JP7697866B2 - 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.

For example, in electrophotographic image forming devices such as facsimiles, printers, and copiers, toner is applied to an electrostatic latent image formed on the surface of a photoconductor to make it visible, and this visible image is then transferred to paper or the like and fixed by applying heat and pressure. From the perspective of achieving high image quality and colorization, so-called two-component developers, which contain a carrier and toner, are widely used as developers.

In a development method using a two-component developer, the carrier and toner are stirred and mixed in the developing device, and the toner is charged to a specified amount by friction. The developer is then supplied to a rotating developing roller, a magnetic brush is formed on the developing roller, and the toner is electrically transferred to the photoconductor via the magnetic brush, making the electrostatic latent image on the photoconductor visible. After the toner has been transferred, the carrier is peeled off from the developing roller and mixed with the toner again in the developing device. For this reason, the carrier's characteristics require a transfer characteristic to the developing roller, a magnetic characteristic to form a magnetic brush, a charging characteristic to impart the desired charge to the toner, and durability in repeated use.

Such carriers are generally made of magnetic particles such as magnetite and various ferrites, the surfaces of which are coated with resin. Magnetic particles as carrier core materials are required to have good magnetic properties as well as good triboelectric charging properties with toner. Various shapes of carrier core materials that satisfy these properties have been proposed.

For example, in order to prevent peeling of the coating resin from the carrier core material due to long-term use, a carrier core material has been proposed that contains a specific amount of at least one of Sr (strontium) and Ca (calcium) and that has a frequency of grains with an average grain length RSm appearing on the particle surface that is equal to or greater than a specific value that is equal to or less than a specific value (Patent Document 1). It has also been proposed to prevent toner spent, which is toner components adhering to the carrier surface, by setting the spacing Sm of the surface irregularities of the carrier core material to a specific value or less and setting the surface roughness Ra to a predetermined value or more (Patent Document 2).

JP 2016-184130 A JP 2007-286092 A

However, in recent years, in order to meet market demand for faster image formation in image forming devices, there has been a trend to increase the rotation speed of the developing roller and the amount of developer supplied to the development area per unit time.

However, if the amount of developer supplied to the development area is increased in line with the increase in image formation speed, the stress on the developer increases, making it more likely that the coating resin will peel off from the carrier core material or wear away. Toner spent is also more likely to occur.

When the coating resin peels off from the carrier core material or wears away, the carrier core material is exposed to the surface, and charge injection occurs from the exposed area, causing the carrier to adhere to the photoreceptor, resulting in what is known as blank areas in the image. In addition, when toner spent occurs on the carrier surface, the carrier's ability to impart charge to the toner can decrease. Furthermore, when the stress on the developer becomes too great, the carrier core material can crack or chip.

To prevent the coating resin from peeling off from the carrier core material, it is possible to increase the pore volume of the carrier core material to exert a strong anchoring effect on the coating resin. However, if the pore volume of the carrier core material is increased, the magnetization per particle of the carrier core material decreases, which may cause carrier scattering.

The object of the present invention is to provide a carrier core material that can suppress toner spent, image voids, and carrier scattering even when the image formation speed is high or after long-term use, as well as an electrophotographic development carrier and an electrophotographic developer that use the same.

The carrier core material of the present invention that achieves the above-mentioned object is a carrier core material composed of ferrite particles, and is characterized in that it has a saturation magnetization _σs of 75 ^Am2 /kg or more and 90 ^Am2 /kg or less, a pore volume of 0.008 ^cm3 /g or more and less than 0.020 ^cm3 /g, a maximum height Rz of 1.4 μm or more and 2.0 μm or less, and a particle irregularity rate measured by the following measurement method is in the range of 10% or more and 50% or less.
(Method for measuring particle irregularity rate)
Measurement device: Injection type image analysis particle size distribution meter Measurement sample amount: 0.07 g
Measurements were performed after dispersion in a screw cap flask (volume 9 cm3 ⁾ containing 9 ^cm3 of polyethylene glycol 400.
(Measurement conditions)
Spacer thickness: 150 μm
Sampling: 20%
Analysis type: relative measurement Measurement volume: 0.95 ^cm3
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 irregularity rate is calculated by dividing the number of particles counted under analysis filter condition II by the number of particles counted under analysis filter condition I.

In the carrier core material having the above configuration, it is preferable that the average length RSm of the grains appearing on the particle surface is 5.0 μm or more and 7.1 μm or less.

In the carrier core material having the above structure, the magnetization σ _1k in a magnetic field of 79.58×10 ³ A/m (1000 Oersted) is preferably 61 Am ² /kg or more and 75 Am ² /kg or less.

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.

The carrier core material of the present invention can suppress toner spent, image voids, and carrier scattering even when the image formation speed is high or after long-term use.

2 is a SEM photograph of the carrier core material of Example 1. 13 is a SEM photograph of the carrier core material of Comparative Example 8. 13 is a SEM photograph of the carrier core material of Comparative Example 9. 1 is a schematic diagram showing an example of a developing device using an electrophotographic developer according to the present invention;

(Carrier core material)
A major feature of the carrier core material according to the present invention is that the saturation magnetization _σs , pore volume, maximum height Rz, and irregularity rate of the particles measured by the above-mentioned measuring method are all simultaneously within specific ranges. More specifically, the saturation magnetization _σs and pore volume being within the specific ranges mainly suppresses carrier scattering. Furthermore, the pore volume and maximum height Rz being within the specific ranges mainly suppresses peeling of the coating resin due to long-term use by exerting an anchor effect. Furthermore, the maximum height Rz and irregularity rate being within the specific ranges mainly suppresses wear of the coating resin due to long-term use. The maximum height Rz and irregularity rate being within the specific ranges mainly suppresses toner spent.

The configuration of the present invention will be described below individually. First, in the present invention, the saturation magnetization _σs of the carrier core material is in the range of 75 ^Am2 /kg or more and 90 ^Am2 /kg or less. If the saturation magnetization _σs is less than 75 ^Am2 /kg, carrier scattering is likely to occur, while if the saturation magnetization _σs exceeds 90 ^Am2 /kg, the magnetic brush formed on the developing roller becomes coarse, which may result in a decrease in image quality. The preferred range of the saturation magnetization _σs is in the range of 80 ^Am2 /kg or more and 90 ^Am2 /kg or less.

Next, the pore volume of the carrier core material in the present invention is in the range of 0.008 ^cm3 /g or more and less than 0.020 ^cm3 /g. If the pore volume is less than 0.008 ^cm3 /g, the anchor effect is not sufficiently obtained and the coating resin may peel off, while if the pore volume is 0.020 ^cm3 /g or more, the magnetization per particle decreases and carrier scattering becomes more likely to occur.

The maximum height Rz of the carrier core material in the present invention is in the range of 1.4 μm or more and 2.0 μm or less. If the maximum height Rz is less than 1.4 μm, the anchor effect is not sufficient and the coating resin may peel off, while if the maximum height Rz exceeds 2.0 μm, the stress on the carrier surface becomes so large that the coating resin wears away, causing whiteouts in the image.

The irregularity rate of the carrier core material measured by the above-mentioned measuring method in the present invention is in the range of 10% to 50%. If the irregularity rate is less than 10%, the toner spent on the carrier surface is difficult to scrape off, which reduces the ability to impart charge to the toner and may lead to poor image quality. On the other hand, if the irregularity rate exceeds 50%, the coating resin may wear away and white spots may occur in the image. The preferable range of the irregularity rate is 15% to 47%.

Next, the average length RSm of the carrier core material in the present invention is preferably in the range of 5.0 μm or more and 7.1 μm or less. By having the average length RSm in this range, excessive sharpening of the grains is suppressed, the mechanical stress on the resin-coated carrier in the magnetic brush is reduced, and wear of the resin coating layer is easily suppressed. A more preferable range for the average length RSm is in the range of 6.0 μm or more and 7.1 μm or less.

In the present invention, the magnetization σ _1k of the carrier core material is preferably 61 Am ² /kg or more and 75 Am ² /kg or less. When the magnetization σ _1k is in this range, carrier scattering is easily suppressed and the developer is easily transported to the development area. The magnetization σ _1k is more preferably in the range of 64 Am ² /kg or more and 70 Am ² /kg or less.

The ferrite particles constituting the carrier core material of the present invention are preferably mainly composed of a material represented by the composition formula (Mn _x Fe _3-x )O ₄ (wherein 0<x<3). Here, the main component refers to a material contained in excess of 50% by mass. Furthermore, it is preferable that Si (silicon) is contained in an amount of 0.06 mol% or more and 0.20 mol% or less with respect to the total molar number of the particle composition. If the Si content is less than 0.06 mol%, the particles may become irregular in shape when the grain size is small, and the sharpening of the grains may be excessively promoted. On the other hand, if the Si content exceeds 0.20 mol%, the decomposition, melting, and diffusion of the particles may be promoted, and the energetically stable spheroidization may be excessively promoted. Note that the Si content can usually be adjusted by the amount of Si raw material added, but it can also be adjusted by the amount of mixed raw material added in which the Si raw material is mixed with other raw materials. In the examples described later, a mixed raw material containing Fe and Si and a mixed raw material containing Mn and Si are used, and the Si content is adjusted by the amount of the mixed raw material added. Note that the amount of the Si raw material added when the mixed raw material is used is calculated from the Si content of the mixed raw material.

The ferrite particles constituting the carrier core material of the present invention may contain 0.01 mol% to 0.50 mol% of Sr (strontium) and may further contain 0.01 mol% to 0.50 mol% of Sn (tin) relative to the total mole number of the particle composition. By containing the above amount of Sr, some Sr ferrite is generated in the firing process, and a magnetoplumbite-type crystal structure is formed, which makes it easier to promote the uneven shape of the carrier core material surface. On the other hand, Sn has a sintering inhibitory effect, and by containing the above amount of Sn together with Sr, excessive deformation of the particles is suppressed compared to when only Sr is contained, and the desired unevenness is formed on the particle surface. A more preferable range of Sr content is 0.10 mol% to 0.50 mol%. A more preferable range of Sn content is 0.10 mol% to 0.50 mol%.

Furthermore, the ferrite particles constituting the carrier core material of the present invention may contain Ca (calcium) in an amount of 0.01 mol% or more and 0.60 mol% or less, based on the total molar number of the particle composition. By containing Ca in the above amount, the electrical properties, magnetic properties, and shape properties of the carrier core material are adjusted to the desired range. The preferred range of the Ca content is 0.40 mol% or more and 0.60 mol% or less.

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.

The apparent density of the carrier core material of the present invention is preferably in the range of 2.00 g/cm ³ or more and 2.50 g/cm ³ or less, and more preferably in the range of 2.10 g/cm ³ or more and 2.40 g/cm ³ or less.

The fluidity (sec/50 g) of the carrier core material of the present invention is preferably in the range of 25 to 40, more preferably in the range of 28 to 40.

(Manufacturing method of carrier core material)
Although there is no particular limitation on the method for producing the carrier core material of the present invention, the production method described below is preferable.

First, the necessary component raw materials are weighed to obtain the desired composition. Also, conventionally known additives are weighed as necessary. For example, _Fe2O3 and the _like are preferably used as the Fe (iron) component raw material. _MnCO3 , _Mn3O4 and _the like are used as the Mn (manganese) component raw material. SrCO3, Sr( _NO3 ) ₂ are used as the Sr component raw material, _SnO2 , SnO are used as the Sn component raw material, and _SiO2 and the _like are preferably used as the Si component raw material. Some Fe component raw materials and Mn component raw materials contain a predetermined amount of Si, and when such Fe component raw materials and Mn component raw materials having a Si content are used, if the amount of Si added is sufficient with the amount of Si contained in the Fe component raw material and the Mn component raw material, the Si component raw material does not need to be mixed. On the other hand, if the amount of Si added is insufficient with the amount of Si contained in the Fe component raw material and the Mn component raw material, the Si component raw material is supplemented and mixed.

Next, the raw materials are put into the dispersion medium to prepare a slurry. Water is a suitable dispersion medium for use in the present invention. In addition to the pre-calcined raw materials, a binder, a dispersant, etc. may be mixed into the dispersion medium as necessary. For example, polyvinyl alcohol is suitable for use as the binder. The amount of the binder to be mixed is preferably about 0.1% by mass to 2% by mass in the slurry. For example, ammonium polycarboxylate is suitable for use as the dispersant. The amount of the dispersant to be mixed 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 mixed. 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 partial ferritization by pre-fire progresses, the amount of gas generated during firing is small, and solid-state reactions progress sufficiently. On the other hand, a temperature of 1000°C or lower is preferable because sintering by pre-fire is weak, and the raw materials can be sufficiently pulverized in the subsequent slurry pulverization process. Furthermore, air is preferable as the atmosphere during pre-fire.

Next, the slurry prepared as described above is wet-pulverized. For example, wet-pulverization is performed for a predetermined time using a ball mill or a vibration mill. The volume average particle size _D50 , which indicates the particle size of 50% in the cumulative particle size distribution of the raw material after pulverization, is preferably 1.0 μm or less. The volume average particle size _D90 , which indicates the particle size of 90% in the cumulative particle size distribution, is preferably 3.0 μm or less. It is preferable that media of a predetermined particle size is contained inside the vibration mill or ball mill. Examples of the material of the media include iron-based chromium steel and oxide-based zirconia, titania, alumina, etc. The form of the pulverization process may be either continuous or batchwise. The particle size of the pulverized product is adjusted by the pulverization time, rotation speed, material and particle size of the media used, etc.

The viscosity of the slurry is preferably 1000 cP or less, and more preferably 100 cP or less.

The prepared slurry is then spray-dried to form granules. Specifically, the slurry is introduced into a spray dryer such as a spray dryer, and sprayed into the atmosphere to form spherical granules. The temperature of the atmosphere 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 obtained granules are classified using a vibrating sieve to prepare granules with a predetermined particle size range. The apparent density AD of the granules is preferably in the range of 1.40 g/cm ³ to 1.75 g/cm ³ , and more preferably in the range of 1.50 g/cm ³ to 1.58 g/cm ^3. By setting the apparent density of the granules in the above range, it becomes easier to adjust the pore volume of the carrier core material to the specified range of the present invention. If the apparent density of the granules is small, the pore volume tends to be large, and if the apparent density of the granules is large, the pore volume tends to be small. The apparent density of the granulated product can be adjusted, for example, by the type (composition) of the raw material, the slurry particle size, the type and amount of the dispersant, etc.

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 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 during sintering is set to 0.3 vol% to 1.2 vol%. In addition, the oxidation state of the ferrite phase may be adjusted by lowering the oxygen concentration during cooling to be lower than the oxygen concentration during sintering. Specifically, it is preferable to control the oxygen concentration during firing to a range of 0.3 vol% to 1.2 vol%, and the oxygen concentration during cooling to a range of 0.1 vol% to 0.8 vol%.

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 volume average 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 350°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.

(Electrophotographic Development Carrier)
The resin that coats the surface of the carrier core material may be any of the conventionally known 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 volume average particle size, and preferably in the range of 30 μm or more and 40 μm or less.

(Electrophotographic developer)
The electrophotographic developer according to the present invention is obtained by mixing 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 to be used. In general, 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, and if the toner concentration exceeds 15% by mass, toner scattering occurs in the developing device, which may cause problems such as dirt inside the device or toner adhesion to the background part of the transfer paper. A more preferable toner concentration is in the range of 3% by mass or more and 10% by mass or less.

Toners that can be used are those manufactured by conventional methods such as polymerization, pulverization and 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. One or more of these may be used in combination.

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 in parallel in the horizontal direction 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 metallic cylindrical body having a surface with a few μm of unevenness, and has a fixed magnet with five magnetic poles arranged in order as magnetic pole generating means: a developing magnetic pole _N1 , a transport magnetic pole _S1 , a peeling magnetic pole _N2 , a pumping magnetic pole _N3 , and a blade magnetic pole _S2 . When the cylindrical body of the developing roller 3 rotates in the direction of the arrow, the developer is pumped up from the screw 1 to the developing roller 3 by the magnetic force of the pumping magnetic pole _N3 . The developer carried on the surface of the developing roller 3 is layer-regulated by a regulating blade 6, and then transported to the development area.

In the development area, a bias voltage consisting of a DC voltage superimposed on an AC voltage is applied from the 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, resulting in development.

The developer on the developing roller 3 is then transported into the device by the transport magnetic pole _S1 , peeled off from the developing roller 3 by the peeling electrode _N2 , and circulated again within the device by the screws 1 and 2, where it is mixed and stirred with the developer 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, 43.55 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ), 8.17 kg _{of Mn3O4} (average particle size: 3.4 μm), 8.33 _{kg of Mn3O4} (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ), 332.9 g of _SrCO3 (average particle size: 0.6 μm), and 273.4 g of _SnO2 (average particle size: 5.6 μm) were dispersed in 20.48 kg of pure water, and 180.1 g of carbon black was added as a reducing agent, 366.3 g of ammonium polycarboxylate-based dispersant as a dispersant, and 42.8 g of 25 mass% ammonia water as a pH adjuster were added to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.17 mol% with respect to the total of Fe, Mn, Sr, Sn, and Si. This mixture was pulverized in a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1110°C at an oxygen concentration of 1.0 vol% over 4.5 hours. It was then fired by holding at 1110°C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. It was then cooled at an oxygen concentration of 0.4 vol% over 6 hours.
The obtained sintered product was disintegrated with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain sintered particles with a volume average particle size of 34.9 μm. The obtained sintered particles were subjected to an oxidation treatment by being held at a temperature of 400° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Example 1.
The apparent density, fluidity, volume average particle diameter (average particle diameter), magnetic properties, static electrical resistance, pore volume, true density, maximum height Rz, average length RSm, and irregularity rate of the obtained carrier core material were measured by the methods described below, and image blanking, carrier scattering, and toner spent were evaluated by the criteria described below. The properties of the carrier core materials in the following examples and comparative examples were also measured by the same methods and evaluated by the same criteria. Figure 1 shows an SEM photograph of the carrier core material of Example 1. The length of the white line in the lower right of Figure 1 is 10 μm (the same applies to Figures 2 and 3).

Example 2
As raw materials, 37.76 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% _SiO2 ) and 14.26 kg _{of Mn3O4} (average particle size: 3.4 μm) were dispersed in 17.37 kg of pure water, and 379.8 g of ammonium polycarboxylate dispersant was added as a dispersant, and 36.4 g of 25 mass% ammonia water was added as a pH adjuster to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.08 mol% relative to the total of Fe, Mn and Si. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1170°C at an oxygen concentration of 1.0 vol% over 4.5 hours. It was then fired by holding at 1170°C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. It was then cooled at an oxygen concentration of 0.4 vol% over 6 hours.
The obtained sintered product was disintegrated with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain sintered particles with a volume average particle size of 35.5 μm. The obtained sintered particles were subjected to an oxidation treatment by being held at a temperature of 350° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Example 2.

Example 3
As raw materials, 37.76 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% _SiO2 ), 14.26 kg _{of Mn3O4} (average particle size: 3.4 μm), and 370.1 g of _CaCO3 (average particle size: 1.1 μm) were dispersed in 17.49 kg of pure water, and 379.8 g of ammonium polycarboxylate dispersant was added as a dispersant, and 36.4 g of 25 mass% ammonia water was added as a pH adjuster to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.08 mol% relative to the total of Fe, Mn, Ca, and Si. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1270° C. at an oxygen concentration of 1.0 vol% over 4.5 hours. It was then fired by being held at 1270° C. at an oxygen concentration of 0.4 vol% to 1.0 vol% for 3 hours. It was then cooled at an oxygen concentration of 0.4 vol% over 6 hours.
The obtained sintered product was disintegrated with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain sintered particles with a volume average particle size of 36.0 μm. The obtained sintered particles were subjected to an oxidation treatment by being held at a temperature of 440° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Example 3.

Example 4
As raw materials, 37.76 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% _SiO2 ), 14.26 kg _{of Mn3O4} (average particle size: 3.4 μm), and 370.1 g of _CaCO3 (average particle size: 1.1 μm) were dispersed in 17.49 kg of pure water, and 379.8 g of ammonium polycarboxylate dispersant was added as a dispersant, and 36.4 g of 25 mass% ammonia water was added as a pH adjuster to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.08 mol% relative to the total of Fe, Mn, Ca, and Si. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1240° C. at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, the material was fired at 1240° C. with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. Then, the material was cooled at an oxygen concentration of 0.4 vol% over 6 hours.
The obtained fired product was pulverized with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain a carrier core material according to Example 4 having a volume average particle size of 36.0 μm.

(Comparative Example 1)
A granulated product was obtained in the same manner as in Example 1, except that coarse particles having a particle size of 54 μm or more were not removed by sieving.
The obtained granules were heated to a temperature of 1300° C. over 4.5 hours at an oxygen concentration of 1.0 vol%, then fired by holding at 1300° C. with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours, and cooled at an oxygen concentration of 0.4 vol% for 6 hours, in the same manner as in Example 1 to obtain fired particles according to Comparative Example 1 having a volume average particle size of 35.4 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 445° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Comparative Example 1.

(Comparative Example 2)
A granulated product was obtained in the same manner as in Example 1, except that coarse particles having a particle size of 54 μm or more were not removed by sieving.
The obtained granules were heated to a temperature of 1250° C. over 4.5 hours at an oxygen concentration of 1.0 vol%, then fired by holding at 1250° C. with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours, and cooled at an oxygen concentration of 0.4 vol% for 6 hours, in the same manner as in Example 1 to obtain fired particles according to Comparative Example 2 having a volume average particle size of 34.9 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 440° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Comparative Example 2.

(Comparative Example 3)
A granulated product was obtained in the same manner as in Example 1, except that coarse particles having a particle size of 54 μm or more were not removed by sieving.
The obtained granules were heated to a temperature of 1200° C. over 4.5 hours at an oxygen concentration of 1.0 vol%, then fired by holding at 1200° C. with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours, and cooled at an oxygen concentration of 0.4 vol% for 6 hours, in the same manner as in Example 1 to obtain fired particles according to Comparative Example 3 having a volume average particle size of 35.7 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 450° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Comparative Example 3.

(Comparative Example 4)
As raw materials, 28.94 _kg of _Fe2O3 (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ) and 11.30 _kg of _Mn3O4 (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ) 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. At this time, the Si content of the mixture was adjusted to 0.28 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1200° C. at an oxygen concentration of 0.5 vol% over 4.5 hours, then fired by holding at 1200° C. at an oxygen concentration of 0.5 vol% for 2.5 hours, then cooled at an oxygen concentration of 0.5 vol% over 6 hours.
The obtained sintered product was disintegrated 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 obtain sintered particles with a volume average particle size of 34.4 μm. The obtained sintered particles were subjected to an oxidation treatment by being held at a temperature of 425° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Comparative Example 4.

(Comparative Example 5)
As raw materials, 23.23 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% _SiO2 ), 8.71 kg _{of Mn3O4} (average particle size: 3.4 μm), and 107.7 g of _CaCO3 (average particle size: 1.1 μm) were dispersed in 10.82 kg of pure water, and 95.8 g of carbon black was added as a reducing agent, 194.8 g of polycarboxylate ammonium dispersant as a dispersant, and 22.6 g of 25 mass% ammonia water as a pH adjuster to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.08 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1300° C. at an oxygen concentration of 1.0 vol% over 4.5 hours. It was then fired by being held at 1300° C. for 3 hours at an oxygen concentration of 1.0 vol% to 0.4 vol%. It was then cooled at an oxygen concentration of 0.4 vol% over 6 hours.
The obtained sintered product was disintegrated with 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 obtain sintered particles with a volume average particle size of 36.0 μm. The obtained sintered particles were subjected to an oxidation treatment by being held at a temperature of 460° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Comparative Example 5.

(Comparative Example 6)
As raw materials, 14.34 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of SiO2), 4.62 kg _{of Mn3O4} (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ), and 1.04 kg of MgO were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were pre-fired in a rotary firing furnace at 850°C under air atmosphere conditions. The mixture was pulverized for 6 hours using a dry bead mill to obtain a pre-fired powder.
The calcined powder and 149.5 g of CaCO ₃ (average particle size: 1.1 μm) were dispersed in 7.12 kg of pure water, and 219.7 g of an aqueous solution containing 21% methacrylic acid-based polymer was added as a dispersant to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.23 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1150° C. at an oxygen concentration of 1.6 vol% over 4.5 hours, then fired at 1300° C. with an oxygen concentration of 1.6 vol% to 0.64 vol% for 3 hours, and then cooled at an oxygen concentration of 0.64 vol% over 6 hours.
The obtained fired product was pulverized with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain a carrier core material according to Comparative Example 6 having a volume average particle size of 35.4 μm.

(Comparative Example 7)
As raw materials, 14.52 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ), 5.44 kg _{of Mn3O4} (average particle size: 3.4 μm), 111.4 g of _SrCO3 (average particle size: 0.6 μm), and 91.0 g of _SnO2 (average particle size: 5.6 μm) were dispersed in 6.90 kg of pure water, and 60.5 g of carbon black was added as a reducing agent, 121.0 g of a polycarboxylate ammonium dispersant as a dispersant, and 14.0 g of 25 mass% ammonia water as a pH adjuster to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.08 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1230° C. at an oxygen concentration of 1.0 vol% over 4.5 hours. It was then fired by holding at 1230° C. and an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. It was then cooled at an oxygen concentration of 0.4 vol% over 6 hours.
The obtained fired product was disintegrated with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain fired particles with a volume average particle size of 35.3 μm. The obtained fired particles were subjected to an oxidation treatment by being held at a temperature of 450° C. in the atmosphere for 1 hour, to obtain a carrier core material according to Comparative Example 7.

(Comparative Example 8)
As raw materials, 20.86 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ), 8.57 kg _{of Mn3O4} (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ), 0.74 kg of MgO, and 131.0 _{g of} CaCO3 were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were pre-fired in a rotary firing furnace at 850°C under air atmosphere conditions. The mixture was pulverized for 6 hours using a dry bead mill to obtain a pre-fired powder.
The calcined powder was dispersed in 10.55 kg of pure water, and 122.0 g of carbon black as a reducing agent and 180.0 g of a polycarboxylate ammonium dispersant as a dispersant were added to prepare a mixture. At this time, the Si content of the mixture was adjusted to 0.27 mol%. The mixture was pulverized in a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 33 μm or less and coarse particles with a particle size of 43 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1110°C at an oxygen concentration of 0.5 vol% over 3 hours, then fired at 1300°C with an oxygen concentration of 0.5 vol% to 0.65 vol% for 3 hours, and then cooled at an oxygen concentration of 0.65 vol% over 6 hours.
The obtained fired product was disintegrated 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 obtain fired particles with a volume average particle size of 33.0 μm. The obtained fired particles were subjected to an oxidation treatment by being held at a temperature of 385° C. in air for 1 hour, thereby obtaining a carrier core material according to Comparative Example 8. An SEM photograph of the carrier core material of Comparative Example 8 is shown in FIG. 2.

(Comparative Example 9)
As raw materials, 20.13 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ), 3.47 _kg of _Mn3O4 (average particle size: 3.4 μm), 6.45 _{kg of Mn3O4} (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ), and 220.1 g of _SrCO3 (average particle size: 0.6 μm) were dispersed in 7.43 kg of pure water, and 180.0 g of polycarboxylate ammonium dispersant as a dispersant, 180.0 g of 25 mass% ammonia water as a pH adjuster, and 6.25 g of colloidal silica with a solid content of 48 wt% were added to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.28 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 90 μm. Fine particles with a particle size of 37 μm or less and coarse particles with a particle size of 50 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1110°C at an oxygen concentration of 2.50 vol% over 3 hours. It was then fired by holding at 1110°C with an oxygen concentration of 2.50 vol% to 1.25 vol% for 3 hours. It was then cooled at an oxygen concentration of 1.25 vol% over 6 hours.
The obtained fired product was disintegrated 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 obtain fired particles with a volume average particle size of 37.3 μm. The obtained fired particles were subjected to an oxidation treatment by being held at a temperature of 405° C. in air for 1 hour, thereby obtaining a carrier core material according to Comparative Example 9. An SEM photograph of the carrier core material of Comparative Example 9 is shown in FIG. 3.

(Comparative Example 10)
As raw materials, 7.98 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ) and 2.02 kg of MgO were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were pre-fired in a rotary firing furnace at 850°C under air atmosphere conditions. The mixture was pulverized in a dry bead mill for 6 hours to obtain a pre-fired powder.
The obtained calcined powder, 18.62 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ), 9.08 kg _{of Mn3O4} (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ), and 203.6 g of _CaCO3 (average particle size: 1.1 μm) were dispersed in 12.70 kg of pure water, and 188.0 g of polycarboxylate ammonium dispersant was added as a dispersant to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.21 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1210° C. over a period of 4.5 hours while in standby. Thereafter, the material was fired by being held at 1210° C. for 4 hours while in standby. Thereafter, the material was cooled in the air over a period of 8 hours.
The obtained fired product was pulverized with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain a carrier core material according to Comparative Example 10 having a volume average particle size of 36.0 μm.

(Comparative Example 11)
As raw materials, 9.58 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ) and 2.42 kg of MgO were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were pre-fired in a rotary firing furnace at 850°C under air atmosphere conditions. The mixture was pulverized for 6 hours using a dry bead mill to obtain a pre-fired powder.
The obtained calcined powder, 22.33 _{kg of Fe2O3} (average particle size: 0.6 μm, containing 0.08 mass% of _SiO2 ), and 10.89 kg _{of Mn3O4} (average particle size: 2.1 μm, containing 0.55 mass% of _SiO2 ) were dispersed in 15.15 kg of pure water, and 226.7 g of ammonium polycarboxylate dispersant was added as a dispersant to obtain a mixture. At this time, the Si content of the mixture was adjusted to 0.21 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry.
This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain dried granules with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from the granules using a sieve.
The granulated material was placed in an electric furnace and heated to 1210° C. over a period of 4.5 hours while in standby. Thereafter, the material was fired by being held at 1210° C. for 4 hours while in standby. Thereafter, the material was cooled in the air over a period of 8 hours.
The obtained fired product was pulverized with a hammer mill ("Hammer Crusher NH-34S" manufactured by Sansho Industry Co., Ltd., screen opening: 0.3 mm) and classified with a vibrating sieve to obtain a carrier core material of Comparative Example 11 having a volume average particle size of 35.6 μm.

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

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

(Volume average particle size (average particle size) and the proportion of particles with a particle size of 22 μm or less)
The volume average particle size and the proportion of particles with particle sizes of 22 μm or less of the carrier core material were measured using a laser diffraction particle size distribution measuring device (Microtrac Model 9320-X100, manufactured by Nikkiso Co., Ltd.).

(Magnetic properties)
Using a room temperature dedicated vibrating sample magnetometer (VSM) ("VSM-P7" manufactured by Toei Kogyo 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 , remanent magnetization σ _r , coercive force H _c , and magnetization σ _1k (Am ² /kg) in a magnetic field of 79.58×10 ³ A/m (1,000 Oersted).

(Static Electrical Resistance)
Two 2 mm thick brass plates with electrolytically polished surfaces were used as electrodes, arranged with an electrode distance 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. DC voltages of 100 V, 500 V, and 1000 V were applied between the electrodes, and the current flowing through the carrier core material was measured using the four-terminal method to obtain the resistance value.

(Pore volume)
The evaluation device used was a POREMASTER-60GT manufactured by Quantachrome. Specifically, the measurement conditions were as follows:
Cell Stem Volume: ^0.5cm3 ,
Headpressure: 20PSIA,
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
The measurement was performed by weighing out 1.500 g of the sample and filling it into a 0.5 ^cm3 cell. The pore volume was determined by subtracting the volume A ( ^cm3 /g) at 60 PSI from the volume B ( ^cm3 /g) at 10,000 PSI.

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

(Measurement of maximum height Rz and 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, the carrier core material was 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 carrier core material was 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. These position data in the X, Y, and Z directions were connected to obtain the three-dimensional shape of the carrier core material surface. In addition, an auto-photography function was used to capture the three-dimensional shape of the carrier core material surface.
Each parameter was measured using particle roughness inspection software (manufactured by Mitani Shoji). First, as a pretreatment, particle recognition and shape selection were performed on the three-dimensional shape of the obtained carrier core surface. Particle recognition was performed in the following manner. 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 a 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 square with a side length of 15.0 μm was drawn with the center of gravity determined from the particle outline recognized by the above method as the center. 21 parallel lines were drawn within the square, and 21 roughness curves corresponding to the line segments were extracted.

Because the carrier core material is approximately spherical, the roughness curve extracted has a certain curvature as the background. For this reason, an optimal quadratic curve was fitted to correct the background 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.

The maximum height 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 50 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 of the roughness curve, with each combination of a valley and a peak defined as one element. To calculate the average length RSm, the average value of 50 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).

(deformity rate)
The evaluation device used was an injection type image analysis particle size distribution meter (Jasco International Co., Ltd., model: IF-3200). Specifically, 0.07 g of the sample was weighed out and dispersed in a screw cap bottle (volume 9 cm ³ ) containing 9 cm ³ of polyethylene glycol 400, and then the measurement was performed.
(Measurement conditions)
Spacer thickness: 150 μm
Sampling: 20%
Analysis type: relative measurement Measurement volume: 0.95 ^cm3
Analysis: Dark Detection Threshold: 169 (fill holes)
O-Roughness filter: 0.5
In addition, O-Roughness = the ratio of the part that is removed until the surface becomes smooth Filter conditions:
ISO Area Diameter: minimum value 5, maximum value 100, inner range Note that ISO Area Diameter = diameter of a circle having a pixel image equal to the particle projected area (analysis condition)
Analysis filter condition I:
ISO Circularity: Minimum value 25, maximum value 55, inner range Analysis filter condition II:
ISO Circularity: minimum value 25, maximum value 55, inner range ISO Solidity: minimum value 0.98, maximum value 1, outer range Note that ISO Solidity = ratio of particle area to area of convex hull surrounding a particle Ell. Ratio: minimum value 0.8, maximum value 1, inner range Note that Ell. Ratio = ratio of minor axis to major axis of ellipse of inertia 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 ratio of irregularly shaped particles.

(viscosity)
The slurry viscosity was measured using a digital viscometer DV-I Prime LV manufactured by BROOKFIELD.

(Evaluation of white spots in images)
The prepared two-component developer was loaded into a developing device having the structure shown in FIG. 4 (circumferential speed of developing roller _v1 : 406 mm/sec, peripheral speed of photosensitive drum _v2 : 205 mm/sec, distance between photosensitive drum and developing roller: 0.3 mm), and after driving the developing device for a time equivalent to printing 100,000 sheets, a solid black image was printed, and the degree of whiteout in the solid black area was visually evaluated according to the following criteria.
"A": No white spots were observed, and the image was good.
"◯": Less than 5 white spots; "Δ": 5 to 10 white spots; "×": More than 10 white spots are clearly present.

(Evaluation of Carrier Scattering)
The prepared two-component developer was put into a developing device having the structure shown in Figure 4 (circumferential speed of developing roller _v1 : 406 mm/sec, peripheral speed of photoconductor drum _v2 : 205 mm/sec, distance between photoconductor drum and developing roller: 0.3 mm), 10 blank originals were developed, and the number of carrier particles adhering to the photoconductor surface was counted in 5 fields of view using a magnifying glass, and the average number of carrier particles adhering per 100 ^cm2 was taken as carrier adhesion. This evaluation was performed after driving the developing device for a time equivalent to the time required to print 100k sheets of A4 portrait paper (time equivalent to printing 100k sheets).
"◎": Less than 10 pieces "○": 10 or more but less than 30 pieces "△": 30 or more but less than 50 pieces "×": 50 or more pieces

(Toner Spent)
After the developer was stirred for 36 hours, the carrier was extracted from the developer and observed under a scanning electron microscope (Model JSM-6510LA, manufactured by JEOL Ltd.) while measuring the number and percentage of carrier particles with fused toner on the surface.
"Excellent": The proportion of carrier particles to which the toner was fused was less than 0.5%.
"Good": The ratio of the number of carrier particles to which the toner was fused was 0.5% or more and less than 1.0%.
"B": The ratio of the number of carrier particles to which the toner was fused was 1.0% or more and less than 5.0%.
"X": The proportion of carrier particles to which the toner was fused was 5.0% or more.

As shown in Table 2, the developers using the carrier core materials of Examples 1 to 3, which satisfy the composition defined in the present invention, did not cause any problems in practical use in the evaluations of image voids, carrier scattering, and toner spent.

In contrast, image voids occurred in the developer using the carrier core materials of Comparative Examples 1 and 2, which have a smaller pore volume, a larger maximum height Rz, and a larger irregularity rate than the range specified in the present invention. Image voids also occurred in the developer using the carrier core materials of Comparative Examples 3 and 5, which have a smaller pore volume and a larger irregularity rate than the range specified in the present invention, and in the developer using the carrier core material of Comparative Example 4, which has a smaller pore volume.

Carrier scattering occurred in the developer using the carrier core material of Comparative Example 6, which has a saturation magnetization _σs smaller than the range specified in the present invention. Carrier scattering also occurred in the developer using the carrier core material of Comparative Example 7, which has a pore volume larger than the range specified in the present invention.

In the developer using the carrier core material of Comparative Example 8, which has a smaller saturation magnetization _σs and a smaller irregularity rate than the range specified in the present invention, carrier scattering and toner spent occurred, while in the developer using the carrier core material of Comparative Example 9, carrier scattering did not pose a problem in practical use, but toner spent, which is a problem in practical use, occurred.

The developer using the carrier core material of Comparative Example 9, which had a smaller saturation magnetization _σs and pore volume and a larger irregularity rate than the specified range of the present invention, and the developer using the carrier core material of Comparative Example 9, which had a smaller saturation magnetization _σs and pore volume than the specified range of the present invention, both caused image voids and carrier scattering.

The carrier core material of the present invention can suppress toner spent, image voids, and carrier scattering even when the image formation speed is high or after long-term use.

3 developing roller 5 photosensitive drum

Claims (5)

A carrier core material composed of ferrite particles,
the ferrite particles are made of a material represented by a composition formula (Mn _x Fe _3-x )O ₄ (where 0<x<3);
The ferrite particles contain 0.06 mol% or more and 0.20 mol% or less of Si relative to the total molar number of the composition of the ferrite particles,
The saturation magnetization σ _s is 75 Am ² /kg or more and 90 Am ² /kg or less,
The pore volume is 0.008 cm ³ /g or more and less than 0.020 cm ³ /g,
The maximum height Rz is 1.4 μm or more and 2.0 μm or less,
A carrier core material having a particle irregularity rate of 10% or more and 50% or less, as measured by the following measuring method.
(Method for measuring particle irregularity rate)
Measurement device: Injection type image analysis particle size distribution meter Measurement sample amount: 0.07 g
Measurements were performed after dispersion in a screw cap flask (volume 9 cm3 ⁾ containing 9 ^cm3 of polyethylene glycol 400.
(Measurement conditions)
Spacer thickness: 150 μm
Sampling: 20%
Analysis type: relative measurement Measurement volume: 0.95 ^cm3
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 irregularity rate is calculated by dividing the number of particles counted under analysis filter condition II by the number of particles counted under analysis filter condition I. The carrier core material according to claim 1, wherein the average length RSm of the grains appearing on the particle surface is 5.0 μm or more and 7.1 μm or less. 3. The carrier core material according to claim 1, wherein the magnetization σ _1k in a magnetic field of 79.58×10 ³ A/m (1000 oersted) is 61 Am ² /kg or more and 75 Am ² /kg or less. An electrophotographic development carrier, characterized in that the surface of the carrier core material according to any one of claims 1 to 3 is coated with a resin. An electrophotographic developer comprising the electrophotographic development carrier according to claim 4 and a toner.