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

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a carrier 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 carrier.

2. Prior Art

[0002] Various carriers 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.

[0003] Further, resin-coated carriers 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. Materials such as nonmetals, 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.

[0004] Although a toner having a mean particle diameter of about 10 to 20 µm has hitherto been used as a developer, there is a tendency that the demand for a high image quality makes it necessary to reduce the particle diameter of the toner. With a reduction in the particle diameter of the toner, it is necessary to increase the specific surface area of the carrier with a view to increasing the capability of imparting a charge to the toner. In the conventional carrier, 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 is unsatisfactory.

[0005] Although carriers 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 is unsatisfactory.

[0006] In order to improve the fluidity, an attempt has been made on the sphering of the carrier, 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:

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to provide a carrier 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 somewhat changes, 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.

[0008] A toner is greatly influenced by the fluidity and specific surface area of the carrier. Specifically, the better the fluidity and the larger the specific surface area of the carrier, the greater the capability of the carrier 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.

[0009] The above-described objects of the present invention can be attained by making use of a carrier having particular properties satisfying the above-described characteristics requirements.

[0010] Specifically, the carrier for an electrophotographic developer of the present invention is characterized by having 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.

DETAILED EXPLANATION OF THE INVENTION

[0011] There is no particular limitation on the raw material for the carrier used in the present invention, and any of the raw materials known in the art may be used. Examples thereof include iron, ferrite, cobalt, copper, nickel and carborundum, among which iron and ferrite are particularly preferred.

[0012] The mean particle diameter of the carrier of the present invention is 15 to 50 µm, preferably 15 to 40 µm. When it is less than 15 µm, the magnetization of the carrier particle becomes so low that the carrier scatters. On the other hand, when it exceeds 50 µm, the specific surface area if the carrier lowers, which causes the toner to scatter. Therefore, both the above cases are unfavorable.

[0013] The magnetization of the carrier of the present invention should be 30 emu/g or more at 3000 Oe. When it is less than 30 emu/g, the carrier tends to scatter. A problem which occurs when the particle size of the carrier is reduced to the above-described value is that the magnetization per carrier becomes low due to a reduction in the weight (volume) per carrier. This makes it necessary to bring the magnetization to a certain value or above. The magnetization at 3000 Oe can be increased to 30 emu/g or more by selecting the composition of raw materials for the carrier. It is also possible to regulate the magnetization through control of an atomosphere in which the carrier is sphered and recovered. A carrier having a magnetization exceeding 190 emu/g at 3000 Oe cannot be practically produced except for a special purpose.

[0014] The apparent density of the carrier of the present invention is 1.3 to 4.2 g/cm³, preferably 1.8 to 4.0 g/cm³. When it is less than 1.3 g/cm³, the fluidity of the carrier tends to lower. On the other hand, when it exceeds 4.2 g/cm³, ears formed on a magnetic brush become so hard that the fluidity of the carrier on the magnetic brush becomes poor. Further, in this case, the stress applied to the toner is increased, which shortens the service life of the developer.

[0015] In the present invention, the carrier should be spherical. A high fluidity can be obtained through the sphering of the carrier. The term "spherical carrier" as used herein is intended to mean one wherein when the carrier is 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.

[0016] Preferred examples of the carrier used in the present invention include a spherical carrier made of iron and having a mean particle diameter of 25 to 40 µm, a magnetization of 70 to 190 emu/g at 3000 Oe, and an apparent density of 3.0 to 4.0 g/cm³ and a spherical carrier made of ferrite and having an average particle diameter of 15 to 50 µm, a magnetization of 30 to 95 emu/g at 3000 Oe and an apparent density of 1.3 to 3.0 g/cm³.

[0017] In the carrier 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 to have a capability of imparting a sufficient property of charging the toner.

[0018] As described above, the fluidity and specific surface area of the carrier 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 with a resin.

[0019] In the conventional carrier 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 having a small mean particle diameter of the present invention, however, as compared with the conventional coating carrier, 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.

[0020] The carrier of the present invention is prepared by melting the above-described raw material for the carrier by the plasma method. More particularly, the carrier is prepared by the plasma method which comprises throwing a raw material for the carrier into a plasma in an inert or oxidizing atomosphere to instantaneously melt the raw material and cooling and recovering the melt when it is sphered by the surface tension. Then reason why the plasma method is employed for the preparation of a carrier is that desired properties, i.e., desired particle diameter, magnetization, apparent density, etc., can be obtained by this method.

[0021] 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 the 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 a raw material. When the direct current plasma method is conducted in an inert atomosphere, it is preferred to conduct it under a reduced pressure of 500 Torr or less from the viewpoint of the properties of the carrier obtained.

[0022] A developer prepared together with a toner through the use of the carrier thus obtained has a very excellent performance.

[0023] As described above, the carrier of the present invention is prepared by melting a raw material for a carrier through a plasma method such as a direct current, high frequency or hybrid plasma method and has 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 PREFERRED EMBODIMENT

[0024] The present invention will now be described in more detail by referring to Examples and so forth.

Example 1

[0025] A vessel purged with an inert atmosphere was reduced to 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 Oe, an apparent density of 3.56 g/cm³ and a specific surface area of 420 cm²/g.

[0026] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0027] A developer was prepared by making use of the above-prepared carrier and a toner (a styrene-acrylic resin) and subjected to a test through the use of a commercially available actual copying machine.

[0028] 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, ⓞ↕

↕ △, and X represent that the properties are excellent, good, slightly poor and poor, respectively.

Example 2

[0029] The surface of the carrier prepared in Example 1 was coated with an acrylic resin.

[0030] A developer was prepared by making use of the resin-coated carrier thus prepared and the toner used in Example 1 and subjected to a test through the use of a commercially available actual copying machine.

[0031] The toner concentration was 12%. The image properties are given in Table 1.

Example 3

[0032] An irregular form of iron powder having a mean particle diameter of 40 µm was thrown into a direct current arc plasma flame in the air to prepare a carrier having a mean particle diameter of 28.2 µm. As shown in Table 1, this carrier had a magnetization of 91 emu/g at 3000 Oe, an apparent density of 2.95 g/cm³ and a specific surface area of 502 cm²/g.

[0033] The carrier thus prepared was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0034] 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.

[0035] The toner concentration was 16%. The image properties are given in Table 1.

Example 4

[0036] A vessel purged with an inert atmosphere was reduced to 30 Torr. An irregular form of Cu-Zn ferrite powder having a magnetization of 65 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 36.4 µm. As shown in Table 1, this carrier had a magnetization of 63 emu/g at 3000 Oe, an apparent density of 2.93 g/cm³ and specific surface area of 489 cm²/g.

[0037] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% more.

[0038] 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. The image properties are given in Table 1.

Example 5

[0039] 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 a carrier having a mean particle diameter of 30.4 µm. As shown in Table 1, this carrier had a magnetization of 175 emu/g at 3000 Oe, an apparent density of 3.72 g/cm³ and a specific surface area of 418 cm²/g.

[0040] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0041] 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. The image properties are given in Table 1.

Comparative Example 1

[0042] 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.

[0043] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0044] 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

[0045] 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.

[0046] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0047] 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

[0048] 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.

[0049] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0050] 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

[0051] 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.

[0052] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 80% or more.

[0053] 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

[0054] 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.

[0055] 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.

[0056] 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

[0057] 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

[0058] 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.

[0059] The toner concentration was 2.5%. The image properties are given in Table 1.

Comparative Example 8

[0060] The surface of the carrier prepared in Comparative Example 7 was coated with an acrylic resin.

[0061] 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.

[0062] The toner concentration was 2.5%. The image properties are given in Table 1.

Comparative Example 9

[0063] 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.

[0064] 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.

[0065] 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 6

[0066] 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 a carrier having a mean particle diameter of 30.5 µm. As shown in Table 1, this carrier had a magnetization of 175 emu/g at 3000 Oe, an apparent density of 3.56 g/cm³ and a specific surface area of 420 cm²/g.

[0067] The above-described carrier was observed under a scanning electrons microscope and found to have a percentage sphericity of 90% or more.

[0068] 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.

[0069] The toner concentration was 15%. The image properties are given in Table 1.

Example 7

[0070] The surface of the carrier used in Example 6 was coated with the acrylic resin used in Example 2.

[0071] A developer was prepared by making use of the resin-coated carrier and the toner used in Example 1 and then subjected to a test by making use of an actual copying machine.

[0072] The toner concentration was 15%. The image properties are given in Table 1.

Example 8

[0073] In a vessel of an inert nitrogen atmosphere, an irregular form of magnetite powder having a mean particle diameter of 40 µm was thrown into a high frequency plasma flame to prepare a carrier having a mean particle diameter of 33 µm.

[0074] As shown in Table 1, this carrier had a magnetization of 88 emu/g at 3000 Oe, an apparent density of 2.50 g/cm³ and a specific surface area of 540 cm²/g.

[0075] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0076] A test was conducted by making use of an actual copying machine and the carrier thus prepared and the toner used in Example 1.

[0077] The toner concentration was 17%. The image properties are given in Table 1.

Example 9

[0078] In a vessel of an inert argon atmosphere, an irregular form of Cu-Zn ferrite having a mean particle diameter of 40 µm and a magnetization 65 emu/g was thrown into a high frequency plasma flame to prepare a carrier having a mean particle diameter of 35 µm. As shown in Table 1, this carrier had a magnetization of 63 emu/g at 3000 Oe, an apparent density of 2.76 g/cm³ and a specific surface area of 510 cm²/g.

[0079] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0080] The surface of the carrier was coated with the acrylic resin used in Example 2.

[0081] 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.

[0082] The toner concentration was 16%. The image properties are given in Table 1.

Example 10

[0083] 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 a carrier having a mean particle diameter of 26 µm. As shown in Table 1, this carrier had a magnetization of 160 emu/g at 3000 Oe, an apparent density of 3.54 g/cm³ and a specific surface area of 550 cm²/g.

[0084] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0085] The surface of the carrier was coated with the acrylic resin used in Example 2.

[0086] 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.

[0087] The toner concentration was 20%. The image properties are given in Table 1.

Example 11

[0088] 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 a carrier having a mean particle diameter of 29 µm. As shown in Table 1, this carrier had a magnetization of 172 emu/g at 3000 Oe, an apparent density of 3.59 g/cm³ and a specific surface area of 435 cm²/g.

[0089] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 95% or more.

[0090] The surface of the carrier was coated with the acrylic resin used in Example 2.

[0091] 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.

[0092] The toner concentration was 15%. The image properties are given in Table 1.

Comparative Example 10

[0093] 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.

[0094] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0095] 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

[0096] 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.

[0097] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0098] 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

[0099] 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.

[0100] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0101] 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

[0102] 

[0103] 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.

[0104] The above-described carrier was observed under a scanning electron microscope and found to have a percentage sphericity of 90% or more.

[0105] 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.

[0106] As is apparent from the comparison of the Examples with the Comparative Examples given in Table 1, the use of the carrier 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 a carrier, a developer comprising a carrier prepared by the high frequency plasma method is generally superior in the image property to a developer comprising a carrier prepared by the direct plasma method. The developer comprising a carrier prepared by the hybrid plasma method exhibited the best image properties. Therefore, the developer comprising the carrier of the present invention can remarkably improve the performance.

Claims

1. 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.

2. A carrier according to claim 1, which has a surface coated with a resin.

3. A carrier according to claim 1 or 2, wherein the specific surface area is 350 cm²/g or more as determined by an air permeation method.

4. A carrier according to claim 1, 2 or 3, which is made of iron and having a mean particle diameter of 25 to 40 µm, a magnetization of 70 to 190 emu/g at 3000 Oe and an apparent density of 3.0 to 4,0 g/cm³.

5. A carrier according to claim 1, 2 or 3, which is made of ferrite and having a mean particle diameter of 15 to 50 µm, a magnetization of 30 to 95 emu/g at 3000 Oe and an apparent density of 1.3 to 3.0 g/cm³.

6. A process for preparing a carrier for an electrophotographic developer, which comprises melting raw material for a carrier by a direct plasma method to prepare a carrier according to any one of claims 1 to 5.

7. A process for preparing a carrier for an electrophotographic developer, which comprises melting a raw material for a carrier by a high frequency plasma method to prepare a carrier according to any one of claims 1 to 5.

8. A process for preparing a carrier for an electrophotographic developer, which comprises melting a raw material for a carrier by a hybrid plasma method to prepare a carrier according to any one of claims 1 to 5.

9. An electrophotographic developer comprising a carrier according to any one of claims 1 to 5.