Carrier for electrophotographic developer, process for preparing the same and developer prepared by using said carrier

Abstract: The present invention relates to a carrier for an electrophotographic developer, which has a mean particle diameter of 15 to 50 µm, a magnetization of 30 to 190 emu/g at 3000 Oe, an apparent density of 1.3 to 4.2 g/cm³ and a percentage sphericity of 80% or more; a process for preparing the carrier by the plasma method; and an electrophotographic developer comprising the same.

Description: The present invention relates to iron carrier particles for an electrophotographic developer used in a copying machine, a printer, etc., for electrophotography, a process for preparing the same and a developer prepared by using said iron carrier particles. Various carrier particles have hitherto been proposed, and examples thereof include those disclosed in U.S. Patent Nos. 2,618,551, 2,638,416, 2,618,552, 3.526,533, 3,533,835 and 3,591,503. Further, resin-coated carrier particles comprising carrier nucleus particles coated with various resins have been proposed for the purpose of prolonging the service life and regulating the amount of charge and resistance of the carrier particles. Materials such as non-metals, metals and metal alloys, such as sand, cobalt, iron, copper, nickel, zinc, aluminum, brass, glass and ferrite, and composite metal oxides have been used as the carrier nucleus particle to be coated with the above-described resins. Although toner particles having a mean particle diameter of about 10 to 20 µm have hitherto been used as a developer, there is a tendency that the demand for a high image quality makes it necessary to reduce the diameter of the toner particles. With a reduction in the diameter of the toner particles, it is necessary to increase the specific surface area of the carrier particles with a view to increasing the capability of imparting a charge to the toner particles. In the conventional carrier particles, since the mean particle diameter is about 50 to 150 µm and the surface area is small, the increase in the capability of imparting a charge to the toner particles is unsatisfactory. Although carrier particles in an irregular form having a mean particle diameter of 35 to 50 µm have been known, they are remarkably poor in the fluidity due to their irregular forms, so that the capability of imparting a charge to the toner particles is unsatisfactory. In order to improve the fluidity, an attempt has been made on the sphering of the carrier particles, and atomizing granulation or the like is known as a means for attaining this purpose. However, it has been practically impossible to prepare a spherical powder having a size of 50 µm or less in a high yield. Japanese Patent Application Laid-Open Gazette No. 60-35764 discloses electrophotographic developer mixtures consisting of toner particles and carrier particles in which the carrier particles are spherically insulated having 5 to 15 µm particle diameters, a magnetization of up to 100 emu/g and an apparent density of up to 4.5 g/cm³. WO-A-86/01314 discloses a ferrite toner carrier and core composition. A particulate feedstock, containing spherical ferrite particles, preferably fly ash, is used. An object of the present invention is to provide carrier particles which, in order to cope with a reduction in the particle size of a toner, can increase the capability of imparting a charge to the toner and further increase the toner concentration, is less susceptible to a change in the image quality of a copy when the toner concentration changes somewhat, and can dispense with the toner concentration control device essential to the conventional copying machine; and a process for preparing the same. Further, a final object of the present invention is to improve the performance of an electrophotographic developer. A toner is greatly influenced by the fluidity and specific surface area of the carrier particles. Specifically, the better the fluidity and the larger the specific surface area of the carrier particles, the greater the capability of the carrier particles to impart a charge to the toner. In particular, the fluidity of the toner per se becomes very poor with a reduction in the particle size of the toner, which makes it very important to take a measure through an improvement in the carrier particles. According to the present invention there is provided iron carrier particles for an electrophotographic developer, the carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m ( 10³ / 4π A/m = 1 Oe), an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. The invention also comprises an electrophotographic developer comprising toner particles and resin coated carrier particles prepared by coating the surface of the carrier particles of the invention with a resin. Also in accordance with the present invention there is provided a process for preparing iron carrier particles for an electrophotographic developer, which comprises melting iron particles by a direct plasma method in an inert atmosphere under a reduced pressure of 66661Pa (500 Torr) or less, cooling and recovering said iron particles to prepare iron carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. Further in accordance with the present invention there is provided a process for preparing iron carrier particles for an electrophotographic developer, which comprises melting iron particles by a high frequency plasma method, cooling and recovering said iron particles to prepare iron carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. Still further in accordance with the present invention there is provided a process for preparing iron carrier particles for an electrophotographic developer, which comprises melting iron particles by a hybrid plasma method, cooling and recovering said iron particles to prepare iron carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. In the present invention, the carrier particles should be spherical. A high fluidity can be obtained through the sphering of the carrier particles. The term "spherical carrier" as used herein is intended to mean one wherein when the carrier particles are observed under a scanning electron microscope, particles having a major axis to minor axis ratio of 1.0 to 1.25 amount to 80% or more of the carrier, that is, one having a percentage sphericity of 80% or more. In the carrier particles used in the present invention, it is desired that the specific surface area determined by the air permeation method (for example, by making use of SS-100 manufactured by Shimadzu Seisakusho Ltd.) be 350 cm²/g or more, and this enables the carrier particles to have a capability of imparting a sufficient property of charging the toner. As described above, the fluidity and specific surface area of the carrier particles have a great influence on the charge imparting property of the toner. However, it is possible to greatly change the absolute value through the coating of the surface of the carrier particles with a resin. In the conventional carrier particles having a large mean particle diameter, it has been a usual practice to conduct thick film coating by making use of a large amount of a resin for the purpose of improving the image quality through an enhancement in the resistance. In the carrier particles having a small mean particle diameter of the present invention, however, as compared with the conventional coating carrier particles, it is easy to conduct coating and obtain a high image quality for reasons including that, even when the coating layer is thin, the resistance is increased due to an increase in the number of carrier particles. There is no particular limitation on the resin to be used, and examples of the resin include those known as a coating resin for a carrier nucleus particle in the art, such as natural resins, thermoplastic resins and partially cured thermosetting resins. The carrier particles for the present invention are prepared by melting the above-described raw material for the carrier particles by the plasma method. More particularly, the carrier particles are prepared by the plasma method which comprises melting iron particles for the carrier particles by a plasma method in an inert or oxidizing atmosphere and cooling and recovering the melt when it is sphered by the surface tension. The reason why the plasma method is employed for the preparation of carrier particles is that desired properties, i.e. desired particle diameter, magnetization, apparent density, etc., can be obtained by this method. Examples of the above-described plasma method include a direct current plasma method wherein use is made of a direct current arc plasma flame, a high frequency plasma method wherein use is made of a high frequency plasma flame, and a hybrid plasma method wherein use is made of a hybrid plasma flame comprising a direct current plasma with a high frequency plasma. Any of the above methods may be used in the present invention. The high frequency plasma method and hybrid plasma method are preferred from a viewpoint of attaining a high percentage sphericity, and the hybrid plasma method is more preferred from the viewpoint of stably sphering a large amount of iron. When the direct current plasma method is conducted in an inert atmosphere, the method is conducted under a reduced pressure of 66661 Pa (500 Torr) or less from the viewpoint of the properties of the carrier obtained. A developer prepared together with a toner through the use of the carrier particles thus obtained has a very excellent performance. As described above, the carrier particles of the present invention are prepared by melting iron particles through a plasma method such as a direct current, high frequency or hybrid plasma method and has a specific properties, so that according to a reduction in the particle size of the toner, the capability of imparting a charge to the toner can be increased, the toner concentration can be increased, a slight change in the toner concentration hardly brings about a change in the image quality of the copy and further the toner concentration control device essential to the conventional copying machine can be omitted. The present invention will now be described in more detail by referring to Examples and so forth. Example 1 A vessel purged with an inert atmosphere was reduced to 6666 Pa (50 Torr). An irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter of 31.2 µm. As shown in Table 1, this carrier had a magnetization of 170 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.56 g/cm³ and a specific surface area of 420 cm²/g. The above-described carrier particles were observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more. A developer was prepared by making use of the above-prepared carrier particles and toner particles (a styrene-acrylic resin) and subjected to a test through the use of a commercially available actual copying machine. The toner concentration was 15%. The image properties (image density, fogging, scratching mark, scattering of carrier, service life, and overall evaluation) are given in Table 1. In Table 1, o ○ , ○, Δ, and X represent that the properties are excellent, good, slightly poor and poor, respectively. Example 2 The surfaces of the carrier particles prepared in Example 1 were coated with an acrylic resin. A developer was prepared by making use of the resin-coated carrier particles thus prepared and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. The toner concentration was 12%. The image properties are given in Table 1. Example 3 A vessel purged with an inert atmosphere was reduced to 100 Torr. An irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare carrier particles having a mean particle diameter of 30.4 µm. As shown in Table 1, these carrier particles had a magnetization of 175 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.72 g/cm³ and a specific surface area of 418 cm²/g. The above-described carrier particles were observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more. A developer was prepared by making use of the above-prepared carrier particles and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. The image properties are given in Table 1. Comparative Example 1 A vessel purged with an inert atmosphere was reduced to 100 Torr. An irregular form of iron powder having a mean particle diameter of 22 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter of 14.5 µm. As shown in Table 1, this carrier had a magnetization of 170 emu/g at 3000 Oe, an apparent density of 2.2 g/cm³ and a specific surface area of 671 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more. A developer was prepared by making use of the above-prepared carrier and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. As a result, a deposition of the carrier on the image was observed. The image properties are given in Table 1. Comparative Example 2 A vessel purged with an inert atmosphere was reduced to 100 Torr. An irregular form of iron powder having a mean particle diameter of 60 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter 53 µm. As shown in Table 1, this carrier had a magnetization of 175 emu/g at 3000 Oe, an apparent density of 3.92 g/cm³ and a specific surface area of 343 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more. A developer was prepared by making use of the above-prepared carrier and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. As a result, the toner scattered, and staining occurred on a non-image area. The image properties are given in Table 1. Comparative Example 3 A vessel purged with an inert atmosphere was reduced to 50 Torr. A Cu-Zn ferrite having a magnetization of 32 emu/g and a mean particle diameter of 40 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter of 36.2 µm. As shown in Table 1, this carrier had a magnetization of 28 emu/g at 3000 Oe, an apparent density of 2.81 g/cm³ and a specific surface area of 524 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more. A developer was prepared by making use of the above-prepared carrier and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. As a result, the carrier scattered on the image area. The image properties are given in Table 1. Comparative Example 4 A vessel purged with an inert atmosphere was reduced to 100 Torr. An irregular form of atomized iron powder having a mean particle diameter of 49 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter of 40 µm. As shown in Table 1, this carrier had a magnetization of 175 emu/g at 3000 Oe, an apparent density of 4.4 g/cm³ and a specific surface area of 355 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more. A developer was prepared by making use of the above-prepared carrier and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. As a result, the image density was sufficient but scratch marks occurred. The image properties are given in Table 1. Comparative Example 5 A vessel purged with an inert atmosphere was reduced to 680 Torr. An irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter of 32.5 µm. As shown in Table 1, this carrier had a magnetization of 178 emu/g at 3000 Oe, an apparent density of 3.69 g/cm³ and a specific surface area of 429 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity as low as about 50% or more. A developer was prepared by making use of the above-prepared carrier and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. As a result, ears on the magnetic brush were heterogeneous, the image was significantly rough, and the fluidity of the developer became remarkably poor. The image properties are given in Table 1. Comparative Example 6 A developer was prepared by making use of Cu-Zn ferrite carrier F-150 (a spherical powder having a mean particle diameter of 80 µm) (a product of Powdertec Co., Ltd.) having properties specified in Table 1 by the granulation method and the toner used in Example 1, and subjected to a test through the use of a commercially available actual copying machine. As a result, the image density and homogeneity of the solid area were inferior to those of the developer wherein use was made of the carrier of Example 1. The toner concentration was 4%. The image properties are given in Table 1. Comparative Example 7 A developer was prepared by making use of spherical iron carrier ASR-1020 (mean particle diameter: 100 µm) (a product of Powdertec Co., Ltd.) having properties specified in Table 1 by the atomization method and the toner used in Example 1, and subjected to a test through the use of a commercially available actual copying machine. The toner concentration was 2.5%. The image properties are given in Table 1. Comparative Example 8 The surface of the carrier prepared in Comparative Example 7 was coated with an acrylic resin. A developer was prepared by making use of the resin-coated carrier thus prepared and the toner used in Example 1, and then subjected to a test through the use of a commercially available actual copying machine. The toner concentration was 2.5%. The image properties are given in Table 1. Comparative Example 9 A vessel purged with an inert atmosphere was reduced to 550 Torr. An irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a direct current arc plasma flame produced in the vessel to prepare a carrier having a mean particle diameter of 32 µm. As shown in Table 1, this carrier had a magnetization of 175 emu/g at 3000 Oe, an apparent density of 3.65 g/cm³ and a specific surface area of 435 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity as low as about 70% or more. A developer was prepared by making use of the above-prepared carrier and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. As a result, ears on the magnetic brush were heterogeneous, and the fluidity of the developer was poor as well. The image properties are given in Table 1. Example 4 In a vessel of an inert argon atmosphere, an irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a high frequency plasma flame to prepare carrier particles having a mean particle diameter of 30.5 µm. As shown in Table 1, these carrier particles had a magnetization of 175 emu/g at 3000 10³ / 4π A/m an apparent density of 3.56 g/cm³ and a specific surface area of 420 cm²/g. The above-described carrier particles were observed under a scanning electrons microscope and found to have a percentage sphericity of 90% or more. A developer was prepared by making use of the above-prepared carrier particles and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine. The toner concentration was 15%. The image properties are given in Table 1. Example 5 The surfaces of the carrier particles used in Example 4 were coated with the acrylic resin used in Example 2. A developer was prepared by making use of the resin-coated carrier particles and the toner used in Example 1 and then subjected to a test by making use of an actual copying machine. The toner concentration was 15%. The image properties are given in Table 1. Example 6 In a vessel of an inert argon atmosphere, an irregular form of iron powder having a mean particle diameter of 35 µm was thrown into a high frequency plasma flame to prepare carrier particles having a mean particle diameter of 26 µm. As shown in Table 1, these carrier particles had a magnetization of 160 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.54 g/cm³ and a specific surface area of 550 cm²/g. The above-described carrier particles were observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more. The surfaces of the carrier particles were coated with the acrylic resin used in Example 2. A developer was prepared by making use of the coated carrier thus prepared and the toner used in Example 1, and a test was conducted by making use of an actual copying machine and the developer thus prepared. The toner concentration was 20%. The image properties are given in Table 1. Example 7 In a vessel of an inert argon atmosphere, an irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a hybrid plasma flame to prepare carrier particles having a mean particle diameter of 29 µm. As shown in Table 1, these carrier particles had a magnetization of 172 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.59 g/cm³ and a specific surface area of 435 cm²/g. The above-described carrier particles were observed under a scanning electron microscope and found to have a percentage sphericity of 95% or more. The surfaces of the carrier particles was coated with the acrylic resin used in Example 2. A developer was prepared by making use of the coated carrier particles thus prepared and the toner used in Example 1, and a test was conducted by making use of an actual copying machine and the developer thus prepared. The toner concentration was 15%. The image properties are given in Table 1. Comparative Example 10 In a vessel of an inert argon atmosphere, an irregular form of iron powder having a mean particle diameter of 22 µm was thrown into a high frequency plasma flame to prepare a carrier having a mean particle diameter of 13.5 µm. As shown in Table 1, this carrier had a magnetization of 140 emu/g at 3000 Oe, an apparent density of 2.2 g/cm³ and a specific surface area of 682 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more. A developer was prepared by making use of the carrier thus prepared and the toner used in Example 1, and a test was conducted by making use of an actual copying machine and the developer thus prepared. As a result, deposition of the carrier was observed on an image. The image properties are given in Table 1. Comparative Example 11 In a vessel of an inert argon atmosphere, an irregular form of iron powder having a mean particle diameter of 60 µm was thrown into a high frequency plasma flame to prepare a carrier having a mean particle diameter of 55 µm. As shown in Table 1, this carrier had a magnetization of 175 emu/g at 3000 Oe, an apparent density of 3.91 g/cm³ and a specific surface area of 343 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more. A developer was prepared by making use of the carrier thus prepared and the toner used in Example 1, and a test was conducted by making use of an actual copying machine and the developer thus prepared. As a result, the toner scattered, and staining occurred on a non-image area. The image properties are given in Table 1. Comparative Example 12 In a vessel of an inert argon atmosphere, a Cu-Zn ferrite having a mean particle diameter of 40 µm and a magnetization of 32 emu/g was thrown into a high frequency plasma flame to prepare a carrier having a mean particle diameter of 35.4 µm. As shown in Table 1, this carrier had a magnetization of 29 emu/g at 3000 Oe, an apparent density of 2.80 g/cm³ and a specific surface area of 531 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more. A developer was prepared by making use of the carrier thus prepared and the toner used in Example 1, and a test was conducted by making use of an actual copying machine and the developer thus prepared. As a result, the carrier scattered on an image area. The image properties are given in Table 1. Comparative Example 13 In a vessel of an inert argon atmosphere, an irregular form of iron powder having a mean particle diameter of 49 µm was thrown into a high frequency plasma flame to prepare a carrier having a mean particle diameter of 39 µm. As shown in Table 1, this carrier had a magnetization of 176 emu/g at 3000 Oe, an apparent density of 4.4 g/cm³ and a specific surface area of 348 cm²/g. The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more. A developer was prepared by making use of the carrier thus prepared and the toner used in Example 1, and a test was conducted by making use of a commercially available actual copying machine and the developer thus prepared. As a result, although the image density was satisfactory, a brush mark occurred. The image properties are given in Table 1. As is apparent from the comparison of the Examples with the Comparative Examples given in Table 1, the use of the carrier particles of the present invention enables good image properties to be obtained in a test on the developer through the use of an actual copying machine. Regarding the difference in the methods of preparing the carrier particles, a developer comprising carrier particles prepared by the high frequency plasma method is generally superior in the image property to a developer comprising carrier particles prepared by the direct plasma method. The developer comprising carrier particles prepared by the hybrid plasma method exhibited the best image properties. Therefore, the developer comprising the carrier particles of the present invention can remarkably improve the performance.

Claims: Iron carrier particles for an electrophotographic developer, the carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. Iron carrier particles according to claim 1, having the surface coated with a resin. An electrophotographic developer comprising toner particles and resin coated carrier particles according to claim 2. A process for preparing iron carrier particles for an electrophotographic developer, which comprises melting iron particles by a direct plasma method in an inert atmosphere under a reduced pressure 66661Pa (500 Torr) or less, cooling and recovering said iron particles to prepare iron carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. A process for preparing iron carrier particles for an electrophotographic developer, which comprises melting iron particles by a high frequency plasma method, cooling and recovering said iron particles to prepare iron carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method. A process for preparing iron carrier particles for an electrophotographic developer, which comprises melting iron particles by a hybrid plasma method, cooling and recovering said iron particles to prepare iron carrier particles having a mean particle size of 25 to 40 µm, a magnetization of 160 to 190 emu/g at 3000 10³ / 4π A/m, an apparent density of 3.0 to 4.2 g/cm³, a percentage sphericity of at least 80%, and a specific surface area of at least 350 cm²/g as determined by an air permeation method.

Inventors: HONJO TOSHIO, KAYAMOTO KANAO, OGATA MASAHIRO, SATO YUJI, HONJO, TOSHIO, KAYAMOTO, KANAO, OGATA, MASAHIRO, SATO, YUJI, HONJO, TOSHIO, C/O POWDERTECH CO. LTD., KAYAMOTO, KANAO, C/O POWDERTECH CO. LTD., OGATA, MASAHIRO, C/O POWDERTECH CO. LTD., SATO, YUJI, C/O POWDERTECH CO. LTD.
Applicants: POWDERTECH CO LTD, POWDERTECH CO. LTD.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9