JP7743741B2 - Electrostatic image developing carrier, electrostatic image developer, process cartridge, image forming apparatus and image forming method - Google Patents

Description

The present invention relates to a carrier for developing electrostatic images, an electrostatic image developer, a process cartridge, an image forming apparatus, and an image forming method.

Patent Document 1 discloses a carrier for developing electrostatic latent images, which comprises magnetic core particles and a coating layer that coats the core particles, has a shape factor SF-2 of 115 to 150, and a bulk density of 1.8 g/cm ³ to 2.4 g/cm ³ , wherein the shape factor SF-2 of the core particles is 120 to 160, and the arithmetic mean surface roughness Ra of the core particles is 0.5 μm to 1.0 μm, and the coating layer contains a resin and a filler in a ratio of 50 to 500 parts by mass of the filler per 100 parts by mass of the resin.

JP 2013-057817 A

The object of the present invention is to provide a carrier for developing electrostatic images that produces images with less density unevenness even under high-load printing conditions, compared to carriers in which particles A are present on the carrier surface in an amount of less than 0.02% by mass or more than 0.5% by mass relative to the total mass of the carrier, or in which the residual rate X of particles A in the carrier after printing 5,000 full-page A4 paper images with 30% halftone at 120 pages per minute is less than 0.3 or more than 1.0 in terms of fluorescent X-ray NET intensity, or in which the moisture content of particles A is less than 7% by mass or more than 12% by mass relative to the total mass of particles A.

Means for solving the above problems include the following aspects.
<1> A carrier for developing electrostatic images, the carrier having particles A on its surface in an amount of 0.02% by mass or more and 0.5% by mass or less, based on the total mass of the carrier, wherein a residual ratio X of the particles A in the carrier after printing 5,000 full-page A4 paper images with 30% halftone at 120 pages per minute is 0.3 or more and 1.0 or less, as measured by fluorescent X-ray NET intensity, and the water content of the particles A is 7% by mass or more and 12% by mass or less, based on the total mass of the particles A.
<2> The carrier for developing electrostatic images according to <1>, wherein the particles A are inorganic oxide particles.
<3> The carrier for developing electrostatic images according to <2>, wherein the inorganic oxide particles are silica particles.
<4> The electrostatic image developing carrier according to any one of <1> to <3>, wherein the particles A have an arithmetic mean particle size of 60 nm to 160 nm.
<5> The carrier for developing an electrostatic image according to any one of <1> to <4>, wherein the particle A has a volume resistivity of 10 ¹² Ω or more and 10 ¹⁵ Ω or less.
<6> The carrier for developing electrostatic images according to any one of <1> to <5>, wherein SF1 of the carrier is 110 or more and 125 or less.
<7> The carrier for developing electrostatic images according to any one of <1> to <5>, wherein SF1 of the carrier is 126 or more and 140 or less.
<8> The carrier for developing electrostatic images according to any one of <1> to <7>, wherein the particle size distribution of the particles A has a standard deviation of 8 to 12.
<9> The carrier for developing electrostatic images according to any one of <1> to <8>, wherein the true specific gravity of the particles A is 1.1 g/cm ³ or more and 2.0 g/cm ³ or less.
<10> The electrostatic image developer according to any one of <1> to <9>, wherein the carrier has magnetic particles and a resin coating layer that coats the magnetic particles.
<11> The electrostatic image developer according to <10>, wherein the weight average molecular weight of the resin contained in the resin coating layer is less than 300,000.
<12> The electrostatic image developing carrier according to <11>, wherein the weight average molecular weight of the resin contained in the resin coating layer is less than 250,000.
<13> An electrostatic image developer comprising a toner for developing electrostatic images and the carrier for developing electrostatic images according to any one of <1> to <12>.
<14> A process cartridge detachably mounted to an image forming apparatus, which contains the electrostatic image developer according to <13> and includes a developing unit that develops an electrostatic image formed on the surface of an image carrier using the electrostatic image developer into a toner image.
<15> An image forming apparatus comprising: an image carrier; charging means for charging the surface of the image carrier; electrostatic image forming means for forming an electrostatic image on the charged surface of the image carrier; developing means that contains the electrostatic image developer according to <13> and develops the electrostatic image formed on the surface of the image carrier as a toner image using the electrostatic image developer; transfer means that transfers the toner image formed on the surface of the image carrier to the surface of a recording medium; and fixing means that fixes the toner image transferred to the surface of the recording medium.
<16> An image forming method comprising: a charging step of charging the surface of an image carrier; an electrostatic image forming step of forming an electrostatic image on the charged surface of the image carrier; a developing step of developing the electrostatic image formed on the surface of the image carrier as a toner image using the electrostatic image developer according to <13>; a transfer step of transferring the toner image formed on the surface of the image carrier to the surface of a recording medium; and a fixing step of fixing the toner image transferred to the surface of the recording medium.

According to the inventions related to <1>, <2>, <6>, <7>, or <10>, there is provided an electrostatic image developing carrier that can provide images with less density unevenness even under high-load printing conditions, compared to a carrier having particles A on the carrier surface in an amount of less than 0.02% by mass or more than 0.5% by mass, relative to the total mass of the carrier, a carrier having a residual rate X of particles A in the carrier after printing 5,000 full-page A4 paper images with 30% halftone at 120 pages/minute, as measured by fluorescent X-ray NET intensity, of less than 0.3 or more than 1.0, or a carrier having a moisture content of particles A of less than 7% by mass or more than 12% by mass, relative to the total mass of particles A.
According to the invention related to <3>, there is provided a carrier for developing electrostatic images that can produce images with less density unevenness even under high-load printing conditions, compared to when the inorganic oxide particles are titania particles.
According to the invention related to <4>, there is provided a carrier for developing electrostatic images that can produce images with less density unevenness even under high-load printing conditions, compared to when the average particle size of the particles A is less than 60 nm or more than 160 nm.
According to the invention related to <5>, ^there is provided a carrier for developing electrostatic images, which can produce images with less density unevenness even under high-load printing conditions, compared to when the volume resistivity of the particles A is less than 10 ¹² Ω or more than 10 15 Ω.
According to the invention related to <8>, there is provided a carrier for developing electrostatic images that can produce images with less density unevenness even under high-load printing conditions, compared to when the standard deviation in the particle size distribution of the particles A is less than 8 or more than 12.
According to the invention related to <9>, there is provided a carrier for developing electrostatic images that can produce images with less density unevenness even under high-load printing conditions, compared to when the true specific gravity ^of the particles A is less than 1.1 g/cm3 or more than ^2.0 g/cm3.
According to the invention related to <11>, there is provided a carrier for developing electrostatic images that can produce images with less density unevenness even under high-load printing conditions, compared to when the weight-average molecular weight of the resin contained in the resin coating layer is 300,000 or more.
According to the invention related to <12>, there is provided a carrier for developing electrostatic images that can produce images with less density unevenness even under high-load printing conditions, compared to when the weight-average molecular weight of the resin contained in the resin coating layer is 250,000 or more.
According to the inventions pertaining to <13> to <16>, there are provided an electrostatic image developer, a process cartridge, an image forming apparatus, or an image forming method which can provide images with less density unevenness even under high-load printing conditions, compared to a case in which particles A are present on the carrier surface in an amount of less than 0.02% by mass or more than 0.5% by mass, relative to the total mass of the carrier, or in which a residual rate X of particles A in the carrier after printing 5,000 full-page A4 paper images with 30% halftone at 120 pages per minute is less than 0.3 or more than 1.0, as measured by fluorescent X-ray NET intensity, or in which the water content of particles A is less than 7% by mass or more than 12% by mass, relative to the total mass of particles A.

1 is a schematic configuration diagram illustrating an example of an image forming apparatus according to an embodiment of the present invention. 1 is a schematic diagram illustrating an example of a process cartridge that is detachably mounted to an image forming apparatus according to an exemplary embodiment of the present invention.

Embodiments of the present disclosure are described below. These descriptions and examples are intended to illustrate the embodiments and are not intended to limit the scope of the embodiments.

In this disclosure, numerical ranges indicated using "~" indicate ranges that include the numerical values before and after "~" as the minimum and maximum values, respectively.

In the numerical ranges described in stages in this disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in the numerical ranges described in this disclosure, the upper or lower limit value of that numerical range may be replaced with the value shown in the examples.

In this disclosure, the term "process" refers not only to an independent process, but also to a process that cannot be clearly distinguished from other processes, as long as the intended purpose of that process is achieved.

When embodiments of this disclosure are described with reference to drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited to these.

In this disclosure, each component may contain multiple types of corresponding substances. When referring to the amount of each component in a composition in this disclosure, if the composition contains multiple substances corresponding to that component, this refers to the total amount of those multiple substances present in the composition, unless otherwise specified.

In this disclosure, multiple types of particles corresponding to each component may be contained. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for a mixture of those multiple types of particles present in the composition, unless otherwise specified.

In this disclosure, "(meth)acrylic" means at least one of acrylic and methacrylic, and "(meth)acrylate" means at least one of acrylate and methacrylate.

In this disclosure, "toner for developing electrostatic images" is also referred to as "toner," "carrier for developing electrostatic images" is also referred to as "carrier," and "electrostatic image developer" is also referred to as "developer."

(Electrostatic image developing carrier)
The carrier for developing electrostatic images according to the present embodiment (hereinafter also simply referred to as "carrier") has particles A on its surface in an amount of 0.02% by mass or more and 0.5% by mass or less, based on the total mass of the carrier, and after printing 5,000 A4 size images with a halftone of 30% on the entire surface at 120 sheets per minute, the residual ratio X of particles A in the carrier is 0.3 to 1.0, as measured by fluorescent X-ray NET intensity, and the moisture content of the particles A is 7% by mass or more and 12% by mass or less, based on the total mass of the particles A.

In this embodiment, carbon black is not an inorganic particle.

The carrier according to this embodiment produces images with minimal density unevenness, even under high-load printing conditions (also known simply as "density unevenness suppression"). The mechanism behind this is thought to be as follows:

As printing continues, the carrier state changes from the initial development state, which is likely to manifest as changes in image density and density unevenness. To date, carriers coated with silicone resins or crosslinked resins have been proposed as a method for suppressing these changes. However, these have low charge-imparting capabilities and low affinity with toner, which makes toner density uneven on the magnetic brush. Another proposed method is to attach low-resistivity particles to the carrier, thereby creating a deteriorated state in the early stages and thereby reducing the extent of charge change. However, the present inventors have found that this method is prone to fluctuations in the transfer of charge between the carrier and toner, resulting in an unstable charge state, which can lead to image density unevenness under high-load conditions such as high-speed printing.
Furthermore, if the toner remains in the developing means for a long time, charge exchange between the toner and the carrier continues, which presumably changes the external additive state of the toner, resulting in a change in the charge.
When the carrier for developing electrostatic images according to the present embodiment is used, the presence of particles A on the carrier surface in the above-mentioned amount range is unlikely to hinder charging and suppresses charge injection, which is presumed to stabilize the charge of the toner. Furthermore, particles A stabilize the separation of the toner from the carrier and suppress uneven density of the image. Furthermore, when printing is continued, the above-mentioned range ensures that the same effect is maintained even under high-load printing conditions (for example, printing 5,000 A4 full-page images with 30% halftone at 120 sheets/minute). Furthermore, when particles A have the above moisture content, excess charge is transferred to the carrier side, which suppresses mutual charge transfer between new toner and toner subjected to high load when new toner is supplied, which is presumed to suppress uneven density of the image even under high-load printing conditions.

The configuration of the carrier according to this embodiment is explained in detail below.

<Particle A>
The carrier for developing electrostatic images according to this embodiment has particles A on its surface in an amount of 0.02% by mass or more and 0.5% by mass or less, based on the total mass of the carrier. After printing 5,000 A4 size images with a halftone of 30% on the entire surface at a speed of 120 sheets per minute, the remaining ratio X of the particles A in the carrier is 0.3 to 1.0, as measured by NET fluorescent X-ray intensity. The water content of the particles A is 7% by mass or more and 12% by mass or less, based on the total mass of the particles A.

Examples of particles A include particles of silica, alumina, titanium oxide (titania), barium titanate, magnesium titanate, calcium titanate, strontium titanate, iron oxide, copper oxide, zinc oxide, tin oxide, chromium oxide, cerium oxide, antimony trioxide, magnesium oxide, zirconium oxide, barium sulfate, barium carbonate, and calcium carbonate.
Among these, from the viewpoint of suppressing image density unevenness after high-load printing, particles A are preferably inorganic particles, more preferably inorganic oxide particles, still more preferably silica particles, titanium oxide particles, or silica-titanium oxide composite particles, and particularly preferably silica particles.
The silica particles are preferably wet silica particles formed by a wet method such as a precipitation method or a sol-gel method.

The particles A may be particles that have been subjected to a hydrophobic treatment.
Furthermore, the particles A preferably contain inorganic oxide particles that have been hydrophobized with a hydrophobizing agent, and more preferably contain silica particles that have been hydrophobized.

Examples of the hydrophobic treatment agent include known surface treatment agents, and specific examples include silane coupling agents and silicone oils.
Examples of silane coupling agents include hexamethyldisilazane, trimethylsilane, trimethylchlorosilane, dimethyldichlorosilane, methyltrichlorosilane, allyldimethylchlorosilane, benzyldimethylchlorosilane, methyltrimethoxysilane, methyltriethoxysilane, isobutyltrimethoxysilane, dimethyldimethoxysilane, dimethyldiethoxysilane, trimethylmethoxysilane, hydroxypropyltrimethoxysilane, phenyltrimethoxysilane, n-butyltrimethoxysilane, n-hexadecyltrimethoxysilane, n-octadecyltrimethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, γ-methacryloxypropyltrimethoxysilane, and vinyltriacetoxysilane.
Examples of silicone oils include dimethylpolysiloxane, methylhydrogenpolysiloxane, and methylphenylpolysiloxane.

--Amount of particles A on the carrier surface--
Particles A are externally added to the surface of the carrier for developing electrostatic images in this embodiment, and it is assumed that most of the particles A are present in a state of being attached to or embedded in the carrier surface, and some particles A may also be present in a state where they can be released.
The carrier for developing electrostatic images according to this embodiment has particles A on its surface in an amount of 0.02% by mass or more and 0.5% by mass or less, relative to the total mass of the carrier. From the viewpoint of suppressing uneven image density after high-load printing, the amount is preferably 0.03% by mass or more and 0.2% by mass or less, more preferably 0.03% by mass or more and 0.15% by mass or less, and particularly preferably 0.03% by mass or more and 0.1% by mass or less.

The amount of particles A on the carrier surface is measured by the following method.
A calibration curve for Si is created using the NET intensity of fluorescent X-rays, and the content is calculated from the measured Si-NET intensity of the carrier.

- Residual rate of particle A X -
In the carrier for developing electrostatic images according to the present embodiment, the residual ratio X of particles A in the carrier after printing 5,000 full-page A4 size images at 120 sheets per minute with a halftone of 30% is, in terms of fluorescent X-ray NET intensity, 0.3 or more and 1.0 or less, and from the viewpoint of suppressing unevenness in image density after high-load printing, is preferably 0.4 or more and 1.0 or less, more preferably 0.5 or more and 1.0 or less, and particularly preferably 0.7 or more and 1.0 or less.

The residual rate X of particles A is measured by the following method.
Before printing or after printing any image for up to 100 sheets, the developer is passed through a 16 μm mesh to remove the toner, and carrier A is extracted. Removal methods include using a 16 μm mesh cage and air blowing. Similarly, after printing 5,000 full-page A4 paper images with a 30% halftone at 120 sheets per minute, the toner is removed from the developer and carrier B is extracted. The fluorescent X-ray net intensity of both is measured, and the residual ratio X is calculated from the net strength α of carrier A and the net strength β of carrier B using the equation: X = β/α.
It is also preferable to use the toner 1 in the examples described below.

- Moisture content of particle A -
In the carrier for developing electrostatic images according to this embodiment, the water content of the particles A is 7% by mass or more and 12% by mass or less, relative to the total mass of the particles A, and from the viewpoint of suppressing uneven image density after high-load printing, it is preferably 9% by mass or more and 11% by mass or less.

The method for measuring the moisture content of particles A in this embodiment will be described below.
After leaving the particles for 48 hours under conditions of 28°C and 85% RH, the moisture content of the particles A is determined by measuring the weight difference before and after heating at 150°C for 20 minutes using a heated Mettler.

-Volume Resistivity of Particle A-
From the viewpoint of suppressing uneven image density after high-load printing, the volume resistivity of the particles A is preferably 10 ¹² Ω or more and 10 ¹⁵ Ω or less, more preferably 5 × 10 ¹² Ω or more and 5 × 10 ¹⁴ Ω or less, and particularly preferably 10 ¹³ Ω or more and 10 ¹⁴ Ω or less.

The method for measuring the volume resistivity of particles A in this embodiment is as follows.
The volume resistivity of particles A is measured by the following method, in an environment of a temperature of 20° C. and a humidity of 50% RH.
Two electrodes are placed parallel to each other with a width of 1 mm, 0.20 g of ferrite particles are placed between them, and they are held in place by a magnet with a cross-sectional area of 2.4 ^cm2 . A voltage of 1,000 V is applied and the current value is measured. The resistance value is calculated from the obtained current value.

-Arithmetic mean particle size of particle A-
From the viewpoint of suppressing unevenness in image density after high-load printing, the arithmetic mean particle size of the particles A is preferably 10 nm or more and 200 nm or less, more preferably 60 nm or more and 160 nm or less, and particularly preferably 90 nm or more and 140 nm or less.

- Standard deviation in particle size distribution of particle A -
The standard deviation in the particle size distribution of the particles A is preferably 5 or more and 20 or less, and more preferably 8 or more and 12 or less, from the viewpoint of suppressing unevenness in image density after high-load printing.

In this embodiment, the arithmetic average particle size of particles A is the diameter of a circle having the same area as the particle image (so-called circle-equivalent diameter), and is determined by taking an electron microscope image of the carrier and analyzing the image of at least 300 particles A on the carrier. The particles A are imported into an image processing analyzer and subjected to image analysis. 300 particles A (primary particles) in the resin coating layer are randomly selected, and their circle-equivalent diameters (nm) are determined and the arithmetic average is taken, which is the average particle size (nm) of particles A.
Further, the standard deviation is calculated from the particle size distribution of the particles A obtained by the above measurement.

- True specific gravity of particle A -
From the viewpoint of suppressing unevenness in image density after high-load printing, the true specific gravity of the particles A is preferably 1.1 g/cm ³ or more and 2.0 g/cm ³ or less, more preferably 1.3 g/cm ³ or more and 1.9 g/cm ³ or less, and particularly preferably 1.5 g/cm ³ or more and 1.9 g/cm ³ or less.

In this embodiment, the true specific gravity of particle A is measured using an automatic powder/granular true specific gravity measuring instrument (product name "AutoTrueDensor MAT-7000") using the pycnometer method, manufactured by Seishin Enterprise Co., Ltd.

-Method of adding particles A externally-
The method for externally adding the particles A to the carrier surface is not particularly limited, and any known external addition method can be used, but a preferred example is the method of external addition using a V blender.
The external addition conditions using a V-blender are preferably 20 rpm and 20 minutes.

<Magnetic particles>
From the viewpoint of the residual properties of particles A and the suppression of uneven image density after high-load printing, the carrier for developing electrostatic images according to this embodiment preferably includes magnetic particles and a resin coating layer that coats the magnetic particles.

From the viewpoint of suppressing uneven image density after high-load printing, the volume average particle size of the magnetic particles is preferably 20 μm or more and 50 μm or less, more preferably 25 μm or more and 34 μm or less, even more preferably 26 μm or more and 33 μm or less, and particularly preferably 28 μm or more and 32 μm or less.
The volume average particle diameters of the magnetic particles and carrier in this embodiment are values measured using a laser diffraction particle size distribution measuring device LA-700 (manufactured by Horiba, Ltd.) Specifically, the particle size distribution obtained by the measuring device is divided into particle size ranges (channels), and the particle diameter at which the cumulative volume distribution is subtracted from the small particle size side and cumulative 50% is taken as the volume average particle diameter.
A suitable method for separating the magnetic particles from the carrier is to dissolve the resin coating layer with an organic solvent and then separate the magnetic particles.
More specifically, as a method for separating the magnetic particles from the carrier, for example, 20 g of resin-coated carrier is placed in 100 mL of toluene. Ultrasonic waves are applied at 40 kHz for 30 seconds. The magnetic particles and resin solution are separated using any filter paper appropriate for the particle size. 20 mL of toluene is poured over the magnetic particles remaining on the filter paper to wash them. Next, the magnetic particles remaining on the filter paper are recovered. The recovered magnetic particles are similarly placed in 100 mL of toluene and ultrasonic waves are applied at 40 kHz for 30 seconds. The particles are similarly filtered, washed with 20 mL of toluene, and then recovered. This is repeated 10 times in total. Finally, the recovered magnetic particles are dried.

As the material of the magnetic particles, known materials used as the core material of carriers are applied.
Specific examples of magnetic particles include particles of magnetic metals such as iron, nickel, and cobalt; particles of magnetic oxides such as ferrite and magnetite; resin-impregnated magnetic particles in which porous magnetic powder is impregnated with resin; and magnetic powder-dispersed resin particles in which magnetic powder is dispersed in resin. In this embodiment, ferrite particles are preferred as the magnetic particles.

Furthermore, from the viewpoints of chargeability, chargeability in a high-temperature, high-humidity environment, and suppression of concentration changes, the magnetic particles preferably contain strontium, more preferably are ferrite particles containing strontium, and particularly preferably are ferrite particles containing iron, manganese, magnesium, and strontium.
When the magnetic particles contain strontium, the dielectric constant increases, the capacity of the carrier improves, and the charge amount increases, allowing for good images to be obtained even in high-temperature, high-humidity environments, while also providing better suppression of density changes.

From the viewpoint of image quality stability and density change suppression under high temperature and high humidity conditions, the content of strontium element in the magnetic particles is preferably 0.001% by mass or more and less than 2% by mass, more preferably 0.005% by mass or more and less than 1.5% by mass, and particularly preferably 0.01% by mass or more and less than 1% by mass.

The content of strontium element contained in the magnetic particles is measured by fluorescent X-ray analysis. The fluorescent X-ray analysis of the ferrite particles is carried out by the following method.
Qualitative and quantitative analysis is performed using an X-ray fluorescence analyzer (Shimadzu Corporation, XRF1500) under the following conditions: X-ray output: 40 V/70 mA, measurement area: 10 mm diameter, measurement time: 15 minutes. The elements to be analyzed are selected based on the elements detected in the qualitative analysis. The main elements selected are iron (Fe), manganese (Mn), magnesium (Mg), calcium (Ca), strontium (Sr), oxygen (O), and carbon (C). The mass percentage (%) of each element is calculated by referring to separately prepared calibration curve data.

The arithmetic mean height Ra (JIS B0601:2001) of the roughness curve of the magnetic particles is preferably 0.1 μm or more and 1 μm or less, and more preferably 0.2 μm or more and 0.8 μm or less.
The arithmetic mean height Ra of the roughness curve of the magnetic particles is determined by observing the magnetic particles at an appropriate magnification (for example, 1000x magnification) using a surface profile measuring device (for example, the "Ultra-Deep Color 3D Profile Measuring Microscope VK-9700" manufactured by Keyence Corporation), obtaining a roughness curve with a cutoff value of 0.08 mm, and extracting a reference length of 10 μm from the roughness curve in the direction of the mean line. The Ra of 100 magnetic particles is then arithmetically averaged.

The magnetic force of the magnetic particles is preferably 50 emu/g or more, more preferably 60 emu/g or more, in terms of saturation magnetization in a magnetic field of 3,000 oersteds. The saturation magnetization is measured using a vibrating sample magnetic measuring device VSMP10-15 (manufactured by Toei Kogyo Co., Ltd.). The measurement sample is placed in a cell with an inner diameter of 7 mm and a height of 5 mm and set in the device. Measurement is performed by applying a magnetic field and sweeping up to a maximum of 3,000 oersteds. The applied magnetic field is then reduced, and a hysteresis curve is created on recording paper. The saturation magnetization, remanent magnetization, and coercive force are determined from the curve data.

The volume electrical resistance (volume resistivity) of the magnetic particles is preferably 1×10 ⁵ Ω·cm or more and 1×10 ⁹ Ω·cm or less, and more preferably 1×10 ⁷ Ω·cm or more and 1×10 ⁹ Ω·cm or less.
The volume resistivity (Ω·cm) of magnetic particles is measured as follows. The object to be measured is placed flat on the surface of a circular jig equipped with a 20 ^cm2 electrode plate to a thickness of 1 mm to 3 mm, forming a layer. The 20 ^cm2 electrode plate is placed on top of this to sandwich the layer. To eliminate gaps between the objects to be measured, a 4 kg load is applied to the electrode plate placed on the layer, and then the layer thickness (cm) is measured. The electrodes above and below the layer are connected to an electrometer and a high-voltage power supply generator. A high voltage is applied to both electrodes so that the electric field becomes 103.8 V/cm, and the current value (A) flowing at this time is read. The measurement environment is a temperature of 20°C and a relative humidity of 50%. The formula for calculating the volume resistivity (Ω·cm) of the object to be measured is as shown below.
R=E×20/(I−I ₀ )/L
In the above formula, R represents the volume resistivity (Ω·cm) of the object to be measured, E represents the applied voltage (V), I represents the current value (A), I ₀ represents the current value (A) at an applied voltage of 0 V, and L represents the layer thickness (cm). The coefficient 20 represents the area of the electrode plate (cm ² ).

<Resin coating layer>
From the viewpoints of the residual properties of particles A and the suppression of uneven image density after high-load printing, the carrier for developing electrostatic images according to this embodiment preferably has a resin coating layer that coats the magnetic particles, and more preferably the resin coating layer contains inorganic particles.

In this embodiment, the average thickness of the resin coating layer is preferably 0.6 μm or more and 1.4 μm or less, more preferably 0.8 μm or more and 1.2 μm or less, and particularly preferably 0.8 μm or more and 1.1 μm or less, from the viewpoint of suppressing uneven image density after high-load printing.

Examples of inorganic particles contained in the resin coating layer include metal oxide particles such as silica, titanium oxide, zinc oxide, and tin oxide; metal compound particles such as barium sulfate, aluminum borate, and potassium titanate; and metal particles such as gold, silver, and copper.
Among these, silica particles are preferred from the viewpoint of suppressing concentration changes.

From the viewpoint of suppressing uneven image density after high-load printing, the arithmetic mean particle size of the inorganic particles in the resin coating layer is preferably 5 nm or more and 90 nm or less, more preferably 5 nm or more and 70 nm or less, even more preferably 5 nm or more and 50 nm or less, and particularly preferably 8 nm or more and 50 nm or less.

In this embodiment, the average particle size of the inorganic particles contained in the resin coating layer and the average thickness of the resin coating layer are determined by the following method.
The carrier is embedded in epoxy resin and cut with a microtome to prepare a carrier cross section. The SEM image of the carrier cross section taken with a scanning electron microscope (SEM) is imported into an image processing analyzer for image analysis. One hundred inorganic particles (primary particles) in the resin coating layer are randomly selected, and their equivalent circle diameters (nm) are determined and arithmetically averaged to obtain the average particle size (nm) of the inorganic particles. Furthermore, the thickness (μm) of the resin coating layer is measured at ten randomly selected locations per carrier particle. Further measurements are taken for 100 carrier particles, and the arithmetic average of all measurements is obtained to obtain the average thickness (μm) of the resin coating layer.

The surfaces of the inorganic particles may be subjected to a hydrophobic treatment. Examples of hydrophobic treatment agents include known organosilicon compounds having an alkyl group (e.g., methyl, ethyl, propyl, butyl, etc.), and specific examples include alkoxysilane compounds, siloxane compounds, and silazane compounds. Of these, silazane compounds are preferred as hydrophobic treatment agents, with hexamethyldisilazane being preferred. The hydrophobic treatment agents may be used alone or in combination of two or more.

Methods for hydrophobizing inorganic particles with a hydrophobizing agent include, for example, a method using supercritical carbon dioxide to dissolve the hydrophobizing agent in supercritical carbon dioxide and adhere the hydrophobizing agent to the surfaces of the inorganic particles; a method in which a solution containing the hydrophobizing agent and a solvent that dissolves the hydrophobizing agent is applied (e.g., sprayed or painted) to the surfaces of inorganic particles in the atmosphere to adhere the hydrophobizing agent to the surfaces of the inorganic particles; and a method in which a solution containing the hydrophobizing agent and a solvent that dissolves the hydrophobizing agent is added to an inorganic particle dispersion in the atmosphere, the mixture is maintained, and then the inorganic particle dispersion and the mixed solution are dried.

From the viewpoint of suppressing uneven image density after high-load printing, the content of inorganic particles contained in the resin coating layer is preferably 10% by mass or more and 60% by mass or less, more preferably 15% by mass or more and 55% by mass or less, and even more preferably 20% by mass or more and 50% by mass or less, relative to the total mass of the resin coating layer.
From the viewpoint of suppressing uneven image density after high-load printing, the content of silica particles contained in the resin coating layer is preferably 10% by mass or more and 60% by mass or less, more preferably 15% by mass or more and 55% by mass or less, and even more preferably 20% by mass or more and 50% by mass or less, relative to the total mass of the resin coating layer.

Examples of resins that constitute the resin coating layer include styrene-acrylic acid copolymers; polyolefin resins such as polyethylene and polypropylene; polyvinyl or polyvinylidene resins such as polystyrene, acrylic resin, polyacrylonitrile, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyvinyl carbazole, polyvinyl ether, and polyvinyl ketone; vinyl chloride-vinyl acetate copolymers; straight silicone resins consisting of organosiloxane bonds or modified products thereof; fluororesins such as polytetrafluoroethylene, polyvinyl fluoride, polyvinylidene fluoride, and polychlorotrifluoroethylene; polyesters; polyurethanes; polycarbonates; amino resins such as urea-formaldehyde resins; and epoxy resins.
In particular, the resin constituting the resin coating layer preferably contains an acrylic resin from the viewpoints of electrostatic chargeability, control of external additive adhesion, and suppression of image density unevenness, more preferably contains 50% by mass or more of the acrylic resin relative to the total mass of the resin in the resin coating layer, and particularly preferably contains 80% by mass or more of the acrylic resin relative to the total mass of the resin in the resin coating layer.

From the viewpoint of suppressing image density unevenness, the resin coating layer preferably contains an acrylic resin having an alicyclic structure. The polymerization component of the acrylic resin having an alicyclic structure is preferably a lower alkyl ester of (meth)acrylic acid (for example, a (meth)acrylic acid alkyl ester having an alkyl group with 1 to 9 carbon atoms), and specific examples thereof include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. These monomers may be used alone or in combination of two or more.
The acrylic resin having an alicyclic structure preferably contains cyclohexyl(meth)acrylate as a polymerization component. The content of the monomer unit derived from cyclohexyl(meth)acrylate contained in the acrylic resin having an alicyclic structure is preferably 75% by mass or more and 100% by mass or less, more preferably 85% by mass or more and 100% by mass or less, and even more preferably 95% by mass or more and 100% by mass or less, based on the total mass of the acrylic resin having an alicyclic structure.

From the viewpoint of suppressing image density unevenness, the weight-average molecular weight of the resin contained in the resin coating layer is preferably less than 300,000, more preferably less than 250,000, even more preferably 5,000 or more but less than 250,000, and particularly preferably 10,000 or more but less than 100,000.

The resin coating layer may contain conductive particles for the purpose of controlling static charge and resistance. Examples of conductive particles include carbon black and the aforementioned inorganic particles that have conductivity.

Methods for forming a resin coating layer on the surface of magnetic particles include, for example, wet and dry processes. A wet process uses a solvent to dissolve or disperse the resin that makes up the resin coating layer. A dry process, on the other hand, does not use the solvent.

Examples of wet manufacturing methods include a dipping method in which magnetic particles are dipped in a resin liquid for forming a resin coating layer to coat them, a spraying method in which a resin liquid for forming a resin coating layer is sprayed onto the surfaces of the magnetic particles, a fluidized bed method in which magnetic particles are fluidized in a fluidized bed and then sprayed with a resin liquid for forming a resin coating layer, and a kneader coater method in which magnetic particles and a resin liquid for forming a resin coating layer are mixed in a kneader coater and the solvent is removed. These manufacturing methods may be repeated or combined.
The resin liquid for forming the resin coating layer used in the wet manufacturing method is prepared by dissolving or dispersing the resin, inorganic particles, and other components in a solvent. The solvent is not particularly limited, and examples thereof include aromatic hydrocarbons such as toluene and xylene, ketones such as acetone and methyl ethyl ketone, and ethers such as tetrahydrofuran and dioxane.

An example of a dry manufacturing method is to heat a mixture of magnetic particles and a resin for forming a resin coating layer in a dry state to form a resin coating layer. Specifically, for example, the magnetic particles and the resin for forming the resin coating layer are mixed in a gas phase and heated to melt, forming a resin coating layer.

The exposed area ratio of the magnetic particles on the carrier surface is preferably 5% to 30%, more preferably 7% to 25%, and even more preferably 10% to 25%. The exposed area ratio of the magnetic particles on the carrier can be controlled by the amount of resin used to form the resin coating layer; the greater the amount of resin relative to the amount of magnetic particles, the smaller the exposed area ratio.

The exposed area ratio of the magnetic particles on the carrier surface is a value determined by the following method.
A target carrier and magnetic particles from which the resin coating layer has been removed are prepared. Methods for removing the resin coating layer from the carrier include, for example, a method of removing the resin coating layer by dissolving the resin component in an organic solvent, and a method of removing the resin coating layer by heating at about 800°C to eliminate the resin component. The carrier and magnetic particles are each used as measurement samples, and the Fe concentration (atomic %) on the sample surface is quantified by XPS, and the exposed area ratio (%) of the magnetic particles is calculated as (Fe concentration of the carrier) ÷ (Fe concentration of the magnetic particles) × 100.

From the viewpoint of suppressing concentration changes, the volume average particle size of the carrier is preferably 25 μm or more and 36 μm or less, more preferably 26 μm or more and 35 μm or less, and particularly preferably 28 μm or more and 34 μm or less.

The carrier shape factor SF1 is preferably 110 or greater and 140 or less, and from the standpoint of image stability and durability, it is more preferably 110 or greater and 125 or less, and from the standpoint of chargeability, it is more preferably 126 or greater and 140 or less.

In this embodiment, the shape factor SF1 of the carrier is determined by capturing an optical microscope image of the carrier dispersed on the surface of a slide glass via a video camera into a Luzex image analyzer, measuring the maximum length (ML) and projected area (A) of 100 or more carriers, and calculating the average value of ML ² /A.

(Electrostatic image developer)
The developer according to this embodiment is a two-component developer containing the electrostatic image developing carrier according to this embodiment and a toner. The toner contains toner particles and, if necessary, an external additive.

The mixture ratio (mass ratio) of carrier to toner in the developer is preferably 100:1 to 100:30, and more preferably 100:3 to 100:20.

<Toner particles>
The toner particles are composed of, for example, a binder resin, and, if necessary, a colorant, a release agent, and other additives.

- Binder resin -
Examples of binder resins include homopolymers of monomers such as styrenes (e.g., styrene, parachlorostyrene, α-methylstyrene, etc.), (meth)acrylic acid esters (e.g., methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, lauryl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, lauryl methacrylate, 2-ethylhexyl methacrylate, etc.), ethylenically unsaturated nitriles (e.g., acrylonitrile, methacrylonitrile, etc.), vinyl ethers (e.g., vinyl methyl ether, vinyl isobutyl ether, etc.), vinyl ketones (e.g., vinyl methyl ketone, vinyl ethyl ketone, vinyl isopropenyl ketone, etc.), and olefins (e.g., ethylene, propylene, butadiene, etc.), and vinyl resins made of copolymers of two or more of these monomers.
Examples of the binder resin include non-vinyl resins such as epoxy resins, polyester resins, polyurethane resins, polyamide resins, cellulose resins, polyether resins, and modified rosin, mixtures of these with the vinyl resins, and graft polymers obtained by polymerizing vinyl monomers in the presence of these.
These binder resins may be used alone or in combination of two or more.

As the binder resin, a polyester resin is preferable.
Examples of polyester resins include known amorphous polyester resins. A crystalline polyester resin may be used in combination with the amorphous polyester resin. The content of the crystalline polyester resin is preferably in the range of 2% by mass to 40% by mass (preferably 2% by mass to 20% by mass) relative to the total binder resin.

The "crystalline" nature of a resin refers to the presence of a clear endothermic peak rather than a stepwise change in endothermic amount in differential scanning calorimetry (DSC), and specifically refers to the half-value width of the endothermic peak being 10°C or less when measured at a heating rate of 10 (°C/min).
On the other hand, the term "amorphous" for a resin means that the half-width exceeds 10°C, that the endothermic amount exhibits a stepwise change, or that no clear endothermic peak is observed.

Amorphous Polyester Resin Examples of the amorphous polyester resin include a condensation polymer of a polycarboxylic acid and a polyhydric alcohol. As the amorphous polyester resin, a commercially available product or a synthesized product may be used.

Examples of polycarboxylic acids include aliphatic dicarboxylic acids (e.g., oxalic acid, malonic acid, maleic acid, fumaric acid, citraconic acid, itaconic acid, glutaconic acid, succinic acid, alkenylsuccinic acid, adipic acid, and sebacic acid), alicyclic dicarboxylic acids (e.g., cyclohexanedicarboxylic acid), aromatic dicarboxylic acids (e.g., terephthalic acid, isophthalic acid, phthalic acid, and naphthalenedicarboxylic acid), anhydrides thereof, and lower alkyl esters thereof (e.g., having 1 to 5 carbon atoms). Among these, aromatic dicarboxylic acids are preferred as polycarboxylic acids.
The polycarboxylic acid may be a trivalent or higher carboxylic acid having a crosslinked or branched structure in combination with a dicarboxylic acid. Examples of the trivalent or higher carboxylic acid include trimellitic acid, pyromellitic acid, anhydrides thereof, and lower (e.g., carbon number 1 to 5) alkyl esters thereof.
The polycarboxylic acids may be used alone or in combination of two or more.

Examples of polyhydric alcohols include aliphatic diols (e.g., ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, butanediol, hexanediol, neopentyl glycol, etc.), alicyclic diols (e.g., cyclohexanediol, cyclohexanedimethanol, hydrogenated bisphenol A, etc.), and aromatic diols (e.g., ethylene oxide adducts of bisphenol A, propylene oxide adducts of bisphenol A, etc.). Among these, aromatic diols and alicyclic diols are preferred as polyhydric alcohols, and aromatic diols are more preferred.
As the polyhydric alcohol, a trihydric or higher polyhydric alcohol having a crosslinked or branched structure may be used in combination with the diol. Examples of trihydric or higher polyhydric alcohols include glycerin, trimethylolpropane, and pentaerythritol.
The polyhydric alcohols may be used alone or in combination of two or more.

The glass transition temperature (Tg) of the amorphous polyester resin is preferably 50°C or higher and 80°C or lower, and more preferably 50°C or higher and 65°C or lower.
The glass transition temperature is determined from a DSC curve obtained by differential scanning calorimetry (DSC), and more specifically, is determined from the "extrapolated glass transition onset temperature" described in the method for determining glass transition temperature in JIS K7121:1987 "Method for measuring transition temperatures of plastics."

The weight average molecular weight (Mw) of the amorphous polyester resin is preferably 5,000 or more and 1,000,000 or less, and more preferably 7,000 or more and 500,000 or less.
The number average molecular weight (Mn) of the amorphous polyester resin is preferably 2,000 or more and 100,000 or less.
The molecular weight distribution Mw/Mn of the amorphous polyester resin is preferably 1.5 or more and 100 or less, and more preferably 2 or more and 60 or less.
The weight average molecular weight and number average molecular weight are measured by gel permeation chromatography (GPC). Molecular weight measurement by GPC is performed using a Tosoh GPC HLC-8120GPC measuring device, a Tosoh TSKgel Super HM-M (15 cm) column, and a THF solvent. The weight average molecular weight and number average molecular weight are calculated from the measurement results using a molecular weight calibration curve prepared with monodisperse polystyrene standard samples.

The amorphous polyester resin can be obtained by a known production method, for example, by carrying out the reaction at a polymerization temperature of 180°C or higher and 230°C or lower, reducing the pressure in the reaction system as necessary, and removing water and alcohol generated during the condensation.
If the raw material monomers are not soluble or compatible at the reaction temperature, a high-boiling solvent may be added as a solubilizer to dissolve them. In this case, the polycondensation reaction is carried out while distilling off the solubilizer. If a monomer with poor compatibility is present in the copolymerization reaction, it is advisable to first condense the poorly compatible monomer with the acid or alcohol to be polycondensed, and then polycondense it with the main component.

Crystalline Polyester Resin Examples of the crystalline polyester resin include polycondensates of polycarboxylic acids and polyhydric alcohols. As the crystalline polyester resin, commercially available products or synthesized products may be used.
Here, the crystalline polyester resin is preferably a polycondensate using a linear aliphatic polymerizable monomer rather than a polymerizable monomer having an aromatic ring, since it easily forms a crystalline structure.

Examples of polycarboxylic acids include aliphatic dicarboxylic acids (e.g., oxalic acid, succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, 1,9-nonanedicarboxylic acid, 1,10-decanedicarboxylic acid, 1,12-dodecanedicarboxylic acid, 1,14-tetradecanedicarboxylic acid, 1,18-octadecanedicarboxylic acid, etc.), aromatic dicarboxylic acids (e.g., dibasic acids such as phthalic acid, isophthalic acid, terephthalic acid, and naphthalene-2,6-dicarboxylic acid), anhydrides thereof, and lower (e.g., having 1 to 5 carbon atoms) alkyl esters thereof.
The polycarboxylic acid may be a trivalent or higher carboxylic acid having a crosslinked or branched structure in combination with a dicarboxylic acid. Examples of the tricarboxylic acid include aromatic carboxylic acids (e.g., 1,2,3-benzenetricarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,2,4-naphthalenetricarboxylic acid, etc.), anhydrides thereof, and lower alkyl esters thereof (e.g., having 1 to 5 carbon atoms).
As the polycarboxylic acid, a dicarboxylic acid having a sulfonic acid group and a dicarboxylic acid having an ethylenic double bond may be used in combination with these dicarboxylic acids.
The polycarboxylic acids may be used alone or in combination of two or more.

Examples of polyhydric alcohols include aliphatic diols (for example, straight-chain aliphatic diols having 7 to 20 carbon atoms in the main chain). Examples of aliphatic diols include ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, 1,10-decanediol, 1,11-undecanediol, 1,12-dodecanediol, 1,13-tridecanediol, 1,14-tetradecanediol, 1,18-octadecanediol, and 1,14-eicosanedecanediol. Among these, 1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol are preferred as aliphatic diols.
The polyhydric alcohol may be used in combination with a diol and a trihydric or higher alcohol having a crosslinked or branched structure. Examples of the trihydric or higher alcohol include glycerin, trimethylolethane, trimethylolpropane, and pentaerythritol.
The polyhydric alcohols may be used alone or in combination of two or more.

Here, the polyhydric alcohol should have an aliphatic diol content of 80 mol% or more, preferably 90 mol% or more.

The melting temperature of the crystalline polyester resin is preferably 50°C or higher and 100°C or lower, more preferably 55°C or higher and 90°C or lower, and even more preferably 60°C or higher and 85°C or lower.
The melting temperature is determined from a DSC curve obtained by differential scanning calorimetry (DSC) by using the "melting peak temperature" described in the method for determining the melting temperature in JIS K7121:1987 "Method for measuring transition temperatures of plastics."

The weight average molecular weight (Mw) of the crystalline polyester resin is preferably 6,000 or more and 35,000 or less.

Crystalline polyester resins can be obtained, for example, by known manufacturing methods, similar to those used for amorphous polyesters.

The binder resin content is preferably 40% by mass or more and 95% by mass or less, more preferably 50% by mass or more and 90% by mass or less, and even more preferably 60% by mass or more and 85% by mass or less, based on the total amount of toner particles.

- Coloring agent -
Examples of colorants include carbon black, chrome yellow, Hansa Yellow, benzidine yellow, threne yellow, quinoline yellow, pigment yellow, permanent orange GTR, pyrazolone orange, Balkan orange, watch young red, permanent red, brilliant carmine 3B, brilliant carmine 6B, DuPont oil red, pyrazolone red, lithol red, rhodamine B lake, lake red C, pigment red, rose bengal, aniline blue, Examples of suitable dyes include pigments such as ultramarine blue, chalco oil blue, methylene blue chloride, phthalocyanine blue, pigment blue, phthalocyanine green, and malachite green oxalate; and dyes such as acridine-based, xanthene-based, azo-based, benzoquinone-based, azine-based, anthraquinone-based, thioindigo-based, dioxazine-based, thiazine-based, azomethine-based, indigo-based, phthalocyanine-based, aniline black-based, polymethine-based, triphenylmethane-based, diphenylmethane-based, and thiazole-based dyes.
The colorant may be used alone or in combination of two or more kinds.

If necessary, the colorant may be surface-treated, or may be used in combination with a dispersant. Multiple types of colorants may also be used in combination.

The colorant content is preferably 1% by mass or more and 30% by mass or less, and more preferably 3% by mass or more and 15% by mass or less, based on the total toner particles.

- Release agent -
Examples of the release agent include hydrocarbon waxes, natural waxes such as carnauba wax, rice wax, and candelilla wax, synthetic or mineral/petroleum waxes such as montan wax, and ester waxes such as fatty acid esters and montanic acid esters. However, the release agent is not limited to these.

The melting temperature of the release agent is preferably 50°C or higher and 110°C or lower, and more preferably 60°C or higher and 100°C or lower.
The melting temperature is determined from a DSC curve obtained by differential scanning calorimetry (DSC) by using the "melting peak temperature" described in the method for determining the melting temperature in JIS K7121:1987 "Method for measuring transition temperatures of plastics."

The release agent content is preferably 1% by mass or more and 20% by mass or less, and more preferably 5% by mass or more and 15% by mass or less, based on the total toner particles.

-Other additives-
Examples of other additives include known additives such as magnetic materials, charge control agents, inorganic powders, etc. These additives are contained in the toner particles as internal additives.

- Characteristics of toner particles -
The toner particles may be toner particles having a single layer structure, or may be toner particles having a so-called core-shell structure composed of a core (core particle) and a coating layer (shell layer) that coats the core.
The toner particles having a core-shell structure may be composed of, for example, a core containing a binder resin and, if necessary, other additives such as a colorant and a release agent, and a coating layer containing the binder resin.

The volume average particle size (D50v) of the toner particles is preferably 2 μm or more and 10 μm or less, and more preferably 4 μm or more and 8 μm or less.
The volume average particle size (D50v) of the toner particles is measured using a Coulter Multisizer II (manufactured by Beckman Coulter), and the electrolyte is measured using an ISOTON-II (manufactured by Beckman Coulter).
For the measurement, 0.5 mg to 50 mg of a measurement sample is added to 2 ml of a 5% by mass aqueous solution of a surfactant (preferably sodium alkylbenzene sulfonate) as a dispersant, and this is then added to 100 ml to 150 ml of an electrolyte.
The electrolyte solution containing the suspended sample is subjected to dispersion treatment in an ultrasonic disperser for 1 minute, and the particle size distribution of particles with a particle size range of 2 μm to 60 μm is measured using a Coulter Multisizer II with an aperture diameter of 100 μm. The number of particles sampled is 50,000. The volume-based particle size distribution is plotted from the smallest diameter side, and the particle size at which the cumulative 50% is reached is defined as the volume average particle size D50v.

The average circularity of the toner particles is preferably 0.94 or more and 1.00 or less, and more preferably 0.95 or more and 0.98 or less.
The average circularity of toner particles is calculated by (circular equivalent perimeter)/(perimeter) [(perimeter of a circle having the same projected area as the particle image)/(perimeter of the particle projected image)]. Specifically, this is a value measured by the following method.
First, toner particles to be measured are sucked and collected, and a flat flow is formed. A strobe is instantaneously activated to capture a particle image as a still image, and the particle image is analyzed using a flow-type particle image analyzer (FPIA-3000 manufactured by Sysmex Corporation). The number of samples to be sampled when determining the average circularity is 3,500.
When the toner contains an external additive, the toner (developer) to be measured is dispersed in water containing a surfactant, and then ultrasonic treatment is performed to obtain toner particles from which the external additive has been removed.

-Method for producing toner particles-
The toner particles may be produced by any of a dry production method (e.g., a kneading and pulverization method) and a wet production method (e.g., an aggregation and coalescence method, a suspension polymerization method, a dissolution and suspension method). There are no particular limitations on these production methods, and any known production method may be used. Among these, it is preferable to obtain toner particles by the aggregation and coalescence method.

Specifically, for example, when toner particles are produced by the aggregation and coalescence method, toner particles are produced through the following steps: preparing a resin particle dispersion in which resin particles that will become the binder resin are dispersed (resin particle dispersion preparation step); aggregating the resin particles (and other particles, if necessary) in the resin particle dispersion (in a dispersion after mixing other particle dispersions, if necessary) to form aggregated particles (aggregated particle formation step); and heating the aggregated particle dispersion in which the aggregated particles are dispersed to fuse and coalesce the aggregated particles to form toner particles (fusion and coalescence step).

Each step will be described in detail below.
In the following description, a method for obtaining toner particles containing a colorant and a release agent will be described, but the colorant and the release agent are used as needed. Of course, other additives than the colorant and the release agent may also be used.

-Resin particle dispersion preparation process-
Along with a resin particle dispersion in which resin particles serving as a binder resin are dispersed, for example, a colorant particle dispersion in which colorant particles are dispersed and a release agent particle dispersion in which release agent particles are dispersed are prepared.

Resin particle dispersions are prepared, for example, by dispersing resin particles in a dispersion medium using a surfactant.

Examples of the dispersion medium used in the resin particle dispersion include aqueous media.
Examples of aqueous media include water such as distilled water and ion-exchanged water, alcohols, etc. These may be used alone or in combination of two or more.

Examples of surfactants include anionic surfactants such as sulfate ester salts, sulfonate salts, phosphate esters, and soap-based surfactants; cationic surfactants such as amine salts and quaternary ammonium salts; and nonionic surfactants such as polyethylene glycols, alkylphenol ethylene oxide adducts, and polyhydric alcohols. Among these, anionic surfactants and cationic surfactants are particularly preferred. Nonionic surfactants may be used in combination with anionic surfactants or cationic surfactants.
The surfactants may be used alone or in combination of two or more.

Methods for dispersing resin particles in a dispersion medium in a resin particle dispersion include common dispersion methods such as those using a rotary shear homogenizer, a ball mill with media, a sand mill, or a Dynomill. Depending on the type of resin particles, the resin particles may also be dispersed in a dispersion medium using a phase inversion emulsification method. Phase inversion emulsification involves dissolving the resin to be dispersed in a hydrophobic organic solvent in which the resin is soluble, adding a base to the organic continuous phase (O phase) to neutralize it, and then introducing an aqueous medium (W phase) to invert the phase from W/O to O/W, thereby dispersing the resin in particulate form in the aqueous medium.

The volume average particle size of the resin particles dispersed in the resin particle dispersion is, for example, preferably from 0.01 μm to 1 μm, more preferably from 0.08 μm to 0.8 μm, and even more preferably from 0.1 μm to 0.6 μm.
The volume average particle size of the resin particles is measured using a particle size distribution obtained by measurement with a laser diffraction particle size distribution analyzer (for example, LA-700 manufactured by Horiba, Ltd.), and the cumulative distribution for the volume of the divided particle size ranges (channels) is calculated from the small particle size side, and the particle size at which the cumulative 50% of all particles is measured is defined as the volume average particle size D50v. The volume average particle sizes of particles in other dispersions are also measured in the same manner.

The content of resin particles in the resin particle dispersion is preferably 5% by mass or more and 50% by mass or less, and more preferably 10% by mass or more and 40% by mass or less.

For example, colorant particle dispersions and release agent particle dispersions are also prepared in the same manner as resin particle dispersions. That is, the volume average particle size, dispersion medium, dispersion method, and particle content of the particles in the resin particle dispersion are the same for the colorant particles dispersed in the colorant particle dispersion and the release agent particles dispersed in the release agent particle dispersion.

-Agglomerated particle formation process-
Next, the resin particle dispersion, the colorant particle dispersion, and the release agent particle dispersion are mixed together.
Then, in the mixed dispersion, the resin particles, colorant particles, and release agent particles are hetero-aggregated to form aggregated particles containing the resin particles, colorant particles, and release agent particles and having a diameter close to that of the target toner particles.

Specifically, for example, an aggregating agent is added to the mixed dispersion, and the pH of the mixed dispersion is adjusted to an acidic value (for example, a pH of 2 or more and 5 or less), and a dispersion stabilizer is added as needed. Thereafter, the mixed dispersion is heated to a temperature close to the glass transition temperature of the resin particles (specifically, for example, the glass transition temperature of the resin particles minus 30°C or more and the glass transition temperature minus 10°C or less), and the particles dispersed in the mixed dispersion are aggregated to form aggregated particles.
In the aggregate particle formation step, for example, the mixed dispersion may be stirred with a rotary shear homogenizer, an aggregating agent may be added at room temperature (e.g., 25°C), the pH of the mixed dispersion may be adjusted to an acidic value (e.g., pH 2 or more and 5 or less), a dispersion stabilizer may be added as needed, and then the mixture may be heated.

Examples of the flocculant include a surfactant having an opposite polarity to that of the surfactant contained in the mixed dispersion, an inorganic metal salt, and a divalent or higher metal complex. When a metal complex is used as the flocculant, the amount of surfactant used can be reduced, and the charging characteristics can be improved.
If necessary, an additive that forms a complex or a similar bond with the metal ions of the flocculant may be used together with the flocculant, and a chelating agent is preferably used as this additive.

Examples of inorganic metal salts include metal salts such as calcium chloride, calcium nitrate, barium chloride, magnesium chloride, zinc chloride, aluminum chloride, and aluminum sulfate; and inorganic metal salt polymers such as polyaluminum chloride, polyaluminum hydroxide, and calcium polysulfide.
The chelating agent may be a water-soluble chelating agent, such as hydroxycarboxylic acid (e.g., tartaric acid, citric acid, gluconic acid), or aminocarboxylic acid (e.g., iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), or ethylenediaminetetraacetic acid (EDTA).
The amount of the chelating agent added is preferably 0.01 parts by mass or more and 5.0 parts by mass or less, and more preferably 0.1 parts by mass or more and less than 3.0 parts by mass, per 100 parts by mass of the resin particles.

-Fusion/unification process-
Next, the aggregated particle dispersion liquid in which the aggregated particles are dispersed is heated, for example, to a temperature equal to or higher than the glass transition temperature of the resin particles (for example, a temperature 10°C to 30°C higher than the glass transition temperature of the resin particles) to fuse and coalesce the aggregated particles and form toner particles.

Through the above steps, toner particles are obtained.
After obtaining the aggregated particle dispersion liquid in which the aggregated particles are dispersed, the toner particles may be manufactured through the following steps: a step of further mixing the aggregated particle dispersion liquid with a resin particle dispersion liquid in which resin particles are dispersed, and aggregating the aggregated particles so that further resin particles adhere to the surfaces of the aggregated particles to form second aggregated particles; and a step of heating the second aggregated particle dispersion liquid in which the second aggregated particles are dispersed, and fusing and coalescing the second aggregated particles to form toner particles having a core-shell structure.

After the fusion and coalescence process is completed, the toner particles formed in the solution are subjected to the known washing process, solid-liquid separation process, and drying process to obtain dried toner particles. From the perspective of chargeability, the washing process should involve thorough substitution washing with ion-exchanged water. From the perspective of productivity, the solid-liquid separation process should involve suction filtration, pressure filtration, etc. From the perspective of productivity, the drying process should involve freeze drying, flash drying, fluidized bed drying, vibration-type fluidized bed drying, etc.

The toner according to this embodiment is produced, for example, by adding external additives to the resulting dry toner particles and mixing them. Mixing can be performed using, for example, a V-blender, a Henschel mixer, a Loedige mixer, or the like. Furthermore, if necessary, coarse particles may be removed from the toner using a vibrating sieve, an air sieve, or the like.

-External additives-
Examples of external additives include inorganic particles such as _SiO2 , TiO2, _Al2O3 , _CuO , ZnO, _SnO2 , _CeO2 , _Fe2O3 , _MgO , BaO, _CaO , _K2O , _Na2O , _ZrO2 , _CaO.SiO2 , _K2O .( _{TiO2 ) n} , _Al2O3.2SiO2 , _CaCO3 , _MgCO3 , _BaSO4 , _{and MgSO4} .

The surfaces of inorganic particles as external additives are preferably subjected to a hydrophobic treatment. The hydrophobic treatment is carried out, for example, by immersing the inorganic particles in a hydrophobic treatment agent. The hydrophobic treatment agent is not particularly limited, and examples thereof include silane coupling agents, silicone oils, titanate coupling agents, and aluminum coupling agents. These may be used alone or in combination of two or more.
The amount of the hydrophobic treatment agent is usually, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the inorganic particles.

External additives include resin particles (resin particles such as polystyrene, polymethyl methacrylate, and melamine resin), cleaning agents (for example, metal salts of higher fatty acids such as zinc stearate, and particles of fluorine-based polymers), etc.

The amount of external additive added is preferably 0.01% by mass or more and 5% by mass or less, and more preferably 0.01% by mass or more and 2.0% by mass or less, relative to the toner particles.

In the electrostatic image developer according to the present embodiment, the ratio ( ^SC /ST) of the volume average particle diameter ^ST of the electrostatic image developing toner to the volume average particle diameter ^SC of the electrostatic image developing carrier is preferably 4 or more ^and 8 or less, and more preferably 5 or more and 7 or less.

<Image forming apparatus, image forming method>
The image forming apparatus according to the present embodiment includes an image carrier, a charging unit that charges the surface of the image carrier, an electrostatic image forming unit that forms an electrostatic image on the surface of the charged image carrier, a developing unit that contains an electrostatic image developer and develops the electrostatic image formed on the surface of the image carrier as a toner image using the electrostatic image developer, a transfer unit that transfers the toner image formed on the surface of the image carrier to the surface of a recording medium, and a fixing unit that fixes the toner image transferred to the surface of the recording medium. The electrostatic image developer according to the present embodiment is used as the electrostatic image developer.

The image forming apparatus according to this embodiment carries out an image forming method (the image forming method according to this embodiment) that includes a charging step of charging the surface of an image carrier, an electrostatic image forming step of forming an electrostatic image on the surface of the charged image carrier, a developing step of developing the electrostatic image formed on the surface of the image carrier as a toner image using the electrostatic image developer according to this embodiment, a transfer step of transferring the toner image formed on the surface of the image carrier to the surface of a recording medium, and a fixing step of fixing the toner image transferred to the surface of the recording medium.

The image forming apparatus according to the present embodiment may be any of known image forming apparatuses, such as a direct transfer type apparatus that directly transfers a toner image formed on the surface of an image carrier to a recording medium; an intermediate transfer type apparatus that primarily transfers a toner image formed on the surface of an image carrier to the surface of an intermediate transfer medium, and then secondarily transfers the toner image transferred to the surface of the intermediate transfer medium to the surface of a recording medium; an apparatus equipped with a cleaning means that cleans the surface of the image carrier after the transfer of the toner image but before charging; and an apparatus equipped with a discharging means that irradiates the surface of the image carrier with discharging light to discharge it after the transfer of the toner image but before charging.
When the image forming apparatus according to the present embodiment is an apparatus of the intermediate transfer type, the transfer means is configured to have, for example, an intermediate transfer body onto whose surface a toner image is transferred, a primary transfer means for primarily transferring the toner image formed on the surface of the image carrier onto the surface of the intermediate transfer body, and a secondary transfer means for secondarily transferring the toner image transferred onto the surface of the intermediate transfer body onto the surface of the recording medium.

In the image forming apparatus according to this embodiment, for example, the portion including the developing means may have a cartridge structure (process cartridge) that is detachably attached to the image forming apparatus. A suitable process cartridge is, for example, a process cartridge that contains the electrostatic image developer according to this embodiment and is equipped with the developing means.

Below, an example of an image forming apparatus according to this embodiment is shown, but the present invention is not limited to this. In the following explanation, the main parts shown in the figure will be explained, and other parts will not be explained.

FIG. 1 is a schematic diagram showing the configuration of an image forming apparatus according to this embodiment.
The image forming apparatus shown in Fig. 1 includes first to fourth electrophotographic image forming units 10Y, 10M, 10C, and 10K (image forming means) that output images in the respective colors of yellow (Y), magenta (M), cyan (C), and black (K) based on color-separated image data. These image forming units (hereinafter sometimes simply referred to as "units") 10Y, 10M, 10C, and 10K are arranged side by side horizontally spaced a predetermined distance apart from one another. These units 10Y, 10M, 10C, and 10K may be process cartridges that are detachably attached to the image forming apparatus.

An intermediate transfer belt (an example of an intermediate transfer body) 20 is provided above each of the units 10Y, 10M, 10C, and 10K and extends through each unit. The intermediate transfer belt 20 is provided wrapped around a drive roll 22 and a support roll 24, and runs in a direction from the first unit 10Y to the fourth unit 10K. A force is applied to the support roll 24 by a spring or the like (not shown) in a direction away from the drive roll 22, and tension is applied to the intermediate transfer belt 20 wrapped around them. An intermediate transfer body cleaning device 30 is provided on the image carrier side of the intermediate transfer belt 20, facing the drive roll 22.
The developing devices (examples of developing means) 4Y, 4M, 4C, and 4K of the units 10Y, 10M, 10C, and 10K are supplied with yellow, magenta, cyan, and black toner contained in toner cartridges 8Y, 8M, 8C, and 8K, respectively.

Since the first to fourth units 10Y, 10M, 10C, and 10K have the same configuration and operation, we will explain here the first unit 10Y, which forms yellow images and is arranged upstream in the direction of travel of the intermediate transfer belt.

The first unit 10Y has a photoreceptor 1Y that acts as an image carrier. Around the photoreceptor 1Y, there are arranged in this order: a charging roll (an example of a charging means) 2Y that charges the surface of the photoreceptor 1Y to a predetermined potential; an exposure device (an example of an electrostatic image forming means) 3 that exposes the charged surface to a laser beam 3Y based on a color-separated image signal to form an electrostatic image; a developing device (an example of a developing means) 4Y that supplies charged toner to the electrostatic image to develop it; a primary transfer roll 5Y (an example of a primary transfer means) that transfers the developed toner image onto an intermediate transfer belt 20; and a photoreceptor cleaning device (an example of a cleaning means) 6Y that removes toner remaining on the surface of the photoreceptor 1Y after the primary transfer.
The primary transfer roll 5Y is disposed inside the intermediate transfer belt 20 and is provided at a position facing the photoreceptor 1Y. A bias power supply (not shown) that applies a primary transfer bias is connected to the primary transfer rolls 5Y, 5M, 5C, and 5K of each unit. Each bias power supply changes the value of the transfer bias applied to each primary transfer roll under the control of a control unit (not shown).

The operation of forming a yellow image in the first unit 10Y will be described below.
First, prior to operation, the surface of the photosensitive member 1Y is charged to a potential of −600V to −800V by the charging roll 2Y.
The photoreceptor 1Y is formed by laminating a photosensitive layer on a conductive substrate (for example, a volume resistivity of 1×10 ⁻⁶ Ωcm or less at 20°C). This photosensitive layer normally has high resistance (the resistance of ordinary resins), but when irradiated with a laser beam, the resistivity of the portion irradiated with the laser beam changes. Therefore, the charged surface of the photoreceptor 1Y is irradiated with a laser beam 3Y from the exposure device 3 in accordance with image data for yellow sent from a control unit (not shown). As a result, an electrostatic charge image of a yellow image pattern is formed on the surface of the photoreceptor 1Y.

An electrostatic image is an image formed on the surface of the photosensitive element 1Y by charging it; the laser beam 3Y reduces the resistivity of the irradiated portion of the photosensitive layer, causing the charged charges on the surface of the photosensitive element 1Y to flow, while the charges remain in the portions not irradiated by the laser beam 3Y, forming a so-called negative latent image.
The electrostatic image formed on the photoreceptor 1Y rotates to a predetermined development position as the photoreceptor 1Y travels. At this development position, the electrostatic image on the photoreceptor 1Y is developed into a toner image by the developing device 4Y and made visible.

Developing device 4Y contains an electrostatic image developer containing, for example, at least yellow toner and a carrier. The yellow toner becomes frictionally charged as it is stirred inside developing device 4Y, and is held on a developer roll (an example of a developer holder) with a charge of the same polarity (negative polarity) as the charge on photoreceptor 1Y. As the surface of photoreceptor 1Y passes through developing device 4Y, yellow toner electrostatically adheres to the discharged latent image portion on the surface of photoreceptor 1Y, and the latent image is developed with yellow toner. Photoreceptor 1Y, with the yellow toner image formed, continues to travel at a predetermined speed, and the toner image developed on photoreceptor 1Y is transported to a predetermined primary transfer position.

When the yellow toner image on the photoreceptor 1Y is transported to the primary transfer position, a primary transfer bias is applied to the primary transfer roll 5Y, and an electrostatic force from the photoreceptor 1Y toward the primary transfer roll 5Y acts on the toner image, causing the toner image on the photoreceptor 1Y to be transferred onto the intermediate transfer belt 20. The transfer bias applied at this time has a (+) polarity opposite to the (-) polarity of the toner, and is controlled to, for example, +10 μA by a control unit (not shown) in the first unit 10Y.
On the other hand, the toner remaining on the photoreceptor 1Y is removed and collected by the photoreceptor cleaning device 6Y.

The primary transfer bias applied to the primary transfer rolls 5M, 5C, and 5K of the second unit 10M and subsequent units is also controlled in accordance with the first unit.
In this way, the intermediate transfer belt 20 onto which the yellow toner image has been transferred by the first unit 10Y is conveyed sequentially through the second to fourth units 10M, 10C, and 10K, and the toner images of each color are transferred onto the intermediate transfer belt 20 in a superimposed manner.

The intermediate transfer belt 20, onto which the four-color toner images have been multiply transferred through the first through fourth units, reaches a secondary transfer section consisting of the intermediate transfer belt 20, a support roll 24 in contact with the inner surface of the intermediate transfer belt, and a secondary transfer roll (an example of a secondary transfer means) 26 positioned on the image bearing surface side of the intermediate transfer belt 20. Meanwhile, recording paper (an example of a recording medium) P is fed via a supply mechanism into the gap between the secondary transfer roll 26 and the intermediate transfer belt 20 at a predetermined timing, and a secondary transfer bias is applied to the support roll 24. The applied transfer bias has a negative polarity, the same as the negative polarity of the toner. Electrostatic force from the intermediate transfer belt 20 toward the recording paper P acts on the toner image, transferring the toner image from the intermediate transfer belt 20 to the recording paper P. The secondary transfer bias is determined based on the resistance detected by a resistance detection means (not shown) that detects the resistance of the secondary transfer section, and is voltage-controlled.

The recording paper P is then fed into the pressure contact area (nip area) between a pair of fixing rolls in the fixing device (an example of a fixing means) 28, where the toner image is fixed onto the recording paper P, forming a fixed image.

Examples of the recording paper P onto which the toner image is transferred include plain paper used in electrophotographic copying machines, printers, etc. In addition to the recording paper P, examples of the recording medium include overhead projector sheets and the like.
To further improve the smoothness of the image surface after fixing, it is preferable that the surface of the recording paper P is also smooth. For example, coated paper in which the surface of ordinary paper is coated with a resin or the like, or art paper for printing, etc., is preferably used.

Once the color image has been fixed, the recording paper P is transported toward the discharge section, completing the color image formation process.

<Process cartridge>
The process cartridge according to this embodiment is a process cartridge that is detachably attached to an image forming apparatus and that contains the electrostatic image developer according to this embodiment and is equipped with a developing means that develops an electrostatic image formed on the surface of an image carrier using the electrostatic image developer into a toner image.

The process cartridge according to this embodiment is not limited to the above configuration, and may also be configured to include a developing unit and, as necessary, at least one other unit selected from the group consisting of an image carrier, a charging unit, an electrostatic image forming unit, and a transfer unit.

Below, an example of a process cartridge according to this embodiment is shown, but the present invention is not limited to this. In the following explanation, the main parts shown in the figure will be described, and other parts will not be described.

FIG. 2 is a schematic diagram showing the configuration of the process cartridge according to this embodiment.
The process cartridge 200 shown in Figure 2 is configured to be a cartridge formed by integrally combining and holding a photosensitive member 107 (an example of an image carrier), a charging roll 108 (an example of a charging means) provided around the photosensitive member 107, a developing device 111 (an example of a developing means), and a photosensitive member cleaning device 113 (an example of a cleaning means), for example, in a housing 117 provided with mounting rails 116 and an opening 118 for exposure.
In FIG. 2, 109 denotes an exposure device (an example of an electrostatic image forming means), 112 denotes a transfer device (an example of a transfer means), 115 denotes a fixing device (an example of a fixing means), and 300 denotes recording paper (an example of a recording medium).

The following examples illustrate embodiments of the invention in detail, but the invention is not limited to these examples. In the following description, "parts" and "%" are by mass unless otherwise specified.

In the following description, the volume average particle size refers to the particle size D50v, which is the cumulative 50% particle size from the smallest diameter side in the volume-based particle size distribution.

<Toner Production>
- Preparation of Colorant Particle Dispersion 1 -
Cyan pigment (copper phthalocyanine B15:3 (manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.)): 50 parts by mass Anionic surfactant: Neogen SC (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) 5 parts by mass Ion-exchanged water: 200 parts by mass The above ingredients were mixed and dispersed for 5 minutes using an Ultra-Turrax manufactured by IKA Corporation, and then for 10 minutes using an ultrasonic bath, to obtain colorant particle dispersion 1 having a solids content of 21%. The volume average particle size was measured using a particle size analyzer LA-700 manufactured by Horiba, Ltd., and was found to be 160 nm.

- Preparation of release agent particle dispersion 1 -
Paraffin wax: HNP-9 (manufactured by Nippon Seiro Co., Ltd.): 19 parts by mass Anionic surfactant: NEOGEN SC (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.): 1 part by mass Ion-exchanged water: 80 parts by mass The above ingredients were mixed in a heat-resistant container, heated to 90°C, and stirred for 30 minutes. Next, the molten liquid was passed through the bottom of the container into a Gaulin homogenizer, and a circulation operation equivalent to three passes was carried out under a pressure condition of 5 MPa. After that, the pressure was increased to 35 MPa, and a circulation operation equivalent to another three passes was carried out. The emulsion thus produced was cooled to 40°C or less in the heat-resistant solution, and release agent particle dispersion 1 was obtained. The volume average particle size was measured using a particle size analyzer LA-700 manufactured by Horiba, Ltd., and was found to be 240 nm.

-Resin particle dispersion liquid 1-
[Oil layer]
Styrene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.): 30 parts by mass n-Butyl acrylate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.): 10 parts by mass β-carboxyethyl acrylate (manufactured by Rhodia Nicca Co., Ltd.): 1.3 parts by mass Dodecanethiol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.): 0.4 parts by mass

[Water layer 1]
Ion-exchanged water: 17 parts by mass Anionic surfactant (Dowfax, manufactured by Dow Chemical Company): 0.4 parts by mass

[Aqueous layer 2]
Ion-exchanged water: 40 parts by mass Anionic surfactant (Dowfax, manufactured by The Dow Chemical Company): 0.05 parts by mass Ammonium peroxodisulfate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.): 0.4 parts by mass

The oil layer components and the aqueous layer 1 components were placed in a flask and mixed with stirring to form a monomer emulsion dispersion. The aqueous layer 2 components were placed in a reaction vessel, the atmosphere in the vessel was thoroughly purged with nitrogen, and the reaction system was heated in an oil bath with stirring until the temperature reached 75°C. The monomer emulsion dispersion was gradually added dropwise to the reaction vessel over 3 hours to carry out emulsion polymerization. After completion of the dropwise addition, the polymerization was continued at 75°C and terminated after 3 hours.
The volume average particle diameter D50v of the obtained resin particles was measured using a laser diffraction particle size distribution analyzer LA-700 (manufactured by Horiba, Ltd.) and was found to be 250 nm. The glass transition point of the resin was measured using a differential scanning calorimeter (DSC-50, manufactured by Shimadzu Corporation) at a heating rate of 10°C/min and was found to be 53°C. The number average molecular weight (polystyrene equivalent) was measured using a molecular weight analyzer (HLC-8020, manufactured by Tosoh Corporation) and THF as a solvent and was found to be 13,000. This resulted in a resin particle dispersion having a volume average particle diameter of 250 nm, a solids content of 42%, a glass transition point of 52°C, and a number average molecular weight Mn of 13,000.

- Preparation of Toner 1 -
Resin particle dispersion: 150 parts by mass Colorant particle dispersion: 30 parts by mass Release agent particle dispersion: 40 parts by mass Polyaluminum chloride: 0.4 parts by mass The above components were thoroughly mixed and dispersed in a stainless steel flask using an Ultra-Turrax manufactured by IKE Corporation, and then the flask was heated to 48° C. with stirring in a heating oil bath. After maintaining at 48° C. for 80 minutes, 70 parts by mass of the same resin particle dispersion as above was slowly added thereto.
The pH of the system was then adjusted to 6.0 using a 0.5 mol/L aqueous sodium hydroxide solution. The stainless steel flask was then sealed, the stirring shaft was magnetically sealed, and the mixture was heated to 97°C and maintained at this temperature for 3 hours while continuing to stir. After the reaction was complete, the mixture was cooled at a rate of 1°C/min, filtered, thoroughly washed with ion-exchanged water, and then subjected to solid-liquid separation using Nutsche suction filtration. The mixture was then redispersed in 3,000 parts by weight of ion-exchanged water at 40°C, stirred and washed for 15 minutes at 300 rpm. This washing procedure was repeated five more times, and when the filtrate reached a pH of 6.54 and an electrical conductivity of 6.5 μS/cm, solid-liquid separation was performed using Nutsche suction filtration with No. 5A filter paper. Vacuum drying was then continued for 12 hours to obtain toner base particles.
The volume average particle diameter D50v of the toner base particles was measured using a Coulter counter and found to be 6.2 μm, and the volume average particle size distribution index GSDv was 1.20. Shape observation using a Luzex image analyzer manufactured by Luzex Corporation revealed that the particle shape factor SF1 was 135 and that the particles were potato-shaped. The glass transition point of the toner was 52°C. Furthermore, silica (SiO ₂ ) particles with an average primary particle diameter of 40 nm that had been surface-hydrophobized with hexamethyldisilazane (hereinafter sometimes abbreviated as "HMDS") and metatitanic acid compound particles with an average primary particle diameter of 20 nm, which were a reaction product of metatitanic acid and isobutyltrimethoxysilane, were added to this toner so that the surface coverage of the toner particles was 40%, and the mixture was mixed using a Henschel mixer to produce Toner 1.

<Preparation of Magnetic Particle 1>
magnetic particles 1
_1,318 parts by mass of _Fe2O3 , 586 parts by mass of Mn(OH) ₂ , 96 parts by mass of Mg(OH) ₂ , and 12 parts by mass of _SrCO3 were mixed, and a dispersant, water, and zirconia beads with a media diameter of 1 mm were added, followed by crushing and mixing in a sand mill. The zirconia beads were filtered and dried, and then further processed in a rotary kiln at 20 rpm and 900 ° C to produce a mixed oxide. Next, a dispersant and water were added, and 6.6 parts by mass of polyvinyl alcohol were added, and the mixture was pulverized in a wet ball mill until the volume average particle size reached 1.2 μm. Next, the mixture was granulated and dried in a spray dryer to a dry particle size of 34 μm. The mixture was then fired in an electric furnace at 1420 ° C for 5 hours in an oxygen-nitrogen mixed atmosphere with an oxygen concentration of 1%. The obtained particles were subjected to a crushing step and a classification step, then heated in a rotary kiln at 15 rpm and 900°C for 2 hours, and similarly subjected to a classification step to obtain magnetic particles 1. The volume average particle size of magnetic particles 1 was 32 μm and SF1 was 112.

<Preparation of Magnetic Particles 2>
_1,318 parts by mass of _Fe2O3 , 586 parts by mass of Mn(OH) ₂ , 96 parts by mass of Mg(OH) ₂ , and 12 parts by mass of _SrCO3 were mixed, and a dispersant, water, and zirconia beads with a media diameter of 1 mm were added, followed by crushing and mixing in a sand mill. The zirconia beads were then filtered and dried. Next, a dispersant and water were added, and 6.6 parts by mass of polyvinyl alcohol was added, followed by pulverization in a wet ball mill until the volume average particle size reached 3 μm. Next, the mixture was granulated and dried in a spray dryer until the dry particle size reached 34 μm. The mixture was then fired in an electric furnace at 1230°C for 8 hours in an oxygen-nitrogen mixed atmosphere with an oxygen concentration of 1.1%. The resulting particles were then subjected to a crushing process and a classification process, followed by a classification process to obtain magnetic particles 2. The volume average particle size of magnetic particles 2 was 32 μm and the SF1 was 138.

<Particles A to be added to the carrier resin coating layer>
As particles A, particles 1 to 11 shown in Table 2 were prepared.

[Preparation of Coating Liquid 1]
Cyclohexyl acrylate resin (weight average molecular weight 50,000): 36 parts by mass Carbon black VXC72 (manufactured by Cabot Corporation): 4 parts by mass Toluene: 250 parts by mass Isopropyl alcohol: 50 parts by mass The above components and glass beads (particle size: 1 mm, same amount as toluene) were placed in a sand mill manufactured by Kansai Paint Co., Ltd., and stirred at a rotation speed of 1,200 rpm for 30 minutes to prepare coating solution 1 with a solids content of 11%.

(Examples 1 to 14 and Comparative Examples 1 to 6)
<Preparation of Resin-Coated Carrier>
- Preparation of Carrier 1 -
2,000 parts of ferrite particles 1 were placed in a vacuum degassing kneader, and 560 parts of coating liquid 1 were then added. The mixture was stirred at 60°C and reduced pressure to -200 mmHg, and mixed for 15 minutes. The mixture was then heated and reduced pressure to 94°C/-720 mmHg, and stirred and dried for 30 minutes to obtain coated particles. The mixture was then sieved through a 75 μm mesh sieve to obtain a carrier. The SF1 of the obtained carrier was 113.
Next, 500 parts of this carrier and 0.5 parts of particles 1 as particles A were charged into a V-blender and mixed at 20 rpm for 20 minutes to obtain carrier 1.

- Preparation of carriers 2 to 20 -
Carriers 2 to 20 were obtained in the same manner as Carrier 1, except that the type of magnetic particles and the type and content (addition amount) of Particle A were changed as shown in Table 3.

<Preparation of Electrostatic Image Developer>
--Preparation of Developer 1--
500 parts of carrier 1 and 40 parts of toner were charged into a V-blender and mixed at 20 rpm for 20 minutes to obtain developer 1.

--Preparation of Developers 2 to 20--
Developers 2 to 20 were obtained in the same manner as in the preparation of developer 1, except that the carrier was changed to one shown in Table 3.

<Measurement of the average particle size of inorganic particles in the resin coating layer>
The carrier was embedded in epoxy resin and cut with a microtome to prepare a carrier cross section. SEM images of the carrier cross section were taken with a scanning transmission electron microscope (S-4100, manufactured by Hitachi, Ltd.), and the images were imported into an image processing analyzer (Luzex AP, manufactured by Nireco Corporation) for image analysis. 100 silica particles (primary particles) were randomly selected from the resin coating layer, and their respective circle-equivalent diameters (nm) were determined and arithmetically averaged to obtain the average particle diameter (nm) of the inorganic particles.

<Measurement of Average Thickness of Resin Coating Layer>
The SEM image was imported into an image processing analyzer (Luzex AP, manufactured by Nireco Corporation) and subjected to image analysis. The thickness (μm) of the resin coating layer was measured at 10 randomly selected locations per carrier particle, and then measurements were taken for 100 carrier particles. The arithmetic mean of all measurements was calculated and this was defined as the average thickness (μm) of the resin coating layer.

<Measurement of Residual Ratio X of Particles A>
Before printing or after printing any image on up to 100 sheets, the developer was passed through a 16 μm mesh to remove the toner, and carrier A was extracted. A 16 μm mesh cage was used to remove the toner by air blowing. Similarly, after printing 5,000 full-page A4 size images with a 30% halftone at 120 sheets per minute, the toner was removed from the developer, and carrier B was extracted. The fluorescent X-ray net intensities of both were measured, and the residual ratio X was calculated from the net intensity α of carrier A and the net intensity β of carrier B using the equation: X=β/α.

<Method for separating particles A from carrier>
To separate particles A from the carrier, the toner is removed from the developer by air blowing, as described above. The resulting carrier is placed in ion-exchange water and treated with ultrasound at 40 Hz for 15 seconds, after which the carrier is filtered and separated. The resulting filtrate is dried, and freeze-dried while the particles are still not completely dry. The resulting particles are further dried for one week at 15°C and 15% RH.

<Measurement of Moisture Content of Particles A>
After leaving the particles for 48 hours under conditions of 28°C and 85% RH, the moisture content of the particles A was determined by measuring the weight difference before and after heating at 150°C for 20 minutes using a heated Mettler.

<Measurement of Volume Resistivity of Particles A>
The volume resistivity of particles A was measured by the following method, where the measurement environment was a temperature of 20° C. and a humidity of 50% RH.
Two electrodes were placed parallel to each other with a width of 1 mm, 0.20 g of ferrite particles were placed between them, and the electrodes were held in place by a magnet with a cross-sectional area of 2.4 ^cm2 . A voltage of 1,000 V was applied, and the current value was measured. The resistance value was calculated from the obtained current value.

<Measurement of standard deviation of particles A>
The particle size distribution was measured by image analysis from field emission scanning electron microscope (FE-SEM) images of particles A on the carrier.

<Collection of magnetic particles from developer>
The carrier was separated from the developer using a 16 μm mesh. The coating layer of the separated carrier was dissolved using, for example, toluene to extract the magnetic particles. The solvent was changed arbitrarily depending on the coating resin. The dissolution was varied by heating, applying ultrasound, etc. depending on the solvent.

<Volume average particle size of magnetic particles>
The volume average particle size (D50) of the magnetic particles was measured using a laser particle size distribution measuring device LA-700 (manufactured by Horiba Ltd.).

<Evaluation of suppression of image density unevenness>
The obtained developer was charged into a modified DocuCentre 400 machine (capable of 120 ppm) manufactured by Fuji Xerox Co., Ltd., and a 15 cm x 15 cm solid print was printed under conditions of 10°C and 15% RH to obtain print A. Print A was evaluated according to the following evaluation criteria (evaluation of initial image density unevenness suppression ability).
Next, 5,000 A4 full-page images with a 30% halftone were printed at a rate of 120 sheets per minute. Next, a 15 cm x 15 cm solid image was printed to obtain print B. Print B was evaluated according to the following evaluation criteria (evaluation of image density unevenness suppression after high-load printing).
The evaluation criteria are as follows:
D: A clear difference in shading can be seen in the image when viewed visually. C: A slight difference in shading can be seen in the image when viewed visually, and unevenness can be seen. B: No unevenness can be seen in the image when viewed visually, but unevenness can be seen when enlarged 10 times. A: No unevenness can be seen even when enlarged.

The evaluation results are summarized in Table 4.

The above results show that this embodiment produces images with less density unevenness, even under high-load printing conditions, compared to the comparative example.

1Y, 1M, 1C, 1K: photoconductor (an example of an image carrier)
2Y, 2M, 2C, 2K: charging roll (an example of a charging means)
3. Exposure device (an example of an electrostatic image forming means)
3Y, 3M, 3C, 3K: Laser beams 4Y, 4M, 4C, 4K: Developing device (an example of developing means)
5Y, 5M, 5C, 5K: Primary transfer roll (an example of a primary transfer means)
6Y, 6M, 6C, 6K: Photoconductor cleaning device (an example of a cleaning means)
8Y, 8M, 8C, 8K toner cartridges 10Y, 10M, 10C, 10K image forming unit 20 intermediate transfer belt (an example of an intermediate transfer body)
22 Drive roll 24 Support roll 26 Secondary transfer roll (an example of a secondary transfer means)
28 Fixing device (an example of fixing means)
30 Intermediate transfer body cleaning device P Recording paper (an example of a recording medium)
107 Photoreceptor (an example of an image carrier)
108 Charging roll (an example of charging means)
109 Exposure device (an example of an electrostatic image forming means)
111 Developing device (an example of developing means)
112 Transfer device (an example of transfer means)
113 Photosensitive member cleaning device (an example of a cleaning means)
115 Fixing device (an example of fixing means)
116 Mounting rail 117 Housing 118 Opening for exposure 200 Process cartridge 300 Recording paper (an example of a recording medium)

Claims (14)

The carrier has particles A on its surface in an amount of 0.02% by mass or more and 0.5% by mass or less based on the total mass of the carrier,
the particles A are silica particles,
a residual rate X of the particles A in the carrier after printing 5,000 full-page A4 paper images of 30% halftone at 10°C, 15% RH, and 120 pages per minute using a modified DocuCentre 400 manufactured by Fuji Xerox Co., Ltd. that is capable of printing at 120 ppm, is 0.3 or more and 1.0 or less in terms of fluorescent X-ray NET intensity;
The electrostatic image developing carrier, wherein the water content of the particles A is 7% by mass or more and 12% by mass or less based on the total mass of the particles A. 2. The carrier for developing electrostatic images according to claim 1 , wherein the arithmetic mean particle size of the particles A is 60 nm or more and 160 nm or less. 3. The carrier for developing electrostatic images according to claim 1, wherein the particle A has a volume resistivity of ^10.sup.12 .OMEGA. or more and ^10.sup.15 .OMEGA . or less. 4. The carrier for developing electrostatic images according to claim 1 , wherein SF1 of the carrier is 110 or more and 125 or less. 5. The carrier for developing electrostatic images according to claim 1 , wherein SF1 of the carrier is 126 or more and 140 or less. 6. The carrier for developing electrostatic images according to claim 1 , wherein the particle size distribution of the particles A has a standard deviation of 8 or more and 12 or less. 7. The carrier for developing electrostatic images according to claim 1 , wherein the particles A have a true specific gravity of 1.1 g/cm ^<3> or more and 2.0 g/cm ^<3> or less. 8. The carrier for developing electrostatic images according to claim 1 , wherein the carrier comprises magnetic particles and a resin coating layer that coats the magnetic particles. 9. The carrier for developing electrostatic images according to claim 8 , wherein the weight average molecular weight of the resin contained in the resin coating layer is less than 300,000. 10. The carrier for developing electrostatic images according to claim 9 , wherein the weight average molecular weight of the resin contained in the resin coating layer is less than 250,000. a toner for developing an electrostatic image;
An electrostatic image developer comprising the carrier for developing electrostatic images according to any one of claims 1 to 10 . a developing unit containing the electrostatic image developer according to claim 11 and developing an electrostatic image formed on a surface of an image carrier into a toner image by using the electrostatic image developer,
A process cartridge is detachably mounted in an image forming apparatus. an image carrier;
a charging means for charging the surface of the image carrier;
an electrostatic image forming means for forming an electrostatic image on the charged surface of the image carrier;
a developing unit containing the electrostatic image developer according to claim 11 and developing the electrostatic image formed on the surface of the image carrier into a toner image by using the electrostatic image developer;
a transfer means for transferring the toner image formed on the surface of the image carrier to the surface of a recording medium;
a fixing means for fixing the toner image transferred onto the surface of the recording medium;
An image forming apparatus comprising: a charging step of charging the surface of the image carrier;
an electrostatic image forming step of forming an electrostatic image on the charged surface of the image carrier;
a developing step of developing the electrostatic image formed on the surface of the image carrier into a toner image by using the electrostatic image developer according to claim 11 ;
a transfer step of transferring the toner image formed on the surface of the image carrier to the surface of a recording medium;
a fixing step of fixing the toner image transferred onto the surface of the recording medium;
An image forming method comprising the steps of: