Carrier Core Material, Electrophotographic Developer Using the Same, and Electrophotographic Developer

Abstract

To provide a carrier core material capable of suppressing toner spent, white spots in images, and carrier scattering even when image forming speed is fast or when used for a prolonged period of time.SOLUTION: A carrier core material provided herein is composed of ferrite particles and features a saturation magnetization σs of 75 Am2 / kg to 90 Am2 / kg, inclusive, a pore volume of 0.008 cm3 / g or more and less than 0.020 cm3 / g, a maximum height Rz of 1.4 μm to 2.0 μm, inclusive, and a particle deformity rate of 10 to 50%, inclusive, as measured by a predetermined measurement method.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a carrier core material, an electrophotographic developer using the same, and an electrophotographic developer.

Background Art

[0002] For example, in an image forming apparatus such as a facsimile machine, a printer, or a copying machine using an electrophotographic method, toner is attached to an electrostatic latent image formed on the surface of a photoreceptor to visualize it, and after this visible image is transferred to paper or the like, it is fixed by heating and pressurization. From the viewpoints of high image quality and colorization, as a developer, a so-called two-component developer containing a carrier and toner is widely used.

[0003] In a developing method using a two-component developer, the carrier and the toner are agitated and mixed in a developing device, and the toner is charged to a predetermined amount by friction. Then, the developer is supplied to a rotating developing roller, a magnetic brush is formed on the developing roller, and the toner moves electrically to the photoreceptor through the magnetic brush, and the electrostatic latent image on the photoreceptor is visualized. The carrier after toner movement is peeled off from the developing roller and mixed with the toner again in the developing device. Therefore, as characteristics of the carrier, movement characteristics to the developing roller, magnetic characteristics for forming a magnetic brush, charging characteristics for imparting a desired charge to the toner, and durability in repeated use are required.

[0004] As such a carrier, those in which the surface of magnetic particles such as magnetite and various ferrites is coated with a resin are generally used. The magnetic particles as the carrier core material are required to have good magnetic characteristics and good triboelectric charging characteristics with respect to the toner. Various shapes of carrier core materials satisfying such characteristics have been proposed.

[0005] For example, for the purpose of suppressing the peeling of the coating resin from the carrier core material due to long-term use, a carrier core material containing at least a specific amount of at least one of Sr (strontium) and Ca (calcium) and having a frequency of grains with an average length RSm of the grains appearing on the particle surface being less than or equal to a specific value has been proposed (Patent Document 1). Also, for the purpose of suppressing toner spent in which toner components adhere to the carrier surface, etc., it has also been proposed to set the interval Sm of the surface unevenness of the carrier core material to be less than or equal to a specific value and the surface roughness Ra to be greater than or equal to a predetermined value (Patent Document 2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] By the way, in recent years, in order to meet the market demand for higher image forming speeds in image forming apparatuses, there is a tendency to increase the rotational speed of the developing roller and increase the supply amount of the developer per unit time to the developing area.

[0008] However, when the supply amount of the developer to the developing area is increased with the increase in the image forming speed, the stress applied to the developer increases, and the peeling of the coating resin from the carrier core material and the wear of the coating resin are likely to occur. Also, toner spent is likely to occur.

[0009] When the coating resin peels off from the carrier core material or the coating resin wears, the carrier core material is exposed on the surface, and charge injection occurs from the exposed portion, causing carriers to adhere to the photoreceptor, which may result in so-called image blanking. In addition, when toner spending occurs on the carrier surface, the ability of the carrier to impart charge to the toner may decrease. And when the stress applied to the developer increases, cracks or chips may occur in the carrier core material.

[0010] To suppress the peeling of the coating resin from the carrier core material, it is conceivable to increase the pore volume of the carrier core material to strongly exert the anchor effect on the coating resin. However, increasing the pore volume of the carrier core material may reduce the magnetization per particle of the carrier core material, increasing the risk of carrier scattering.

[0011] Therefore, an object of the present invention is to provide a carrier core material that can suppress toner spending, image blanking, and carrier scattering even when the image formation speed is high or during long-term use, as well as an electrophotographic developer using the same and an electrophotographic developer.

Means for Solving the Problems

[0012] The carrier core material according to the present invention that achieves the above object is a carrier core material composed of ferrite particles, and the saturation magnetization σ s is 75 Am 2 / kg or more and 90 Am 2 / kg or less, the pore volume is 0.008 cm 3 / g or more and less than 0.020 cm 3 / g, the maximum height Rz is 1.4 μm or more and 2.0 μm or less, and the particle shape ratio measured by the following measurement method is in the range of 10% or more and 50% or less. (Measurement Method of Particle Shape Ratio) Measuring device: Injection type image analysis particle size distribution meter Measured sample amount: 0.07 g 9 cm of polyethylene glycol 400 3 was put into a screw tube bottle (capacity 9 cm 3 and the measurement was performed after dispersion. (Measurement Conditions) Spacer Thickness: 150 μm Sampling: 20% Analysis Type: Relative Measurement Measurement Quantity: 0.95 cm 3 Analysis: Dark Detection Threshold: 169 (Filling the hole) O-Roughness Filter: 0.5 Filter Conditions: ISO Area Diameter: Minimum 5, Maximum 100, Inner Range (Analysis Conditions) Analysis Filter Condition I: ISO Area Diameter: Minimum 25, Maximum 55, Inner Range Analysis Filter Condition II: ISO Area Diameter: Minimum 25, Maximum 55, Inner Range ISO Solidity: Minimum 0.98, Maximum 1, Outer Range Ell.Ratio: Minimum 0.8, Maximum 1, Inner Range The ratio of the number of particles counted under Analysis Filter Condition II to the number of particles counted under Analysis Filter Condition I is divided to calculate the irregularity ratio.

[0013] In the carrier core material having the above configuration, the average length RSm of the grains appearing on the particle surface is preferably 5.0 μm or more and 7.1 μm or less.

[0014] Also, in the carrier core material having the above configuration, the magnetization σ at a magnetic field of 79.58×10 3 A / m (1000 Oersteds) 1k is preferably 61 Am 2 / kg or more and 75 Am 2 / kg or less.

[0015] Further, according to the present invention, there is provided an electrophotographic developer carrier characterized in that the surface of the carrier core material described in any of the above is coated with a resin.

[0016] According to the present invention, there is also provided an electrophotographic developer containing the electrophotographic carrier and toner described above.

Advantages of the Invention

[0017] According to the carrier core material of the present invention, even when the image formation speed is high or during long-term use, toner spent, image blanking, and carrier scattering can be suppressed.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0019] (Carrier Core Material) A major feature of the carrier core material according to the present invention is that the saturation magnetization σ s , the pore volume, the maximum height Rz, and the particle shape irregularity rate measured by the above measurement method are simultaneously satisfied within specific ranges. More specifically, the saturation magnetization σ s and the pore volume being within specific ranges mainly suppress carrier scattering. Also, when the pore volume and the maximum height Rz are within specific ranges, the anchor effect is exerted, mainly suppressing the peeling of the coating resin due to long-term use. And further, when the maximum height Rz and the shape irregularity rate are within specific ranges, mainly the wear of the coating resin due to long-term use is suppressed. When the maximum height Rz and the shape irregularity rate are within specific ranges, mainly toner spent is suppressed.

[0020] The configuration of the present invention will be individually described below. First, in the present invention, the saturation magnetization σ of the carrier core material s is 75 Am2 90 Am / kg or more of 90Am 2 is in the range of 75 Am / kg or less. The saturation magnetization σ s is 75 Am 2 / kg or less. When the saturation magnetization σ is less than 75 Am / kg, carrier scattering is likely to occur. On the other hand, when the saturation magnetization σ s is 90 Am 2 / kg or more, the magnetic brush formed on the developing roller may become rough and the image quality may deteriorate. The preferable range of the saturation magnetization σ s is 80 Am 2 / kg or more and 90 Am 2 / kg or less.

[0021] Next, the pore volume of the carrier core material in the present invention is 0.008 cm 3 / g or more and 0.020 cm 3 / g or less. When the pore volume is less than 0.008 cm / g, the anchor effect may not be sufficiently obtained and the coating resin may peel off. On the other hand, when the pore volume is 0.020 cm 3 / g or more, the magnetization per particle decreases and carrier scattering is likely to occur. 3

[0022] The maximum height Rz of the carrier core material in the present invention is in the range of 1.4 μm or more and 2.0 μm or less. When the maximum height Rz is less than 1.4 μm, the anchor effect may not be sufficiently obtained and the coating resin may peel off. On the other hand, when the maximum height Rz exceeds 2.0 μm, the stress on the carrier surface increases and the coating resin may wear and image white spots may occur.

[0023] The irregularity rate of the carrier core material measured by the measurement method in the present invention is in the range of 10% or more and 50% or less. When the irregularity rate is less than 10%, it becomes difficult to scrape off the toner spent on the carrier surface, resulting in a decrease in the ability to impart charge to the toner and a possible deterioration in image quality. On the other hand, when the irregularity rate exceeds 50%, the coating resin may wear and image white spots may occur. The preferable range of the irregularity rate is 15% or more and 47% or less.

[0024] Next, the average length RSm of the carrier core material in the present invention is preferably in the range of 5.0 μm or more and 7.1 μm or less. When the average length RSm is within this range, the excessive sharpening of the grains is suppressed, the mechanical stress on the resin-coated carrier in the magnetic brush is reduced, and the wear of the resin-coated layer is likely to be suppressed. A more preferable range of the average length RSm is in the range of 6.0 μm or more and 7.1 μm or less.

[0025] Also, the magnetization σ of the carrier core material in the present invention 1k is preferably 61 A·m 2 / kg or more and 75 A·m 2 / kg or less. When the magnetization σ 1k is within this range, carrier scattering is likely to be suppressed and the developer is likely to be sufficiently transported to the development region. A more preferable range of the magnetization σ 1k is in the range of 64 A·m 2 / kg or more and 70 A·m 2 / kg or less.

[0026] The ferrite particles constituting the carrier core material of the present invention are preferably those mainly composed of a material represented by the composition formula (Mn x Fe 3-x )O4 (where 0 < x < 3). Here, the main component means a material contained in an amount exceeding 50% by mass. Further, with respect to the total number of moles of the particle composition, it is preferable that Si (silicon) is contained in an amount of 0.06 mol% or more and 0.20 mol% or less. If Si is less than 0.06 mol%, the particle shape may change with a small grain size, and the excessive sharpening of the grains may be promoted. On the other hand, if Si exceeds 0.20 mol%, the decomposition, melting, and diffusion of the particles may be promoted, and the spheroidization that is energetically stable may be excessively promoted. Usually, the Si content can be adjusted by the addition amount of the Si raw material, but it can also be adjusted by the addition amount of the mixed raw material in which the Si raw material and other raw materials are mixed. In the examples described later, a mixed raw material containing Fe and Si and a mixed raw material containing Mn and Si are used, and the Si content is adjusted by the addition amount of the mixed raw material. Note that the addition amount of the Si raw material when using the mixed raw material is calculated from the Si content rate of the mixed raw material.

[0027] In addition, the ferrite particles constituting the carrier core material of the present invention may contain Sr (strontium) in an amount of 0.01 mol% or more and 0.50 mol% or less, and further may contain Sn (tin) in an amount of 0.01 mol% or more and 0.50 mol% or less, based on the total number of moles of the particle composition. When Sr is contained in the above amount, Sr ferrite is partially generated in the firing process, and a magnetoplumbite-type crystal structure is formed, facilitating the formation of the uneven shape on the surface of the carrier core material. On the other hand, Sn has an inhibitory effect on sintering. When both Sr and Sn are contained in the above amount, the excessive deformation of the particles is suppressed compared to the case where only Sr is contained, and the desired unevenness is formed on the particle surface. A more preferable content range of Sr is 0.10 mol% or more and 0.50 mol% or less. Also, a more preferable content of Sn is in the range of 0.10 mol% or more and 0.50 mol% or less.

[0028] Furthermore, the ferrite particles constituting the carrier core material of the present invention may contain Ca (calcium) in an amount of 0.01 mol% or more and 0.60 mol% or less, based on the total number of moles of the particle composition. When Ca is contained in the above amount, the electrical properties, magnetic properties, and shape properties of the carrier core material are adjusted to the desired ranges. The preferable content range of Ca is 0.40 mol% or more and 0.60 mol% or less.

[0029] The volume average particle diameter of the carrier core material of the present invention is preferably in the range of 25 μm or more and less than 50 μm, and more preferably in the range of 30 μm or more and 40 μm or less.

[0030] The apparent density of the carrier core material of the present invention is preferably in the range of 2.00 g / cm 3 or more and 2.50 g / cm 3 or less, and more preferably in the range of 2.10 g / cm 3 or more and 2.40 g / cm 3 or less.

[0031] Also, the fluidity (sec / 50g) of the carrier core material of the present invention is preferably in the range of 25 or more and 40 or less, more preferably in the range of 28 or more and 40 or less.

[0032] (Method for manufacturing carrier core material) The manufacturing method of the carrier core material of the present invention is not particularly limited, but the manufacturing method described below is suitable.

[0033] First, the necessary component raw materials are weighed so as to have a desired composition. Also, if necessary, conventionally known additives are weighed. For example, as the Fe (iron) component raw material, Fe2O3 etc. are preferably used. As the Mn (manganese) component raw material, MnCO3, Mn3O4 etc. are used. Also, as the Sr component raw material, SrCO3, Sr(NO3)2 are used, as the Sn component raw material, SnO2, SnO are used, and as the Si component raw material, SiO2 etc. are preferably used. Note that some of the Fe component raw material and Mn component raw material contain a predetermined amount of Si. When using such Fe component raw material and Mn component raw material having an Si content, if the Si addition amount is sufficient with the Si amount contained in the Fe component raw material and Mn component raw material, the Si component raw material may not be blended. Conversely, when the Si addition amount is insufficient with the Si amount contained in the Fe component raw material and Mn component raw material, the Si component raw material is supplemented and blended.

[0034] Next, the raw materials are put into a dispersion medium to prepare a slurry. Water is preferable as the dispersion medium used in the present invention. In the dispersion medium, in addition to the calcined raw materials, a binder, a dispersant, etc. may be blended as necessary. As the binder, for example, polyvinyl alcohol can be preferably used. The blending amount of the binder is preferably such that the concentration in the slurry is about 0.1 mass% to 2 mass%. Further, as the dispersant, for example, ammonium polycarboxylate etc. can be preferably used. The blending amount of the dispersant is preferably such that the concentration in the slurry is about 0.1 mass% to 2 mass%. In addition, a reducing agent such as carbon black, a pH adjuster such as ammonia, a lubricant, a sintering accelerator, etc. may be blended. The solid content concentration of the slurry is desirably in the range of 50 mass% to 90 mass%. More preferably, it is 60 mass% to 80 mass%. If it is 60 mass% or more, there are few pores inside the particles in the granulated product, and insufficient sintering during firing can be prevented.

[0035] In addition, after weighing and mixing the raw materials, calcining them temporarily and deflocculating them, they may be put into a dispersion medium to prepare a slurry. The temperature of the temporary calcination is preferably in the range of 750°C to 1000°C. If it is 750°C or higher, partial ferrite formation due to the temporary calcination proceeds, the amount of gas generated during firing is small, and the solid-state reaction proceeds sufficiently, which is preferable. On the other hand, if it is 1000°C or lower, the sintering due to the temporary calcination is weak, and the raw materials can be sufficiently pulverized in the subsequent slurry pulverization process, which is preferable. Also, the atmosphere during the temporary calcination is preferably an air atmosphere.

[0036] Next, the slurry prepared as described above is wet-pulverized. For example, it is wet-pulverized for a predetermined time using a ball mill or a vibration mill. The volume average particle size D 50 showing 50% of the particle size in the cumulative particle size distribution of the pulverized raw materials is preferably 1.0 μm or less. Also, the volume particle size D 90The value is preferably 3.0 μm or less. For a vibration mill or a ball mill, it is preferable to incorporate media of a predetermined particle size. Examples of the material of the media include iron-based chromium steel and oxide-based zirconia, titania, alumina, and the like. The form of the pulverization process may be either continuous or batch. The particle size of the pulverized material is adjusted by factors such as the pulverization time, the rotation speed, the material and particle size of the media used.

[0037] The viscosity of the slurry is preferably 1000 cP or less, and more preferably 100 cP or less.

[0038] Then, the produced slurry is spray-dried to granulate. Specifically, the slurry is introduced into a spray dryer such as a spray dryer and sprayed into the atmosphere to granulate into a spherical shape. The atmosphere temperature during spray drying is preferably in the range of 100°C to 300°C. Thereby, spherical granulated products with a particle size of 10 μm to 200 μm are obtained. Next, if necessary, the obtained granulated products are classified using a vibrating sieve to produce granulated products within a predetermined particle size range. The apparent density AD of the granulated product is 1.40 g / cm 3 ~1.75 g / cm 3 The range is preferable, and 1.50 g / cm 3 ~1.58 g / cm 3 The range is more preferable. By setting the apparent density of the granulated product within the above range, it becomes easier to adjust the pore volume of the carrier core material to the specified range of the present invention. When the apparent density of the granulated product is small, the pore volume tends to be large, and when the apparent density of the granulated product is large, the pore volume tends to be small. The apparent density of the granulated product can be adjusted, for example, by the type (composition) of the raw material, the particle size of the slurry, the type and amount of the dispersant, and the like.

[0039] Next, the granulated product is put into a furnace heated to a predetermined temperature and fired by a general method for synthesizing ferrite particles to generate ferrite particles. The firing temperature is preferably in the range of 1100°C to 1350°C. When the firing temperature is 1100°C or lower, phase transformation is less likely to occur and sintering also progresses less easily. Also, when the firing temperature exceeds 1350°C, there is a risk of generating oversize grains due to over-sintering. The heating rate until reaching the firing temperature is preferably in the range of 250°C / h to 500°C / h. The holding time at the firing temperature is preferably 2 hours or more. The unevenness on the surface of the ferrite particles can also be adjusted by the oxygen concentration in the firing process. Specifically, the oxygen concentration during firing is set to 0.3 vol% to 1.2 vol%. Also, by making the oxygen concentration during cooling lower than the oxygen concentration during firing, the oxidation state of the ferrite phase may be adjusted. Specifically, it is preferable to control the oxygen concentration during firing in the range of 0.3 vol% to 1.2 vol% and the oxygen concentration during cooling in the range of 0.1 vol% to 0.8 vol%.

[0040] The fired product thus obtained is disintegrated if necessary. Specifically, for example, the fired product is disintegrated by a hammer mill or the like. The form of the disintegration process may be either continuous or batch. Also, after the disintegration treatment, classification may be performed as necessary to align the particle size within a predetermined range. As the classification method, conventionally known methods such as air classification and screening can be used. Also, after primary classification with an air classifier, the particle size may be aligned within a predetermined range using a vibrating screen or an ultrasonic screen. Further, after the classification step, non-magnetic particles may be removed by a magnetic separator. The volume average particle size of the ferrite particles is preferably 25 μm or more and less than 50 μm.

[0041] Thereafter, if necessary, the classified ferrite particles may be heated in an oxidizing atmosphere to form an oxide film on the particle surface to increase the resistance of the ferrite particles (resistance increasing treatment). The oxidizing atmosphere may be either an air atmosphere or a mixed atmosphere of oxygen and nitrogen. Further, the heating temperature is preferably in the range of 200°C or higher and 800°C or lower, and more preferably in the range of 350°C or higher and 550°C or lower. The heating time is preferably in the range of 0.5 hour or longer and 5 hours or shorter. From the viewpoint of homogenizing the surface and the interior of the ferrite particles, it is desirable that the heating temperature be low.

[0042] The ferrite particles produced as described above are used as the carrier core material of the present invention. Then, in order to obtain a desired chargeability or the like, the outer periphery of the carrier core material is coated with a resin to form an electrophotographic developer carrier.

[0043] (Electrophotographic developer carrier) As the resin for coating the surface of the carrier core material, conventionally known resins can be used. For example, polyethylene, polypropylene, polyvinyl chloride, poly-4-methylpentene-1, polyvinylidene chloride, ABS (acrylonitrile-butadiene-styrene) resin, polystyrene, (meth)acrylic resin, polyvinyl alcohol resin, and thermoplastic elastomers such as polyvinyl chloride-based, polyurethane-based, polyester-based, polyamide-based, polybutadiene-based, and fluorosilicone-based resins can be mentioned.

[0044] To coat the surface of the carrier core material with resin, a resin solution or dispersion may be applied to the carrier core material. As the solvent for the coating solution, one or more of the following can be used: aromatic hydrocarbon solvents such as toluene and xylene; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; cyclic ether solvents such as tetrahydrofuran and dioxane; alcohol solvents such as ethanol, propanol, and butanol; cellosolve solvents such as ethyl cellosolve and butyl cellosolve; ester solvents such as ethyl acetate and butyl acetate; amide solvents such as dimethylformamide and dimethylacetamide. The concentration of the resin component in the coating solution is generally preferably in the range of 0.001% by mass or more and 30% by mass or less, particularly 0.001% by mass or more and 2% by mass or less.

[0045] As a method for coating the resin on the carrier core material, for example, a spray drying method, a fluidized bed method, a spray drying method using a fluidized bed, an immersion method, etc. can be used. Among these, the fluidized bed method is particularly preferable in terms of being able to efficiently coat with a small amount of resin. The amount of resin coating can be adjusted, for example, in the case of the fluidized bed method, by the amount of the resin solution sprayed and the spraying time.

[0046] The particle size of the carrier is generally preferably in the range of 25 μm or more and less than 50 μm in terms of volume average particle size, particularly in the range of 30 μm or more and 40 μm or less.

[0047] (Electrophotographic developer) The electrophotographic developer according to the present invention is formed by mixing the carrier produced as described above and toner. There is no particular limitation on the mixing ratio of the carrier and the toner, and it may be appropriately determined according to the developing conditions of the developing device to be used. Generally, the toner concentration in the developer is preferably in the range of 1% by mass or more and 15% by mass or less. When the toner concentration is less than 1% by mass, the image density becomes too low. On the other hand, when the toner concentration exceeds 15% by mass, toner scattering may occur in the developing device, resulting in problems such as internal contamination of the machine and toner adhesion to the background portion of the transfer paper. A more preferable toner concentration is in the range of 3% by mass or more and 10% by mass or less.

[0048] As the toner, those manufactured by conventionally known methods such as polymerization method, pulverization classification method, melt granulation method, spray granulation method, etc. can be used. Specifically, those containing a colorant, a release agent, a charge control agent, etc. in a binder resin mainly composed of a thermoplastic resin can be preferably used.

[0049] Generally, the particle size of the toner is preferably in the range of 5 μm or more and 15 μm or less, more preferably in the range of 7 μm or more and 12 μm or less, in terms of volume average particle size measured by a Coulter counter.

[0050] If necessary, a modifier may be added to the toner surface. Examples of the modifier include silica, alumina, zinc oxide, titanium oxide, magnesium oxide, polymethyl methacrylate, etc. One or a combination of two or more of these can be used.

[0051] For mixing the carrier and the toner, conventionally known mixing devices can be used. For example, a Henschel mixer, a V-type mixer, a tumbler mixer, a hybridizer, etc. can be used.

[0052] Although there is no particular limitation on the developing method using the developer of the present invention, the magnetic brush developing method is preferable. Fig. 4 shows a schematic diagram showing an example of a developing device for performing magnetic brush development. The developing device shown in Fig. 4 includes a rotatable developing roller 3 having a plurality of magnetic poles, a regulating blade 6 for regulating the amount of developer on the developing roller 3 conveyed to the developing section, two screws 1 and 2 arranged in parallel in the horizontal direction and stirring and conveying the developer in opposite directions to each other, and a partition plate 4 formed between the two screws 1 and 2, enabling the movement of the developer from one screw to the other screw at both ends of both screws and preventing the movement of the developer except at both ends.

[0053] The two screws 1 and 2 are formed with spiral blades 13 and 23 having the same inclination angle on the shaft portions 11 and 21, and are rotated in the same direction by a drive mechanism (not shown) to convey the developer in opposite directions to each other. Then, the developer moves from one screw to the other screw at both ends of the screws 1 and 2. As a result, the developer composed of toner and carrier always circulates and is agitated in the apparatus.

[0054] On the other hand, the developing roller 3 has a fixed magnet in which five magnetic poles, namely a developing magnetic pole N1, a conveying magnetic pole S1, a peeling magnetic pole N2, a pumping magnetic pole N3, and a blade magnetic pole S2, are arranged in order as magnetic pole generating means inside a metal cylindrical body having irregularities of several micrometers on its surface. When the cylindrical body of the developing roller 3 rotates in the direction of the arrow, the developer is pumped from the screw 1 to the developing roller 3 by the magnetic force of the pumping magnetic pole N3. The developer carried on the surface of the developing roller 3 is layer-regulated by the regulating blade 6 and then conveyed to the developing area.

[0055] In the developing area, a bias voltage obtained by superimposing an AC voltage on a DC voltage is applied from the transfer voltage power source 8 to the developing roller 3. The DC voltage component of the bias voltage is set to a potential between the background potential and the image potential on the surface of the photosensitive drum 5. Also, the background potential and the image potential are set to potentials between the maximum value and the minimum value of the bias voltage. The peak-to-peak voltage of the bias voltage is preferably in the range of 0.5 kV to 5 kV, and the frequency is preferably in the range of 1 kHz to 10 kHz. Also, the waveform of the bias voltage may be any of a rectangular wave, a sine wave, a triangular wave, etc. Thereby, the toner and the carrier vibrate in the developing area, and the toner adheres to the electrostatic latent image on the photosensitive drum 5 to perform development.

[0056] Thereafter, the developer on the developing roller 3 is conveyed into the apparatus by the conveying magnetic pole S1, peeled from the developing roller 3 by the peeling electrode N2, and recirculated and conveyed in the apparatus by the screws 1 and 2, and is mixed and agitated with the developer that has not been used for development. Then, a new developer is supplied from the screw 1 to the developing roller 3 by the pumping pole N3.

[0057] In the embodiment shown in FIG. 4, there were five magnetic poles built into the developing roller 3. However, in order to further increase the amount of movement of the developer in the developing area or to further improve the pumping property or the like, the number of magnetic poles may of course be increased to 8 poles, 10 poles, or 12 poles.

Example

[0058] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.

[0059] (Example 1) As raw materials, 43.55 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), 8.17 kg of Mn3O4 (average particle size: 3.4 μm), 8.33 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55% by mass of SiO2), 332.9 g of SrCO3 (average particle size: 0.6 μm), and 273.4 g of SnO2 (average particle size: 5.6 μm) were dispersed in 20.48 kg of pure water. 180.1 g of carbon black was added as a reducing agent, 366.3 g of a polycarboxylic acid ammonium-based dispersant was added as a dispersant, and 42.8 g of 25% by mass aqueous ammonia was added as a pH adjuster to obtain a mixture. At this time, the Si in the mixture was adjusted so as to be 0.17 mol% with respect to the total of Fe, Mn, Sr, Sn, and Si. This mixture was pulverized by a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210° C. using a spray dryer to obtain a dried granulated product having a particle size of 10 μm to 75 μm. Fine particles having a particle size of 25 μm or less and coarse particles having a particle size of 54 μm or more were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1110° C. at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, firing was performed by holding at 1110° C. with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. Then, it was cooled at an oxygen concentration of 0.4 vol% over 6 hours. The obtained fired product was granulated using a hammer mill (HAMMER CRUSHER NH-34S manufactured by Sanjo Industries Co., Ltd., screen opening: 0.3 mm), classified using a vibrating sieve, and fired particles with a volume average particle size of 34.9 μm were obtained. The obtained fired particles were subjected to oxidation treatment by holding them at 400 °C in the atmosphere for 1 hour to obtain the carrier core material according to Example 1. The apparent density, fluidity, volume average particle size (average particle size), magnetic properties, static electrical resistance, pore volume, true density, maximum height Rz, average length RSm, and irregularity rate of the obtained carrier core material were measured by the methods shown below, and image whiteout, carrier scattering, and toner spent were evaluated according to the criteria shown below. The physical properties of the carrier core materials according to the following Examples and Comparative Examples were also measured by the same method and evaluated according to the same criteria. Also, an SEM photograph of the carrier core material of Example 1 is shown in Fig. 1. The length of the white line in the lower right of Fig. 1 is 10 μm (the same applies to Figs. 2 and 3).

[0060] (Example 2) As raw materials, 37.76 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2) and 14.26 kg of Mn3O4 (average particle size: 3.4 μm) were dispersed in 17.37 kg of pure water, 379.8 g of a polycarboxylic acid ammonium-based dispersant was added as a dispersant, and 36.4 g of 25% by mass aqueous ammonia was added as a pH adjuster to obtain a mixture. At this time, the Si in the mixture was adjusted so as to be 0.08 mol% with respect to the total of Fe, Mn, and Si. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1170 °C at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, firing was performed by holding at 1170 °C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. Then, it was cooled at an oxygen concentration of 0.4 vol% over 6 hours. The obtained fired product was granulated with a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industry Co., Ltd., screen opening: 0.3 mm), classified using a vibrating sieve, and fired particles with a volume average particle size of 35.5 μm were obtained. The obtained fired particles were subjected to an oxidation treatment by holding them at a temperature of 350 °C in the air for 1 hour to obtain the carrier core material according to Example 2.

[0061] (Example 3) As raw materials, 37.76 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), 14.26 kg of Mn3O4 (average particle size: 3.4 μm), and 370.1 g of CaCO3 (average particle size: 1.1 μm) were dispersed in 17.49 kg of pure water. 379.8 g of a polycarboxylic acid ammonium-based dispersant was added as a dispersant, and 36.4 g of 25% by mass aqueous ammonia was added as a pH adjuster to form a mixture. At this time, the Si in the mixture was adjusted so that it was 0.08 mol% with respect to the total of Fe, Mn, Ca, and Si. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Minute particles with a particle size of 25 μm or less were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1270 °C at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, firing was performed by holding it at 1270 °C at an oxygen concentration of 0.4 vol% to 1.0 vol% for 3 hours. Then, it was cooled at an oxygen concentration of 0.4 vol% over 6 hours. The obtained fired product was granulated with a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industry Co., Ltd., screen opening: 0.3 mm), classified using a vibrating sieve, and fired particles with a volume average particle size of 36.0 μm were obtained. The obtained fired particles were subjected to an oxidation treatment by holding them at a temperature of 440 °C in the air for 1 hour to obtain the carrier core material according to Example 3.

[0062] (Example 4) As raw materials, 37.76 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), 14.26 kg of Mn3O4 (average particle size: 3.4 μm), and 370.1 g of CaCO3 (average particle size: 1.1 μm) were dispersed in 17.49 kg of pure water. 379.8 g of an ammonium polycarboxylate-based dispersant was added as a dispersant, and 36.4 g of 25% by mass aqueous ammonia was added as a pH adjuster to form a mixture. At this time, the Si in the mixture was adjusted so that it was 0.08 mol% with respect to the total of Fe, Mn, Ca, and Si. This mixture was pulverized by a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1240 °C at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, firing was carried out by holding at 1240 °C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. Thereafter, it was cooled at an oxygen concentration of 0.4 vol% over 6 hours. The obtained fired product was disintegrated with a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industries Co., Ltd., screen opening: 0.3 mm) and classified using a vibrating sieve to obtain a carrier core material according to Example 4 with a volume average particle size of 36.0 μm.

[0063] (Comparative Example 1) A granulated product was obtained in the same procedure as in Example 1 except that coarse particles with a particle size of 54 μm or more were not removed by a sieve. The obtained granulated product was heated to 1300 °C at an oxygen concentration of 1.0 vol% over 4.5 hours, then fired by holding at 1300 °C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours, and cooled at an oxygen concentration of 0.4 vol% over 6 hours in the same manner as in Example 1 to obtain fired particles according to Comparative Example 1 with a volume average particle size of 35.4 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 445 °C in the air for 1 hour to obtain a carrier core material according to Comparative Example 1.

[0064] (Comparative Example 2) A granulated product was obtained in the same procedure as in Example 1, except that coarse particles with a particle size of 54 μm or more were not removed by sieving. The obtained granulated product was heated from an oxygen concentration of 1.0 vol% to 1250 °C over 4.5 hours, and then fired by holding at 1250 °C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours, and cooled over 6 hours with an oxygen concentration of 0.4 vol%. Except for this, fired particles according to Comparative Example 2 with a volume average particle size of 34.9 μm were obtained in the same manner as in Example 1. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 440 °C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 2.

[0065] (Comparative Example 3) A granulated product was obtained in the same procedure as in Example 1, except that coarse particles with a particle size of 54 μm or more were not removed by sieving. The obtained granulated product was heated from an oxygen concentration of 1.0 vol% to 1200 °C over 4.5 hours, and then fired by holding at 1200 °C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours, and cooled over 6 hours with an oxygen concentration of 0.4 vol%. Except for this, fired particles according to Comparative Example 3 with a volume average particle size of 35.7 μm were obtained in the same manner as in Example 1. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 450 °C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 3.

[0066] (Comparative Example 4) As raw materials, 28.94 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08 mass% of SiO2) and 11.30 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55 mass% of SiO2) were dispersed in 13.62 kg of pure water, 111.6 g of carbon black was added as a reducing agent, and 250.8 g of a polycarboxylic acid ammonium-based dispersant was added as a dispersant to form a mixture. At this time, the mixture was adjusted so that Si was 0.28 mol%. This mixture was pulverized by a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210°C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1200°C at an oxygen concentration of 0.5 vol% over 4.5 hours. Then, firing was carried out by holding at 1200°C with an oxygen concentration of 0.5 vol% for 2.5 hours. After that, it was cooled at an oxygen concentration of 0.5 vol% over 6 hours. The obtained fired product was disintegrated using a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industries, screen opening: 0.3 mm) and classified using a vibrating sieve to obtain fired particles with a volume average particle size of 34.4 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 425°C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 4.

[0067] (Comparative Example 5) As raw materials, 23.23 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08 mass% of SiO2), 8.71 kg of Mn3O4 (average particle size: 3.4 μm), and 107.7 g of CaCO3 (average particle size: 1.1 μm) were dispersed in 10.82 kg of pure water. 95.8 g of carbon black was used as a reducing agent, 194.8 g of a polycarboxylic acid ammonium-based dispersant was used as a dispersant, and 22.6 g of 25 mass% ammonia water was used as a pH adjuster and added to form a mixture. At this time, the mixture was adjusted so that Si was 0.08 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210°C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1300°C at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, firing was carried out by holding at 1300°C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. After that, it was cooled at an oxygen concentration of 0.4 vol% over 6 hours. The obtained fired product was granulated using a hammer mill (HAMMER CRUSHER NH-34S manufactured by Sanjo Industry Co., Ltd., screen opening: 0.3 mm), classified using a vibrating sieve, and fired particles with a volume average particle size of 36.0 μm were obtained. The obtained fired particles were subjected to an oxidation treatment by holding them at 460°C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 5.

[0068] (Comparative Example 6) As raw materials, 14.34 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), 4.62 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55% by mass of SiO2), and 1.04 kg of MgO were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were calcined in a rotary kiln at 850°C under an air atmosphere. They were pulverized for 6 hours using a dry bead mill to obtain a calcined powder. The obtained calcined powder and 149.5 g of CaCO3 (average particle size: 1.1 μm) were dispersed in 7.12 kg of pure water, and 219.7 g of a 21% aqueous solution containing a methacrylic acid-based polymer was added as a dispersant to form a mixture. At this time, the mixture was adjusted so that the Si content was 0.23 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210°C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from this granulated product using a sieve. This granulated product was charged into an electric furnace and heated from room temperature to 1150°C at an oxygen concentration of 1.6 vol% over 4.5 hours. Then, firing was performed by holding at 1300°C with an oxygen concentration of 1.6 vol% to 0.64 vol% for 3 hours. Then, it was cooled at an oxygen concentration of 0.64 vol% over 6 hours. The obtained fired product was granulated using a hammer mill (HAMMER CRUSHER NH-34S manufactured by Sanjo Industry Co., Ltd., screen opening: 0.3 mm), classified using a vibrating sieve, and a carrier core material according to Comparative Example 6 with a volume average particle size of 35.4 μm was obtained.

[0069] (Comparative Example 7) As raw materials, 14.52 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), 5.44 kg of Mn3O4 (average particle size: 3.4 μm), 111.4 g of SrCO3 (average particle size: 0.6 μm), and 91.0 g of SnO2 (average particle size: 5.6 μm) were dispersed in 6.90 kg of pure water. 60.5 g of carbon black was added as a reducing agent, 121.0 g of an ammonium polycarboxylate-based dispersant was added as a dispersant, and 14.0 g of 25% by mass ammonia water was added as a pH adjuster to form a mixture. At this time, the mixture was adjusted so that Si was 0.08 mol%. This mixture was pulverized by a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1230 °C at an oxygen concentration of 1.0 vol% over 4.5 hours. Then, firing was carried out by holding at 1230 °C with an oxygen concentration of 1.0 vol% to 0.4 vol% for 3 hours. Thereafter, it was cooled at an oxygen concentration of 0.4 vol% over 6 hours. The obtained fired product was disintegrated using a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industries Co., Ltd., screen opening: 0.3 mm) and classified using a vibrating sieve to obtain fired particles with a volume average particle size of 35.3 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 450 °C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 7.

[0070] (Comparative Example 8) As raw materials, 20.86 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), 8.57 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55% by mass of SiO2), 0.74 kg of MgO, and 131.0 g of CaCO3 were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were calcined preliminarily in a rotary kiln at 850 °C under atmospheric conditions. They were pulverized for 6 hours using a dry bead mill to obtain a preliminarily calcined powder. The obtained calcined powder was dispersed in 10.55 kg of pure water, and 122.0 g of carbon black as a reducing agent and 180.0 g of an ammonium polycarboxylate-based dispersant as a dispersant were added to form a mixture. At this time, the mixture was adjusted so that the Si content was 0.27 mol%. This mixture was pulverized by a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 33 μm or less and coarse particles with a particle size of 43 μm or more were removed from this granulated product using a sieve. This granulated product was charged into an electric furnace and heated to 1110 °C at an oxygen concentration of 0.5 vol% over 3 hours. Then, firing was performed by holding at 1300 °C at an oxygen concentration of 0.5 vol% to 0.65 vol% for 3 hours. Thereafter, it was cooled at an oxygen concentration of 0.65 vol% over 6 hours. The obtained fired product was disintegrated using a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industry Co., Ltd., screen opening: 0.3 mm) and classified using a vibrating sieve to obtain fired particles with a volume average particle size of 33.0 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 385 °C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 8. An SEM photograph of the carrier core material of Comparative Example 8 is shown in Figure 2.

[0071] (Comparative Example 9) As raw materials, 20.13 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08 mass% of SiO2), 3.47 kg of Mn3O4 (average particle size: 3.4 μm), 6.45 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55 mass% of SiO2), and 220.1 g of SrCO3 (average particle size: 0.6 μm) were dispersed in 7.43 kg of pure water, and 180.0 g of an ammonium polycarboxylate-based dispersant as a dispersant, 180.0 g of 25 mass% aqueous ammonia as a pH adjuster, and 6.25 g of colloidal silica with a solid content of 48 wt% were added to form a mixture. At this time, the mixture was adjusted so that the Si content was 0.28 mol%. This mixture was pulverized by a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 90 μm. Fine particles with a particle size of 37 μm or less and coarse particles with a particle size of 50 μm or more were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated to 1110 °C at an oxygen concentration of 2.50 vol% over 3 hours. Then, firing was carried out by holding at 1110 °C for 3 hours at an oxygen concentration of 2.50 vol% to 1.25 vol%. Then, it was cooled at an oxygen concentration of 1.25 vol% over 6 hours. The obtained fired product was disintegrated using a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industries, screen opening: 0.3 mm) and classified using a vibrating sieve to obtain fired particles with a volume average particle size of 37.3 μm. The obtained fired particles were subjected to an oxidation treatment by holding at a temperature of 405 °C in the atmosphere for 1 hour to obtain a carrier core material according to Comparative Example 9. An SEM photograph of the carrier core material of Comparative Example 9 is shown in Figure 3.

[0072] (Comparative Example 10) As raw materials, 7.98 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08 mass% of SiO2) and 2.02 kg of MgO were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were calcined preliminarily in a rotary kiln at 850 °C under atmospheric conditions. They were pulverized for 6 hours using a dry bead mill to obtain a preliminarily calcined powder. The obtained preliminarily calcined powder, 18.62 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08 mass% of SiO2), 9.08 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55 mass% of SiO2), and 203.6 g of CaCO3 (average particle size: 1.1 μm) were dispersed in 12.70 kg of pure water, and 188.0 g of a polycarboxylic acid ammonium-based dispersant was added as a dispersant to form a mixture. At this time, the mixture was adjusted so that the Si was 0.21 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated up to 1210 °C over 4.5 hours while waiting. Then, firing was carried out by holding at 1210 °C for 4 hours while waiting. After that, it was cooled in the atmosphere over 8 hours. The obtained fired product was disintegrated using a hammer mill (Hammer Crusher NH-34S manufactured by Sanjo Industries Co., Ltd., screen opening: 0.3 mm) and classified using a vibrating sieve to obtain a carrier core material according to Comparative Example 10 with a volume average particle size of 36.0 μm.

[0073] (Comparative Example 11) As raw materials, 9.58 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2) and 2.42 kg of MgO were mixed. This mixture was pelletized using a roller compactor. The obtained pellets were calcined preliminarily in a rotary kiln at 850 °C under atmospheric conditions. It was pulverized for 6 hours using a dry bead mill to obtain a preliminarily calcined powder. The obtained preliminarily calcined powder, 22.33 kg of Fe2O3 (average particle size: 0.6 μm, containing 0.08% by mass of SiO2), and 10.89 kg of Mn3O4 (average particle size: 2.1 μm, containing 0.55% by mass of SiO2) were dispersed in 15.15 kg of pure water, and 226.7 g of a polycarboxylic acid ammonium-based dispersant was added as a dispersant to form a mixture. At this time, it was adjusted so that Si in the mixture was 0.21 mol%. This mixture was pulverized using a wet ball mill (media diameter 3 mm) to obtain a mixed slurry. This mixed slurry was sprayed into hot air at about 210 °C using a spray dryer to obtain a dried granulated product with a particle size of 10 μm to 75 μm. Fine particles with a particle size of 25 μm or less and coarse particles with a particle size of 54 μm or more were removed from this granulated product using a sieve. This granulated product was put into an electric furnace and heated up to 1210 °C over 4.5 hours while waiting. Then, firing was carried out by holding at 1210 °C for 4 hours while waiting. After that, it was cooled in the atmosphere over 8 hours. The obtained fired product was granulated using a hammer mill (HAMMER CRUSHER NH-34S manufactured by Sanjo Industries Co., Ltd., screen opening: 0.3 mm), classified using a vibrating sieve, and a carrier core material according to Comparative Example 11 with a volume average particle size of 35.6 μm was obtained.

[0074] (Apparent density, AD) The apparent densities of the carrier core material and the granulated product were measured in accordance with JIS Z 2504.

[0075] (Flowability) The flowability of the carrier core material was measured in accordance with JIS Z 2502.

[0076] (Volume average particle size (average particle size) and ratio of particles with a particle size of 22 μm or less) The volume average particle size and the ratio of particles with a particle size of 22 μm or less of the carrier core material were measured using a laser diffraction particle size distribution analyzer (MICROTRAC Model9320-X100 manufactured by Nikkiso Co., Ltd.).

[0077] (Magnetic properties) Using a room-temperature dedicated vibrating sample magnetometer (VSM) (VSM-P7 manufactured by Toei Industry Co., Ltd.), an external magnetic field was continuously applied in the range of 0 to 79.58×10 4 A / m (10000 Oe) for one cycle to measure the saturation magnetization σ s , the residual magnetization σ r , the coercive force H c and the magnetization σ 3 at a magnetic field of 79.58×10 1k (A / m 2 / kg), respectively.

[0078] (Static electrical resistance) Two brass plates with a thickness of 2 mm and an electrolytically polished surface were used as electrodes and arranged so that the distance between the electrodes was 2 mm. After loading 200 mg of the carrier core material into the gap between the two electrode plates, a cross-sectional area of 240 mm 2With the magnet placed to form a bridge of the powder to be measured between the electrodes, DC voltages of 100V, 500V, and 1000V were applied between the electrodes, and the current value flowing through the carrier core material was measured by the four-terminal method to obtain the resistance value.

[0079] (Pore volume) The evaluation apparatus used POREMASTER-60GT manufactured by Quantachrome Corporation. Specifically, as the measurement conditions, Cell Stem Volume: 0.5 cm 3 、 Headpressure: 20 PSIA, Surface tension of mercury: 485.00 erg / cm 2 、 Contact angle of mercury: 130.00 degrees, High-pressure measurement mode: Fixed Rate, Moter Speed: 1, High-pressure measurement range: 20.00~10000.00 PSI were set. 1.500 g of the sample was weighed and filled into a 0.5 cm 3 cell for measurement. Also, the value obtained by subtracting the volume A (cm 3 / g) at 60 PSI from the volume B (cm 3 / g) at 10000 PSI was taken as the pore volume.

[0080] (True density) The true density of the carrier core material was measured using "ULTRA PYCNOMETER 1000" manufactured by Quantachrome Corporation.

[0081] (Measurement of maximum height Rz and average length RSm) The surface was observed using a super-depth color 3D shape measurement microscope (manufactured by Keyence Corporation, model "VK-X100") with a 100x objective lens. Specifically, first, the carrier core material was fixed to a flat adhesive tape on the surface. After determining the measurement field of view with a 100x objective lens, the focus was adjusted to the adhesive tape surface using the autofocus function. A laser beam was irradiated onto the flat adhesive tape surface to which the carrier core material was fixed, and scanning was performed in the X and Y directions of the surface. Also, Z-direction data was obtained by connecting the height positions of the lens when the intensity of the reflected light from the surface was maximized. By connecting these position data in the X, Y, and Z directions, the three-dimensional shape of the carrier core material surface was obtained. Note that the autofocus shooting function was used to capture the three-dimensional shape of the carrier core material surface. For the measurement of each parameter, particle roughness inspection software (manufactured by Mitani Shoko Co., Ltd.) was used. First, as a pre-treatment, particle recognition and shape selection of the three-dimensional shape of the obtained carrier core material surface were performed. Particle recognition was carried out by the following method. Among the three-dimensional shapes obtained by shooting, the maximum value in the Z direction was set as 100%, the minimum value was set as 0%, and the range between the maximum value and the minimum value was divided into 100 equal parts. The region corresponding to 100 - 35% was extracted, and the contour of the independent region was recognized as the particle contour. Next, in shape selection, particles such as coarse, fine, and aggregated particles were excluded. By performing this shape selection, the error during the subsequent extreme ratio correction can be reduced. Specifically, particles corresponding to an equivalent area diameter of 28 μm or less, 38 μm or more, and an aspect ratio of 1.15 or more were excluded. Here, the aspect ratio is a parameter calculated from the ratio of the maximum length of the particle / diagonal width, and the diagonal width represents the shortest distance between two straight lines when the particle is sandwiched by two straight lines parallel to the maximum length. Next, the part to be used for analysis was extracted from the three-dimensional shape of the surface. First, a square with a side length of 15.0 μm was drawn with the center of gravity obtained from the particle contour recognized by the above method as the center. Twenty-one parallel lines were drawn in the drawn square, and twenty-one roughness curves corresponding to those line segments were extracted.

[0082] Since the carrier core material is approximately spherical, the extracted roughness curve has a certain curvature as the background. Therefore, as a background correction, an optimal quadratic curve was fitted and subtracted from the roughness curve. In this case, a low-pass filter was applied at an intensity of 1.5 μm, and the cut-off value λ was set to 80 μm.

[0083] The maximum height Rz was determined as the sum of the height of the highest peak and the depth of the deepest valley in the roughness curve. For the calculation of the maximum height Rz, the average value of 50 particles was used as the average value of each parameter.

[0084] The average length RSm is defined as one element by combining valleys and peaks in the roughness curve, and is the average of the lengths of each element. For the calculation of the average length RSm, the average value of 50 particles was used as the average value of each parameter.

[0085] The measurement of the maximum height Rz and the average length RSm described above is carried out in accordance with JIS B0601 (2001 edition).

[0086] (Irregularity ratio) An injection-type image analysis particle size distribution meter (Jasco International Co., Ltd., model: IF-3200) was used as the evaluation device. Specifically, 0.07 g of the sample was weighed, and polyethylene glycol 400 was added to 9 cm 3 into a screw tube bottle (capacity 9 cm 3 ) and measured after dispersion. (Measurement conditions) Spacer thickness: 150 μm Sampling: 20% Analysis type: Relative measurement Measurement amount: 0.95 cm 3 Analysis: Dark detection Threshold value: 169 (fill holes) O-Roughness filter: 0.5 Note that O-Roughness = the ratio of the part removed until the surface becomes smooth Filter conditions: ISO Area Diameter: Minimum value 5, maximum value 100, inner range Note that ISO Area Diameter is the diameter of a circle with a pixel image equal to the particle projected area (Analysis conditions) Analysis filter condition I: ISO Circularity: Minimum value 25, maximum value 55, inner range Analysis filter condition II: ISO Circularity: Minimum value 25, maximum value 55, inner range ISO Solidity: Minimum value 0.98, maximum value 1, outer range Note that ISO Solidity is the ratio of the particle area to the area of the convex hull surrounding the particle Ell.Ratio: Minimum value 0.8, maximum value 1, inner range Note that Ell.Ratio is the ratio of the minor axis to the major axis of the inertia ellipse The ratio of the number of particles counted under analysis filter condition II to the number of particles counted under analysis filter condition I was divided to calculate the irregularity rate, which is the ratio of irregular particles

[0087] (Viscosity) The slurry viscosity was measured using a BROOKFIELD digital viscometer DV-I Prime LV

[0088] (Evaluation of image blanking) A two-component developer prepared on a developing device with the structure shown in Fig. 4 (peripheral speed v1 of the developing roller: 406 mm / sec, peripheral speed v2 of the photosensitive drum: 205 mm / sec, distance between the photosensitive drum and the developing roller: 0.3 mm) was introduced, and after driving the developing device for a time equivalent to printing 100,000 sheets, a solid black image was printed, and the degree of blanking in the solid black part was visually evaluated according to the following criteria 「◎」: No blanking was confirmed, and the image was good 「○」: Less than 5 blanks 「△」: 5 to 10 blanks 「×」: Clearly more than 10 blanks were present

[0089] (Evaluation of Carrier Scattering) A two-component developer was put into a developing device having the structure shown in Fig. 4 (peripheral speed v1 of the developing roller: 406 mm / sec, peripheral speed v2 of the photosensitive drum: 205 mm / sec, distance between the photosensitive drum and the developing roller: 0.3 mm). Ten white paper originals were developed, and the number of carriers adhering to the surface of the photosensitive body was counted in 5 fields of view by loupe observation, and the number of carriers adhering per 100 cm 2 on average was taken as the carrier adhesion. This evaluation was carried out after driving the developing device for a time corresponding to the time required for printing 100,000 sheets of A4 vertical paper (time corresponding to 100,000-sheet printing). “◎”: Less than 10 “○”: 10 or more and less than 30 “△”: 30 or more and less than 50 “×”: 50 or more

[0090] (Toner Spent) After stirring the developer for 36 hours, the carrier was extracted from the developer and observed with a scanning electron microscope (JSM-6510LA type, manufactured by JEOL Ltd.), and the percentage of the number of carriers with toner fused on the surface was measured. “◎”: The percentage of the number of carriers with toner fused was less than 0.5%. “○”: The percentage of the number of carriers with toner fused was 0.5 or more and less than 1.0%. “△”: The percentage of the number of carriers with toner fused was 1.0 or more and less than 5.0%. “×”: The percentage of the number of carriers with toner fused was 5.0% or more.

[0091]

Table 1

[0092]

Table 2

[0093] As shown in Table 2, in the developers using the carrier core materials of Examples 1 to 3 that satisfy the configuration defined in the present invention, there were no problems in actual use in each evaluation of image blanking, carrier scattering, and toner spent.

[0094] On the other hand, in the developers using the carrier core materials of Comparative Examples 1 and 2, where the pore volume is smaller, the maximum height Rz is larger, and the shape factor is larger than the defined range of the present invention, image blanking occurred. Also, in the developers using the carrier core materials of Comparative Examples 3 and 5, where the pore volume is smaller and the shape factor is larger than the defined range of the present invention, and in the developer using the carrier core material of Comparative Example 4, where the pore volume is small, image blanking also occurred.

[0095] In the developer using the carrier core material of Comparative Example 6, where the saturation magnetization σ s is smaller than the defined range of the present invention, carrier scattering occurred. Also, in the developer using the carrier core material of Comparative Example 7, where the pore volume is larger than the defined range of the present invention, carrier scattering occurred.

[0096] In the developer using the carrier core material of Comparative Example 8, where the saturation magnetization σ s is smaller and the shape factor is smaller than the defined range of the present invention, carrier scattering and toner spent occurred. In the developer using the carrier core material of Comparative Example 9, carrier scattering was not a problem in actual use, but toner spent, which is a problem in actual use, occurred.

[0097] In the developer using the carrier core material of Comparative Example 9, where the saturation magnetization σ s and the pore volume are small and the shape factor is large, and in the developer using the carrier core material of Comparative Example 9, where the saturation magnetization σ s and the pore volume are small, image blanking and carrier scattering occurred in all cases.

Industrial Applicability

[0098] According to the carrier core material of the present invention, toner spent, image blanking, and carrier scattering can be suppressed even when the image formation speed is high or during long-term use.

Description of Symbols

[0099] 3 Developing roller 5 Photoconductor drum

Claims

1. A carrier core material composed of ferrite particles, wherein the ferrite particles are composed of a material represented by the composition formula (MnxFe3−x)O4 (where 0 < x < 3). Si is contained in an amount of 0.06 mol% or more and 0.20 mol% or less based on the total number of moles of the composition of the ferrite particles, Saturation magnetization σ ｓ is 75 A·m ２ / kg or more and 90 A·m ２ / kg or less, and The pore volume is 0.008 cm ３ / g or more and less than 0.020 cm ３ / g, and the maximum height Rz is 1.4 μm or more and 2.0 μm or less, and the irregularity ratio of the particles measured by the following measurement method is in the range of 10% or more and 50% or less A carrier core material characterized by the above. (Measurement method for the irregularity ratio of particles) Measuring device: Injection type image analysis particle size distribution meter Measured sample amount: 0.07 g 9 cm of polyethylene glycol 400 ３ was put into a screw tube bottle (capacity 9 cm ３ and measurements were taken after dispersion therein). (Measurement conditions) Spacer thickness: 150 μm Sampling: 20% Analysis type: Relative measurement Measured quantity: 0.95 cm ３ Analysis: Dark detection Threshold value: 169 (filling holes) O-Roughness filter: 0.5 Filter conditions: ISO Area Diameter: Minimum value 5, maximum value 100, inner range (Analysis conditions) Analysis filter condition I: ISO Area Diameter: Minimum value 25, maximum value 55, inner range Analysis filter condition II: ISO Area Diameter: Minimum value 25, maximum value 55, inner range ISO Solidarity: Minimum value 0.98, maximum value 1, outer range Ell. Ratio: Minimum value 0.8, maximum value 1, inner range The irregularity ratio is calculated by dividing the number of particles counted under analysis filter condition II by the number of particles counted under analysis filter condition I.

2. The carrier core material according to claim 1, wherein the average length RSm of the grains appearing on the particle surface is 5.0 μm or more and 7.1 μm or less.

3. Magnetic field 79.58×10 ３ Magnetization σ at A / m (1000 Oersteds) １ｋ is 61 Am ２ / kg or more and 75 Am ２ / kg or less, the carrier core material according to claim 1 or 2

4. An electrophotographic developer characterized in that the surface of the carrier core material according to any one of claims 1 to 3 is coated with a resin.

5. An electrophotographic developer comprising the electrophotographic carrier according to claim 4 and toner.

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