JP2016189024A - Resin-coated carrier for electrophotographic developer, and electrophotographic developer using electrophotographic developer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin-coated carrier for an electrophotographic developer, in which carrier deposition is prevented while reduction in weight of the carrier is achieved, and the carrier particles have high fracture strength and show stabilized charging characteristics in long-term printing, and to provide an electrophotographic developer using the resin-coated carrier.SOLUTION: There are provided a non-resin-filled type resin-coated carrier for an electrophotographic developer having a resin-coated layer on a surface of a porous ferrite core material, where the porous ferrite core material has a pore volume of 55-160 mm/g and a peak pore size of 0.2-0.7 μm, and the carrier has a carrier strength which is a change rate of a volume average particle size before and after stirring and is represented by formula (1) of less than 5.0%; and an electrophotographic developer using the resin-coated carrier.SELECTED DRAWING: None

Description

  The present invention relates to a resin-coated carrier for an electrophotographic developer used for a two-component electrophotographic developer used in a copying machine, a printer, and the like, and an electrophotographic developer using the resin-coated carrier.

  The electrophotographic development method is a method in which toner particles in a developer are attached to an electrostatic latent image formed on a photoreceptor and developed, and the developer used in this method is composed of toner particles and carrier particles. The two-component developer and the one-component developer using only toner particles.

  Among these developers, as a developing method using a two-component developer composed of toner particles and carrier particles, the cascade method has been used in the past, but at present, the magnetic brush method using a magnet roll is the mainstream. It is.

  In the two-component developer, the carrier particles are agitated together with the toner particles in the developing box filled with the developer, thereby imparting a desired charge to the toner particles, and thus being charged. A carrier material for transporting toner particles to the surface of the photoreceptor to form a toner image on the photoreceptor. The carrier particles remaining on the developing roll holding the magnet are returned to the developing box from the developing roll, mixed and stirred with new toner particles, and used repeatedly for a certain period.

  Unlike the one-component developer, the two-component developer has the function of mixing and stirring the carrier particles with the toner particles, charging the toner particles, and further transporting the toner particles. Good controllability. Therefore, the two-component developer is suitable for a full-color developing device that requires high image quality and a device that performs high-speed printing that requires image maintenance reliability and durability.

  In the two-component developer used in this manner, image characteristics such as image density, fog, vitiligo, gradation, and resolving power show predetermined values from the initial stage, and these characteristics are in the printing life period. It needs to be kept stable without fluctuating inside. In order to maintain these characteristics stably, it is necessary that the characteristics of the carrier particles contained in the two-component developer are stable.

  Conventionally, various types of iron powder carriers, ferrite carriers, resin-coated ferrite carriers, magnetic powder-dispersed resin carriers, and the like have been used as carrier particles for forming a two-component developer.

  Recently, the networking of offices has progressed and evolved from the single-function copying machine to the multifunctional machine, and the service system has been changed from a system in which contracted service personnel regularly perform maintenance and replace developer etc. However, there has been a shift to the era of maintenance-free systems, and the demand for further extending the life of the developer is increasing from the market.

  Under such circumstances, for the purpose of reducing the weight of carrier particles and extending the developer life, Patent Document 1 (Japanese Patent Laid-Open No. 5-40367) discloses that fine magnetic fine particles are dispersed in a resin. Many magnetic powder dispersed carriers have also been proposed.

  Such a magnetic powder-dispersed carrier can reduce the true density by reducing the amount of magnetic fine particles, and can reduce stress due to stirring, so it can prevent the film from being scraped or peeled off, and is stable over a long period of time. Image characteristics can be obtained.

  However, the magnetic powder dispersion type carrier is obtained by solidifying magnetic fine particles with a binder resin, and the magnetic fine particles are detached due to agitation stress or impact in the developing machine, or the conventional iron powder carrier or ferrite carrier is used. In some cases, the mechanical strength may be inferior, or the carrier particles may be broken. The detached magnetic fine particles and broken carrier particles may adhere to the photoreceptor and cause image defects.

  Furthermore, since the magnetic powder-dispersed carrier uses fine magnetic fine particles, there are disadvantages that the residual magnetization and the coercive force are increased and the fluidity of the developer is deteriorated. In particular, when a magnetic brush is formed on a magnet roll, since the residual magnetization and coercive force are high, the ears of the magnetic brush become hard and it is difficult to obtain high image quality. In addition, even when the magnet roll is separated, the magnetic aggregation of the carrier is not loosened and the toner is not quickly mixed with the replenished toner, so that the charge amount rises poorly and causes image defects such as toner scattering and fogging. was there.

In addition to such a magnetic powder-dispersed carrier, there has been proposed a hollow carrier in which pores are formed inside the carrier core particle as a lighter carrier particle. For example, in Patent Document 2 (Japanese Patent Application Laid-Open No. 2008-310104), the core particle has at least one void of 20% or more and 65% or less on the basis of the cross-sectional area, and the total porosity based on the cross-sectional area. Is 20% or more and 70% or less. Further, Patent Document 3 (JP 2009-244572), 0.1 when the outer diameter of the hollow portion was _{d 2} the outer diameter of the carrier core material _{d 1,} present in the inner core material _{<d 2} It is described that / d ₁ <0.9 is desirable.

  Although the carriers described in these documents are certainly lighter in weight, each has a very large hole, so the mechanical strength is still weaker than that of a conventional ferrite carrier that does not have a hollow shape. The carrier particles were crushed by the agitation stress and the impact in the developing machine, and the crushed particles adhered to the photoconductor, causing image defects. For this reason, it has not been satisfactory at all for the long service life that has recently been required.

  Further, as an alternative to such a magnetic powder-dispersed carrier or a hollow carrier, a resin-filled carrier in which a void is filled in a porous carrier core material has been proposed.

  Patent Document 4 (Japanese Patent Laid-Open No. 2006-337579) discloses a resin-filled carrier obtained by filling a ferrite core material having a porosity of 10 to 60% with a resin, as disclosed in Patent Document 5 (Japanese Patent Laid-Open No. 2007-57943). (Patent Publication) proposes a resin-filled carrier having a three-dimensional laminated structure. Furthermore, Patent Document 6 (Japanese Patent Application Laid-Open No. 2009-175666) defines the pore volume, pore diameter, and pore distribution characteristics of a porous ferrite core material filled with a resin, and has a high dielectric breakdown voltage and Resin-filled carriers with improved carrier particle breaking strength have also been proposed.

  In the resin-filled carriers described above, the resin is filled into the porous ferrite particles to form a three-dimensional laminated structure. In particular, in Patent Document 6, since the pore distribution characteristics are controlled with higher accuracy, it is desirable that the variation in the degree of filling of the resin is reduced and that the resin coating be applied to the surface of the filled resin. As a result, the weight of the carrier particles has been reduced and the breaking strength has been improved, but the carrier particles themselves have a higher resistance, and since they have a three-dimensional laminated structure of resin and ferrite, they have a capacitor-like function. At the time of working and printing, there is a problem that the charge amount increases due to the accumulation of electric charges, it is difficult to secure the image density, and it becomes difficult to obtain a high-quality image. For this reason, it has not been satisfactory with respect to the recently demanded extension of life, like the hollow carrier.

  On the other hand, the realization of the weight reduction also raises a problem that the image permeation of the carrier from the magnetic brush to the photosensitive member, that is, the carrier adhesion is likely to occur because the magnetization per carrier particle becomes low.

  Therefore, while achieving weight reduction in response to the demand for longer life, on the other hand, it prevents image defects such as carrier adhesion, further improves carrier strength, and stabilizes charging characteristics at the time of printing. Carriers for electrophotographic developers are desired.

JP-A-5-40367 JP 2008-310104 A JP 2009-244572 A JP 2006-337579 A JP 2007-57943 A JP 2009-175666 A

  Thus, there has been a demand for a carrier for an electrophotographic developer that prevents image defects such as carrier adhesion while achieving weight reduction, has a high carrier breaking strength, and has little fluctuation in charge amount during printing.

  Accordingly, an object of the present invention is to provide a resin coating for an electrophotographic developer that prevents carrier adhesion while achieving a reduction in the weight of the carrier, has a high particle breaking strength, and has stable charging characteristics during printing durability. An object is to provide a carrier and an electrophotographic developer using the resin-coated carrier.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have formed a resin coating layer on the surface of a porous ferrite core material (particle) having specific pore characteristics, so that the carrier surface A closed space is formed inside the particle, lightening and breaking strength are improved, and no resin is present inside the particle. As a result, the present invention has been achieved.

That is, the present invention relates to a resin-uncoated resin-coated carrier for an electrophotographic developer having a coating resin layer on the surface of a porous ferrite core material, the porous ferrite core material having a pore volume of 55 to 160 mm. ³ / g, the peak pore diameter is 0.2 to 0.7 μm, the apparent density is 1.0 to 2.0 g / cm ³ , and the carrier has a change rate of the volume average particle diameter before and after stirring. And the carrier intensity | strength shown by following formula (1) is less than 5.0%, The resin coated carrier for electrophotographic developers characterized by the above-mentioned is provided.

  In the resin-coated carrier for an electrophotographic developer according to the present invention, in the pore diameter distribution of the porous ferrite core material, the pore diameter variation dv represented by the following formula (2) is 1.0 or less. desirable.

  In the resin-coated carrier for an electrophotographic developer according to the present invention, particles that are present on the surface of the porous ferrite core material and have a particle size of 2 μm or less and the same composition as the porous ferrite core material are used as ultrafine powder. When the presence ratio of the ultrafine powder calculated by the following formula (3) from the projected area of the core material and the fine powder projected area calculated by image analysis is defined as the fine powder coverage, the fine powder coverage is 6.0% or less. It is desirable to be.

  In the resin-coated carrier for an electrophotographic developer according to the present invention, the coating amount of the coating resin layer is desirably 0.5 to 5 parts by weight with respect to 100 parts by weight of the porous ferrite core material.

  The resin-coated carrier for an electrophotographic developer according to the present invention preferably has an average particle size of 20 to 60 μm.

The resin-coated carrier for an electrophotographic developer according to the present invention preferably has a saturation magnetization of 30 to 80 Am ² / kg.

The resin-coated carrier for an electrophotographic developer according to the present invention preferably has an apparent density of 1.0 to 2.0 g / cm ³ .

  The present invention also provides an electrophotographic developer comprising the resin-coated carrier and a toner.

  The electrophotographic developer according to the present invention is also used as a replenishment developer.

  The resin-coated carrier for an electrophotographic developer according to the present invention is obtained by applying a resin coating layer to the surface of a porous ferrite core material having a specific pore volume, a peak pore diameter and an apparent density, and not filling the voids with resin. A space closed from the surface of the carrier can be formed inside the particles to reduce the weight, improve the breaking strength, and stabilize the charging characteristics during printing. As a result, the electrophotographic developer using this resin-coated carrier is excellent in durability and can achieve a long life.

Hereinafter, modes for carrying out the present invention will be described.
<Resin-coated carrier for electrophotographic developer according to the present invention>
The resin-coated carrier for an electrophotographic developer according to the present invention is obtained by resin-coating the surface of a porous ferrite core material, that is, porous ferrite particles. The porous ferrite core material preferably contains at least one selected from Mn, Mg, Li, Ca, Sr, Cu, and Zn. Considering the recent trend of reducing environmental burdens including waste regulations, it is preferable not to include heavy metals such as Cu, Zn and Ni beyond the range of inevitable impurities (accompanying impurities).

The pore volume of the porous ferrite core material 55~160mm ³ / g, preferably 55~100mm ³ / g, more desirably 60 to 80 mm ³ / g. If the pore volume of the porous ferrite core material is less than 55 mm ³ / g, weight cannot be reduced because sufficient pores cannot be secured inside the carrier after resin coating. On the other hand, when the pore volume of the porous ferrite core material exceeds 160 mm ³ / g, the volume of pores formed inside the carrier after resin coating is large, and the strength of the carrier cannot be maintained.

  The peak pore diameter of the porous ferrite core material is 0.2 to 0.7 μm, desirably 0.3 to 0.6 μm. If the peak pore diameter of the porous ferrite core material is less than 0.2 μm, the surface of the core material is less uneven and the carrier surface after resin coating becomes smooth, so that frictional charging with the toner is not performed efficiently. Therefore, for the carrier having a low specific gravity, sufficient stress with the toner is not given, and the rising of charging is deteriorated. Moreover, when the peak pore diameter of the porous ferrite core material exceeds 0.7 μm, it becomes difficult to coat the resin only on the surface, the resin is impregnated to the inside of the pores, and the core convexity is exposed or the resin film thickness. In the initial stage, the target of resistance and charging characteristics is not achieved, charge leakage and printing durability are not preferable because they cause resistance fluctuations and charging fluctuations due to wear on the convex portions.

  Thus, since the pore volume and the peak pore diameter are in the above ranges, a resin-coated carrier that is free from the above-described problems and is appropriately reduced in weight can be obtained.

[Pore diameter and pore volume of porous ferrite core material]
The pore diameter and pore volume of the porous ferrite core material are measured as follows. That is, it measured using mercury porosimeter Pascal140 and Pascal240 (ThermoFisher Scientific company make). CD3P (for powder) was used as the dilatometer, and the sample was put in a commercially available gelatin capsule having a plurality of holes and placed in the dilatometer. After degassing with Pascal 140, it was filled with mercury and the low pressure region (0 to 400 Kpa) was measured to obtain 1st Run. Next, deaeration and measurement of the low pressure region (0 to 400 Kpa) were performed again to obtain 2nd Run. After 2nd Run, the combined weight of the dilatometer, mercury, capsule and sample was measured. Next, the high pressure region (0.1 Mpa to 200 Mpa) was measured with Pascal240. The pore volume, the pore size distribution, and the peak pore size of the porous ferrite core material were determined from the mercury intrusion amount obtained by the measurement of the high pressure part. Further, when determining the pore diameter, the surface tension of mercury was 480 dyn / cm and the contact angle was 141.3 °.

In the pore diameter distribution of the porous ferrite core material, the pore diameter variation dv is desirably 1.0 or less. Here, assuming that the total mercury intrusion amount in the high pressure region is 100%, the pore diameter calculated from the pressure applied to mercury when the indentation amount reaches 84% is d ₈₄ , and the mercury when the indentation amount reaches 16%. the pore size calculated from the pressure applied to that the d _16. The dv value was calculated by the following formula.

  If the pore diameter variation dv of the porous ferrite core material exceeds 1.0, it means that the unevenness between the particles and the variation of the core material shape become large. Therefore, if the dv value exceeds the desired range, the particle shape and the aggregation due to the resin coating are likely to cause inter-particle variation, and as a result, the rise of charge is deteriorated and the charge fluctuation is increased. It becomes.

The apparent density of the porous ferrite core material used in the resin-coated carrier for an electrophotographic developer according to the present invention is preferably 1.0 to 2.0 g / cm ³ , more preferably 1.3 to 1. 8 g / cm ^3, most preferably from 1.5~1.7g / cm ^3. When the apparent density exceeds 2.0 g / cm ³ , the weight of the carrier is not sufficiently reduced and the durability is inferior. On the other hand, when the apparent density is less than 1.0 g / cm ³ , the carrier is too light and the charge imparting ability tends to be lowered.

[Apparent density]
The apparent density is measured in accordance with JIS-Z2504 (Apparent density test method for metal powder).

  The fine powder coverage of the porous ferrite core material used in the resin-coated carrier for an electrophotographic developer according to the present invention is preferably 6.0% or less, and more preferably 3.0% or less. The fine powder coverage of the porous ferrite core material (porous ferrite particles) referred to herein is the proportion of ultrafine powder of about 1 to 2 μm or less present on the surface of the porous ferrite particles. When such ultrafine powder is present on the surface of the porous ferrite particles, it is present even after the resin coating, which causes carrier adhesion. If the fine powder coverage exceeds 6.0%, image defects due to carrier adhesion will exceed an allowable range in actual use.

[Fine powder coverage of porous ferrite]
The fine powder coverage of this porous ferrite core material (porous ferrite particles) is measured as follows. First, the porous ferrite particles are photographed with a scanning electron microscope (JSM-6100 type, manufactured by JEOL Ltd.) at a magnification of 450 times. For the obtained SEM image, the core material projected area and the fine powder projected area were calculated by image analysis, and the fine powder coverage was calculated by the following formula.

  The resin-coated carrier for an electrophotographic developer according to the present invention coats the resin only on the surface of the porous ferrite core material. The coating amount of the resin is preferably 0.5 to 5 parts by weight, more preferably 1 to 4 parts by weight, and most preferably 1 to 3 parts by weight with respect to 100 parts by weight of the porous ferrite core material. If the coating amount of the resin is less than 0.5 parts by weight, the surface of the porous ferrite core material cannot be sufficiently covered with the resin, and voids are not formed from the coating resin layer, so that sufficient weight reduction is achieved. I can't figure it out. On the other hand, when the resin coating amount exceeds 5 parts by weight, aggregation between particles occurs and the aggregation is released by stress applied to the carrier at the time of printing, which causes problems such as charging fluctuation.

  The resin to be coated is not particularly limited as long as the resin can be coated on the surface without impregnating the pores of the porous ferrite core material, and the type can be appropriately selected depending on the toner to be combined, the environment in which it is used, etc. It is possible to use a commercially available resin. For example, fluorine resin, acrylic-styrene resin, acrylic resin, fluorine acrylic resin, polyamide resin, polyester resin, unsaturated polyester resin, silicone resin, epoxy resin, urea resin, phenol resin, melamine resin, or acrylic resin, polyester resin, Examples thereof include modified silicone resins modified with resins such as epoxy resins, polyamide resins, alkyd resins, urethane resins, and fluororesins. As the resin coating method, as will be described later, a resin that can be used as resin fine particles and that can be mixed with a porous ferrite core material and then formed by heating and melting is preferable.

  A conductive agent can be added to the coating resin for the purpose of controlling the electrical resistance, charge amount, and charging speed of the carrier. Since the conductive agent itself has a low electric resistance, an excessive amount of the conductive agent tends to cause a rapid charge leak. Accordingly, the addition amount is 0.25 to 20.0% by weight, preferably 0.5 to 15.0% by weight, particularly preferably 1.0 to 10.0% by weight, based on the solid content of the coating resin. It is. Examples of the conductive agent include conductive carbon, oxides such as titanium oxide and tin oxide, and various organic conductive agents.

  In addition, the coating resin can contain a charge control agent. Examples of the charge control agent include various charge control agents generally used for toners and various silane coupling agents. The types of charge control agents and coupling agents that can be used are not particularly limited, but charge control agents such as nigrosine dyes, quaternary ammonium salts, organometallic complexes, and metal-containing monoazo dyes, aminosilane coupling agents, and fluorine-based silane couplings. An agent or the like is preferable.

  The volume average particle size of the resin-coated carrier for an electrophotographic developer according to the present invention is desirably 20 to 60 μm. In this range, carrier adhesion is prevented and good image quality is obtained. A volume average particle size of less than 20 μm is not preferable because it causes carrier adhesion. On the other hand, if the volume average particle diameter exceeds 60 μm, the image quality tends to deteriorate, which is not preferable.

[Volume average particle size (Microtrack)]
This volume average particle diameter is measured as follows. That is, it is measured using a Nikkiso Co., Ltd. Microtrac particle size analyzer (Model 9320-X100). Water was used as the dispersion medium. Place 10 g of sample and 80 ml of water in a 100 ml beaker and add 2-3 drops of dispersant (sodium hexametaphosphate). Subsequently, using an ultrasonic homogenizer (UH-150 type manufactured by SMT Co Ltd), the output level was set to 4 and dispersion was performed for 20 seconds. Thereafter, bubbles formed on the beaker surface were removed, and the sample was put into the apparatus.

The resin-coated carrier for an electrophotographic developer according to the present invention preferably has a saturation magnetization of 30 to 80 Am ² / kg, and more preferably 50 to 70 Am ² / kg. If the saturation magnetization is less than 30 Am ² / kg, carrier adhesion tends to be induced, and if it exceeds 80 Am ² / kg, the ears of the magnetic brush become high, and it is difficult to obtain high image quality.

[Saturation magnetization]
Here, the magnetization was measured using an integral BH tracer BHU-60 type (manufactured by Riken Denshi Co., Ltd.). A magnetic field measuring H coil and a magnetization measuring 4πI coil are placed between the electromagnets. In this case, the sample is placed in a 4πI coil. The outputs of the H coil and the 4πI coil whose magnetic field H is changed by changing the current of the electromagnet are respectively integrated, and the H output is drawn on the X axis, the output of the 4πI coil is drawn on the Y axis, and a hysteresis loop is drawn on the recording paper. As measurement conditions, sample filling amount: about 1 g, sample filling cell: inner diameter 7 mmφ ± 0.02 mm, height 10 mm ± 0.1 mm, 4πI coil: measured with 30 turns.

The apparent density of the resin-coated carrier for an electrophotographic developer according to the present invention is preferably 1.0 to 2.0 g / cm ³ , more preferably 1.3 to 1.8 g / cm ³ , most preferably. Is 1.5 to 1.7 g / cm ³ . If the apparent density is less than 1.0 g / cm ³ , the carrier is too light and the charge imparting ability tends to be lowered. When the apparent density exceeds 2.0 g / cm ³ , the weight of the carrier is not sufficiently reduced and the durability is inferior.
This measurement is the same as the above-described measurement of the apparent density of the porous ferrite.

<Method for producing resin-coated carrier for electrophotographic developer according to the present invention>
A method for producing a resin-coated carrier for an electrophotographic developer according to the present invention will be described.

  In the method for producing a resin-coated carrier for an electrophotographic developer according to the present invention, in order to produce a porous ferrite core material, first, an appropriate amount of raw materials are weighed and then 0.5 hours or longer using a ball mill or a vibration mill. , Preferably pulverized and mixed for 1 to 20 hours. The raw material is not particularly limited, but is preferably selected so as to have a composition containing the above-described elements.

  The pulverized material thus obtained is pelletized using a pressure molding machine or the like, and then calcined at a temperature of 700 to 1200 ° C. You may grind | pulverize without using a pressure molding machine, add water to make a slurry, and granulate using a spray dryer. After calcination, the mixture is further pulverized with a ball mill or a vibration mill, and then water and, if necessary, a dispersant and a binder are added. After adjusting the viscosity, the mixture is granulated with a spray dryer and granulated. When pulverizing after calcination, water may be added and pulverized by a wet ball mill, a wet vibration mill or the like.

  The pulverizer such as the above-mentioned ball mill and vibration mill is not particularly limited, but in order to disperse the raw materials effectively and uniformly, it is preferable to use fine beads having a particle diameter of 1 mm or less for the medium to be used. Further, the degree of grinding can be controlled by adjusting the diameter, composition and grinding time of the beads used.

  Thereafter, the obtained granulated product is held at a temperature of 800 to 1500 ° C. for 1 to 24 hours in an atmosphere in which the oxygen concentration is controlled to perform main firing. At that time, a rotary electric furnace, a batch electric furnace or a continuous electric furnace is used, and an atmosphere at the time of firing is oxygenated by implanting an inert gas such as nitrogen or a reducing gas such as hydrogen or carbon monoxide. The concentration may be controlled. In the case of a rotary electric furnace, firing may be performed many times by changing the atmosphere and firing temperature.

  The fired product thus obtained is pulverized and classified. As a classification method, the particle size is adjusted to a desired particle size using an existing air classification, mesh filtration method, sedimentation method, or the like.

  Then, if necessary, the surface can be heated at a low temperature to perform an oxide film treatment to adjust the electric resistance. The oxide film treatment can be performed by heat treatment at, for example, 300 to 700 ° C. using a general rotary electric furnace, batch electric furnace or the like. The thickness of the oxide film formed by this treatment is preferably 0.1 nm to 5 μm. If the thickness is less than 0.1 nm, the effect of the oxide film layer is small, and if it exceeds 5 μm, the magnetization is lowered or the resistance becomes too high, so that it is difficult to obtain desired characteristics. Moreover, you may reduce | restore before an oxide film process as needed. In this way, a porous ferrite core material (particle) having a pore volume and a peak pore diameter in a specific range is prepared.

  As a method of controlling the pore volume, peak pore diameter, pore diameter variation and apparent density of the porous ferrite core material as described above, the raw material type to be blended, the pulverization degree of the raw material, the presence or absence of calcination, calcination It can be carried out by various methods such as temperature, calcining time, amount of binder during granulation by spray dryer, baking method, baking temperature, baking time, reduction with hydrogen gas, carbon monoxide gas, or the like. These control methods are not particularly limited, but an example is shown below.

  That is, the use of hydroxide or carbonate as the raw material species to be blended tends to increase the pore volume and the apparent density, compared to the case where an oxide is used. In addition, the pore volume tends to be large and the apparent density tends to be small when the calcination is not performed, or when the calcination temperature is lower, or the main calcination temperature is lower and the calcination time is shorter.

  As for the peak pore diameter, the degree of pulverization of the raw material to be used, particularly the raw material after calcination, is strengthened, and the smaller the primary particle diameter of the pulverization tends to be smaller. Further, it is possible to reduce the peak pore diameter by introducing a reducing gas such as hydrogen or carbon monoxide rather than using an inert gas such as nitrogen during the main firing.

  Furthermore, with regard to the variation in pore diameter, it is possible to reduce the variation by uniformly advancing the sinterability of the raw material during the main firing. Specifically, it is preferable to use a rotary electric furnace that can uniformly heat the raw material, rather than using a tunnel-type continuous furnace. In addition, the variation in pore diameter can be reduced by increasing the degree of pulverization of the raw material used, particularly the raw material after calcination, and sharpening the distribution of the pulverized particle diameter.

  By using these control methods singly or in combination, a porous ferrite core material having a desired pore volume, peak pore diameter, and variation in pore diameter can be obtained.

  The porous ferrite core material thus obtained is coated with a resin. Carrier characteristics, particularly electrical characteristics such as charging characteristics, are often affected by materials and properties existing on the carrier surface. Therefore, the desired carrier characteristics can be adjusted with high accuracy by coating the surface with an appropriate resin. As a method for coating, known methods such as a brush coating method, a dry method, a spray drying method using a fluidized bed, a rotary drying method, an immersion drying method using a universal stirrer, etc. are known. It is desirable to use a dry coating method in which the resin coating can be efficiently applied to the surface without impregnating the resin. The dry coating method does not use a solvent, uses resin fine particles as the coating resin, mixes the resin fine particles and the porous ferrite core material, and then heats to melt the coating resin. In this method, the surface of the material is coated with a resin. Since the viscosity of the molten resin is higher than that of the diluted resin solution, impregnation into the pores can be prevented, and the surface can be efficiently coated with the resin. As a manufacturing apparatus, a heating kneader, a heating Henschel mixer, a heating UM mixer, a planetary mixer, a heating fluidized rolling bed, or a heating kiln can be used. Although the heating temperature varies depending on the resin used, a temperature equal to or higher than the melting point or glass transition point is necessary, and in the case of a thermosetting resin or a condensation-crosslinking resin, it is necessary to raise it to a temperature at which curing proceeds sufficiently.

<Electrophotographic developer according to the present invention>
Next, the electrophotographic developer according to the present invention will be described.
The electrophotographic developer according to the present invention comprises the above-described resin-coated carrier for an electrophotographic developer and a toner.

  The toner particles constituting the electrophotographic developer of the present invention include pulverized toner particles produced by a pulverization method and polymerized toner particles produced by a polymerization method. In the present invention, toner particles obtained by any method can be used.

  The pulverized toner particles are, for example, a binder resin, a charge control agent, and a colorant are sufficiently mixed with a mixer such as a Henschel mixer, then melt-kneaded with a twin screw extruder or the like, cooled, pulverized, classified, After adding the external additive, it can be obtained by mixing with a mixer or the like.

  The binder resin constituting the pulverized toner particles is not particularly limited, but polystyrene, chloropolystyrene, styrene-chlorostyrene copolymer, styrene-acrylic acid ester copolymer, styrene-methacrylic acid copolymer, Furthermore, rosin modified maleic acid resin, epoxy resin, polyester resin, polyurethane resin and the like can be mentioned. These may be used alone or in combination.

  Any charge control agent can be used. For example, nigrosine dyes and quaternary ammonium salts can be used for positively charged toners, and metal-containing monoazo dyes can be used for negatively charged toners.

  As the colorant (colorant), conventionally known dyes and pigments can be used. For example, carbon black, phthalocyanine blue, permanent red, chrome yellow, phthalocyanine green, etc. can be used. In addition, external additives such as silica powder and titania for improving the fluidity and aggregation resistance of the toner can be added according to the toner particles.

  The polymerized toner particles are toner particles produced by a known method such as a suspension polymerization method, an emulsion polymerization method, an emulsion aggregation method, an ester elongation polymerization method, or a phase inversion emulsification method. Such polymerized toner particles are prepared by, for example, mixing and stirring a colored dispersion in which a colorant is dispersed in water using a surfactant, a polymerizable monomer, a surfactant, and a polymerization initiator in an aqueous medium. Then, the polymerizable monomer is emulsified and dispersed in an aqueous medium, polymerized while stirring and mixing, and then a salting-out agent is added to salt out the polymer particles. Polymerized toner particles can be obtained by filtering, washing and drying the particles obtained by salting out. Thereafter, if necessary, an external additive is added to the dried toner particles.

  Further, in the production of the polymerized toner particles, in addition to the polymerizable monomer, the surfactant, the polymerization initiator, and the colorant, a fixability improving agent and a charge control agent can be blended and obtained. Various characteristics of the polymerized toner particles can be controlled and improved. A chain transfer agent can be used to improve the dispersibility of the polymerizable monomer in the aqueous medium and adjust the molecular weight of the resulting polymer.

  The polymerizable monomer used for the production of the polymerized toner particles is not particularly limited. For example, styrene and its derivatives, ethylene unsaturated monoolefins such as ethylene and propylene, vinyl halides such as vinyl chloride, Α-methylene aliphatic monocarboxylic acids such as vinyl esters such as vinyl acetate, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, 2-ethylhexyl methacrylate, dimethylamino acrylate and diethylaminoester methacrylate Examples include esters.

  Conventionally known dyes and pigments can be used as the colorant (coloring material) used in the preparation of the polymerized toner particles. For example, carbon black, phthalocyanine blue, permanent red, chrome yellow, phthalocyanine green, and the like can be used. Moreover, the surface of these colorants may be modified using a silane coupling agent, a titanium coupling agent, or the like.

  As the surfactant used in the production of the polymerized toner particles, an anionic surfactant, a cationic surfactant, an amphoteric surfactant and a nonionic surfactant can be used.

  Here, examples of the anionic surfactant include fatty acid salts such as sodium oleate and castor oil, alkyl sulfates such as sodium lauryl sulfate and ammonium lauryl sulfate, alkyl benzene sulfonates such as sodium dodecyl benzene sulfonate, and alkyl naphthalene sulfonic acids. Salt, alkyl phosphate ester salt, naphthalene sulfonic acid formalin condensate, polyoxyethylene alkyl sulfate ester salt and the like. Examples of the nonionic surfactant include polyoxyethylene alkyl ether, polyoxyethylene fatty acid ester, sorbitan fatty acid ester, polyoxyethylene alkylamine, glycerin, fatty acid ester, and oxyethylene-oxypropylene block polymer. . Furthermore, examples of the cationic surfactant include alkylamine salts such as laurylamine acetate, and quaternary ammonium salts such as lauryltrimethylammonium chloride and stearyltrimethylammonium chloride. Examples of amphoteric surfactants include aminocarboxylates and alkylamino acids.

  The surfactant as described above can be used usually in an amount in the range of 0.01 to 10% by weight with respect to the polymerizable monomer. The amount of such a surfactant used affects the dispersion stability of the monomer and also affects the environmental dependency of the obtained polymerized toner particles. It is preferably used in an amount within the above range that is ensured and does not exert an excessive influence on the environment dependency of the polymerized toner particles.

  For the production of polymerized toner particles, a polymerization initiator is usually used. The polymerization initiator includes a water-soluble polymerization initiator and an oil-soluble polymerization initiator, and any of them can be used in the present invention. Examples of the water-soluble polymerization initiator that can be used in the present invention include persulfates such as potassium persulfate and ammonium persulfate, water-soluble peroxide compounds, and oil-soluble polymerization initiators. Examples thereof include azo compounds such as azobisisobutyronitrile and oil-soluble peroxide compounds.

  When a chain transfer agent is used in the present invention, examples of the chain transfer agent include mercaptans such as octyl mercaptan, dodecyl mercaptan, tert-dodecyl mercaptan, and carbon tetrabromide.

  Further, when the polymerized toner particles used in the present invention contain a fixability improver, a natural wax such as carnauba wax, an olefin wax such as polypropylene or polyethylene can be used as the fixability improver. .

  Further, when the polymerized toner particles used in the present invention contain a charge control agent, the charge control agent to be used is not particularly limited, and nigrosine dyes, quaternary ammonium salts, organometallic complexes, metal-containing monoazo dyes, etc. Can be used.

  Examples of the external additive used for improving the fluidity of polymerized toner particles include silica, titanium oxide, barium titanate, fluororesin fine particles, and acrylic resin fine particles. Can be used in combination.

  Furthermore, examples of the salting-out agent used for separating the polymer particles from the aqueous medium include metal salts such as magnesium sulfate, aluminum sulfate, barium chloride, magnesium chloride, calcium chloride, and sodium chloride.

  The average particle size of the toner particles produced as described above is in the range of 2 to 15 μm, preferably 3 to 10 μm, and the polymerized toner particles have higher particle uniformity than the pulverized toner particles. If the toner particles are smaller than 2 μm, the charging ability is lowered and fog and toner scattering are liable to occur, and if it exceeds 15 μm, the image quality is deteriorated.

  An electrophotographic developer can be obtained by mixing the carrier and toner manufactured as described above. The mixing ratio of the carrier and the toner, that is, the toner concentration is preferably set to 3 to 15% by weight. If it is less than 3% by weight, it is difficult to obtain a desired image density. If it exceeds 15% by weight, toner scattering and fogging are likely to occur.

  A developer obtained by mixing the carrier and toner manufactured as described above can be used as a replenishment developer. In this case, the toner is mixed at a ratio of 2 to 50 parts by weight of the toner with respect to 1 part by weight of the carrier and toner.

  The electrophotographic developer according to the present invention prepared as described above uses an electrostatic latent image formed on a latent image holding member having an organic photoconductor layer, while applying a bias electric field to the toner and the carrier. The present invention can be used in digital copiers, printers, fax machines, printers, and the like that use a developing method in which reversal development is performed using a two-component developer magnetic brush. Further, the present invention can also be applied to a full color machine using an alternating electric field, which is a method of superimposing an AC bias on a DC bias when a developing bias is applied from the magnetic brush to the electrostatic latent image side.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example etc., this invention is not limited at all by this.

MnO: 38mol%, MgO: 9.5mol %, Fe 2 O 3: 52mol% and SrO: materials were weighed so that 0.5 mol%, dry media mill (vibration mill, 1/8-inch diameter stainless steel The resulting pulverized product was formed into pellets of about 1 mm square using a roller compactor. Trimanganese tetraoxide was used as the MnO raw material, magnesium hydroxide was used as the MgO raw material, and strontium carbonate was used as the SrO raw material. The coarse powder was removed from the pellets with a vibrating sieve having a mesh opening of 3 mm, and then the fine powder was removed with a vibrating sieve having a mesh opening of 0.5 mm, followed by heating at 1000 ° C. for 3 hours to perform preliminary firing.

Next, after crushing to an average particle size of 5.1 μm using a dry media mill (vibration mill, 1/8 inch diameter stainless steel beads), water was added, and a wet media mill (vertical bead mill, 1 / 16-inch diameter stainless steel beads) for 3 hours. The slurry particle size (primary particle diameter of the grinding) results measured at Microtrac, D ₅₀ was 2.41Myuemu. An appropriate amount of a dispersant is added to the slurry, and the strength of the granulated particles is ensured, and in order to realize a reducing atmosphere in the next firing step, PVA (10% solution) is used as a binder to the solid content of the slurry 12% by weight, and then granulated and dried with a spray dryer, and the particle size of the obtained particles was adjusted.

  First, the granulated product obtained as described above was subjected to main firing in a rotary electric furnace while keeping the inside of the furnace at a positive pressure and kept in a reducing atmosphere at a temperature of 1000 ° C. for 1 hour. Moreover, the reducing atmosphere utilized the thermal decomposition gas of the dispersing agent and binder which were added when granulating with a spray dryer.

Thereafter, the mixture was pulverized, further classified to adjust the particle size, and the low-magnetic force product was separated by magnetic separation, thereby obtaining a core material composed of porous ferrite particles. The porous ferrite core material has a pore volume of 70.8 mm ³ / g, a peak pore diameter of 0.53 μm, a pore diameter variation dv of 0.32, and an apparent density of 1.68 g / cm ³ , The fine powder coverage was 2.8%.

  Next, 3 parts by weight of silicone resin 1 (condensation-crosslinked silicone resin powder, KR220L manufactured by Shin-Etsu Chemical Co., Ltd.) is blended with 100 parts by weight of the porous ferrite core material. Stir and mix for minutes.

  Next, it is put into a heating type kneader, heated from normal temperature to 240 ° C. at a rate of 5 ° C./min, stirred and kneaded for 2 hours, turned off the heater, cooled with stirring for 30 minutes, Discharged.

  Thereafter, the particles were agglomerated with a vibrating sieve having an opening of 200 M, and the non-magnetic material was removed using a magnetic separator. Thereafter, coarse particles were removed again with a vibration sieve to obtain a resin-coated carrier in which the surface of the porous ferrite core material was coated with a resin.

  Porous in the same manner as in Example 1 except that acrylic resin (acrylic resin powder (MMA), BR-73 manufactured by Mitsubishi Rayon Co., Ltd.) was used as the coating resin, and the temperature of the heating kneader was 140 ° C. A resin-coated carrier in which the surface of the ferrite core material was coated with a resin was obtained.

As pulverization conditions after pre-firing, the time for pulverization with a wet media mill (vertical bead mill, 1/16 inch diameter stainless steel beads) was changed to 10 hours. As a result, the slurry particle size (primary particle size of pulverization) of _{D 50} became finely up to 1.62μm. Then, the obtained granulated material is heated at 700 ° C. for 2 hours in a rotary electric furnace to remove organic components such as a dispersant and a binder. Further, as a condition for main firing, in a tunnel electric furnace, The firing temperature was 1050 ° C. and the atmosphere was held in a nitrogen gas atmosphere for 5 hours. Thereafter, the mixture was pulverized, further classified to adjust the particle size, and the low-magnetic force product was separated by magnetic separation, thereby obtaining a core material composed of porous ferrite particles.

This porous ferrite core material has a pore volume of 121.3 mm ³ / g, a peak pore diameter of 0.54 μm, a pore diameter variation dv of 0.26, and an apparent density of 1.48 g / cm ³ , The fine powder coverage was 0.7%.

  Thereafter, in the same manner as in Example 2, a resin-coated carrier in which the surface of the porous ferrite core material was coated with a resin was obtained.

A resin-coated carrier in which the surface of the porous ferrite core material was coated with a resin was obtained in the same manner as in Example 2 except that the temperature was 900 ° C. as the conditions for the main firing. The obtained porous ferrite core material has a pore volume of 155.3 mm ³ / g, a peak pore diameter of 0.55 μm, a pore diameter variation dv of 0.47, and an apparent density of 1.31 g. / Cm ³ , and the fine powder coverage was 5.8%.

A resin-coated carrier in which the surface of the porous ferrite core material was coated with a resin was obtained in the same manner as in Example 2 except that the temperature was set to 1040 ° C. as the conditions for the main firing. The obtained porous ferrite core material has a pore volume of 56.8 mm ³ / g, a peak pore diameter of 0.49 μm, a pore diameter variation dv of 0.48, and an apparent density of 1.79 g. / Cm ³ , and the fine powder coverage was 2.5%.

Comparative example

[Comparative Example 1]
As pulverization conditions after pre-firing, the time for pulverization with a wet type media mill (vertical bead mill, 1/16 inch diameter stainless steel beads) was changed to 0.5 hours. D ₅₀ of (diameter) became rough to 4.3 μm. Then, the obtained granulated material is heated at 700 ° C. for 2 hours in a rotary electric furnace to remove organic components such as a dispersant and a binder. Further, as a condition for main firing, in a tunnel electric furnace, The calcination temperature was 1100 ° C. and a nitrogen gas atmosphere was maintained for 5 hours. Thereafter, the mixture was pulverized, further classified to adjust the particle size, and the low-magnetic force product was separated by magnetic separation, thereby obtaining a core material composed of porous ferrite particles. The porous ferrite core material has a pore volume of 112.5 mm ³ / g, a peak pore diameter of 1.56 μm, a pore diameter variation dv of 1.32, and an apparent density of 1.31 g / cm ³ . The fine powder coverage was 1.0%. Thereafter, in the same manner as in Example 1, a resin-coated carrier coated with a resin was obtained.

[Comparative Example 2]
100 parts by weight of the same porous ferrite core material as in Example 1 and a silicone resin 2 (condensation-crosslinked silicone resin comprising T units and D units, weight average molecular weight: about 8000) were prepared. Aminosilane coupling agent (γ-aminopropyltrimethoxysilane) is added to 10 parts by weight with respect to the resin solids by weight (9 parts by weight as the solid content because the resin solution concentration is 20%, dilution solvent: toluene). The resin was infiltrated and filled into the porous ferrite core material while mixing and stirring at 60 ° C. under a reduced pressure of 4.0 kPa to evaporate toluene.

  After confirming that toluene was fully volatilized, stirring was continued for another 30 minutes. After toluene was almost completely removed, the toluene was taken out from the filling device, placed in a container, and placed in a hot air heating type oven at 220 ° C. for 2 hours. The heat treatment was performed.

  Then, it cooled to room temperature, the ferrite particle | grains by which resin was hardened were taken out, the aggregation of particle | grains was released with the vibration sieve of 200M opening, and the nonmagnetic substance was removed using the magnetic separator. Thereafter, coarse particles were again removed with a vibrating sieve to obtain a resin-filled carrier filled with resin.

[Comparative Example 3]
A resin-coated carrier in which the surface of the porous ferrite core material was coated with a resin was obtained in the same manner as in Example 2 except that the temperature was set to 800 ° C. as the conditions for the main firing. The pore volume of the obtained porous ferrite core material was 229.5 mm ³ / g, the peak pore diameter was 0.49 μm, the pore diameter variation dv was 0.21, and the apparent density was 0.98 g. / Cm ³ , and the fine powder coverage was 12.8%.

[Comparative Example 4]
Using the same granulated material as in Example 2, it was heated at 700 ° C. for 2 hours in a rotary electric furnace to remove organic components such as a dispersant and a binder. And held at a firing temperature of 1200 ° C. under a nitrogen gas atmosphere for 5 hours. Thereafter, the mixture was pulverized, further classified to adjust the particle size, and the low-magnetic force product was separated by magnetic separation, thereby obtaining a core material composed of porous ferrite particles. The porous ferrite core material has a pore volume of 18.4 mm ³ / g, a peak pore diameter of 0.65 μm, a pore diameter variation dv of 0.73, and an apparent density of 2.21 g / cm ³ . The fine powder coverage was 0.3%. Thereafter, in the same manner as in Example 2, a resin-coated carrier coated with a resin was obtained.

  Pore volume, peak pore diameter, pore diameter variation dv of Examples 1 to 5 and Comparative Examples 1 to 4, apparent density of porous ferrite core material, fine powder coverage, coating or filling, and filling or coating resin (kind And the amount added) are shown in Table 1. Table 1 shows the volume average particle diameter, saturation magnetization, apparent density, forced stirring test (charge amount before and after forced stirring, charge change rate) and carrier strength obtained in Examples 1 to 5 and Comparative Examples 1 to 4. Show. A forced stirring test (charge amount before and after forced stirring, charge change rate) and a method for measuring carrier strength are shown below. Other measurement methods are as described above.

[Charge amount]
The charge amount was determined by measuring a mixture of carrier and toner with a suction charge amount measuring device (Epping q / m-meter, manufactured by PES-Laboratorium). The toner used is a commercially available negative polarity toner (cyan toner, for DocuPrint C3530 manufactured by Fuji Xerox Co., Ltd .; average particle size of about 5.8 μm) used in a full-color printer. It was adjusted. The adjusted developer is put in a 50 cc glass bottle, and the glass bottle is stored and fixed in a cylindrical holder having a diameter of 130 mm and a height of 200 mm, and stirred for 30 minutes with a tumbler mixer manufactured by Shinmaru Enterprise Co., Ltd., 635M The amount of electrification was measured using the net.

[Forced stirring test]
Using the same commercially available negative-polarity toner as the above-mentioned toner (cyan toner, for DocuPrint C3530 manufactured by Fuji Xerox Co., Ltd .; average particle size of about 5.8 μm), the developer amount is adjusted to 20 g, the toner concentration is adjusted to 10 wt%, The glass bottle was stirred for 10 hours with a paint shaker manufactured by Asada Tekko. After the stirring, the developer was taken out, the toner was sucked using a 635M net, and only the carrier was taken out. The charge amount of the obtained carrier was measured by the above-described charge amount measurement method to obtain the charge amount after forced stirring.

[Charge amount change rate]
The charge amount change rate was calculated by the following formula.

[Carrier strength]
20 g of the carrier was placed in a 50 cc glass bottle, and the glass bottle was stirred for 10 hours with a paint shaker. When the carrier particles are cracked, scraped, or fine particles are generated by the stress due to stirring, the average particle size of the carrier after stirring becomes small. As the carrier is weaker, scraping and fine particles are generated and the average particle size becomes smaller. Therefore, the change rate of the average particle size before and after stirring was used as an index of carrier strength. The average particle size was determined as the volume average particle size measured using the aforementioned Nikkiso Co., Ltd. Microtrac particle size analyzer (Model 9320-X100), and the particle size change rate was determined as follows.

  Based on the above measurement results, weight reduction (apparent density), fine powder coverage, carrier strength characteristics and charging characteristics were evaluated. This evaluation was performed in four stages: ◎: Excellent, ○: Good, Δ: Acceptable, ×: Impossible. The results are shown in Table 2.

[Evaluation criteria for weight reduction (apparent density)]
^◎: less than ^{1.30g / cm 3 ~1.80g / cm 3} ○: less than 1.0g / cm ³ ~1.30g / cm 3 or 1.80g ^/ cm 3 ~2.00g ^/ cm less than ³ △: 0 .90g ^/ cm ³ ~1.00g ^/ cm 3 or less than 2.00g ^/ cm ³ ~2.10g ^/ cm 3 less ×: 0.90g / cm ³ or less than 2.10 g / cm ³ or more

[Evaluation criteria for fine powder coverage]
◎: Less than 3.0% ○: 3.0% to less than 6.0% △: 6.0% to less than 10.0% ×: 10.0% or more

[Evaluation criteria for electrification fluctuation characteristics]
The evaluation criteria for the charging fluctuation characteristics are as follows.
◎: 90% or more and less than 110% ○: 80% or more and less than 90%, 110% or more and less than 130% △: 70% or more and less than 80%, 130% or more and less than 150% ×: Less than 70%, 150% or more

[Evaluation criteria for carrier strength characteristics]
The evaluation criteria for carrier strength characteristics are as follows.
◎: Less than 0.5% ○: 0.5% to less than 1.0% △: 1.0% to less than 5.0% ×: More than 5.0%

  As is clear from the results shown in Tables 1 and 2, since the resin-coated carriers shown in Examples 1 to 5 maintain the desired ranges of the apparent density and the carrier strength, weight reduction can be realized, In addition, since there is little charge fluctuation and fine powder coverage after forced stirring, the charging characteristics during printing durability are stabilized even in actual use, and good image quality without image defects such as carrier adhesion, toner scattering, and fog is stable. Is easily assumed to be obtained.

  On the other hand, the carriers shown in Comparative Examples 1 to 4 were inferior in evaluation regarding weight reduction, fine powder coverage, carrier strength, and electrification fluctuation because each characteristic of the porous ferrite core material was not in an appropriate range. For this reason, in actual use, image defects such as toner adhesion and fogging due to carrier adhesion, electrification fluctuations at the time of printing, chipping of carriers, cracks, and fogging can be promoted, and good image quality cannot be stably maintained. Is easily assumed.

  The resin-coated carrier for an electrophotographic developer according to the present invention can reduce the weight, prevent carrier adhesion, improve the breaking strength, and stabilize the charging characteristics during printing durability. As a result, the electrophotographic developer using this resin-coated carrier is excellent in durability and can achieve a long life.

  Therefore, the present invention can be widely used in the fields of full-color machines that require high image quality and high-speed machines that require image maintenance reliability and durability.

Claims (9)

A resin-uncoated resin-coated carrier for an electrophotographic developer having a coating resin layer on the surface of a porous ferrite core material,
The porous ferrite core material has a pore volume of 55 to 160 mm ³ / g, a peak pore diameter of 0.2 to 0.7 μm, and an apparent density of 1.0 to 2.0 g / cm ³ . ,
The carrier is a resin-coated carrier for an electrophotographic developer, which has a volume average particle size change rate before and after stirring and has a carrier strength represented by the following formula (1) of less than 5.0%.

2. The resin-coated carrier for an electrophotographic developer according to claim 1, wherein in the pore diameter distribution of the porous ferrite core material, the pore diameter variation dv represented by the following formula (2) is 1.0 or less.

A core material projected area and a fine powder projected area calculated by image analysis using particles having a particle size of 2 μm or less and having the same composition as the porous ferrite core material present on the surface of the porous ferrite core material as ultrafine powder The fine powder coverage of the porous ferrite core material is 6.0% or less, when the presence ratio of the ultrafine powder calculated by the following formula (3) is defined as the fine powder coverage: The resin-coated carrier for an electrophotographic developer as described.

  4. The resin-coated carrier for an electrophotographic developer according to claim 1, wherein a coating amount of the coating resin layer is 0.5 to 5 parts by weight with respect to 100 parts by weight of the porous ferrite core material.   The resin-coated carrier for an electrophotographic developer according to any one of claims 1 to 4, having an average particle size of 20 to 60 µm. The resin-coated carrier for an electrophotographic developer according to claim 1, wherein the saturation magnetization is 30 to 80 Am ² / kg. The resin-coated carrier for an electrophotographic developer according to claim 1, which has an apparent density of 1.0 to 2.0 g / cm ³ .   An electrophotographic developer comprising the resin-coated carrier according to claim 1 and a toner.   The electrophotographic developer according to claim 8, which is used as a replenishment developer.