm. A preferable upper limit of the maximum height Rz is 2.00 μm. On the other hand, a more preferable upper limit of the average length RSm of the carrier core material is 6.5 μm. A preferable lower limit of the average length RSm is 6.0 μm. The maximum height Rz and average length RSm of the carrier core material can be adjusted, for example, by the baking temperature, baking time, and the amount of Sr component raw material and Zr component raw material added.

The pore volume of the carrier core material according to the present invention is preferably 0.015 cm ³ /g or less. If the pore volume of the carrier core material exceeds 0.015 cm ³ /g, i.e., if the internal voids of the carrier core material become large, the magnetization per particle of the carrier core material becomes small, and carrier scattering becomes more likely to occur. A more preferable upper limit of the pore volume of the carrier core material is 0.014 cm ³ /g. The pore volume of the carrier core material can be adjusted, for example, by the type of each component raw material of the carrier core material, the firing temperature, the firing time, etc. A preferable lower limit of the pore volume of the carrier core material is 0.001 cm ³ /g.

The true density of the carrier core material according to the present invention is preferably in the range of 4.800 g/ ^cm3 or more and 5.000 g/ ^cm3 or less. When the true density of the carrier core material is in this range, the magnetization per particle of the carrier core material becomes large, so that carrier scattering is easily suppressed. A more preferable lower limit of the true density of the carrier core material is 4.870 g/ ^cm3 . On the other hand, a more preferable upper limit of the true density of the carrier core material is 4.950 g/ ^cm3 . The true density of the carrier core material can be adjusted, for example, by the type of each component raw material of the carrier core material.

The particle volume magnetization of the carrier core material according to the present invention calculated from the formula (1) is preferably in the range of 0.284 Am ² /cm ³ or more and 0.320 Am ² /cm ³ or less. If the particle volume magnetization of the carrier core material is less than 0.284 Am ² /cm ³ , the magnetization per particle of the carrier core material is small, so that carrier scattering and carrier adhesion are likely to occur. On the other hand, if the particle volume magnetization exceeds 0.320 Am ² /cm ³ , development memory is likely to occur. A more preferable lower limit of the particle volume magnetization of the carrier core material is 0.290 Am ² /cm ^3. A more preferable upper limit of the particle volume magnetization of the carrier core material is 0.310 Am ² /cm ^3. The particle volume magnetization of the carrier core material can be adjusted, for example, by the type of each component raw material of the carrier core material, the firing temperature, the firing time, etc.

The volume average particle size of the carrier core material of the present invention is preferably in the range of 25 μm or more and less than 50 μm, and more preferably in the range of 30 μm or more and 40 μm or less.

In addition, in order to adjust the magnetic properties, electrical properties, shape, and powder properties of the carrier core material (and carrier) of the present invention, minor components can be added in an amount of up to 1 mass % to improve various properties. Examples of minor components include Na, K, Mg, Ca, Ti, Ba, Cu, Zn, Sn, Ni, and Co.

There are no particular limitations on the manufacturing method of the carrier core material of the present invention, but the manufacturing method described below is preferred.

First, Fe component raw material, Mn component raw material, Sr component raw material, Zr component raw material, and additives as necessary are weighed. As the Fe component raw material, _Fe2O3 or the _like is preferably used. As the Mn component raw material, _MnCO3 , _{Mn3O4 ,} or the like is preferably used. As the Sr component raw material, _SrCO3 , Sr( _NO3 ) ₂ , or the like is preferably used. As the Zr component raw material, _ZrO2 or the like is preferably used.

Next, the raw materials are put into the dispersion medium to prepare a slurry. The dispersion medium used in the present invention is preferably water. In addition, a binder, a dispersant, etc. may be added as necessary. For example, polyvinyl alcohol can be used as a binder. The amount of binder to be added is preferably about 0.1% by mass to 2% by mass in the slurry. In addition, for example, ammonium polycarboxylate can be used as a dispersant. The amount of dispersant to be added is preferably about 0.1% by mass to 2% by mass in the slurry. In addition, a reducing agent such as carbon black, a pH adjuster such as ammonia, a lubricant, a sintering accelerator, etc. may be added. The solid content concentration of the slurry is preferably in the range of 50% by mass to 90% by mass. More preferably, it is 60% by mass to 80% by mass. If it is 60% by mass or more, there are few pores in the particles in the granulated material, and insufficient sintering during firing can be prevented.

The weighed raw materials may be mixed, pre-fired, and broken down into particles, and then poured into a dispersion medium to produce a slurry. The pre-fire temperature is preferably in the range of 750°C to 1000°C. A temperature of 750°C or higher is preferable because the pre-fired process promotes partial ferritization, the amount of gas generated during firing is small, and solid-state reactions proceed sufficiently. On the other hand, a temperature of 1000°C or lower is preferable because the pre-fired process results in weak sintering, allowing the raw materials to be sufficiently pulverized in the subsequent slurry pulverization process. Furthermore, air is preferable as the atmosphere during pre-fired.

Next, the slurry prepared as described above is wet-milled. For example, wet-milling is performed for a predetermined time using a ball mill or a vibration mill. The average particle size of the raw material after milling is preferably 5 μm or less, more preferably 1 μm or less. It is preferable to incorporate media of a predetermined particle size in the vibration mill or ball mill. Examples of media materials include iron-based chrome steel and oxide-based zirconia, titania, alumina, etc. The milling process may be either continuous or batch-based. The particle size of the milled product is adjusted by the milling time, rotation speed, material and particle size of the media used, etc.

The pulverized slurry is then spray-dried to form granules. Specifically, the slurry is introduced into a spray dryer or other spray drying machine, and sprayed into the atmosphere to form spherical granules. The atmospheric temperature during spray drying is preferably in the range of 100°C to 300°C. This results in spherical granules with a particle size of 10 μm to 200 μm. Next, if necessary, the resulting granules are classified using a vibrating sieve to produce granules in a specified particle size range.

Next, the granulated material is put into a furnace heated to a predetermined temperature and sintered by a general method for synthesizing ferrite particles to produce ferrite particles. The sintering temperature is preferably in the range of 1100°C to 1350°C. If the sintering temperature is 1100°C or lower, phase transformation is unlikely to occur and sintering is also unlikely to proceed. If the sintering temperature exceeds 1350°C, excessive sintering may cause excessive grains to be generated. The heating rate up to the sintering temperature is preferably in the range of 250°C/h to 500°C/h. The holding time at the sintering temperature is preferably 2 hours or more. The unevenness of the ferrite particle surface can also be adjusted by the oxygen concentration in the sintering process. Specifically, the oxygen concentration is set to 0.05% to 10%. In addition, the oxidation state of the ferrite phase may be adjusted by setting the oxygen concentration during cooling lower than the oxygen concentration during sintering. Specifically, the oxygen concentration is set to the range of 0.05% to 1.5%. It is preferable to control the oxygen concentration during heating, sintering, and cooling to a range of 0.05% to 10%.

The sintered product thus obtained is disintegrated as necessary. Specifically, the sintered product is disintegrated, for example, by a hammer mill or the like. The disintegration process may be either continuous or batchwise. After the disintegration process, classification may be performed to adjust the particle size to a predetermined range, if necessary. As a classification method, a conventionally known method such as wind classification or sieve classification may be used. After primary classification using a wind classifier, the particle size may be adjusted to a predetermined range using a vibrating sieve or ultrasonic sieve. Furthermore, after the classification process, non-magnetic particles may be removed using a magnetic separator. The particle size of the ferrite particles is preferably 25 μm or more and less than 50 μm.

If necessary, the classified ferrite particles may then be heated in an oxidizing atmosphere to form an oxide film on the particle surface and increase the resistance of the ferrite particles (resistance-increasing treatment). The oxidizing atmosphere may be either an air atmosphere or a mixed atmosphere of oxygen and nitrogen. The heating temperature is preferably in the range of 200°C to 800°C, more preferably in the range of 360°C to 550°C. The heating time is preferably in the range of 0.5 hours to 5 hours. From the viewpoint of homogenizing the surface and interior of the ferrite particles, a low heating temperature is desirable.

The ferrite particles prepared as described above are used as the carrier core material of the present invention. Then, in order to obtain the desired charging properties, the outer periphery of the carrier core material is coated with resin to form a carrier for electrophotographic development.

Resins that can be used to coat the surface of the carrier core material include conventional resins, such as polyethylene, polypropylene, polyvinyl chloride, poly-4-methylpentene-1, polyvinylidene chloride, ABS (acrylonitrile-butadiene-styrene) resin, polystyrene, (meth)acrylic resins, polyvinyl alcohol resins, as well as thermoplastic elastomers such as polyvinyl chloride, polyurethane, polyester, polyamide, and polybutadiene, and fluorosilicone resins.

To coat the surface of the carrier core material with a resin, a solution or dispersion of the resin may be applied to the carrier core material. As the solvent for the coating solution, one or more of the following may be used: aromatic hydrocarbon solvents such as toluene and xylene; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; cyclic ether solvents such as tetrahydrofuran and dioxane; alcohol solvents such as ethanol, propanol, and butanol; cellosolve solvents such as ethyl cellosolve and butyl cellosolve; ester solvents such as ethyl acetate and butyl acetate; and amide solvents such as dimethylformamide and dimethylacetamide. The resin component concentration in the coating solution is generally in the range of 0.001% by mass to 30% by mass, and particularly 0.001% by mass to 2% by mass.

Methods for coating the carrier core with resin include, for example, the spray-drying method, the fluidized bed method, the spray-drying method using a fluidized bed, and the immersion method. Among these, the fluidized bed method is particularly preferred because it allows efficient application with a small amount of resin. In the case of the fluidized bed method, for example, the amount of resin coating can be adjusted by the amount of resin solution sprayed and the spraying time.

The particle size of the carrier is generally in the range of 25 μm or more and less than 50 μm in terms of volume average particle size, and preferably in the range of 30 μm or more and 40 μm or less.

The electrophotographic developer according to the present invention is a mixture of the carrier and toner prepared as described above. There is no particular limitation on the mixture ratio of the carrier and toner, and it may be appropriately determined based on the development conditions of the developing device used. In general, the toner concentration in the developer is preferably in the range of 1% by mass to 15% by mass. If the toner concentration is less than 1% by mass, the image density becomes too thin, while if the toner concentration exceeds 15% by mass, toner scattering may occur in the developing device, causing problems such as dirt inside the device and toner adhesion to the background parts of transfer paper, etc. A more preferable toner concentration is in the range of 3% by mass to 10% by mass.

Toners that can be used are those manufactured by conventional methods such as polymerization, pulverization classification, melt granulation, and spray granulation. Specifically, toners that contain colorants, release agents, charge control agents, etc. in a binder resin that is mainly composed of a thermoplastic resin are preferably used.

The particle size of the toner is generally preferably in the range of 5 μm to 15 μm, and more preferably in the range of 7 μm to 12 μm, as measured by a Coulter counter.

If necessary, a modifier may be added to the toner surface. Examples of modifiers include silica, alumina, zinc oxide, titanium oxide, magnesium oxide, polymethyl methacrylate, etc. These may be used alone or in combination of two or more.

The carrier and toner can be mixed using a conventional mixing device. For example, a Henschel mixer, a V-type mixer, a tumbler mixer, a hybridizer, etc. can be used.

There is no particular limitation on the developing method using the developer of the present invention, but a magnetic brush development method is preferable. Figure 4 shows a schematic diagram of an example of a developing device that performs magnetic brush development. The developing device shown in Figure 4 is equipped with a rotatable developing roller 3 that incorporates multiple magnetic poles, a regulating blade 6 that regulates the amount of developer on the developing roller 3 that is transported to the development section, two screws 1 and 2 that are arranged horizontally in parallel and stir and transport the developer in opposite directions, and a partition plate 4 that is formed between the two screws 1 and 2 and allows the developer to move from one screw to the other screw at both ends of the two screws, while preventing the developer from moving anywhere other than at both ends.

The two screws 1 and 2 have helical blades 13 and 23 formed on shafts 11 and 21 at the same inclination angle, and are rotated in the same direction by a drive mechanism (not shown), transporting developer in opposite directions. Developer moves from one screw to the other at both ends of the screws 1 and 2. This causes the developer, which is made up of toner and carrier, to constantly circulate and be stirred within the device.

Meanwhile, the developing roller 3 is a metal cylinder with a surface roughness of several μm, and has a fixed magnet with five magnetic poles arranged in order as magnetic pole generating means: developing pole N1, transport pole S1, peeling pole N2, pumping pole N3, and blade pole S2. When the cylindrical body of the developing roller 3 rotates in the direction of the arrow, the magnetic force of the pumping pole N3 pumps the developer from the screw 1 to the developing roller 3. The developer carried on the surface of the developing roller 3 is layer-regulated by the regulating blade 6, and then transported to the development area.

In the development area, a bias voltage in which an AC voltage is superimposed on a DC voltage is applied from a transfer voltage power source 8 to the development roller 3. The DC voltage component of the bias voltage is set to a potential between the background potential and the image potential on the surface of the photoconductor drum 5. The background potential and the image potential are set to potentials between the maximum and minimum values of the bias voltage. The peak-to-peak voltage of the bias voltage is preferably in the range of 0.5 kV to 5 kV, and the frequency is preferably in the range of 1 kHz to 10 kHz. The waveform of the bias voltage may be any of a square wave, a sine wave, a triangular wave, and the like. This causes the toner and carrier to vibrate in the development area, and the toner adheres to the electrostatic latent image on the photoconductor drum 5 to develop it.

The developer on the developing roller 3 is then transported inside the device by the transport magnetic pole S1, peeled off from the developing roller 3 by the peeling electrode N2, and circulated again through the device by the screws 1 and 2, where it is mixed and stirred with the developer that is not being used for development. Then, new developer is supplied from the screw 1 to the developing roller 3 by the pumping pole N3.

In the embodiment shown in FIG. 4, the developing roller 3 has five magnetic poles, but it is of course possible to increase the number of magnetic poles to eight, ten, or twelve in order to increase the amount of developer movement in the development area or to improve the pumping performance.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

Example 1
As raw materials, 36.29 kg _{of Fe2O3} (average particle size: 0.6 μm), 13.61 kg _{of Mn3O4} (average particle size: 3.4 μm), 278.4 g of _SrCO3 (average particle size: 0.6 μm), and 115.5 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 17.32 kg of pure water, and 151.3 g of carbon black as a reducing agent, 363 g of polycarboxylate ammonium dispersant as a dispersant, and 35.39 g of ammonia water as a pH adjuster were added to obtain a mixture. This mixture was pulverized by a wet ball mill (media diameter 2 mm) to obtain a mixed slurry.

This mixed slurry was sprayed into hot air at about 140°C using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. The granules were placed in an electric furnace and heated to 1300°C at an oxygen concentration of 1.0% over 4.5 hours. Then, the mixture was fired by holding at 1300°C and an oxygen concentration of 0.4% to 1.0% for 3 hours. Then, the mixture was cooled at an oxygen concentration of 0.4% for 6 hours. The fired product was pulverized using a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified to a volume average particle size of 35 μm using a vibrating sieve, and then oxidized by holding at a temperature of 460°C in the atmosphere for 1 hour to obtain a carrier core material. The apparent density AD, fluidity FR, volume average particle diameter (average particle diameter) _D50 , pore volume, specific surface area, true density, magnetic properties, resistance when a voltage of 1000 V is applied, maximum height Rz, average length RSm, irregularity rate, development memory, carrier adhesion, and carrier scattering of the obtained carrier core material were measured or evaluated by the methods shown below. The carrier core materials of the following examples and comparative examples were also measured or evaluated by the same methods. SEM photographs of the carrier core materials and cross sections of the core materials of Example 1, Comparative Example 2, and Comparative Example 8 are shown in Figures 1, 2, and 3.

Example 2
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 83.5 g of _SrCO3 (average particle size: 0.6 μm), and 55.4 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 5.2 kg of pure water, and 45.4 g of carbon black as a reducing agent, 108.9 g of polycarboxylate ammonium dispersant as a dispersant, and 10.62 g of ammonia water as a pH adjuster were added to obtain a mixture. This mixture was pulverized by a wet ball mill (media diameter 2 mm) to obtain a mixed slurry.
The mixed slurry was sprayed into hot air at about 140°C using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. The granules were placed in an electric furnace and heated to 1300°C at an oxygen concentration of 1.0% over 4.5 hours. Then, the mixture was fired by holding at 1300°C for 3 hours at an oxygen concentration of 0.4% to 1.0%. Then, the mixture was cooled at an oxygen concentration of 0.4% over 6 hours. The fired product was pulverized using a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified to a volume average particle size of 35 μm using a vibrating sieve, and then oxidized by holding at a temperature of 440°C in the atmosphere for 1 hour to obtain a carrier core material.

Example 3
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 83.5 g of _SrCO3 (average particle size: 0.6 μm), and 62.4 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 5.2 kg of pure water, and 45.4 g of carbon black was added as a reducing agent, 108.9 g of polycarboxylate ammonium dispersant as a dispersing agent, and 10.62 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Example 4
As raw materials, 36.29 kg _{of Fe2O3} (average particle size: 0.6 μm), 13.61 kg _{of Mn3O4} (average particle size: 3.4 μm), 278.4 g of _SrCO3 (average particle size: 0.6 μm), and 231 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 17.32 kg of pure water, and 151.3 g of carbon black was added as a reducing agent, 363 g of ammonium polycarboxylate dispersant as a dispersing agent, and 35.39 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Example 5
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 83.5 g of _SrCO3 (average particle size: 0.6 μm), and 105.3 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 5.2 kg of pure water, and 45.4 g of carbon black was added as a reducing agent, 108.9 g of ammonium polycarboxylate dispersant as a dispersing agent, and 10.62 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Example 6
As raw materials, 36.29 kg of Fe ₂ O ₃ (average particle size: 0.6 μm), 13.61 kg of Mn ₃ O ₄ (average particle size: 3.4 μm), 278.4 g of SrCO ₃ (average particle size: 0.6 μm), and 231 g of ZrO ₂ (average particle size: 1.3 μm) were dispersed in 17.32 kg of pure water, and 151.3 g of carbon black was added as a reducing agent, 363 g of a polycarboxylate ammonium dispersant as a dispersant, and 35.39 g of ammonia water as a pH adjuster to form a mixture. This mixture was pulverized using a wet ball mill (media diameter 2 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 140° C. using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. This granulated product was placed in an electric furnace and heated to 1300° C. over 4.5 hours at an oxygen concentration of 2.0%. Then, the mixture was fired at 1300° C. with an oxygen concentration of 0.4% to 2.0% for 3 hours. Then, the mixture was cooled at an oxygen concentration of 0.4% for 6 hours in the same manner as in Example 2 to obtain a carrier core material having a volume average particle size of 35 μm.

Example 7
As raw materials, 36.29 kg _{of Fe2O3} (average particle size: 0.6 μm), 13.61 kg _{of Mn3O4} (average particle size: 3.4 μm), 278.4 g of _SrCO3 (average particle size: 0.6 μm), and 231 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 17.32 kg of pure water, and 151.3 g of carbon black as a reducing agent, 363 g of polycarboxylate ammonium dispersant as a dispersant, and 35.39 g of ammonia water as a pH adjuster were added to obtain a mixture. This mixture was pulverized by a wet ball mill (media diameter 2 mm) to obtain a mixed slurry.

This mixed slurry was sprayed into hot air at approximately 140°C using a spray dryer to obtain dried granules with particle sizes of 10μm to 75μm. The granules were placed in an electric furnace and heated to 1300°C over 4.5 hours at an oxygen concentration of 5.0%. They were then fired by holding at 1300°C with an oxygen concentration of 0.4% to 5.0% for 3 hours. A carrier core material with a volume average particle size of 35μm was obtained in the same manner as in Example 2, except that they were then cooled over 6 hours at an oxygen concentration of 0.4%.

Example 8
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 83.5 g of _SrCO3 (average particle size: 0.6 μm), and 69.3 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 5.2 kg of pure water, and 68.1 g of carbon black was added as a reducing agent, 108.9 g of polycarboxylate ammonium dispersant as a dispersing agent, and 10.62 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Example 9
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 83.5 g of _SrCO3 (average particle size: 0.6 μm), and 69.3 g of _ZrO2 (average particle size: 1.1 μm) were dispersed in 5.2 kg of pure water, and 45.38 g of carbon black as a reducing agent, 108.9 g of polycarboxylate ammonium dispersant as a dispersant, and 10.62 g of ammonia water as a pH adjuster were added to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Example 10
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 66.8 g of _SrCO3 (average particle size: 0.6 μm), and 69.3 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 5.2 kg of pure water, and 45.4 g of carbon black was added as a reducing agent, 108.9 g of polycarboxylate ammonium dispersant as a dispersing agent, and 10.62 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Example 11
As raw materials, 10.89 kg _{of Fe2O3} (average particle size: 0.6 μm), 4.08 kg _{of Mn3O4} (average particle size: 3.4 μm), 41.8 g of _SrCO3 (average particle size: 0.6 μm), and 69.3 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 5.2 kg of pure water, and 45.4 g of carbon black was added as a reducing agent, 108.9 g of polycarboxylate ammonium dispersant as a dispersing agent, and 10.62 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Comparative Example 1
As raw materials, 28.94 kg of Fe ₂ O ₃ (average particle size: 0.6 μm) and 11.3 kg of Mn ₃ O ₄ (average particle size: 2.1 μm) were dispersed in 13.62 kg of pure water, and 111.6 g of carbon black as a reducing agent and 250.8 g of polycarboxylate ammonium dispersant as a dispersant were added to obtain a mixture. This mixture was pulverized using a wet ball mill (media diameter 2 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 140° C. using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. This granulated product was placed in an electric furnace and heated to 1200° C. at an oxygen concentration of 1.3% over 4.5 hours. Then, firing was performed by holding at 1200° C. at an oxygen concentration of 1.3% for 3 hours. Then, cooling was performed at an oxygen concentration of 0.4% over 6 hours. The obtained fired product was pulverized using a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified using a vibrating sieve to have a volume average particle size of 35 μm. The product was then subjected to an oxidation treatment by being held at a temperature of 380° C. in the atmosphere for 1 hour, to obtain a carrier core material.

Comparative Example 2
As raw materials, 23.95 kg _{of Fe2O3} (average particle size: 0.6 μm) and 8.98 kg _{of Mn3O4} (average particle size: 3.4 μm) were dispersed in 11.31 kg of pure water, and 99.83 g of carbon black as a reducing agent, 199.66 g of a polycarboxylate ammonium dispersant as a dispersant, and 23.12 g of ammonia water as a pH adjuster were added to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2, except that the mixture was prepared by adding 99.83 g of carbon black as a reducing agent, 199.66 g of a polycarboxylate ammonium dispersant as a dispersant, and 23.12 g of ammonia water as a pH adjuster.

Comparative Example 3
As raw materials, 14.52 kg _{of Fe2O3} (average particle size: 0.6 μm), 5.44 kg _{of Mn3O4} (average particle size: 3.4 μm), and 111.36 g of _SrCO3 (average particle size: 0.6 μm) were dispersed in 6.628 kg of pure water, and 60.5 g of carbon black was added as a reducing agent, 121 g of polycarboxylate ammonium dispersant as a dispersing agent, and 14.09 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Comparative Example 4
As raw materials, 36.29 kg _{of Fe2O3} (average particle size: 0.6 μm), 13.61 kg _{of Mn3O4} (average particle size: 3.4 μm), 278.4 g of _SrCO3 (average particle size: 0.6 μm), and 57.8 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 17.32 kg of pure water, and 151.3 g of carbon black was added as a reducing agent, 363 g of ammonium polycarboxylate dispersant as a dispersing agent, and 35.39 g of ammonia water as a pH adjuster to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

Comparative Example 5
As raw materials, 24.12 kg _{of Fe2O3} (average particle size: 0.6 μm), 9.42 kg _{of Mn3O4} (average particle size: 3.4 μm), and 62.9 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 11.37 kg of pure water, and 93 g of carbon black as a reducing agent and 209 g of ammonium polycarboxylate-based dispersant as a dispersant were added to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2, except that the mixture was prepared by adding 93 g of carbon black as a reducing agent and 209 g of ammonium polycarboxylate-based dispersant as a dispersant.

Comparative Example 6
As raw materials, 24.12 kg _{of Fe2O3} (average particle size: 0.6 μm), 9.42 kg _{of Mn3O4} (average particle size: 3.4 μm), and 125.9 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 11.39 kg of pure water, and 93 g of carbon black as a reducing agent and 209 g of ammonium polycarboxylate-based dispersant as a dispersant were added to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2, except that the mixture was prepared by adding 93 g of carbon black as a reducing agent and 209 g of ammonium polycarboxylate-based dispersant as a dispersant.

Comparative Example 7
As raw materials, 24.12 kg _{of Fe2O3} (average particle size: 0.6 μm), 9.42 kg _{of Mn3O4} (average particle size: 3.4 μm), and 251.7 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 11.43 kg of pure water, and 93 g of carbon black as a reducing agent and 209 g of ammonium polycarboxylate-based dispersant as a dispersant were added to form a mixture. A carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2, except that the mixture was prepared by adding 93 g of carbon black as a reducing agent and 209 g of ammonium polycarboxylate-based dispersant as a dispersant.

Comparative Example 8
As raw materials, 36.29 kg of Fe ₂ O ₃ (average particle size: 0.6 μm), 13.61 kg of Mn ₃ O ₄ (average particle size: 3.4 μm), 278.4 g of SrCO ₃ (average particle size: 0.6 μm), and 231 g of ZrO ₂ (average particle size: 1.3 μm) were dispersed in 17.32 kg of pure water, and 151.3 g of carbon black was added as a reducing agent, 363 g of a polycarboxylate ammonium dispersant as a dispersant, and 35.39 g of ammonia water as a pH adjuster to form a mixture. This mixture was pulverized using a wet ball mill (media diameter 2 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 140° C. using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. This granulated product was placed in an electric furnace and heated to 1230° C. over 4.5 hours at an oxygen concentration of 1.0%. Then, the mixture was fired at 1230° C. with an oxygen concentration of 0.4% to 1.0% for 3 hours. Then, the mixture was cooled at an oxygen concentration of 0.4% for 6 hours in the same manner as in Example 2 to obtain a carrier core material having a volume average particle size of 35 μm.

Comparative Example 9
As raw materials, 7.93 kg _{of Fe2O3} (average particle size: 0.6 μm), 3.77 kg _{of Mn3O4} (average particle size: 3.4 μm), 65.31 g of _SrCO3 (average particle size: 0.6 μm), and 54.52 g of _ZrO2 (average particle size: 1.3 μm) were dispersed in 3.88 kg of pure water, and 35.49 g of carbon black as a reducing agent, 85.17 g of polycarboxylate ammonium dispersant as a dispersing agent, and 6.9 g of ammonia water as a pH adjuster were added to form a mixture, and a carrier core material with a volume average particle size of 35 μm was obtained in the same manner as in Example 2.

(Apparent density: AD)
The apparent density of the carrier core material was measured in accordance with JIS Z 2504.

(Flow rate: FR)
The fluidity of the carrier core material was measured in accordance with JIS Z 2502.

(Volume average particle size (average particle size): _D50 )
The volume average particle size of the carrier core material was measured using a laser diffraction particle size distribution measuring device ("Microtrac Model 9320-X100" manufactured by Nikkiso Co., Ltd.).

(Pore volume)
The evaluation device used was a POREMASTER-60GT manufactured by Quantachrome. Specifically, the measurement conditions were Cell Stem Volume: 0.5 mL, Head pressure: 20 PSIA, surface tension of mercury: 485.00 erg/cm ² , contact angle of mercury: 130.00 degrees, high pressure measurement mode: Fixed Rate, Motor Speed: 1, high pressure measurement range: 20.00 to 10000.00 PSI, and 1.500 g of the sample was weighed and filled into a 0.5 mL (cc) cell for measurement. The pore volume was determined by subtracting the volume A (mL/g) at 60 PSI from the volume B (mL/g) at 10000 PSI.

(BET specific surface area)
The evaluation was performed using a BET single-point specific surface area measuring device (Macsorb HM model-1208, manufactured by Mountec Co., Ltd.). Specifically, 6.000 g of the sample was weighed out and filled into a standard cell having a diameter of 12 mm, and the sample was degassed at 200° C. for 30 minutes and then measured.

(True Density)
The true density of the carrier core material was measured using "ULTRA PYCNOMETER 1000" manufactured by Quantachrome.

(Magnetic properties)
Using a room temperature dedicated vibrating sample magnetometer (VSM) ("VSM-P7" manufactured by Toei Industry Co., Ltd.), an external magnetic field in the range of 0 to 79.58×10 ⁴ A/m (10,000 Oersted) was continuously applied for one cycle to measure the saturation magnetization σ _s , magnetization σ _1k , and residual magnetization σ _r .

(Electrical Resistance)
Two brass plates with a thickness of 2 mm and whose surfaces had been electrolytically polished were arranged as electrodes with a distance between the electrodes of 2 mm. 200 mg of carrier core material was loaded into the gap between the two electrode plates, and then a magnet with a cross-sectional area of ²⁴⁰ mm2 was placed behind each electrode plate to form a bridge of the powder to be measured between the electrodes. A DC voltage of 1000 V was applied between the electrodes, and the current flowing through the carrier core material was measured using the four-terminal method to calculate the electrical resistance of the carrier core material.

(Maximum peak/valley depth Rz, average length RSm)
The surface was observed with a 100x objective lens using an ultra-deep color 3D shape measuring microscope ("VK-X100" manufactured by Keyence Corporation). Specifically, first, ferrite particles were fixed to a flat adhesive tape on the surface, and the measurement field of view was determined with a 100x objective lens, and then the focus was adjusted to the adhesive tape surface using the autofocus function. A laser beam was irradiated from the vertical direction (Z direction) to the flat adhesive tape surface on which the ferrite particles were fixed, and the surface was scanned in the X and Y directions. In addition, data in the Z direction was obtained by connecting the height positions of the lens when the intensity of the reflected light from the surface was maximized. The position data in the X, Y, and Z directions were connected to obtain the three-dimensional shape of the ferrite particle surface. In addition, an auto-photography function was used to capture the three-dimensional shape of the ferrite particle surface.
Each parameter was measured using particle roughness inspection software (manufactured by Mitani Shoji). First, as a pretreatment, particle recognition and shape selection of the three-dimensional shape of the obtained ferrite particle surface were performed. Particle recognition was performed by the following method. Of the three-dimensional shape obtained by photography, the maximum value in the Z direction was set to 100%, the minimum value was set to 0%, and the range between the maximum value and the minimum value was divided into 100 equal parts. The region corresponding to 100 to 35% was extracted, and the outline of the independent region was recognized as the particle outline. Next, particles such as coarse, small, and associated particles were excluded by shape selection. By performing this shape selection, it is possible to reduce errors during the curvature correction performed later. Specifically, particles with an area equivalent diameter of 28 μm or less, 38 μm or more, and an acicular ratio of 1.15 or more were excluded. Here, the acicular ratio is a parameter calculated from the ratio of the maximum length/diagonal width of the particle, and the diagonal width represents the shortest distance between two straight lines when the particle is sandwiched between two straight lines parallel to the maximum length.
Next, the portion to be used for analysis was extracted from the three-dimensional shape of the surface. First, a 15.0 μm square was drawn with the center of gravity determined from the particle outline recognized by the above method as its center. 21 parallel lines were drawn within the square, and 21 roughness curves corresponding to the line segments were extracted.
Since the ferrite particles are approximately spherical, the roughness curve has a certain curvature as the background. Therefore, to correct the background, an optimal quadratic curve was fitted and subtracted from the roughness curve. In this case, a low-pass filter was applied with a strength of 1.5 μm, and the cutoff value λ was set to 80 μm.
In addition, the average particle size of the carrier core material used in the analysis was limited to 32 to 34 μm. By limiting the average particle size of the carrier core material to be measured to a narrow range in this way, it is possible to reduce errors due to residues that occur during curvature correction.

The maximum peak-valley depth Rz was calculated as the sum of the height of the highest peak and the depth of the deepest valley in the roughness curve. The average value of 30 particles was used as the average value of each parameter to calculate the maximum height Rz.

The average length RSm is calculated by averaging the length of each element, with a combination of a valley and a peak on the roughness curve defined as one element. To calculate the average length RSm, the average value of 30 particles was used as the average value of each parameter.

The measurements of maximum height Rz and average length RSm described above are performed in accordance with JIS B0601 (2001 edition).

(Rate of irregular shapes)
An injection type image analysis particle size distribution meter (Jasco International Co., Ltd., Model: IF-3200) was used. Specifically, 0.07 g of the sample was weighed out and dispersed in a screw cap bottle (volume 9 mL) containing 9 mL of polyethylene glycol 400, and then the measurement was performed.
(Measurement conditions)
Spacer thickness: 150 μm
Sampling: 20%
Analysis type: relative measurement Measurement amount: 0.95 mL
Analysis: Dark Detection Threshold: 169 (fill holes)
O-Roughness filter: 0.5
Filter by:
ISO Area Diameter: Minimum value 5, maximum value 100, inner range (analysis conditions)
Analysis filter condition I:
ISO Area Diameter: Minimum value 25, maximum value 55, inside range Analysis filter condition II:
ISO Area Diameter: minimum value 25, maximum value 55, inner range ISO Solidity: minimum value 0.98, maximum value 1, outer range Ell. Ratio: minimum value 0.8, maximum value 1, inner range The number of particles counted under analysis filter condition II was divided by the number of particles counted under analysis filter condition I to calculate the irregularity rate, which is the proportion of irregularly shaped particles.

(Development memory)
The surface of the obtained carrier core material was coated with a resin to prepare a carrier. Specifically, 450 parts by mass of silicone resin and 9 parts by mass of (2-aminoethyl)aminopropyltrimethoxysilane were dissolved in 450 parts by mass of toluene as a solvent to prepare a coating solution. This coating solution was applied to 50,000 parts by mass of the carrier core material using a fluidized bed type coating device, and heated in an electric furnace at a temperature of 300°C to obtain a carrier. The carrier was obtained in the same manner for all the following examples and comparative examples. The obtained carrier and toner having an average particle size of about 5.0 μm were mixed for a predetermined time using a pot mill to obtain a two-component electrophotographic developer. In this case, the carrier and toner were adjusted so that the mass of the toner/(mass of the toner and carrier)=5/100. The developer was obtained in the same manner for all the following examples and comparative examples. The obtained developer was put into a developing device having a structure shown in Figure 4 (circumferential speed of developing roller Vs: 406 mm/sec, peripheral speed of photosensitive drum Vp: 205 mm/sec, distance between photosensitive drum and developing roller: 0.3 mm), and an image in which a solid image area and a non-image area are adjacent to each other in the longitudinal direction of the photosensitive drum, followed by a wide area of intermediate tones, was obtained initially and after 200,000 images were formed, and the image density of the area where the solid image of the first rotation of the developing roller was developed and the area where it was not developed on the second rotation of the developing roller was measured using a reflection densitometer (Model TC-6D, manufactured by Tokyo Denshoku Co., Ltd.), and the difference was determined and evaluated according to the following criteria. The results are also shown in Table 2.
"Good": Less than 0.003 "Good": 0.003 or more and less than 0.020 "Poor": 0.020 or more

(Carrier adhesion)
The prepared two-component developer was fed into a developing device having a structure shown in FIG. 4 (circumferential speed of developing roller Vs: 406 mm/sec, peripheral speed of photosensitive drum Vp: 205 mm/sec, distance between photosensitive drum and developing roller: 0.3 mm) to form a solid image on the surface of the photosensitive drum. The solid image on the surface of the photosensitive drum was peeled off with cellophane tape, and the number of blank spots due to carrier adhesion per unit area was evaluated according to the following criteria.
"Good": No carrier adhesion was observed, or slight carrier adhesion was observed but within a range that did not cause any problems in practical use.
"△": Carrier adhesion was observed and the product could not be used.
"X": Strong carrier adhesion was observed and the sample could not be used at all.

(Carrier scattering)
The prepared two-component developer was charged into a developing device having a structure shown in FIG. 4 (circumferential speed of developing roller Vs: 406 mm/sec, peripheral speed of photosensitive drum Vp: 205 mm/sec, distance between photosensitive drum and developing roller: 0.3 mm) and stirred. Black spots on the surface of the photosensitive drum when charged were peeled off with cellophane tape, and the number of black spots per unit area was evaluated according to the following criteria.
"Good": 0 to 5 black dots; "Good": 6 to 10 black dots; "Poor": 11 to 15 black dots

For the electrophotographic developers using the carrier core materials of Examples 1 to 11, the evaluations of development memory, carrier adhesion, and carrier scattering were all "good."

In contrast, the electrophotographic developer using the carrier core material of Comparative Example 1, which did not contain Sr or Zr and had a baking temperature of 1200°C, was rated as "△" for development memory and carrier adhesion.

The carrier core material of Comparative Example 2, which did not contain Sr or Zr and had a firing temperature of 1,300°C, had a small pore volume and large particle volume magnetization, but the spacing between the irregularities on the particle surface was large, and the electrophotographic developer using this carrier core material was rated as "○" for development memory and carrier scattering, but "△" for carrier adhesion.

The electrophotographic developer using the carrier core material of Comparative Example 3, which contains Sr but not Zr, was rated as "△" for carrier adhesion.

The electrophotographic developer using the carrier core material of Comparative Example 4, which has a low Zr content of 0.07 mol%, was rated as "△" for carrier adhesion.

In the electrophotographic developers using the carrier core materials of Comparative Examples 5 to 7, which did not contain Sr and had Zr contents of 0.12 mol%, 0.24 mol%, and 0.48 mol%, the carrier adhesion evaluation was "△" for all of them.

In the carrier core material of Comparative Example 8, which was fired at 1230°C, the irregularities on the particle surface were formed at the desired intervals and heights, but because the pore volume was large and the particle volume magnetization was small, the electrophotographic developer using this carrier core material was rated as "△" for carrier scattering.

The carrier core material of Comparative Example 9 having a low composition ratio of Fe had a low saturation magnetization σ _S of 75.8 Am ² /kg, and the electrophotographic developer using this carrier core material was evaluated as "Δ" for carrier scattering.

The carrier core material of the present invention can suppress development memory and carrier adhesion.

3 developing roller 5 photosensitive drum

Claims (7)

A carrier core material containing a material represented by a composition formula _{MnXFe3 - XO4} (where 0<X<1) as a main component and containing Sr and Zr,
The Sr content is 0.01 mol% or more and 0.50 mol% or less,
The Zr content is 0.10 mol% or more and 0.50 mol% or less,
Saturation magnetization σ _s is 77 Am ² /kg or more and 87 Am ² /kg or less,
Residual magnetization σ _r is 0.2 Am ² /kg or more and 2.0 Am ² /kg or less,
The magnetization σ _1k when a magnetic field of 79.58×10 ³ A/m (1000 Oersted) is applied is 63 Am ² /kg or more and 68 Am ² /kg or less,
A carrier core material comprising: The carrier core material according to claim 1, in which the maximum height Rz is 1.60 μm or more and the average length RSm is 7.0 μm or less. 3. The carrier core material according to claim 1, which has a pore volume of 0.015 cm ³ /g or less. 4. The carrier core material according to claim 1, wherein the true density is 4.800 g/cm ³ or more and 5.000 g/cm ³ or less. 5. The carrier core material according to claim 1, wherein the particle volume magnetization calculated from the following formula (1) is 0.284 Am ² /cm ³ or more and 0.320 Am ² /cm ³ or less.
Particle volume magnetization = magnetization σ _1k (Am ² /kg) x true density (g/cm ³ ) x 10 ^-3 x (100 - excluded volume (%)) / 100... (1)
(wherein magnetization σ _1k : magnetization when a magnetic field of 79.58×10 ³ A/m (1000 Oersted) is applied, and excluded volume=(1−1/(1+pore volume×true density))×100) A carrier for electrophotographic development, characterized in that the surface of the carrier core material according to any one of claims 1 to 5 is coated with a resin. An electrophotographic developer comprising the electrophotographic development carrier according to claim 6 and a toner.