Carrier for two-component type developer

Abstract: Disclosed is a ferrite carrier for a two-component type developer, characterized in that the internal pore ratio, based on the area, of carrier particles is adjusted to 3 to 30%, the electric resistance is adjusted within a certain range, and the developing electric field dependency indicating the change ratio of the electric resistance of the particles caused when the electric field is changed is controlled within a specific range. This carrier has a good durability, and when a two-component developer comprising this ferrite carrier is used, there can be obtained a high-quality image in which a solid image area and a fine line image area are clearly reproduced.

Description: The present invention relates to a carrier for a two-component type developer for use in electrophotography. More particularly, the present invention relates to a carrier for two-component type developer, which has durability and provides an image of high quality on development and which is suitable for use even for high-speed development. In the field of electrophotography, a two-component type magnetic developer is widely used as the means for developing an electrostatic latent image. According to this developing method, the two component composition is stirred to cause frictional charging and to apply the toner to the periphery of carrier particles by charging. The composition is supplied onto a developing sleeve provided with magnets in the interior thereof to form a magnetic brush composed of the composition, and this magnetic brush is brought into sliding contact with an electrophotographic photosensitive surface having an electrostatic latent image to form an electroscopic toner image on a photosensitive plate. In general, a voltage is applied between the photosensitive surface and the developing sleeve, and the voltage difference between the electrostatic latent image on the photosensitive surface and the magnetic brush is especially large. Accordingly, the electroscopic toner particles on the magnetic brush is caused to adhere onto the electrostatic latent image by Coulomb force, whereby the development of the electrostatic latent image is accomplished. In contrast, the magnetic carrier is attracted by the magnets arranged in the sleeve and the charging polarity of the magnetic carrier is the some as the polarity of the charge of the electrostatic latent image, and therefore, the magnetic carrier is left on the sleeve. Accordingly, it is readily understood that the physical characteristics, electric characteristics and magnetic characteristics of the magnetic carrier have great influence on the quality of the developed image in this developing process. An iron powder carrier has been widely used as the magnetic carrier, but recently, the iron powder carrier is being replaced by sintered ferrite particles. The chemical properties and magnetic characteristics of the sintered ferrite particles are more stable than those of the iron powder carrier, and the sintered ferrite particles are characterized in that the particle size and magnetic properties can be easily controlled. In JP-A-5922866 are disclosed ferrite carriers for development in electrophotography, having ≦ 2.0 x 10⁻² cm³/g pore volume, measured by the mercury penetration method. In Japanese Unexamined Patent Publication No. 63-2076, we previously proposed a two-component type developer for the electrophotography, which comprises a ferrite carrier and an electroscopic toner, wherein the ferrite carrier is composed of ferrite particles in which the relation between the applied electric field and the current density is one represented by the Schottky plot, that is, a linear relation is established between the 1/2 power of the electric field intensity and the natural logarithm of the current density. In this prior art technique, by the Schottky effect is meant the phenomenon that with elevation of the applied electric field intensity at the emission of electrons, the saturation electric current is further increased. According to this prior art technique, the control of the electroconductivity by the Schottky emission of electrons from the surface layer of the ferrite having low electric resistance to the core of the ferrite having a high electric resistance is utilized. However, the conventional magnetic carrier composed of sintered ferrite particles shows a large electric resistance at a low electric field in the vicinity of the bias voltage but a small electric resistance at a high electric field in the vicinity of the developing electric field, and the dependency of the electric resistance on the developing electric field is large. Therefore, if the electric resistance at a low electric field is set at a low level, the electric resistance is further reduced on the high electric field side, and a brush mark or the like is formed in a solid image area. On the other hand, if an electric resistance is set at a high level on the low electric field side, the electric resistance is unnecessarily elevated on the high electric field side, and therefore, the edge effect is readily caused and the density is reduced in a solid image area while the toner is excessively developed in a letter or line image area and the consumption of the toner is increased. Furthermore, the toner adheres to the entire surface of a transfer sheet and background fogging is readily caused. Although ferrite particles are more chemically stable than the iron powder carrier, since the ferrite particles are sintered particles, the mechanical strength of the particles is poorer than that of the iron powder carrier, and while the developer is being used, the particles are destroyed and the life of the particles is relatively short. This defect is especially conspicuous when sintered ferrite particles having a relatively large particle size are used for a developer and the development is carried out in the state where the distance between the developing sleeve and photosensitive material (distance D-S) is relatively narrow. It is therefore a primary object of the present invention to provide a carrier for a two-component type developer, which can impart a sufficient density to a solid image area without occurrence of the edge effect and can prevent a toner from being consumed above a necessary level. Another object of the present invention is to provide a carrier for a two-component type developer, in which the life of the carrier is prolonged when the developer is used for the magnetic brush development. According to a first aspect the present invention provides a magnetic carrier to be used together with a toner for a two-component type developer, which is composed of sintered ferrite particles containing at least 80% by weight of particles in which the internal pore ratio based on the area is 3 to 30%. The electric resistance (R2500) at an electric field intensity of 2500 V/cm may be 1 x 10⁷ to 1 x 10¹¹ -cm and the developing electric field dependency defined by the ratio between said electric resistance R(2500) and the electric resistance R(5000) at an electric field intensity of 5000 V/cm may be 1.5 to 20. We previously found that in the case where the development is carried out at an ordinary speed, in order to clearly manifest a solid image area, it is preferred that the average particle size of the carrier particles be in the range of from 30 to 50 µm. We have now found that at the high-speed development recently adopted, it is even more advantageous if the volume median particle size of the carrier particles be in the range of from 70 to 110 µm, and if this requirement is satisfied, a fine line image area can be clearly developed and reproduced. Therefore, according to another aspect the present invention provides a carrier for a two-component type developer in which the volume median particle size is 70 to 110 µm and thus which is capable of clearly developing and reproducing a fine line image area. More specifically, in accordance with the present invention, there is provided a magnetic carrier having an excellent adaptability to high-speed development, to be used together with a toner for a two-component type developer, which is composed of sintered ferrite particles containing at least 80% by weight of particles in which the internal pore ratio based on the area is 3 to 30%, wherein the electric resistance (R2500) at an electric field intensity of 2500 V/cm is 1 x 10⁷ to 1 x 10¹¹ Ω-cm, the developing electric field dependency defined by the ratio between said electric resistance (R2500) and the electric resistance (R5000) at an electric field intensity of 5000 V/cm is 1.5 to 20, and the volume median particles size is 70 to 110 µm. In this carrier, the apparent density is preferably in the range of from 2.0 to 3.0 g/cc. In this carrier, the sintered ferrite particles have preferably a flowability of 20 to 30 sec/50 g as determined according to the method of JIS Z-2502. In this carrier, the sintered ferrite particles have preferably a specific surface area of 0.02 to 0.20 m²/g as determined according to the BET method. This carrier has preferably a saturation magnetization of 40 to 60 emu/g. Furthermore, in accordance with the present invention, there is provided a magnetic carrier to be used together with a toner for a two-component type developer, which is composed of sintered ferrite particles containing at least 80% by weight of particles in which the internal pore ratio based on the area is 3 to 30%, wherein the electric resistance (R1500) at an electric field intensity of 1500 v/cm is 1 x 10⁶ to 1 x 10¹⁰ Ω-cm, the developing electric field dependency defined by the ratio between said electric resistance (R1500) and the electric resistance (R150) at an electric field intensity of 150 V/cm is 5 to 20, and the volume median particle size is 30 to 50 µm. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Figs 1 and 2 are diagrams illustrating the relation between the resistance of a carrier and the electric field intensity. Fig.3 is an electron microscope of the internal particle structure of the carrier of the present invention. We found that if carrier particles having a specific internal pore ratio, a specific electric resistance and a specific particle size are used as a magnetic carrier for a two-component type developer, the above-mentioned defects of the conventional ferrite carrier, such as background fogging, excessive consumption of the toner and formation of brush marks, can be completely eliminated and the life of the carrier can be prolonged, and a sufficient toner density can be given to a solid image area at the development. The present invention has now been completed based on this finding. We also found that in case of high-speed development, if the volume median particle size of a carrier for a two-component type developer is increased to enhance the flowability of the carrier per se and the saturation magnetization quantity per carrier particle is increased, adhesion of the carrier at high-speed development can be prevented and a fine line image area can be clearly reproduced. The present invention has now been completed based on this finding. In the instant specification, by formation of brush marks is meant a phenomenon that many white fine lines are formed in a solid image, and by carrier dragging is meant a phenomenon that an electrostatic latent image is developed not only by the toner but also by the carrier, that is, also the carrier adheres to the toner image. Tailing resembles fogging in the point where the toner adheres to a portion other than the normal image, but tailing means the phenomenon that the toner adheres to the periphery of the normal image. In the present invention, it is important that a magnetic carrier containing a specific amount of ferrite particles having an internal pore ratio of 3 to 30%, especially 5 to 25%, based on the area should be used as the magnetic carrier. The internal pore ratio based on the area is a value determined according to the following formula form a microscope photo of a section of a carrier particle but through the center of the particle: Internal pore ratio (Pr) = sectional area of pores sectional area of entire particle x 100(%) The internal pore ratio based on the area of sintered ferrite particles has influences on the electric resistance of the particles. Fig. 1 is a graph illustrating the dependency of the electric resistance of the magnetic carrier on the applied voltage. In Fig. 1, line A illustrates the dependency of the conventional sintered ferrite carrier (the content of particles having Pr lower than 3% is 50%, the content of particles having Pr higher than 30% is 10% and De is equal to 25) and line B represents the dependency of the sintered ferrite carrier used in the present invention (the content of particles having Pr of 3 to 30% is 90% and De is equal to 5). Fig. 1 shows data of ferrite carrier particles to be used for high-speed development, which have a volume median particle size of 70 to 110 µm. The volume median particle size means the particle size of the particle located at the center of the distribution in the state where particles are distributed in the volume. Fig. 2 shows data of the above-mentioned dependency obtained with respect to carriers having a volume median particle size of 30 to 50 µm. The reason why the measurement region of the electric field intensity is different from that of Fig. 1 is that since the particle size is small, if the measurement is carried out at a high electric field intensity, the particles having the above particle size are scattered and the precise value cannot be obtained. In connection with the electric resistance of the magnetic carrier, in general, the surface resistance of carrier particles makes a larger contribution in the state where the electric field intensity is low, and the internal resistance of carrier particles makes a larger contribution in the state where the electric field intensity is high. Accordingly, in case of ordinary magnetic carrier particles, the surface resistance is mainly a resistance value at a low electric field intensity and the internal resistance is mainly a resistance value at a high electric field intensity. As shown in Figs. 1 and 2, in case of a carrier having an internal pure ratio included within the above-mentioned range, the resistance value is hardly influenced by the electric field intensity, and it is understood that the states of the surface resistance and internal resistance of carrier particles are not changed. Therefore, as shown in Fig. 1, the internal pore ratio has serious influences on the electric resistance of a carrier for a developer and also on the developing electric field dependency. In case of a carrier for a developer for high-speed development, it is important that the volume median particle size should be 70 to 110 µm, especially 80 to 100 µm. The developing electric field dependency is a value defined by the following formula (2): Developing electric field dependency (De) = R₂₅₀₀ R₅₀₀₀ wherein R2500 represents the electric resistance value (Ω-cm) of the magnetic carrier as measured at an electric field intensity of 2500 V/cm, and R5000 represents an electric resistance value (Ω-cm) of the magnetic carrier as measured at an electric field intensity of 5000 V/cm. The developing electric field dependency has the following significance. The reason why the electric resistance of the denominator is determined at an electric field intensity of 5000 V/cm is that the developing electric field intensity applied to the magnetic brush, that is, the electric field intensity between the photosensitive material and the developing sleeve, is substantially equal to the above-mentioned value. The bias voltage is applied to the developing sleeve and this voltage is generally in the range of from 100 to 300 V, and the developing electric field is set at 350 to 700 V. Furthermore, the distance between the photosensitive material and the developing sleeve (distance D-S) is set at 0.05 to 0.13 cm. Accordingly, the developing electric field intensity applied to the magnetic brush at the development becomes the above-mentioned value. the reason why the electric resistance of the numerator is determined at an electric field intensity of 2500 V/cm is that the electric field intensity given only by the bias voltage applied to the developing sleeve is substantially equal to this value and this intensity has influences mainly on the adhesion of the carrier and the background fogging. In the case where the internal pore ratio based on the area of ferrite carrier particles is within the above-mentioned range, the developing electric field dependency of the electric resistance is greatly influenced by the internal pore ratio. Namely, if the internal pore ratio of the particles increases, the denominator R5000 of formula (2) increases while the numerator R2500 decreases. In short, the internal resistance of the sintered ferrite is brought close to the surface resistance of the sintered ferrite. In this feature, this sintered ferrite is conspicuously distinguishable over the conventional sintered ferrite particles where the developing electric field dependency of the electric resistance is very large. It is important that the developing electric field dependency (De) of the carrier for high-speed development should be adjusted to 1.5 to 20. The developing electric field dependency corresponds to the change of the electric resistance of the carrier between the state where an electric field is applied to the carrier and the state where no electric field is applied to the carrier. That is, when the electric field is changed, the charging state of the carrier can be known from this developing electric field dependency. In the present invention, since the developing electric field dependency of the carrier is within the above-mentioned range, reduction of the resistance of the carrier can be attained on the low voltage side and simultaneously, elevation of the resistance of the carrier is attained on, the high voltage side, with the result that background fogging, excessive consumption of the toner (decrease of the transfer ratio of the toner) and formation of brush marks are eliminated and an image having an excellent gradient can be obtained. If the content of particles in which the internal pore ratio (Pr) based on the area is 3 to 30% is lower than 50% and the proportion of particles in which the internal pore ratio is lower than 3% increases, however controlled and adjusted the ferrite substrate may be, it becomes difficult to control the developing electric field dependency (De) within the range specified in the present invention, with the result that formation of brush marks and carrier dragging tend to occur. On the other hand, if the content of particles having an internal pore ratio (Pr) higher than 30% increases, it is difficult to maintain a good image density, and the gradient and the durability of the carrier are degraded. If the content of particles having an internal pore ratio (Pr) of 3 to 30% is lower than 80%, the image density and other characteristics as mentioned above cannot be sufficiently maintained. From the viewpoint of the image quality, in the carrier to be used for high-speed development, it is important that the value of R2500 should be 1 x 10⁷ to 1 x 10¹¹ Ω -cm, especially 1 x 10⁸ to 5 x 10¹⁰ Ω-cm. If this requirement of the electric resistance and the above-mentioned requirement of the internal pore ratio are satisfied, the ranges of the electric resistance value of the carrier on the low electric field side and the electric resistance value of the carrier on the high electric field side are substantially determined, and values included within these ranges are appropriate for the carrier and within these ranges, formation of brush marks, carrier dragging and background fogging are not caused in the developer. If a carrier satisfying the foregoing requirements and having an apparent density of 2.0 to 3.0 g/cc, especially 2.2 to 2.8 g/cc, is used for a two-component type developer, the saturation magnetization quantity per carrier particle is maintained at a high level stably, and the adhesion of the carrier at high-speed development can be prevented and an excellent image quality can be obtained at a high developing speed. The apparent density is generally determined by the density, particle size and shape of particles per se, and the apparent density of the carrier is influenced by the above-mentioned range of the pore ratio of ferrite particles and the above-mentioned range of the particle size and has close relations to these factors. Accordingly, it is understood that the above-mentioned functions to the stirring load and the saturation magnetization are attained by the combination of the foregoing requirements. It is also important that the saturation magnetization of the carrier should be in the range of from 40 to 60 emu/g, especially from 45 to 55 emu/g. In the case where the saturation magnetization is within the above range, the ear of the developer brush formed on the surface of the developing sleeve can be kept soft and tile image quality can be improved. If the saturation magnetization of the carrier is within the above range, since the saturation magnetization quantity is large as pointed out above, carrier dragging can be prevented and the distance D-S can be diminished. Fig. 2 is a graph illustrating the dependency of the electric resistance of the magnetic carrier on the applied voltage. Line A shows data obtained with respect to the conventional sintered ferrite carrier (the content of particles having an internal pore ratio lower than 3% is 50%, the content of particles having an internal pore ratio of 3 to 30% is 45%, the content of particles having an internal pore ratio higher than 30% is 5% and De is equal to 50), and line B shows data obtained with respect to the sintered ferrite carrier used in the present invention (the content of particles having an internal pore ratio lower than 3% is 5%, the content of particles having an internal pore ratio of 3 to 30% is 85%, the content of particles having an internal pore ratio higher than 30% is 10% and De is equal to 11.5). Fig. 2 relates to a carrier for improving the reproducibility of a solid image area, and it is important that this carrier particles should have a volume median particle size of 30 to 50 µm, especially 35 to 45 µm. The developing electric field dependency is a value defined by the following formula: Developing electric field dependency (De) = R₁₅₀ R₁₅₀₀ wherein R1500 represents the electric resistance (Ω-cm) of the magnetic carrier as measured at an electric field intensity of 1500 V/cm and R150 represents the electric resistance (Ω-cm) as measured at an electric field intensity of 150 V/cm. The reason why the developing electric field dependency of the carrier of this type is represented by formula (3) is that the particle size of the carrier particles is small. If the measurement is carried out at a high electric field, migration of the carrier particles on the electric field is caused and the measurement cannot be precisely performed. In the present invention, it is important that the developing electric field dependency (De) of the carrier particles should be adjusted within the range of from 5 to 20. If the value of the developing electric field dependency is smaller than 5, tailing is readily caused in the formed image and the toner is readily scattered during the development. It is considered that the reason is that since the resistance of the carrier at the charging is too low, the charge quantity of the toner is reduced. If the value of the electric field dependency is larger than 20, brush marks are readily formed. The reason is considered to be that since the resistance of the carrier at the application of the bias voltage is too low, leakage of charges on the surface of the photosensitive material is caused. From the viewpoint of the image quality, it is important that the value of R1500 of the carrier should be in the range of from 1 x 10⁶ to 1 x 10¹⁰ Ω-cm, especially from 1 x 10⁷ to 5 x 10⁹ Ω-cm. If the value of R1500 is below the above range, formation of brush marks and carrier dragging are readily caused, and if the value of R1500 exceeds the above range, good development of a solid image area is not attained. In the present invention, it also is important that the carrier should comprise sintered ferrite particles having the above-mentioned characteristics in an amount of at least 80% by weight based on the entire magnetic carrier. If the amount of the sintered ferrite particles is below the above range, the above functions of the present invention cannot be exerted. If fine particles having a particle size smaller than 400 mesh (37 µm) are contained in an amount of 20 to 40% by weight, the ear of the magnetic brush formed on the developing sleeve can be kept soft. If the amount of particles having this particle size exceeds 40% by weight, carrier dragging is readily caused. If the amount of particles having this particle size is smaller than 20% by weight, the ear of the magnetic brush becomes hard and the image quality is poorer than the image quality attained when the amount of particles having the above particle size is within the above-mentioned range. It is preferred that the saturation magnetization of the carrier be 50 to 60 emu/g. If the saturation magnetization of the carrier is within this range, the ear of the magnetic brush formed on the developing sleeve can be kept soft and good image quality can be obtained. By imparting the above-mentioned properties to the carrier particles, the obtained two-component type developer can provide an image having an excellent quality, and the adaptability to the operation in the apparatus, especially the adaptability to high-speed development, can be improved. The two-component type developer comprises a magnetic carrier and a toner as two main components. Preferred embodiments of the two-component type developer and these two main components will now be described. Magnetic Carrier The carrier is composed mainly of sintered ferrite particles, and the ferrite particles have a known ferrite composition. As the magnetic pigment, there can be mentioned, for example, zinc iron oxide (ZnFe₂O₄), yttrium iron oxide (Y₃Fe₅O₁₂), cadmium iron oxide (Cd₃Fe₅O₁₂), gadolinium iron oxide (Gd₃Fe₅O₁₉), copper iron oxide (CuFe₂O₄), lead iron oxide (PbFe₁₂O₁₉), neodium iron oxide (NdFeO₃), barium iron oxide (BaFe₁₂O₁₉), magnesium iron oxide (MgFe₂O₄), manganese iron oxide (MnFe₂O₄) and lanthanum iron oxide (LaFeO₃). These magnetic pigments can be used alone or in the form of a mixture of two or more of them. A soft ferrite containing at least one metal component, preferably at least two metal components, selected from the group consisting of Cu, Zn, Mg and Ni, for example, a copper-zinc-magnesium ferrite, is preferably used as the ferrite. A ferrite comprising 35 to 70 mole% of Fe₂O₃, 5 to 15 mole% of CuO, 5 to 35 mole% of ZnO and 0 to 40 mole% of MnO, is especially preferably used. If the primary particle size of oxide particles constituting a ferrite increases, in general, the internal pore ratio increases. In contrast, if this primary particle size becomes small, the internal pore ratio tends to drop. In the production of ferrite particles by sintering, the higher is the sintering degree, the lower becomes the internal pore ratio. For example, the higher is the sintering temperature, the lower is the internal pore ratio, and the longer is the sintering time, the lower is the internal pore ratio. Of course, if the particle size of the starting ferrite material or intermediate material is fine, the sintering is advanced under milder conditions than in case or the starting material or intermediate material having a coarse particle size. Therefore, the foregoing factors should be combined carefully so that the intended internal pore ratio can be attained. The process for the preparation of this internal pore-containing sintered ferrite is not particularly critical, and for example, there can be adopted a process in which a starting metal oxide material or intermediate material having a primary particle size of 0.1 to 2.0 µm is used and molded into a predetermined particulate shape, and the molded body is sintered at a temperature of 900 to 1500°C for 5 to 50 hours. Fig. 3 is a sketch of a microscope photo (800 magnifications) showing the internal structure of sintered ferrite particles used in the present invention. In the present invention, the preparation is carried out according to the above-mentioned process so that sintered ferrite particles in which the internal pore ratio based on the area is 3 to 30%, especially 5 to 25%, are mainly formed from a ferrite composition and the amount or sintered ferrite particles having the above-mentioned internal pore ratio is at least 80% by weight based on the entire magnetic carrier. Particles having an internal pore ratio higher than 30% are inferior in the durability, and if the occupancy ratio of particles having an internal pore ratio lower than 3% increases, the adjustment of the developing electric field dependency becomes difficult. In the carrier for high-speed development, the average particle size is controlled so that the, volume median particle size is 70 to 110 µm, especially 80 to 100 µm. If the particle size is below the above range, the saturation magnetization quantity at the development is reduced because of this small particle size as well as the apparent density, adhesion of the carrier is readily caused. If the particle size exceeds the above range, the ear of the magnetic brush formed on the developing sleeve at the development becomes hard, and therefore, the image quality is degraded. It is important that the electric resistance or the carrier should be 1 x 10⁷ to 1 x 10¹¹ Ω-cm, especially 1 x 10⁸ to 1 x 10¹⁰ Ω-cm, as measured at an electric field intensity of 2500 V/cm, and it also is important that the developing electric field dependency (De) should be in the range of from 1.5 to 20. The electric resistance value can be adjusted within the above-mentioned range by controlling the mixing ratio of the components in the ferrite composition and changing the internal pore ratio within the above-mentioned range. The apparent density of the carrier is set at 2.0 to 3.0) g/cc, especially 2.2 to 2.8 g/cc. If the apparent density exceeds the above range, the stirring load at the development becomes large and the saturation magnetization is reduced, and adhesion of the carrier cannot be prevented at the high-speed development. It is preferred that the saturation magnetization of the carrier be in the range of from 40 to 60 emu/g, especially from 45 to 55 emu/g. The saturation magnetization of the sintered ferrite particles can be controlled within the above-mentioned range by changing the above-mentioned ferrite composition. If the saturation magnetization is below the above-mentioned range, carrier dragging is caused at the development, and if the saturation magnetization exceeds the above-mentioned range, the ear of the magnetic brush formed on the developing sleeve becomes hard and the image quality is degraded. In order to obtain a high density in a solid image area by the development, it is preferred that the volume median particle size of the carrier be 30 to 70 µm, especially 35 to 50 µm. It also is preferred that fine particles having a particle size smaller than 400 mesh be contained in an amount of 20 to 40% by weight based on the carrier. In this case, it is important that the electric resistance of the carrier should be 1 x 10⁶ to 1 x 10¹⁰ Ω-cm, especially 1 x 10⁶ to 5 x 10⁹ Ω-cm, as measured at an electric field intensity of 1500 V/cm, and it also is important that the developing electric field dependency (De) should be 5 to 20. It is preferred that the saturation magnetization of the carrier be 50 to 60 emu/g and the apparent density of the carrier be 1.8 to 2.5 g/cc, especially 2.0 to 2.3 g/cc. The flowability of the carrier particles can be adjusted to 20 to 30 sec/50 g, especially 20 to 28 sec/50 g, as determined according to JIS Z-2502. If the flowability of the carrier particles is below the above range, the production becomes difficult, and if the flowability exceeds the above range, shapes of the carrier particles become irregular and the pulverization resistance is reduced, and the internal pore strength is adversely influenced. Furthermore, in the present invention, by uniformalizing primary particles of the carrier composition, ferrite particles having a specific surface area adjusted to 0.02 to 0.2 m²/g can be obtained. This specific surface area is one measured by the BET method, if the specific surface area of the particles exceeds the above range, it is apprehended that the above-mentioned internal pores will have influences on the surfaces and the particles will be readily destroyed. Toner Any of coloring toners having electroscopic and fixing properties can be used, and a granular composition formed by dispersing a coloring pigment, a charge-controlling agent and the like into a binder resin, which has a particle size of 5 to 30 µm, is generally used. As the binder resin, there can be used thermoplastic resins, uncured thermosetting resins and precondensates of thermosetting resins. As preferred examples, there can be mentioned, in order of importance, a vinyl aromatic resin such as polystyrene, an acrylic resin, polyvinyl acetal, a polyester, an epoxy resin, a phenolic resin, a petroleum resin and an olefin resin. As the colorant, there can be used carbon black, pigments such as a quinacridone pigment, a rhodamine pigment, a thioindigo pigment, an azo pigment, a phthalocyanine pigment and a nitro pigment, and C.I. Solvent Red 49, C.I. Solvent Red 19, C.I. Solvent Blue 70, C.I. Solvent Yellow 19 and C.I. Solvent Yellow 21. A charge-controlling agent, for example, an oil-soluble dye such as Nigrosine Base (C.I. 50615) or Oil Black (C.I. 26150), a metal-containing azo dye or a metal complex of salicylic acid, naphthoic acid or a fatty acid metal soap, is used according to need. The electroscopic toner particles used in the present invention preferably have such particle size characteristics that the median particle size is 5 to 35 µm, especially 7 to 20 µm. Two-Component Type Developer A magnetic carrier composed of the above-mentioned ferrite particles and a toner are mixed at such a ratio that the content of the magnetic carrier is at least 90% by weight, especially 92 to 98% by weight, and the content of the toner is up to 10% by weight, especially 2 to 8% by weight, whereby a two-component type developer is obtained. The developer having the above mixing ratio is mixed and stirred and is then supplied onto a developing sleeve having magnets arranged in the interior thereof and brought into sliding contact with the surface of a photosensitive material having an electrostatic image to form a toner image. The toner image is transferred onto a copying sheet and is contacted with a heating roller to obtain a copy having a fixed toner image. According to the present invention, by using a sintered ferrite having a specific internal pore ratio based on the area, a specific electric resistance and a specific particle size at a specific mixing ratio as a carrier of a two-component type developer, a sufficiently high density is attained in a solid image area without any edge effect in the formed toner image. Furthermore, the life of the developer is long and unnecessary consumption of the toner is prevented. Moreover, by adjusting the particle size of the ferrite carrier within a specific range, a developer suitable for high-speed development can be provided. The present invention will now be described with reference to the following experiments. Experiment 1 Carrier samples 1-1 through 1-8 having characteristics shown in Table 1 were used. A basic composition of an electroscopic toner comprises 100 parts by weight of a styrene-acrylic polymer, 10 parts by weight of carbon black, 1.0 part by weight of a metal-containing azo dye and 1.5 parts by weight of low-molecular-weight polypropylene. Toner particles having a median diameter of 13 µm were prepared from this electroscopic toner according to customary procedures, and 100 parts by weight of the toner particles were surface-treated with 0.3 part by weight of hydrophobic silica and 0.15 part by weight of alumina. The obtained toner composition was mixed with the carrier sample to obtain a two-component type developer having a toner concentration of 5%. High-speed development was carried out by using the obtained developer in a remodelled machine of Model DC-4555 (supplied by Mita Industrial Co., Ltd.), and the printing test was carried out under a developing voltage difference of 500 V and a bias voltage of 250V to obtain 50000 copies. The obtained results are shown in Table 1. Experiment 2 Carrier samples 2-1 through 2-10 having characteristics shown in Table 2 were used. The same toner composition as used in Experiment 1 was mixed with the carrier sample to obtain a two-component developer having a toner concentration of 9%. By using the obtained developer, the printing test was carried out in a remodelled machine of Model DC-4055 (supplied by Mita Industrial Co., Ltd.) under a developing voltage difference of 570 V and a bias voltage of 250 V to obtain 50000 copies. The obtained results are shown in Table 2.

Claims: A magnetic carrier to be used together with a toner for a two-component type developer, which is composed of sintered ferrite particles containing at least 80% by weight of particles in which the internal pore ratio based on the area is 3 to 30%, wherein the electric resistance (R2500) at an electric field intensity of 2500 V/cm is 1 x 10⁷ to 1 x 10¹¹Ω.cm, the developing electric field dependency defined by the ratio between said electric resistance (R2500) and the electric resistance (R5000) at an electric field intensity of 5000 V/cm is 1.5 to 20, and the volume median particle size is 70 to 110 µm. A carrier as set form in claim 1, wherein the apparent density is in the range of from 2.0 to 3.0 g/cc. A magnetic carrier to be used together with a toner for a two-component type developer, which is composed of sintered ferrite particles containing at least 80% by weight of particles in which the internal pore ratio based on the area is 3 to 30%, wherein the electric resistance (R1500) at an electric field intensity of 1500 V/cm is 1 x 10⁶ to 1 x 10¹⁰Ω.cm, the developing electric field dependency defined by the ratio between said electric resistance (R1500) and the electric resistance (R150) an electric field intensity of 150 V/cm is 5 to 20, and the volume median particle size is 30 to 50 µm. A carrier as set forth in claim 3, wherein particles having a particle size smaller than 400 mesh (37 µm) are contained in an amount of 20 to 40% by weight based on the entire carrier. A carrier as set forth in claim 3 or 4, wherein the apparent density is in the range of from 1.8 to 2.5 g/cc. A carrier as set forth in any preceding claim, wherein the sintered ferrite particles have a flowability of 20 to 30 sec/50 g as measured by the method of JIS Z-2502. A carrier as set forth in any preceding claim, wherein the sintered ferrite particles have a specific surface area of 0.02 to 0.20 m² /g as measured by the BET method. A carrier as set forth in claim 1 or 2, wherein the saturation magnetization is in the range of from 40 to 60 emu/g. A two-component type developer for use in electrophotography or "photocopying", the developer comprising a carrier according to any one of the preceding claims.

Inventors: ASADA HIDENORI, KIMURA TAKAHIKO, KOMATA HIROSHI, TSUJI NOBUYUKI, YAMADA SHIGEKI, ASADA, HIDENORI, KIMURA, TAKAHIKO, KOMATA, HIROSHI, TSUJI, NOBUYUKI, YAMADA, SHIGEKI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9