JP2022090791A - Carrier core material - Google Patents

Abstract

To provide a carrier core material which can simultaneously suppress background part carrier adhesion and a void in an image.SOLUTION: A carrier core material consists of ferrite particles. The static breakdown voltage Vs and the dynamic breakdown voltage Vd satisfy the following formula (1). The internal porosity of the ferrite particle calculated from the following formula (2) is equal to or less than 3.0%. (1): 0(V)≤Vs-Vd≤1000(V). (2): internal porosity(%)=(particle area A-particle area B)/particle area A×100. In the formula, the particle area A is the particle cross-sectional area including the internal gap, and the particle area B is the particle cross-sectional area not including the internal gap.SELECTED DRAWING: Figure 1

Description

The present invention relates to a carrier core material and the like.

For example, in an image forming apparatus such as a facsimile, a printer, or a copier using an electrophotographic method, toner is attached to an electrostatic latent image formed on the surface of a photoconductor to make a visible image, and this visible image is printed on paper. After transferring to, etc., it is fixed by heating and pressurizing. From the viewpoint of high image quality and colorization, a so-called two-component developer containing a carrier and a toner is widely used as a developer.

In a developing method using a two-component developer, the carrier and toner are stirred and mixed in a developing device, and the toner is charged to a predetermined amount by friction. Then, a developer is supplied to the rotating developing roller, a magnetic brush is formed on the developing roller, and toner is electrically transferred to the photoconductor via the magnetic brush to obtain an electrostatic latent image on the photoconductor. Visualize. After the toner is transferred, the carrier remains on the developing roller and is mixed with the toner again in the developing apparatus. Therefore, as the characteristics of the carrier, the magnetic characteristics for forming the magnetic brush, the charging characteristics for imparting a desired charge to the toner, and the durability in repeated use are required.

As such a carrier, a carrier in which the surface of magnetic particles such as magnetite and various ferrites is coated with a resin is generally used. Magnetic particles as a carrier core material are required to have good triboelectric properties with respect to toner as well as good magnetic properties.

However, if the electric resistance of the carrier is designed to be high in order to improve the triboelectric property with respect to the toner, the countercharge is accumulated in the carrier remaining on the developing roller after the toner is transferred to the photoconductor (after development). There may be a problem that a part of the carrier adheres to the non-image portion (background portion) of the photoconductor (background portion carrier adhesion).

Therefore, for example, in Patent Document 1, the carrier core material is made uneven, and after the surface of the carrier core material is coated with resin, a part of the carrier core material is exposed from the surface of the coating resin to accumulate countercharge. Means to suppress it have been proposed.

JP-A-2017-156737A

The development bias tends to be set high with the recent improvement in image quality and the increase in image formation speed. When the development bias is set higher than before, dielectric breakdown of the carrier is likely to occur. Then, an electric charge is injected into the carrier whose dielectric breakdown is broken, and the electric charge has the same polarity as the toner and adheres to the image portion of the photoconductor. The carrier adhering to the image portion of the photoconductor exerts a spacer effect and hinders the transfer of toner around the carrier from the photoconductor to the paper. As a result, there is a possibility that a problem (white spot in the image) may occur in the paper transfer image.

Further, as the image forming speed is increased, the stirring speed of the developer in the developing apparatus is also increased, and the stress continuously applied to the carrier is increased, so that the carrier is liable to be cracked or chipped. If the carrier is cracked or chipped, dielectric breakdown may occur from the cross section of the exposed carrier having low insulating property, and white spots may occur in the image by the same mechanism as described above.

The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a carrier core material capable of simultaneously suppressing defects in a trade-off relationship, that is, background carrier adhesion and white spots in an image. Is to be.

In the carrier core material according to the present invention that achieves the above object, the static dielectric breakdown voltage Vs and the dynamic dielectric breakdown voltage Vd satisfy the following formula (1), and the ferrite particles calculated from the following formula (2). It is characterized in that the internal void ratio is 3.0% or less.
0 (V) ≤ Vs-Vd ≤ 1000 (V) ... (1)
Internal porosity (%) = (particle area A-particle area B) / particle area A × 100 ... (2)
In the formula, particle area A: particle cross-sectional area including internal voids Particle area B: particle cross-sectional area not including internal voids

The static insulation breakdown voltage Vs, the dynamic breakdown voltage Vd, the particle area A, and the particle area B in the present specification are values measured by the measurement method and measurement conditions in the examples described later. Further, in the present specification, "ferrite particles", "carrier core material", "carrier for electrophotographic development", and "developer for electrophotographic development" each mean an aggregate (powder) of individual particles. be.

In the carrier core material, the composition of the ferrite particles contains MnO: 35 mol% or more and 55 mol% or less, Fe ₂ O ₃ : 45 mol% or more and 65 mol% or less, and a part thereof is SrO: 0.1 mol% or more. It may be substituted with 0 mol% or less, SnO: 0.1 mol% or more and 5.0 mol% or less.

Further, in the carrier core material, the maximum mountain valley depth Rz is preferably 1.9 μm or more and 2.6 μm or less. Specific measurement methods and measurement conditions will be shown in Examples described later.

Further, in the carrier core material, it is preferable that the magnetization σ _{1 k} when a magnetic field of 79.58 × 10 ³ A / m (1000 oersted) is applied is 63 Am ² / kg or more and 73 Am ² / kg or less. Specific measurement methods and measurement conditions will be shown in Examples described later.

Further, in the carrier core material, the pore volume is preferably 0.001 cm ³ / g or more and 0.015 cm ³ / g or less. Specific measurement methods and measurement conditions will be shown in Examples described later.

Further, in the carrier core material, the ratio of the particle size of 26 μm or less in the number-based particle size distribution measured by the laser diffraction type particle size distribution measuring device is preferably 25% or less. Specific measurement methods and measurement conditions will be shown in Examples described later.

Further, according to the present invention, there is provided a carrier for electrophotographic development, wherein the surface of the carrier core material according to any one of the above is coated with a resin.

Further, according to the present invention, there is provided an electrophotographic developer containing the above-mentioned electrophotographic developing carrier and toner.

According to the carrier core material of the present invention, good triboelectric characteristics for toner can be obtained, carrier adhesion to the background portion is suppressed, and white spots occur in the image even when used in a high-speed image forming apparatus. It is suppressed.

Further, according to the electrophotographic developing carrier and the electrophotographic developer of the present invention, it can be widely used for a plurality of types of image forming apparatus having different development speeds, development biases and the like.

It is a schematic diagram of the particle cross section of a carrier core material. It is a schematic diagram of the measuring apparatus of dynamic dielectric breakdown voltage Vd. It is a schematic diagram which shows an example of a developing apparatus.

The present inventors have a case where they are used in an image forming apparatus having a high development bias and an image forming apparatus having a high stirring speed of a developer because good triboelectric characteristics with respect to toner can be obtained and carrier adhesion to a background portion is suppressed. However, as a result of diligent studies to obtain a carrier core material that does not cause whiteout in the image described above, if the static dielectric breakdown voltage Vs of the carrier core material and the dynamic dielectric breakdown voltage Vd satisfy a predetermined relationship. I first found a good thing. That is, one of the major features of the carrier core material according to the present invention is that it satisfies the above formula (1).

According to the findings of the present inventors, in the conventional carrier core material, the static breakdown voltage Vs is smaller than the dynamic breakdown voltage Vd. With such a conventional carrier core material, it is difficult to simultaneously suppress background carrier adhesion and white spots in an image. Therefore, when the static breakdown voltage Vs is set higher than the dynamic breakdown voltage Vd, the carrier adhesion in the background and the white spots in the image can be suppressed. In the present invention, the static breakdown voltage Vs and the dynamic breakdown voltage Vd are used. It was determined that the difference between 0 (V) and 1000 (V) or less. When the difference between the static breakdown voltage Vs and the dynamic breakdown voltage Vd is negative, that is, when the dynamic breakdown voltage Vd is larger than the static breakdown voltage Vs, countercharges are likely to accumulate in the carrier. Carrier adheres to the non-image portion (background portion) of the photoconductor Background portion Carrier adhesion is likely to occur. On the other hand, when the difference between the static dielectric breakdown voltage Vs and the dynamic dielectric breakdown voltage Vd is larger than +1000 (V), weakly charged toner that is not sufficiently charged is likely to be generated, which is caused by the weakly charged toner. Toner scattering and fog are likely to occur. The upper limit of the difference between the static breakdown voltage Vs and the dynamic breakdown voltage Vd is more preferably 500 (V). 300 (V) is more preferable. In order to make the static dielectric breakdown voltage Vs higher than the dynamic breakdown voltage Vd, for example, Sn (tin) and Sr (strontium) should be contained in a predetermined amount as described in the method for manufacturing a carrier core material described later. Just do it.

Another major feature of the present invention is that the internal porosity of the ferrite particles calculated from the above formula (2) is 3.0% or less. In the present invention, the internal porosity calculated from the above formula (2) is used as an index of the particle density of the carrier core material.

First, the internal porosity of the carrier core material will be described. FIG. 1 (a) is a schematic diagram of a particle cross section of a carrier core material, and FIG. 1 (b) is a diagram showing a particle area A (particle cross section including internal voids) as a shaded area, and FIG. 1 (c). ) Is a diagram showing the particle area B (particle cross section excluding internal voids) as a shaded area.

The internal void ratio is the area obtained by subtracting the particle area B shown by the diagonal line in FIG. 1 (c) from the particle area A shown by the diagonal line in FIG. 1 (b), that is, colored in gray in FIG. 1 (c). It shows the ratio of the area of the void portion inside the particles to the particle area A. In this way, it is calculated for each particle, and the internal porosity is obtained from the average value of 100 particles. As described above, the cross-section observation image of the core material is used for the measurement, but the maximum length of the cross-section (particle) for which the cross-section observation image is acquired is 50% of the average particle size (μm) of the powder. It shall be of the above length. This is because it is appropriate for porosity and particle evaluation.

In the present invention, the internal porosity of the ferrite particles thus obtained is determined to be 3% or less. If the internal porosity is larger than 3%, the carriers (ferrite particles) may be cracked or chipped due to the influence of high-speed stirring or the like when used in a high-speed image forming apparatus, and white spots may occur in the image. A more preferable range of the internal porosity of the ferrite particles is 1% or less.

In order to reduce the internal porosity of the ferrite particles to 3% or less, for example, the raw material concentration of the slurry is increased in the granulation step and appropriate drying conditions are selected. Select. In addition, sintering inside the particles is sufficiently promoted by selecting a raw material having a small particle size, controlling the particle size of the slurry raw material after wet grinding to be small, and selecting an appropriate firing temperature in the firing step. Conditions should be selected.

The composition of the ferrite particles of the present invention is at least selected from the group consisting of the composition formula MX Fe _{3-X O 4} (where M is Mg, Mn, Ca, Ti, Sr, Cu, Zn, Sn, Ni). One kind of metal element, which is represented by 0 <X <1), is preferable. Among these, it is represented by the general formula (MnO) x (Fe ₂ O ₃ ) y, and x and y are 35 mol% or more and 55 mol% or less, 45 mol% or more and 65 mol% or less, respectively, and a part of MnO is 0 in SrO. .1 mol% or more and 5.0 mol% or less, and SnO 0.1 mol% or more and 5.0 mol% or less are preferable. Further, the range of the total value of the substituted SrO and SnO is preferably in the range of 0.2 mol% or more and 10.0 mol% or less. If the total value of SrO and SnO exceeds 10.0 mol%, the magnetization decreases due to the increase in the non-magnetic component, and there is a concern that the background carrier adhesion deteriorates. If the total value of SrO and SnO is lower than 0.2 mol%, the difference between the static breakdown voltage Vs and the dynamic breakdown voltage Vd becomes negative, and countercharges tend to accumulate in the carrier, and the carrier is the non-image area of the photoconductor. There is a concern that the background carrier adhering to the (background) is likely to occur. A more preferable range of the total value of the substituted SrO and SnO is 0.2 mol% or more and 4.0 mol% or less.

The maximum mountain valley depth Rz in the carrier core material of the present invention is preferably in the range of 1.9 μm or more and 2.6 μm or less. When the maximum mountain valley depth Rz on the surface of the carrier core material is within the above range, the space formed between the carrier core materials becomes large, and more toner is taken into this space and the amount of toner supplied to the developing region. Can be increased, and the image formation speed can be increased. A more preferable range of the maximum mountain valley depth Rz is a range of 2.1 μm or more and 2.5 μm or less. The maximum valley depth Rz on the surface of the ferrite particles can be controlled by the amount of chlorine added to the raw material, the firing conditions in the manufacturing process, and the like.

Further, it is preferable that the magnetization σ _{1 k} in the carrier core material of the present invention when a magnetic field of 79.58 × 10 ³ A / m (1000 oersted) is applied is 63 Am ² / kg or more and 73 Am ² / kg or less. If the magnetization σ _1k is less than 63 Am ² / kg, background carrier adhesion may occur.

The pore volume in the carrier core material of the present invention is preferably in the range of 0.001 cm ³ / g or more and 0.015 cm ³ / g or less. When the pore volume is in the range of 0.001 cm ³ / g or more, the weight per carrier particle is lightened, the carrier friction is alleviated, and the toner spend is reduced. On the other hand, when the pore volume exceeds 0.015 cm ³ / g, the internal voids become too large and the magnetization per carrier particle becomes small, so that background carrier adhesion and white spots in the image are likely to occur. A more preferable range of the pore volume is 0.003 cm ³ / g or more and 0.010 cm ³ / g or less.

The volume average particle size (hereinafter, may be referred to as “average particle size”) measured by the laser diffraction type particle size distribution measuring device of the carrier core material of the present invention is preferably in the range of 30 μm or more and less than 50 μm, more preferably. The range is 30 μm or more and 40 μm or less. Further, the ratio of the particle size of 26 μm or less in the particle size distribution based on the number is preferably 25% or less. If the ratio of the particle size of 26 μm or less exceeds 25%, background carrier adhesion may occur.

The method for producing the carrier core material of the present invention is not particularly limited, but the production method described below is suitable.

In order to produce a carrier core material satisfying the above formula (1) and having an internal porosity of 3% or less, a predetermined amount of Sn (tin) and Sr (strontium) is added to the carrier core material composed of ferrite particles having a predetermined composition. It is preferable to contain it. By containing Sn and Sr in predetermined amounts, the dynamic breakdown voltage Vd tends to be smaller than the static breakdown voltage Vs. This mechanism has not been clarified yet, but the present inventors speculate that it may be from the following mechanism. That is, when Sn and Sr are contained in a predetermined amount, a highly conductive compound composed of Sn, Sr, Fe, O and the like (for example, SrSnO ₃ or SrSnFe ₁₁ O ₁₉ or the like) is produced. When the carrier core material is conveyed and stirred on the surface of the developing roller, the frequency with which the highly conductive compound localized on the particle surface of the carrier core material comes into contact with other carrier core material particles increases, and the dynamic breakdown voltage increases. Vd decreases, and as a result, the dynamic breakdown voltage Vd becomes smaller than the static breakdown voltage Vs unlike the conventional case. That is, the dynamic breakdown voltage Vd decreases while the static breakdown voltage Vs of the main body of the carrier core material itself is maintained high.

Therefore, first, additives such as Fe component raw material, M component raw material, Sn component raw material, and Sr component are weighed. Fe ₂ O ₃ or the like is preferably used as the raw material for the Fe component. As the M component raw material, MgO, Mg (OH) ₂ , MgCO ₃ and the like can be preferably used for Mg, MnCO ₃ , Mn ₃ O ₄ and the like can be used for Mn, and CaO, CaO, can be used as the Ca component raw material. Ca (OH) ₂ , CaCO ₃ , etc. can be used, TiO ₂ or the like can be used for Ti, and ZrO ₂ or the like can be used for Zr. Further, SnO ₂ and SnO can be used as the Sn component raw material, and SrCO ₃ and Sr (NO ₃ ) ₂ and the like are preferably used as the Sr component raw material.

Next, the raw material is put into a dispersion medium to prepare a slurry. Water is suitable as the dispersion medium used in the present invention. In addition to the temporary firing raw material, a binder, a dispersant, or the like may be added to the dispersion medium, if necessary. As the binder, for example, polyvinyl alcohol can be preferably used. The amount of the binder to be blended is preferably such that the concentration in the slurry is about 0.1% by mass to 2% by mass. Further, as the dispersant, for example, ammonium polycarboxylate or the like can be preferably used. The amount of the dispersant to be blended is preferably such that the concentration in the slurry is about 0.1% by mass to 2% by mass. In addition, a reducing agent such as carbon black, a pH adjuster such as ammonia, a lubricant, a sintering accelerator and the like may be blended. 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. When it is 60% by mass or more, the pores in the particles are small in the granulated product, and it is possible to prevent insufficient sintering during firing.

The weighed raw materials may be mixed, calcined and pulverized, and then charged into a dispersion medium to prepare a slurry. The temperature of the tentative firing is preferably in the range of 750 ° C to 1000 ° C. When the temperature is 750 ° C. or higher, partial ferrite formation proceeds by calcination, the amount of gas generated during calcination is small, and the reaction between solids proceeds sufficiently, which is preferable. On the other hand, when the temperature is 1000 ° C. or lower, sintering by calcination is weak and the raw material can be sufficiently pulverized in the subsequent slurry pulverization step, which is preferable. Further, the atmosphere at the time of temporary firing is preferably an atmospheric atmosphere.

Next, the slurry produced as described above is wet-ground and pulverized. For example, wet pulverization is performed for a predetermined time using a ball mill or a vibration mill. The average particle size of the raw material after pulverization is preferably 5 μm or less, more preferably 1 μm or less. It is preferable that the vibration mill or the ball mill contains a medium having a predetermined particle size. Examples of the material of the media include iron-based chromium steel, oxide-based zirconia, titania, and alumina. The form of the crushing step may be either a continuous type or a batch type. The particle size of the crushed material is adjusted by the crushing time, rotation speed, material and particle size of the media used.

Then, the crushed slurry is spray-dried to be granulated. Specifically, the slurry is introduced into a spray dryer such as a spray dryer and sprayed into the atmosphere to granulate into a spherical shape. The atmospheric temperature during spray drying is preferably in the range of 100 ° C to 300 ° C. As a result, a spherical granule having a particle size of 10 μm to 200 μm can be obtained. Then, if necessary, the obtained granulated product is classified using a vibrating sieve to prepare a granulated product having a predetermined particle size range.

Next, the granulated product is put into a furnace heated to a predetermined temperature and fired by a general method for synthesizing ferrite particles to generate ferrite particles. The firing temperature is preferably in the range of 1100 ° C to 1350 ° C. If the firing temperature is less than 1100 ° C., phase transformation is less likely to occur and sintering is less likely to proceed. Further, if the firing temperature exceeds 1350 ° C., excessive grain may be generated due to excessive sintering. The rate of temperature rise up to the firing temperature is preferably in the range of 250 ° C./h to 500 ° C./h. The holding time at the firing temperature is preferably 2 hours or more. The unevenness of the ferrite particle surface can also be adjusted by the oxygen concentration in the firing process. Specifically, the oxygen concentration is set to 0.05% to 10%. Further, the oxidation state of the ferrite phase may be adjusted by lowering the oxygen concentration at the time of cooling to the oxygen concentration at the time of firing. Specifically, the oxygen concentration is set in the range of 0.05% to 1.5%. The oxygen concentration in raising, sintering, and cooling is preferably controlled in the range of 0.05% to 10%.

The fired product thus obtained is pulverized as necessary. Specifically, for example, the fired product is pulverized by a hammer mill or the like. The form of the pulverization step may be either a continuous type or a batch type. Further, after the pulverization treatment, if necessary, classification may be performed in order to make the particle size within a predetermined range. As the classification method, conventionally known methods such as wind power classification and sieve classification can be used. Further, after the primary classification with a wind power classifier, the particle size may be adjusted to a predetermined range with a vibration sieve or an ultrasonic sieve. Further, after the classification step, the non-magnetic particles may be removed by a magnetic field beneficiation machine. The average particle size of the ferrite particles is preferably 30 μm or more and less than 50 μm.

Then, if necessary, the classified ferrite particles may be heated in an oxidizing atmosphere to form an oxide film on the particle surface to increase the resistance of the ferrite particles (high resistance treatment). The oxidizing atmosphere may be either an atmospheric atmosphere or a mixed atmosphere of oxygen and nitrogen. The heating temperature is preferably in the range of 200 ° C. or higher and 800 ° C. or lower, and more preferably in the range of 360 ° C. or higher and 550 ° C. or lower. The heating time is preferably in the range of 0.5 hours or more and 5 hours or less. From the viewpoint of homogenizing the surface and the inside of the ferrite particles, it is desirable that the heating temperature is low.

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

Conventionally known resins can be used as the resin for coating the surface of the carrier core material, for example, polyethylene, polypropylene, polyvinyl chloride, poly-4-methylpentene-1, polyvinylidene chloride, ABS (acrylonitrile-butadiene-styrene). ) Resins, polystyrenes, (meth) acrylic resins, polyvinyl alcohol-based resins, polyvinyl chloride-based, polyurethane-based, polyester-based, polyamide-based, polybutadiene-based thermoplastic estramers, fluorosilicone-based resins, and the like.

To coat the surface of the carrier core material with resin, a resin solution or dispersion may be applied to the carrier core material. As the solvent for the coating solution, 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; ethanol, propanol and butanol Alcohol-based solvents such as; cellosolve-based solvents such as ethyl cellosolve and butyl cellosolve; ester-based solvents such as ethyl acetate and butyl acetate; amide-based solvents such as dimethylformamide and dimethylacetamide can be used alone or in combination of two or more. .. The concentration of the resin component in the coating solution is generally preferably in the range of 0.001% by mass or more and 30% by mass or less, particularly 0.001% by mass or more and 2% by mass or less.

As a method for coating the carrier core material with the resin, for example, a spray-drying method, a fluidized bed method, a spray-drying method using a fluidized bed, a dipping method, or the like can be used. Among these, the fluidized bed method is particularly preferable because it can be applied efficiently with a small amount of resin. For example, in the case of the fluidized bed method, the resin coating amount can be adjusted by the amount of the resin solution to be sprayed and the spraying time.

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

The developer for electrophotographic processing according to the present invention is made by mixing the carrier and toner produced as described above. The mixing ratio of the carrier and the toner is not particularly limited, and may be appropriately determined from the development conditions of the developing apparatus to be used. Generally, the toner concentration in the developer is preferably in the range of 1% by mass or more and 15% by mass or less. 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 is scattered in the developing device and the toner adheres to the background part such as stains in the machine or transfer paper. This is because there is a possibility that a problem may occur. A more preferable toner concentration is in the range of 3% by mass or more and 10% by mass or less.

As the toner, those manufactured by conventionally known methods such as a polymerization method, a pulverization classification method, a melt granulation method, and a spray granulation method can be used. Specifically, a binder resin containing a thermoplastic resin as a main component containing a colorant, a mold release agent, a charge control agent, or the like can be preferably used.

Generally, the particle size of the toner is preferably in the range of 5 μm or more and 15 μm or less, and more preferably in the range of 7 μm or more and 12 μm or less in terms of the volume average particle size of the Coulter counter.

If necessary, a modifier may be added to the surface of the toner. Examples of the modifier include silica, alumina, zinc oxide, titanium oxide, magnesium oxide, polymethylmethacrylate and the like. One of these or a combination of two or more thereof can be used.

A conventionally known mixing device can be used for mixing the carrier and the toner. For example, a Henschel mixer, a V-type mixer, a tumbler mixer, a hybridizer and the like can be used.

The developing method using the developer of the present invention is not particularly limited, but the magnetic brush developing method is suitable. FIG. 3 shows an outline diagram showing an example of a developing device that performs magnetic brush development. The developing apparatus shown in FIG. 3 is arranged in parallel in the horizontal direction with a rotatable developing roller 3 having a plurality of magnetic poles and a regulating blade 6 for regulating the amount of developer on the developing roller 3 conveyed to the developing unit. It is formed between two screws 1 and 2 and two screws 1 and 2 that stir and convey the developer in opposite directions, and develops from one screw to the other at both ends of both screws. It is provided with a partition plate 4 that allows the agent to move and prevents the developer from moving except at both ends.

The two screws 1 and 2 have spiral blades 13 and 23 formed on the shaft portions 11 and 21 at the same inclination angle, and are rotated in the same direction by a drive mechanism (not shown) to charge a developer. Transport in opposite directions. Then, the developer moves from one screw to the other screw at both ends of the screws 1 and 2. As a result, the developer composed of toner and carrier is constantly circulated and stirred in the apparatus.

On the other hand, the developing roller 3 has a developing magnetic pole N ₁ , a transport magnetic pole S ₁ , a peeling magnetic pole N ₂ , and a pumping magnetic pole N ₃ as means for generating magnetic poles inside a metal cylindrical body having irregularities of several μm on the surface. It has a fixed magnet in which the _five magnetic poles of the blade magnetic pole S2 are arranged in order. When the tubular body of the developing roller 3 rotates in the direction of the arrow, the developer is pumped from the screw 1 to the developing roller ₃ by the magnetic force of the pumping magnetic pole N3. The developer supported on the surface of the developing roller 3 is layer-regulated by the regulating blade 6 and then conveyed to the developing region.

In the developing region, a bias voltage obtained by superimposing an AC voltage on a DC voltage is applied from the transfer voltage power supply 8 to the developing roller 3. The DC voltage component of the bias voltage is a potential between the background potential and the image potential on the surface of the photoconductor drum 5. Further, the background potential and the image potential are 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 a rectangular wave, a sine wave, a triangular wave, or the like. As a result, the toner and the carrier vibrate in the developing region, and the toner adheres to the electrostatic latent image on the photoconductor drum 5 to develop.

After that, the developer on the developing roller 3 is conveyed to the inside of the apparatus by the transport magnetic pole S1, is detached from the developing roller ₃ by the peeling electrode N ₂ , is circulated and conveyed again in the apparatus by the screws 1 and 2, and is used for development. Mix and stir with undeveloped developer. Then, the developing agent is newly supplied from the screw 1 to the developing roller ₃ by the pumping electrode N3.

In the embodiment shown in FIG. 3, the number of magnetic poles built in the developing roller 3 is five, but in order to further increase the amount of movement of the developing agent in the developing region and further improve the pumping property and the like. Of course, the number of magnetic poles may be increased to 8 poles, 10 poles, or 12 poles.

(Example 1)
As raw materials, Fe ₂ O ₃ (average particle size: 0.6 μm) 20.3 kg, Mn ₃ O ₄ (average particle size: 3.4 μm) 7.6 kg, SrCO ₃ (average particle size: 0.6 μm) 0. Disperse 156 kg, SnO ₂ (average particle size: 4.4 μm) 0.128 kg in 9.4 kg of pure water, 69 g of carbon black as a reducing agent, 203 g of an ammonium polycarboxylate dispersant as a dispersant, and aqueous ammonia. (25 wt% aqueous solution) was added in an amount of 20 g to prepare a mixture. This mixture was pulverized with 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. with a spray dryer to obtain dried granules having a particle size of 10 μm to 75 μm. Fine particles having a particle size of 25 μm or less were removed from this granulated product using a sieve.
This granulated product was placed in an electric furnace and heated to 1265 ° C. over 4.5 hours. After that, firing was performed by holding at 1265 ° C. for 3 hours. The oxygen concentration in the electric furnace was adjusted so that it would be 10,000 ppm in the heating stage and 4000 ppm in the cooling stage.
The obtained calcined product was pulverized with a hammer mill and then classified using a vibrating sieve to obtain a calcined product having an average particle size of 34.7 μm.
Next, the obtained fired product was held at 390 ° C. for 1.5 hours in an air atmosphere to perform an oxidation treatment (high resistance treatment) to obtain a carrier core material.

The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

Next, the surface of the carrier core material thus obtained was coated with a resin to prepare a carrier. Specifically, 450 parts by mass of a 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 coated solution. This coating solution was applied to 50,000 parts by mass of a carrier core material using a fluidized bed coating device, and heated in an electric furnace at a temperature of 300 ° C. to obtain carriers. Carriers were obtained in the same manner for 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 the toner were adjusted so that the mass of the toner / (mass of the toner and the carrier) = 5/100. Hereinafter, developing agents were obtained in the same manner for all Examples and Comparative Examples. The obtained developer was evaluated on an actual machine as described later. The evaluation results are shown in Tables 1 and 2.

(Example 2)
A carrier core material having an average particle size of 34.8 μm was obtained in the same manner as in Example 1 except that the electric furnace temperature in the firing step was changed to 1300 ° C.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 3)
A carrier core material having an average particle size of 35.0 μm was obtained in the same manner as in Example 1 except that fine particles having a particle size of 30 μm or less were removed from the granulated product using a sieve.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 4)
A carrier core material having an average particle size of 36.5 μm was obtained in the same manner as in Example 1 except that fine particles having a particle size of 33 μm or less were removed from the granulated product using a sieve.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 5)
A carrier core material having an average particle size of 37.7 μm was obtained in the same manner as in Example 1 except that fine particles having a particle size of 35 μm or less were removed from the granulated product using a sieve.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 6)
A carrier core material having an average particle size of 36.6 μm was obtained in the same manner as in Example 4 except that the oxygen concentration in the electric furnace at the stage of raising the temperature in the firing step was changed to 50,000 ppm.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 7)
A carrier core material having an average particle size of 34.4 μm was obtained in the same manner as in Example 2 except that the electric furnace temperature in the oxidation treatment step was changed to 480 ° C.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 8)
A carrier core material having an average particle size of 36.5 μm was obtained in the same manner as in Example 4 except that the oxygen concentration in the electric furnace at the cooling stage in the firing step was changed to 1000 ppm.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 9)
A carrier core material having an average particle size of 35.6 μm was obtained in the same manner as in Example 2 except that the electric furnace temperature in the oxidation treatment step was changed to 500 ° C.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 10)
As raw materials, Mn ₃ O ₄ (average particle size: 2.0 μm, specific surface area: 2.7 m ² / g) 3.8 kg, Mn ₃ O ₄ (average particle size: 3.4 μm) having different average particle size and specific surface area. , Specific surface area: 9.9 m ² / g) A carrier core material having an average particle size of 36.1 μm was obtained in the same manner as in Example 9 except that 3.8 kg was used.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Example 11)
A carrier core material having an average particle size of 36.4 μm was obtained in the same manner as in Example 4 except that the temperature in the electric furnace in the firing step was changed to 1315 ° C.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Comparative Example 1)
Weigh Fe ₂ O ₃ (average particle size: 0.8 μm) to 50.0 mol% and Mn ₃ O ₄ (average particle size: 2.0 μm) to 50.0 mol% in terms of MnO, and use a roller compactor. It was pelletized. The obtained pellets were tentatively calcined in a rotary calcining furnace at 850 ° C. under atmospheric air conditions. The mixture was pulverized with a dry bead mill for 6 hours to obtain a temporary calcining raw material (average particle size: 2.2 μm). 20.0 kg of this temporary firing raw material and 3.0 kg of SrFe ₁₂ O ₁₉ (average particle size: 1.2 μm) were dispersed in 7.6 kg of pure water, and 139 g of an ammonium polycarboxylate-based dispersant was added as a dispersant. It was made into a mixture. This mixture was pulverized with 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. with a spray dryer to obtain dried granules having a particle size of 10 μm to 75 μm. Fine particles having a particle size of 30 μm or less were removed from this granulated product using a sieve.
This granulated product was placed in an electric furnace and heated to 1120 ° C. over 5 hours. After that, firing was performed by holding at 1120 ° C. for 3 hours. The oxygen concentration in the electric furnace was adjusted so that it would be 100,000 ppm in the heating stage and 2000 ppm in the cooling stage.
The obtained calcined product was pulverized with a hammer mill and then classified using a vibrating sieve to obtain a calcined product having an average particle size of 36.5 μm.
Next, the obtained fired product was held at 390 ° C. for 1.5 hours in an air atmosphere to perform an oxidation treatment (high resistance treatment) to obtain a carrier core material.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Comparative Example 2)
Fe ₂ O ₃ (average particle size: 0.8 μm) 50.0 mol%, Mn ₃ O ₄ with different average particle size and specific surface area (average particle size: 2.0 μm, specific surface area: 2.7 m ² / g) Weighed to 25.0 mol% in terms of MnO and Mn ₃ O ₄ (average particle size: 3.4 μm, specific surface area: 9.9 m ² / g) to 25.0 mol% in terms of MnO, and with a roller compactor. It was pelletized. The obtained pellets were tentatively calcined in a rotary calcining furnace at 850 ° C. under atmospheric air conditions. The mixture was pulverized with a dry bead mill for 6 hours to obtain a temporary calcining raw material (average particle size: 2.2 μm). 20.0 kg of this temporary firing raw material and 3.0 kg of SrFe ₁₂ O ₁₉ (average particle size: 1.2 μm) were dispersed in 7.6 kg of pure water, and 139 g of an ammonium polycarboxylate-based dispersant was added as a dispersant. It was made into a mixture. This mixture was pulverized with 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. with a spray dryer to obtain dried granules having a particle size of 10 μm to 75 μm. Fine particles having a particle size of 30 μm or less were removed from this granulated product using a sieve.
This granulated product was placed in an electric furnace and heated to 1240 ° C. over 4.5 hours. After that, firing was performed by holding at 1240 ° C. for 3 hours. The oxygen concentration in the electric furnace was adjusted so that it would be 100,000 ppm in the heating stage and 2000 ppm in the cooling stage.
The obtained calcined product was pulverized with a hammer mill and then classified using a vibrating sieve to obtain a calcined product having an average particle size of 36.4 μm.
Next, the obtained fired product was held at 370 ° C. for 1.5 hours in an air atmosphere to perform an oxidation treatment (high resistance treatment) to obtain a carrier core material.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Comparative Example 3)
A fired product having an average particle size of 36.1 μm was obtained in the same manner as in Comparative Example 2 except that the electric furnace temperature in the oxidation treatment step was changed to 400 ° C.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Comparative Example 4)
As a raw material, 20.3 kg of Fe ₂ O ₃ (average particle size: 0.6 μm) and 7.6 kg of Mn ₃ O ₄ (average particle size: 2.0 μm) were dispersed in 9.4 kg of pure water and used as a reducing agent. A mixture was prepared by adding 69 g of carbon black, 203 g of an ammonium polycarboxylate dispersant as a dispersant, and 20 g of aqueous ammonia (25 wt% aqueous solution). This mixture was pulverized with 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. with a spray dryer to obtain dried granules having a particle size of 10 μm to 75 μm. Fine particles having a particle size of 25 μm or less were removed from this granulated product using a sieve.
This granulated product was placed in an electric furnace and heated to 1200 ° C. over 4.5 hours. After that, firing was performed by holding at 1200 ° C. for 3 hours. The oxygen concentration in the furnace was adjusted so that the atmosphere in the electric furnace was 7,000 ppm.
The obtained calcined product was pulverized with a hammer mill and then classified using a vibrating sieve to obtain a calcined product having an average particle size of 34.4 μm.
Next, the obtained fired product was held at 390 ° C. for 1.5 hours in an air atmosphere to perform an oxidation treatment (high resistance treatment) to obtain a carrier core material.
The powder characteristics, shape characteristics, magnetic characteristics, etc. of the obtained carrier core material were measured by the method described later. The measurement results are shown in Tables 1 and 2.

(Composition analysis)
(Analysis of Fe)
The carrier core material containing the iron element was weighed and dissolved in a mixed acid water of hydrochloric acid and nitric acid. After evaporating and drying this solution, sulfuric acid water is added and redissolved to volatilize excess hydrochloric acid and nitric acid. Solid Al is added to this solution to reduce all Fe ³⁺ in the solution to Fe ²⁺ . Subsequently, the amount of Fe ²⁺ ions in this solution was quantitatively analyzed by potentiometric titration with a potassium permanganate solution to determine the titration amount of Fe (Fe ²⁺ ).
(Analysis of Mn)
The Mn content of the carrier core material was quantitatively analyzed according to the ferromanganese analysis method (potentiometric titration method) described in JIS G1311-1987. The Mn content of the carrier core material described in the present specification is the amount of Mn obtained by quantitative analysis by this ferromanganese analysis method (potential difference drip method).
(Analysis of Sr)
The Sr content of the carrier core material was analyzed by the following method. The carrier core material was dissolved in an acid solution and quantitative analysis was performed by ICP. The Sr content of the carrier core material described in the present specification is the Sr content obtained by the quantitative analysis by this ICP.
(Analysis of Sn)
The Sn content of the carrier core material was quantitatively analyzed by ICP as in the analysis of Sr.

(Apparent density AD)
The apparent density of the carrier core material was measured according to JIS Z 2504.

(Fluidity FR)
The fluidity of the carrier core material was measured according to JIS Z 2502.

(Average particle size D ₅₀ , ratio of particle size 22 μm or less and ratio of particle size 26 μm or less)
The average particle size _D50 of the carrier core material, the volume ratio of the particle size of 22 μm or less, and the number ratio of the particle size of 26 μm or less are determined by using a laser diffraction type particle size distribution measuring device (“Microtrack Model 9320-X100” manufactured by Nikkiso Co., Ltd.). It was measured.

(Pore volume)
The measurement of the pore volume was performed as follows. As the evaluation device, POREMASTER-60GT manufactured by Quantachrome was used. Specifically, as the measurement conditions, Cell Stem Volume: 0.5 ml, Headpressure: 20PSIA, surface tension of mercury: 485.00 erg / cm ² , mercury contact angle: 130.00 degrees, high-pressure measurement mode: Fixed Rate, Moter Speed: 1, high pressure measurement range: 20.00 to 10000.00 PSI, 1.200 g of sample was weighed and filled in a 0.5 ml (cm ³ ) cell for measurement. Further, the value obtained by subtracting the volume A (ml / g) at 100 PSI from the volume B (ml / g) at 10000.00 PSI was defined as the pore volume.

(BET specific surface area)
Evaluation was performed using a BET one-point method specific surface area measuring device (manufactured by Mountech Co., Ltd., model: Macsorb HM model-1208). Specifically, the sample was weighed at 10.000 g, filled in a cell having a diameter of 15 mm, degassed at 200 ° C. for 30 minutes, and measured.

(Internal porosity)
The carrier core material is dispersed in the resin and vacuum defoamed to fill the carrier core material with the resin, which is then applied to an auxiliary plate and heat-treated at a temperature of 200 ° C. for 20 minutes to cure the resin. rice field. After that, the carrier core material was cut using a cross session polisher (SM-09010, manufactured by JEOL Ltd.). Then, the cross section of the carrier core material was photographed with a scanning electron microscope (JSM-6510LA type manufactured by JEOL Ltd.). The particle area A <including voids, μm ² > and the particle area B <without voids, μm ² > were measured from the captured images using image analysis software (Image-Pro Plus, manufactured by Media Cybernetics). In the cross-sectional observation image, since some of the carrier core materials were cut off in the field of view, particles having a size of 20 μm or more were selected for the measurement. Each area was calculated for each particle, and the average value of 100 particles was taken as the particle area A (μm ² ) and the particle area B (μm ² ) of the carrier core material. Then, the internal porosity was calculated from the following formula (2).
Internal porosity (%) = (particle area A-particle area B) / particle area A × 100 ... (2)

(Measurement method of maximum mountain valley depth Rz)
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, the carrier core material was fixed to the adhesive tape having a flat surface, the measurement field of view was determined by the 100x objective lens, and then the focus was adjusted to the adhesive tape surface by using the autofocus function. The flat adhesive tape surface on which the carrier core material was fixed was irradiated with a laser beam from the vertical direction (Z direction) and scanned in the X direction and Y direction of the surface. In addition, data in the Z direction was acquired by connecting the height positions of the lenses when the intensity of the reflected light from the surface was maximized. These position data in the X, Y and Z directions were joined to obtain a three-dimensional shape of the surface of the carrier core material. An automatic photographing function was used to capture the three-dimensional shape of the surface of the carrier core material.
Each parameter was measured using particle roughness inspection software (manufactured by Mitani Corporation). First, as a pretreatment, particle recognition and shape selection of the three-dimensional shape of the surface of the obtained carrier core material were performed. Particle recognition was performed by the following method. Of the three-dimensional shapes obtained by photographing, the maximum value in the Z direction is set to 100%, the minimum value is set to 0%, and the range from the maximum value to the minimum value is divided into 100 equal parts. The region corresponding to 100 to 35% of this was extracted, and the contour of the independent region was recognized as the particle contour. Next, the shape selection excluded particles such as coarse, micro, and association. By performing this shape selection, it is possible to reduce the error during the subsequent pole factor correction. Specifically, particles having an area equivalent diameter of 28 μm or less, 38 μm or more, and a needle-like ratio of 1.15 or more were excluded. Here, the needle-like ratio is a parameter calculated from the ratio of the maximum length / diagonal width of the particles, and the diagonal width is the shortest distance between the two straight lines when the particles are sandwiched between two straight lines parallel to the maximum length. Represents.
Next, the part used for the analysis was taken out from the three-dimensional shape of the surface. First, a square having a side length of 15.0 μm is drawn around the center of gravity obtained from the particle contour recognized by the above method. Twenty-one parallel lines were drawn in the drawn square, and 21 roughness curves on the line segments were taken out.

Since the carrier core material has a substantially spherical shape, the extracted roughness curve has a constant curvature as a background. Therefore, as a background correction, an optimum quadratic curve was fitted and a correction was performed by subtracting it from the roughness curve. In this case, a low-pass filter was applied with an intensity of 1.5 μm, and the cutoff value λ was set to 80 μm.

The maximum valley depth Rz was calculated as the sum of the height of the highest mountain and the depth of the deepest valley in the roughness curve. The measurement of the maximum height Rz described above is performed in accordance with JIS B0601 (2001 version). For the calculation of the maximum height Rz, the average value of 50 particles was used as the average value of each parameter.

(Magnetic characteristics)
Using a vibrating sample magnetometer (VSM) for room temperature (“VSM-P7” manufactured by Toei Kogyo Co., Ltd.), the external magnetic field is continuously applied for one cycle in the range of 0 to 79.58 × 10 ⁴ A / m (10000 oersted). When a magnetic field of 79.58 × 10 ³ A / m (1,000 oersted) was applied, the magnetization σ _{1 k} , saturation magnetization σ _s , residual magnetization σ _r , and holding force H _c were measured.

(Electrical resistance)
Two brass plates with a thickness of 2 mm whose surfaces are electrolytically polished as electrodes are arranged so that the distance between the electrodes is 2 mm, and 200 mg of a carrier core material is charged in the gap between the two electrode plates, and then each electrode plate is used. A magnet with a cross-sectional area of 240 mm ² is placed behind the electrode, and a 500 V DC voltage is applied between the electrodes with a bridge of the powder to be measured formed between the electrodes, and the current value flowing through the carrier core material is measured by the 4-terminal method. It was measured. The electric resistance of the carrier core material was calculated from the current value, the distance between the electrodes of 2 mm, and the cross-sectional area of 240 mm ² .

(Static breakdown voltage Vs)
For the static insulation breakdown voltage Vs, two brass plates with a thickness of 2 mm whose surfaces are electrolytically polished as electrodes are arranged so that the distance between the electrodes is 2 mm, and a carrier core material of 200 mg is placed in the gap between the two electrode plates. After inserting, a magnet with a cross-sectional area of 240 mm ² is placed behind each electrode plate to form a bridge of the powder to be measured between the electrodes, and a DC voltage is applied between the electrodes to form a carrier core material. The flowing current value was measured by the 4-terminal method. As for the DC voltage, the applied voltage was increased with 100 V as the starting voltage and 100 V-100 seconds as one step. The voltage at which the current value flowing between the electrodes becomes 100 mA or more in the process of increasing the applied voltage was defined as the static dielectric breakdown voltage Vs in the bridge type measuring instrument.

(Dynamic breakdown voltage Vd)
As shown in FIG. 2, the dynamic breakdown voltage Vd was measured using a device consisting of a carrier stirring unit, a developing roller, and an aluminum electrode. 120 g of the carrier core material is filled in the apparatus shown in FIG. 2, and the developing rollers are opposed to the aluminum electrodes at a gap d = 0.07 cm. It was rotated in the same direction at the number of rotations. In this state, the current I when the DC voltage V was applied between the developing roller and the aluminum electrode was measured. As for the DC voltage, the applied voltage was increased with 100 V as the starting voltage and 100 V-10 seconds as one step. The voltage at which the current value flowing between the electrodes becomes 3 mA or more in the process of increasing the applied voltage is defined as the dynamic breakdown voltage Vd in the dynamic resistance measuring instrument.
In this measurement, an aluminum cylinder with a diameter of 30 mm and a length of 100 mm that has been subjected to bead blasting is used as the developing roller, and an aluminum cylinder with a diameter of 30 mm and a length of 100 mm is used as the aluminum electrode, and the distance between the developing roller and the regulation plate is used. Was adjusted to 0.7 mm and measured.

(Evaluation of background carrier adhesion)
Manufactured in a developing device having the structure shown in FIG. 3 (developing roller peripheral speed v ₁ : 406 mm / sec, photoconductor drum peripheral speed v ₂ : 205 mm / sec, photoconductor drum-developing roller distance: 0.3 mm). The above-mentioned two-component developer is added, 10 blank sheets are developed, the number of carriers adhering to the surface of the photoconductor is counted in 5 fields by loupe observation, and the average number of carriers adhering to 100 cm ² is used as the carrier adhering. And said. This evaluation was performed at the initial stage and after the developing apparatus was driven for a time corresponding to the time required for printing 100 k sheets of A4 vertical paper (time equivalent to printing 100 k sheets).
"◎": Less than 10 pieces "○": 10 pieces or more and less than 30 pieces "△": 30 pieces or more and less than 50 pieces "×": 50 pieces or more

(Evaluation of white spots in the image)
Manufactured in a developing device having the structure shown in FIG. 3 (developing roller peripheral speed v ₁ : 406 mm / sec, photoconductor drum peripheral speed v ₂ : 205 mm / sec, photoconductor drum-development roller distance: 0.3 mm). A solid black image was printed after the above-mentioned two-component developer was added and the developing apparatus was driven for an initial period and equivalent to 100k sheet printing, and the degree of white spots in the solid black portion was visually evaluated according to the following criteria.
"◎": No white spots can be confirmed, which is a good image.
"○": Less than 5 white spots "Δ": 5 to 10 white spots "×": Clearly more than 10 white spots.

As is clear from Tables 1 and 2, the difference between the static dielectric breakdown voltage Vs and the dynamic breakdown voltage Vd is 100V or 200V, and the internal void ratio is 2.9% or less in Examples 1 to 11. With the carrier core material, there is no problem in actual use with less than 10 or less than 30 background carrier adhesions at the initial stage and after driving for a time equivalent to 100k sheet printing, and white spots in the image cannot be confirmed or less than 5 pieces. There was no problem in actual use.

On the other hand, the carrier core material of Comparative Example 1 in which the difference between the static dielectric breakdown voltage Vs and the dynamic breakdown voltage Vd is (-100V) and the internal void ratio is as large as 9.1% is 100k from the initial stage. After driving for a time equivalent to sheet printing, background carrier adhesion occurred at a level of 30 or more and less than 50, which was a problem in actual use. Further, after driving for a time equivalent to 100k sheet printing, more than 10 white spots occurred in the image.

In the carrier core material of Comparative Example 2 in which the difference between the static dielectric breakdown voltage Vs and the dynamic dielectric breakdown voltage Vd was (-100V), no white spots were confirmed in the image, but background carrier adhesion was initially observed. It occurred at a level where there was a problem in actual use, with 30 or more and less than 50.

In the carrier core material of Comparative Example 3 in which the difference between the static dielectric breakdown voltage Vs and the dynamic dielectric breakdown voltage Vd is (-600V) and the internal void ratio is as large as 3.6%, the background carrier adhesion is 50 at the initial stage. It occurred violently with more than 30 pieces, and even after driving for a time equivalent to 100k sheet printing, it occurred at a level of 30 or more and less than 50 pieces, which is a problem in actual use. In addition, after driving for a time equivalent to 100k sheet printing, white spots in the image were 5 to 10 at a level that was problematic in actual use.

In the carrier core material of Comparative Example 4 in which the difference between the static dielectric breakdown voltage Vs and the dynamic dielectric breakdown voltage Vd is (-200V), the background carrier adhesion is 30 or more and less than 50 at the initial stage, which is a problem in actual use. Occurred at a certain level.

According to the carrier core material of the present invention, good triboelectric characteristics for toner can be obtained, carrier adhesion to the background portion is suppressed, and white spots occur in the image even when used in a high-speed image forming apparatus. It is suppressed and useful.

3 Develop roller 5 Photoreceptor drum

Claims (8)

A carrier core material composed of ferrite particles.
The static dielectric breakdown voltage Vs and the dynamic breakdown voltage Vd satisfy the following equation (1).
A carrier core material having an internal porosity of ferrite particles calculated from the following formula (2) of 3.0% or less.
0 (V) ≤ Vs-Vd ≤ 1000 (V) ... (1)
Internal porosity (%) = (particle area A-particle area B) / particle area A × 100 ... (2)
In the formula, particle area A: particle cross-sectional area including internal voids Particle area B: particle cross-sectional area not including internal voids The composition of the ferrite particles contains MnO: 35 mol% or more and 55 mol% or less, Fe ₂ O ₃ : 45 mol% or more and 65 mol% or less, and a part thereof is SrO: 0.1 mol% or more and 5.0 mol% or less, SnO :. The carrier core material according to claim 1, wherein the carrier core material is substituted with 0.1 mol% or more and 5.0 mol% or less. The carrier core material according to claim 1 or 2, wherein the maximum mountain valley depth Rz is 1.9 μm or more and 2.6 μm or less. The carrier core material according to any one of claims 1 to 3, wherein the magnetization σ _{1 k} when a magnetic field of 79.58 × 10 ³ A / m (1000 oersted) is applied is 63 Am ² / kg or more and 73 Am ² / kg or less. The carrier core material according to any one of claims 1 to 4, wherein the pore volume is 0.001 cm ³ / g or more and 0.015 cm ³ / g or less. The carrier core material according to any one of claims 1 to 5, wherein the ratio of the particle size of 26 μm or less in the number-based particle size distribution measured by the laser diffraction type particle size distribution measuring device is 25% or less. A carrier for electrophotographic development, wherein the surface of the carrier core material according to any one of claims 1 to 6 is coated with a resin. An electrophotographic developer comprising the electrophotographic developing carrier and toner according to claim 7.

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