EP0704767A1

EP0704767A1 - A two-component type developer

Info

Publication number: EP0704767A1
Application number: EP95305606A
Authority: EP
Inventors: Masatomi C/O Mita Industrial Co. Ltd. Funato; Seijiro C/O Mita Industrial Co. Ltd. Ishimaru; Yoshitake C/O Mita Industrial Co. Ltd. Shimizu; Norio C/O Mita Industrial Co. Ltd. Kubo; Kazuya Nagao; Mamoru Kato; Terumichi Asano; Tomohide Iida; Hideaki Kawata; Nobuaki Kawano; Yoshiteru Hatase; Hidekazu Tamura; Kazuhiko Fujii
Original assignee: Mita Industrial Co Ltd
Current assignee: Kyocera Mita Industrial Co Ltd
Priority date: 1994-08-31
Filing date: 1995-08-11
Publication date: 1996-04-03
Also published as: KR960008443A

Abstract

The invention provides a two-component type developer including toner and a carrier. The toner particles in the toner includes a binder resin and magnetic powder dispersed in the binder resin. The binder resin is made of a composition including a resin having an anionic group. The magnetic powder is included in the toner particles at a proportion of 0.1 through 5 parts by weight per 100 parts by weight of the binder resin. Each of the carrier particles in the carrier has a core particle and a coating layer covering the core particle, and the core particle is made of a magnetic material represented by the following Formula (A):

Formula (A) MOFe₂O₃

wherein M indicates at least one metal selected from the group consisting of Cu, Zn, Fe, Ba, Ni, Mg, Mn, Al and Co.

Description

1. Field of the Invention:

The present invention relates to a two-component type developer used for electrophotography. More particularly, the present invention relates to a two-component type developer, which includes carrier and toner having no charge control agent, suitably used in an electrophotographic image forming apparatus such as an electrostatic copying machine and a laser beam printer.

2. Description of the Related Art:

A two-component type developer is used as one of the developers used for developing an electrostatic latent image on a photosensitive body in an electrophotographic image forming apparatus. The two-component type developer includes toner comprising a binder resin and a coloring agent such as carbon black, and magnetic carrier such as iron powder and ferrite particles.
An electrostatic latent image is developed by the following steps: the developer forms a magnetic brush shape on a developing roller by a magnetic field thereof and is carried out to the photosensitive body. In this step, the toner is charged by friction with the carrier so as to have a desired charge and polarity of charge. Then, the developer is contacted with the photosensitive body by the developing roller, resulting in attaching the toner onto the electrostatic latent image formed thereon. Generally, the toner includes a charge control agent which controls and stabilizes the charge of the toner so as to attach a constant amount of the toner on the electrostatic latent image and provide a good developed image for a long period of time. Negatively charged toner includes a negative charge control agent such as a dye of a metal complex including a metal ion such as chrome(III) (for example, an azo compound - chrome(III) complex), and an oxycarboxylic acid - metal complex (for example, a salicylic acid - metal complex) (Japanese Laid-Open Patent Publication No. 3-67268). Positively charged toner includes a positive charge control agent such as an oil soluble dye including nigrosine and an amine type charge control agent (Japanese Laid-Open Patent Publication No. 56-106249).
Many metal complexes, including a heavy metal ion such as a chrome ion, are used as a conventional charge control agent. They are carefully selected, in terms of environmental safety, so that only those having passed various toxicity tests and safety tests alone are used. Therefore, although they would be safe in themselves or when included in toner, it is more preferable to refrain from using the metal complexes including a heavy metal as the charge control agent. In addition, the charge control agent is expensive as compared with the other materials for toner such as a binder resin and a coloring agent, for example, carbon black. Therefore, although the charge control agent has a content of merely several %, this results in increasing the price of the resultant toner. Accordingly, it is desired to develop toner having no charge control agent of a metal complex.
Furthermore, when conventional toner is used for a long period of time, the toner components tend to attach on a surface of the carrier particle. The attached components are called a spent. The spent makes the carrier charge with the same polarity as the toner, resulting in the disadvantages that the toner can be scattered and transfer efficiency of toner image is decreased.

SUMMARY OF THE INVENTION

The two-component type developer of this invention comprises toner and carrier. The toner includes toner particles, and the toner particles include a binder resin and magnetic powder dispersed in the binder resin. The binder resin is made of a composition including a resin having an anionic group. The magnetic powder is included in the toner particles in the range of 0.1 to 5 parts by weight per 100 parts by weight of the binder resin. Each carrier particle in the carrier has a core particle and a coating layer covering the core particle, and the core particle is made of a magnetic material represented by the following Formula (A):

Formula (A): MOFe₂O₃

wherein M indicates at least one metal selected from the group consisting of Cu, Zn, Fe, Ba, Ni, Mg, Mn, Al and Co.
In one embodiment, an extracted solution obtained by extracting the toner with methanol has substantially no absorption peak in the range of 280 to 350 nm, and has a substantially zero absorbance in the range of 400 to 700 nm.
In one embodiment, the magnetic powder is contained in the range of 0.5 to 3 parts by weight per 100 parts by weight of the binder resin.
In one embodiment, the toner particles have a volume-based average particle diameter of 5 through 15 µm, and spacer particles having a volume-based average particle diameter of 0.05 through 1.0 µm are attached onto the surfaces of the toner particles.
In one embodiment, the coating layer is made of a resin composition including a resin having a cationic group.
In one embodiment the coating layer is made of a resin composition including an alkylated melamine resin and an acryl-modified silicone resin, and the alkylated melamine resin has a weight-average molecular weight M represented by the following Formula (B): $Formula (B): M ≧ 1100C - 400$

wherein C indicates the number of carbon atoms included in an alkyl group contained in the alkylated melamine resin.
In one embodiment, the coating layer is made of a resin composition including a methyl silicone resin and a methylated melamine resin, and the methylated melamine resin has a weight-average molecular weight of 700 or more.
In one embodiment, the coating layer is made of a resin composition including a methyl silicone resin containing a T unit at a proportion of 70 mol% or more.
In one embodiment the coating layer is made of a resin composition including a thermosetting resin and a thermoplastic resin, the thermoplastic resin including a quaternary ammonium group at a concentration of 0.1 through 20 mmole per 100 g of the resin composition.
In one embodiment, the coating layer includes a thermosetting resin and has a curing degree of 85% or more.
In one embodiment, the coating layer has a content of 0.001 through 2.5 parts by weight per 100 parts by weight of the core particle.
In one embodiment, the thermosetting resin is at least one selected from the group consisting of a modified or unmodified silicone resin, a thermosetting acrylic resin, a thermosetting styrene-acrylic resin, a phenol resin, a urethane resin, a thermosetting polyester resin, an epoxy resin and an amino resin.
In one embodiment, the resin having a cationic group is a resin having a basic nitrogen containing group.
In one embodiment, the core particle has a particle diameter of 50 through 150 µm.
Thus, the invention described herein makes possible the advantages of (1) providing a two-component type developer including toner with excellent chargeability including no charge control agent at all; (2) providing a two-component type developer including toner which is not or only slightly scattered in development for realizing a copied image with a high quality; and (3) providing a two-component type developer including toner in which a spent is not caused even when used for a long period of time, and hence, by which an excellent image quality can be maintained and transfer efficiency can be stabilized.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a graph showing absorbance of a methanol extracted solution of the developer according to the present invention in the range of 200 to 700 nm;
Figure 2 is a graph showing absorbance of a methanol extracted solution of toner having a dye of an azo compound - chrome complex as a charge control agent in the range of 200 to 700 nm;
Figure 3 is a graph showing absorbance of a methanol extracted solution of toner having a salicylic acid - metal complex as the charge control agent in the range of 200 to 700 nm;
Figure 4 is a graph showing absorbance of a methanol extracted solution of carrier in a two-component magnetic developer used for a long time in which toner has a dye of an azo compound - chrome complex as the charge control agent and chargeability of carrier is unstabilized by a spent in the range of 200 to 700 nm;
Figure 5 is a graph showing a relationship between shaking time and a spent ratio obtained with regard to two kind of a two-component magnetic developer, one comprising toner having a charge control agent and magnetic carrier and another comprising toner having no charge control agent and magnetic carrier;
Figure 6 is a graph showing a relationship between shaking time and quantity of charge of toner obtained with regard to two kind of a two-component magnetic developer, one comprising toner having a charge control agent and magnetic carrier and another comprising the toner having no charge control agent and magnetic carrier;
Figure 7 is a graph showing a relationship between an amount of spent of carrier and content of a charge control agent in a toner particle;
Figure 8 is a graph showing a relationship between shaking time and amount of spent obtained in the case where each component contained in a toner particle and magnetic carrier are individually mixed and shaken;
Figure 9 illustrates a mechanism of a charge failure caused by a spent in a conventional two-component magnetic developer; and
Figure 10 is a graph showing the relationship among the molecular weight of an alkylated melamine resin, the number of carbon atoms included in an alkyl group contained in the alkylated melamine resin and the roughness on a coating layer formed on a carrier particle using the alkylated melamine resin.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Toner included in a two-component type developer of the present invention has no charge control agent, such as a dye of an azo compound - metal complex and an oxycarboxylic acid - metal complex, at all. Therefore, a spent caused by a charge control agent, which will be described in detail below, scarcely occurs in the present toner, resulting in realizing a high quality copied image for a long period of time. Since the toner included in a two-component type developer of the present invention has no charge control agent, it is impossible to detect any charge control agent, i.e., a dye type compound, from the toner by any chemical or physical method. For example, such a compound cannot be detected in the toner included in a two-component type developer of the present invention by any chemical reaction. Alternatively, absorption peaks owing to such a compound cannot be detected in an organic solvent extracted solution of the toner. For example, when the toner is extracted with an organic solvent such as methanol, the extracted solution has substantially no absorption peak in the range of 280 to 350 nm, and has substantially zero absorbance in the range of 400 to 700 nm. Herein, "to have substantially no absorption peak" means, in an extracted solution obtained by extracting 0.1 g of the toner included in a two-component type developer of the present invention with 50 ml of methanol, absorption peaks are not detected at all, or if detected, values of the absorbance peaks are 0.05 or less. Similarly, "to have substantially zero absorbance" means that values of the absorbance of the extracted solution obtained by extracting 0.1 g of the toner included in a two-component type developer of the present invention with 50 ml of methanol are 0.05 or less.
In the present two-component type developer, the instability in the charge amount due to the lack of a charge control agent in the toner is compensated as follows. First, a binder resin included in the toner particles includes a resin having an anionic group; secondly, the toner particles include magnetic powder at a predetermined proportion; and thirdly, each carrier particle has a coating layer. Further, spacer particles having a predetermined particle diameter are attached onto the surfaces of the toner particles if necessary, thereby improving the transfer efficiency of an image from a photosensitive body onto transfer paper.
In the present developer, it is preferable that the coating layer on a carrier particle includes a resin having a cationic group to enhance the functions of the developer. As a result, the chargeability of the resultant carrier can be stabilized.
In the present developer, it is preferable that the coating layer on a carrier particle includes an alkylated melamine resin having a molecular weight within a predetermined range and an acryl-modified silicone resin. As a result, the chargeability of the resultant carrier can be further stabilized. Furthermore, since the coating layers of the respective carrier particles are prevented from being fused to be attached to one another, each of the resultant carrier particles has a smooth and uniform surface of a coating layer. This results in decreasing the occurrence of a spent, and improving the durability of the carrier, thereby elongating the life time of the developer.
In the present developer, it is preferable that the coating layer on a carrier particle includes a methyl silicone resin and a methylated melamine resin having a molecular weight within a predetermined range. As a result, the toner particles are prevented from scattering during the development, and a so-called fog is prevented from being formed on a copied image. Furthermore, the occurrence of coagulation of the carrier particles and the spent is further suppressed, resulting in providing a developer with a long life time having excellent fluidity and durability.
In the present developer, it is preferable that the coating layer on a carrier particle includes a methyl silicone resin containing 70 mol% or more of a T unit in order to enhance the functions of the developer. As a result, the occurrence of the coagulation of the carrier particles and the spent is further suppressed, thereby providing a developer with a long life time excellent in fluidity and durability.
In the present developer, it is preferable, for the purpose of enhancing the functions of the developer, that the coating layer on a carrier particle includes a thermosetting resin and a thermoplastic resin, and that the thermoplastic resin can include a quaternary ammonium group at a predetermined proportion.
In the present developer, it is preferable, for the purpose of enhancing the functions of the developer, that the coating layer on a carrier particle can includes a thermosetting resin and that the curing degree of the coating layer can be 85% or more. As a result, the occurrence of the spent is further suppressed. In addition, the resultant developer, in which filming of the photosensitive body is not caused, attains an excellent image stability.
The above-mentioned characteristics of the two-component type developer of the present invention will be described in detail.
Figure 1 shows an UV-visible spectrum of a methanol extracted solution of the toner included in a two-component type developer of the present inventionin the range of 200 to 700 nm. As is shown in this spectrum, the extracted solution has no peak, which is otherwise formed because of a charge control agent. Specifically, the solution has substantially no absorption peak in the range of 280 to 350 nm, and the absorbance in the range of 400 to 700 nm is substantially zero. To the contrary, in an absorbance curve of a methanol extracted solution of toner having a dye of an azo compound - chrome complex as a charge control agent shown in Figure 2, absorption peaks are found in the range of 400 to 700 nm, in particular, 550 to 570 nm. Furthermore, in the UV-visible spectrum of a methanol extracted solution of toner having a salicylic acid - metal complex as a charge control agent shown in Figure 3, an absorption peak is found in the range of 280 to 350 nm.
It is because the charge control agent is present on the surfaces of the toner particles at a rather high concentration that the methanol extracted solution of the toner having the charge control agent has absorption peaks due to the charge control agent.
A carrier included in the present developer which has insufficient chargeability owing to occurrence of a spent is extracted with methanol, and then the UV-visible spectrum of the extracted solution is measured to find absorption peaks in the range of 400 to 700 nm derived from a charge control agent. For example, the developer comprising the toner having a dye of an azo compound - chrome complex, whose UV-visible spectrum is shown in Figure 2, was used for a long period of time to cause a spent therein. Then, UV-visible spectrum of a methanol extracted solution of the carrier in this developer was measured to give the spectrum shown in Figure 4. As is shown in Figure 4, absorption peaks are found at the same position as the spectrum in Figure 2. It is conventionally understood that a spent is caused because a binder resin in the toner is attached to the surface of a carrier particle to form a resin film. The comparison between the absorbance curves in Figures 2 and 4, however, reveals that one of the major causes of a spent is the transfer of the charge control agent from the toner particles to the carrier particles.
The present inventors conducted the following experiments in order to find out more about the relationship between a charge control agent and a spent: First, toner comprising toner particles containing 1.5 wt% of the dye of the azo compound - chrome complex was mixed with a carrier to obtain a developer. The toner and the carrier was shaken for a predetermined period of time. Figure 5 shows a relationship between the shaking time and amount of an attachment on the surfaces of the carrier particles. In Figure 5, the amount of attachment is indicated as a spent ratio, that is, a percentage based on a total weight of the carrier particles bearing the attachment. Furthermore, Figure 6 shows the relationship between the shaking time and the amount of charge of the toner. The same procedure was repeated with regard to a developer comprising toner having no charge control agent and carrier. The experimental results of this developer are also shown in Figures 5 and 6, wherein the results obtained by the developer including the toner having the charge control agent are plotted with black circles, and those by the developer including the toner having no charge control agent are plotted with white circles. It is apparent from Figures 5 and 6 that a larger amount of attachment is formed on the carrier particles as the spent and the charge amount of the toner has a greater decrease in the developer including the toner particle having the charge control agent than in the developer including the toner particle having no charge control agent.
Next, the weight of toner components attached on the surfaces of the carrier particles as the spent was measured with time. The results are shown in a graph of Figure 7, wherein the abscissa indicates a measured amount of the spent and the ordinate indicates the content of the charge control agent in the toner particle. The broken line in Figure 7 indicates the amount of the charge control agent calculated in assuming that the toner components attached as the spent are identical to the components in the toner particles. Figure 7 reveals that a large amount of the charge control agent is deposited to be attached on the surfaces of the carrier particles at the initial stage. In Figure 7, as amount of the spent increases, the measured values approximate the calculated values. This is because they are experimental results obtained in a close system having no supply of fresh toner. Therefore, when toner is exchanged as in a copying machine, the difference between the measured values and the calculated values would be much larger.
Furthermore, the present inventors measured the weight of the attachment on the surfaces of the carrier particles resulting from mixing the carrier with each of the toner components, that is, a charge control agent, a binder resin, carbon black as a coloring agent and wax, so as to find out the relationships between the respective toner components and the spent. The results are shown in Figure 8 as a variation with time in the amount of the attachment (i.e., amount of the spent), wherein the results obtained from the mixture with the charge control agent is plotted with white circles, those from the carbon black with black circles, those from the binder resin with squares, and those from the wax with triangles. It is apparent from Figure 8 that the charge control agent causes the largest amount of attachment due to the spent.
Based on the above-mentioned facts, the charge failure caused by the spent in a conventional two-component magnetic developer is explained as follows referring to Figure 9. In the initial stage of the usage of a developer, a carrier particle 1 is positively charged and a toner particle 2 is negatively charged as is shown in an upper portion of Figure 9. In this case, the toner particle works as a negative toner particle 21. When this developer is continued to be used, a component including the charge control agent as a main component in the toner particle is attached on the surface of the carrier particle 1. Attachment 201, which is the spent, is negatively charged. The negatively charged attachment 201 leads to the formation of a toner particle having positive charge, that is, a reversely charged toner particle 22. The reversely charged toner particle 22 is formed on the surface of the carrier particle 1 as is shown in a lower portion of Figure 9, resulting in scattering of the toner and decreasing the transfer efficiency of the toner.
As described above, preferably, the toner does not have a charge control agent not only because the agent can include a heavy metal but also because the agent is the main cause of the spent, scatter of the toner and of a decrease in the transfer efficiency of the toner. Accordingly, the toner included in a two-component type developer of the present inventionhas no charge control agent at all.
The instability of charge of the toner due to the lack of the charge control agent, in particular, the insufficiency in charge amount of the toner is compensated by using a binder resin having an anionic group as mentioned above. The insufficiency in charge amount of the toner particles can be supplemented because the binder resin has a negative charge in itself owing to the anionic group included therein. Since the anionic group is bonded to the main chain of the binder resin, it would never move onto the surface of the carrier particle as the charge control agent does, and hence it never causes the spent. On the contrary, charge around the surface of the toner particle caused by the anionic group of the binder resin is not so large that the electrostatic attraction between the toner particle and the carrier particle owing to the Coulomb force is insufficient when they are conveyed as a magnetic brush for development. Therefore, in a rapid copying operation, the toner cannot be sufficiently prevented from scattering because of insufficient coupling with the carrier particles. The scattered toner stains the inner wall of the copying machine, and can cause so-called a fog on a copied image.
In order to overcome such disadvantages, the toner included in a two-component type developer of the present invention includes magnetic powder at a predetermined proportion, that is, 0.1 to 5 parts by weight on the basis of 100 parts by weight of the binder resin. The insufficiency in the charge amount of the toner particles can be thus compensated for. The magnetic powder contained in the toner particle causes magnetic attraction between the toner particle and the carrier particle. This magnetic attraction between the toner particle and the carrier particle together with electrostatic attraction prevents the toner from scattering. Moreover, since the number of the toner particles to be attached onto an electrostatic latent image is increased as the charge amount of one toner particle is smaller, apparent development sensitivity is increased.
The content of the magnetic powder in the toner particles is in the range of 0.1 to 5 parts by weight per 100 parts by weight of the binder resin as described above. When the content is less than 0.1 parts by weight, the charge amount of the toner particle is insufficient, resulting in insufficient coupling with the carrier particle and causing toner scattering. In this case, a fog can be disadvantageously formed on a copied image. Furthermore, the density of the copied image is low because of the insufficient charge amount. When the contents exceeds 5 parts by weight, the magnetic attraction between the carrier particle and the toner particle becomes so strong that the toner is not sufficiently attached onto an electrostatic latent image, resulting in decreasing the density of the copied image.
Several attempts have been made to improve the resolution of a copied image and the like by including (inclusively adding) magnetic powder as a toner component. For example, Japanese Laid-Open Patent Publication No. 56-106249 discloses a toner particle including 10 wt% of ferrite, and Japanese Laid-Open Patent Publication No. 59-162563 discloses a toner particle including 5 through 35 wt% of a magnetic fine particle. In either case, however, the content of the magnetic powder is excessive, and hence, the density of the copied image is low. Japanese Laid-Open Patent Publication No. 3-67268 discloses toner to which 0.05 to 2 wt% of magnetic powder is externally added. In this case, since the magnetic powder is not included in the toner particle, the powder is likely to be ununiformly attached onto the surface of the toner particle, resulting in insufficient magnetic attraction between the toner particle and the carrier particle. Furthermore, in either of the above-mentioned toners, the spent can be disadvantageously caused because a charge control agent is contained therein.
In the present invention, spacer particles having a particle diameter of 0.05 through 1.0 µm are attached preferably onto the surfaces of the toner particles in order to increase the transfer efficiency of the toner image. The spacer particles can work to enhance fluidity of the toner, and in addition, form a gap between the photosensitive body and the toner particles when the toner is attached onto the electrostatic latent image formed on the photosensitive body. Therefore, the toner can be transferred from the photosensitive body onto the transfer paper with ease even when the toner attains a large quantity of charge through a long copying operation, resulting in a high transfer efficiency of the toner. When the spacer particle is similar to the particle of the magnetic powder included in the toner particle, the magnetic attraction between the toner particle and the carrier particle can be further enhanced, thereby further preventing toner scattering and a fog.
A fine particle having a particle diameter of approximately 0.015 µm is used to enhance fluidity of a conventional toner. Such a small particle cannot form a sufficient gap between the photosensitive body and the toner particles, and cannot work as the spacer particle for the aforementioned purposes.
In the present invention, a carrier particle has a coating layer to further enhance the functions of the resultant developer. The coating layer of the carrier can stabilize the chargeability of the toner. Further, the coating layer forms smooth and uniform surface on the carrier particle. Therefore, the occurrence of the spent is suppressed and the durability of the carrier is increased, resulting in a developer with a long life time.
In one aspect of the invention, it is preferable, for the purpose of enhancing the functions of the developer, that the coating layer on a carrier particle includes a resin having a cationic group. Because of the cationic group, the carrier attains chargeability. Therefore, when such a carrier is mixed with toner including no charge control agent, the chargeability of the toner can be remarkably improved. As a result, the chargeability of the resultant toner can be stabilized. Moreover, since this toner does not include a charge control agent, as is contained in a conventional toner, the occurrence of the spent on the carrier particles by the charge control agent is effectively suppressed, resulting in elongating the life time of the developer.
Examples of the resin having a cationic group include resins having a basic nitrogen containing group such as an amino group. For example, melamine resins, and preferably an alkylated melamine resin can be used.
In another aspect of the invention, it is preferable, in order to enhance the functions of the developer, that the coating layer on a carrier particle includes an acryl-modified silicone resin and an alkylated melamine resin having a molecular weight within a predetermined range, i.e., having a weight-average molecular weight M represented by the following Formula (B): $Formula (B): M ≧ 1100C - 400$

wherein C indicates the number of carbon atoms included in an alkyl group in the alkylated melamine resin.
Since a melamine resin has a large number of amino groups, that is, cationic groups, within its molecule, it positively charges the carrier particle when included in the coating layer. In particular, the alkylated melamine resin is preferred for the following reason. In this resin, at least part of the methylol groups generated by a reaction between a melamine and formaldehyde are further alkylated through a reaction with alcohol (i.e., alkyletherified). As a result, the melting point of the resin is decreased and its solubility to a solvent is improved. Furthermore, the compatibility with the acryl-modified silicone resin is improved as well. Thus, the alkylated melamine resin is excellent in a coating layer forming property and a curing property. Furthermore, since it includes a methylol group and/or an alkylated methylol group in its molecule, it attains a high reactivity and exhibits an excellent curing proporty when combined with the acryl-modified silicone resin. As a result, a dense and rigid coating layer can be formed.
A silicone resin has a water repellent property, an excellent water resisting property and a small friction coefficient. Therefore, this resin is excellent in preventing a spent. Furthermore, when an acryl-modified silicone resin, which is obtained by denaturing a silicone resin with an acrylic resin, is used in the coating layer on a carrier particle contained in a developer, a resultant copied image can attain a high density as well as the spent being effectively prevented. Moreover, when a silicone resin is modified with an acrylic resin, the compatibility with the alkylated melamine resin and the curing reactivity can also be improved.
Furthermore, when the alkylated melamine resin has the weight-average molecular weight represented by Formula (B), the coating layer on a carrier particle attains a smooth and uniform surface without any roughness. In Formula (B), it is preferable that C is in the range between 1 and 4.
A cause of the formation of roughness on the coating layer is regarded to be that the coating layers of the respective carrier particles are melted, thereby forming an attachment of the carrier particles. The coating layers are melted and attached to one another when the coating layers are cured or when the coating layers are cooled after curing. When such a fused resin is cracked, roughness is formed on the surface of the coating layer by breaking the fused portion between resins. When the toner particles are attached onto such an irregular surface of the carrier particle, the spent is caused, resulting in shortening the life time of the carrier. It was found that there is a correlation among the formation of the roughness, the molecular weight of the alkylated melamine resin, and the number of carbon atoms in an alkyl group in the alkylated melamine resin when the coating layer is formed of the alkylated melamine resin. Figure 10 is a graph showing the relationship among the formation of the roughness on a coating layer, the molecular weight of the alkylated melamine resin and the number of carbon atoms in an alkyl group in the alkylated melamine resin. In this graph, the abscissa indicates the number of carbon atoms (C) in an alkyl group, and the ordinate indicates the weight-average molecular weight (M) of the alkylated melamine resin. The case where the roughness was formed on the coating layer is plotted with ×, and the case where no roughness was formed is plotted with ○. As is shown in Figure 10, when the alkylated melamine resin having a molecular weight satisfying Formula (B) is used, namely, when the molecular weight of the alkylated melamine resin used is specified to exceed a predetermined value, the coating layers are prevented from attaching one another. As a result, the roughness is not formed on the coating layer, and the resultant carrier particle attains a smooth surface of the coating layer. When the carrier particle has such a smooth and uniform surface of the coating layer, the occurrence of the spent is remarkably suppressed.
In still another aspect of the invention, it is preferable, in order to enhance the functions of the developer, that the coating layer on a carrier particle includes an alkylated melamine resin and a silicone resin. For example, a combination of a methylated melamine resin having a molecular weight within a predetermined range, i.e., a weight-average molecular weight of 700 or more, and a methyl silicone resin is preferred. Also in this case, since the coating layer on a carrier particle includes a resin having a cationic group, the chargeability of the toner having no charge control agent can be remarkably improved. Furthermore, due to the silicone resin, the occurrence of the spent is suppressed, and the coagulation of the carrier particles is also suppressed, thereby improving the fluidity of the toner. For these purposes, the methyl silicone resin is particularly effective, and a methyl silicone resin having no phenyl group is more preferred. When such a methyl silicone resin is used, the effect of suppressing the spent and the coagulation is further enhanced.
In the aforementioned developer, by limiting the melamine resin to a methylated melamine resin and limiting the molecular weight of the resin to a predetermined value or more, the following advantages can be achieved. Since the compatibility between a silicone resin and a melamine resin is generally poor, masses of particles of the melamine resin are dispersed in the silicone resin in the coating layer formed from these resins. A cured material of the thus specified methylated melamine resin, however, has a hardness as high as that of a cured material of the methyl silicone resin. Therefore, although the masses of the melamine resin particles are dispersed in the silicone resin, the masses would not come off the coating layer when the carrier is repeatedly shaken in a developing device. Accordingly, the surface of the coating layer is prevented from losing its uniformity through shaking. As a result, the coagulation of the carrier particles and the occurrence of the spent are further suppressed, thereby providing a developer with a long life time having excellent fluidity and durability.
In still another aspect of the invention, it is preferable, in order to enhance the functions of the developer, that the coating layer on a carrier particle includes a methyl silicone resin containing a T unit, i.e., a trifunctional unit (RSiO_1.5, wherein R is a methyl group), at a proportion of 70 mol% or more. The methyl silicone resin is excellent in preventing the coagulation of the carrier particles and the occurrence of the spent as described above. When the content of the T unit in the methyl silicone resin is limited to 70 mol% or more, a three-dimensional network structure formed by curing the methyl silicone resin can attain a more dense structure, resulting in improving the hardness and uniformity of the surface of the coating layer of the carrier particle, and further enhancing the effect to prevent the coagulation of the carrier particles and the occurrence of the spent. Therefore, when such a methyl silicone resin is used to form the coating layer, the resultant developer achieves a longer life time and higher fluidity and durability. When the content of the T unit is less than 70 mol%, the proportions of a D unit making no contribution to cross-linkage, i.e., a bifunctional unit (R₂SiO), and an M unit decreasing the molecular weight, i.e., a monofunctional unit (R₃SiO_0.5), are relatively increased, thereby slightly decreasing the hardness of the coating layer.
In still another aspect of the invention, it is preferable, in order to enhance the functions of the developer, that the coating layer on a carrier particle includes a thermosetting resin and a thermoplastic resin, and that the thermoplastic resin includes a quaternary ammonium group at a predetermined proportion. When such resins are included in the coating layer, phase separation between the thermoplastic resin having a quaternary ammonium group and the thermosetting resin is induced in the heat curing process, and hence, the thermoplastic resin moves the surface of the coating layer. Therefore, the carrier having such a coating layer can stabilize the chargeability of the resultant developer when mixed with toner having no charge control agent. Furthermore, since the toner includes no charge control agent as is contained in a conventional developer, the occurrence of the spent on the carrier particles are effectively suppressed, thereby elongating the life time of the developer. Generally, a resin having a quaternary ammonium group can be a thermosetting resin or a thermoplastic resin. In the present invention, however, a quaternary ammonium group is contained in a thermoplastic resin included in a resin composition which forms the coating layer. Therefore, it is possible to apply toner with a stable chargeability without decreasing the durability of the coating layer. The chargeability can be further stabilized by allowing 0.1 through 20 mmole of the quaternary ammonium group to be contained in 100 g of the resin composition.
In still another aspect of the invention, it is preferable, in order to enhance the functions of the developer, that the coating layer on a carrier particle includes a thermosetting resin, and that the curing degree of the coating layer is 85% or more. The thermosetting resin is preferably used for forming the coating layer on a carrier particle in terms of the abrasion resistance, the hardness, the non-stickiness, the heat resistance and the durability of the carrier. By limiting the curing degree to 85% or more, the cured material attains a higher density, the uniformity of the surface of the resultant coating layer is improved, the coating layer is prevented from peeling off, and the effect to prevent the spent is further enhanced. When such a resin is used, since the hardness of the coating layer is increased, the filming of the photosensitive body and the spent scarsely occur.
The curing degree is obtained as follows. A coating layer is formed and cured on a core particle to prepare carrier particles. The resultant carrier particles are washed with a solvent, which can dissolve or disperse an uncured resin composition to be used for forming the coating layer. A ratio of the coating layer remained on the core particle without being washed away by the solvent is taken as the curing degree. This ratio can be calculated by the following formula: $Curing degree (%) = \frac{C in carrier after washing}{C in carrier before washing} x 100$

wherein C indicates an amount of carbon measured through the elemental analysis or the like.
Now, the developer of the present invention will be described. Herein, a "lower alkyl group" indicates alkyl having 1 to 5 carbon atoms.

(Binder resin of a toner particle included in the present depeloper)

The binder resin of the toner particles included in the present developer comprises a composition including a polymer having an anionic group. Such a binder resin is obtained by polymerizing a monomer having an anionic group or a mixture of the monomer having an anionic group with other monomers. The obtained resin can be a homopolymer or a copolymer.
The binder resin used in the toner included in a two-component type developer of the present inventionis preferably a copolymer, such as a randam copolymer, a block copolymer and a grafted copolymer, obtained from a monomer having an anionic group and other monomers.
Examples of the monomer having an anionic group include monomers having a carboxylic acid group, a sulfonic acid group or a phosphoric acid group, and a monomer having a carboxylic acid group is generally used. Examples of the monomer having a carboxylic acid group include ethylenically unsaturated carboxylic acids such as acrylic acid, methacrylic acid, crotonic acid, maleic acid and fumaric acid; monomers that can form a carboxylic acid group such as maleic anhydride; and lower alkyl halfester of dicarboxylic acid such as maleic acid and fumaric acid. Examples of the monomer having a sulfonic acid group include styrene sulfonic acid and 2-acrylamido-2-methylpropane sulfonic acid. Examples of the monomer having a phosphoric acid group include 2-phosphono(oxy)propylmethacrylate, 2-phosphono(oxy) ethylmethacrylate, 3-chloro-2-phosphono(oxy) propylmethacrylate.
Such a monomer having an anionic group can be a free acid, a salt of an alkaline metal such as sodium and potassium, a salt of an alkaline earth metal such as calcium and magnesium, and a salt such as zinc.
The monomer having no anionic group used to prepare the binder resin is selected so that the resultant binder resin has a sufficient fixability and chargeability required of toner, and is one or a combination of an ethylenically unsaturated monomer. Examples of such a monomer include ethylenically unsaturated carboxylic acid ester, monovinyl arene, vinyl ester, vinyl ether, diolefin and monoolefin.
The ethylenically unsaturated carboxylic acid esters are represented by the following Formula (I):

wherein R¹ is a hydrogen atom or a lower alkyl group; and R is a hydrocarbon group having 11 or less carbon atoms or a hydroxyalkyl group having 11 or less carbon atoms.
Examples of such ethylenically unsaturated carboxylic acid esters include methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, phenyl acrylate, methyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, β-hydroxyethylacrylate, γ-hydroxypropylacrylate, δ-hydroxybutylacrylate and β-hydroxyethylmethacrylate.
The monovinyl arenes are represented by the following Formula (II):

wherein R³ is a hydrogen atom, a lower alkyl group or a halogen atom; R⁴ is a hydrogen atom, a lower alkyl group, a halogen atom, an alkoxy group, an amino group or a nitro group; and φ is a phenylene group.
Examples of such monovinyl arene include styrene, α-methylstyrene, vinyltoluene, α-chlorostyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene and p-ethylstyrene.
The vinyl esters are represented by the following Formula (III):

wherein R⁵ is a hydrogen atom or a lower alkyl group.
Examples of such vinyl esters include vinyl formate, vinyl acetate and vinyl propionate.
The vinyl ethers are represented by the following Formula (IV):

Formula (IV): CH₂=CH-O-R⁶

wherein R⁶ is a monovalent hydrocarbon group having 11 or less carbon atoms.
Examples of such vinyl ethers include vinyl methyl ether, vinyl ethyl ether, vinyl n-butyl ether, vinyl phenyl ether and vinyl cyclohexyl ether.
The diolefins are represented by the following Formula (V):

wherein R⁷, R⁸ and R⁹ are independently a hydrogen atom, a lower alkyl group or a halogen atom.
Examples of such diolefins include butadiene, isoprene and chloroprene.
The monoolefins are represented by the following Formula (VI):

wherein R¹⁰ and R¹¹ are independently a hydrogen atom or a lower alkyl group.
Examples of such monoolefins include ethylene, propylene, isobutylene, 1-butene, 1-pentene and 4-methyl-1-pentene.
Specific examples of the polymer having an anionic group, that is, a (co)polymer obtained through the polymerization of the aforementioned monomers, include styrene-acrylic acid copolymers, styrene-maleic acid copolymers and ionomer resins. Furthermore, a polyester resin having an anionic group can be also used. The polymer having an anionic group preferably includes the anionic group at a proportion for attaining an acid value of 2 through 30, and preferably 5 through 15, when the anionic group is present as a free acid. When part or the entire anionic group is neutralized, the anionic group is preferably contained at such a proportion that the acid value would be in the aforementioned range in assuming that it is present as a free acid. When the acid value, i.e., the concentration of the anionic group, of the polymer or the composition is below the aforementioned range, the chargeability of the resultant toner is insufficient. When it exceeds the range, the resultant toner disadvantageously has a hygroscopic property. A preferable binder resin is a copolymer obtained from the monomer having an anionic group and at least one of the ethylenically unsaturated carboxylic acid ester represented by Formula (I) as an indispensable components, and any of the monomers represented by Formulae (II) through (VI) as an optional component to be used if necessary. One or a combination of two or more of the aforementioned monomers is used for preparing the binder resin.
The binder resin used in the invention is made of the composition including the aforementioned polymers, and the composition can further include a polymer having no anionic group as well. In this case, the proportion of the anionic group in the entire composition is preferably within the aforementioned range.

(Magnetic powder)

The magnetic powder contained in (inclusively added to) the toner particles can be any magnetic powder used in a conventional one-component type developer. Examples of the material for the magnetic powder include triiron tetroxide (Fe₃O₄), maghemite (γ-Fe₂O₃), zinc iron oxide (ZnFe₂O₄), yttrium iron oxide (Y₃Fe₅O₁₂), cadmium iron oxide (CdFe₂O₄), gadolinium iron oxide (Gd₃Fe₅O₁₂), copper iron oxide (CuFe₂O₄), lead iron oxide (PbFe₁₂O₁₉), nickel iron oxide (NiFe₂O₄), neodyum iron oxide (NdFeO₃), barium iron oxide (BaFe₁₂O₁₉), magnesium iron oxide (MgFe₂O₄), manganese iron oxide (MnFe₂O₄), lanthanum iron oxide (LaFeO₃), iron (Fe), cobalt (Co) and Nickel (Ni). Particularly preferable magnetic powder is made from triiron tetroxide (magnetite) in the shape of fine particles. The particle of preferable magnetite is in the shape of a regular octahedron with a particle diameter of 0.05 through 1.0 µm. Such a magnetite particle can be subjected to a surface treatment with a silane coupling agent or a titanium coupling agent. The particle diameter of the magnetic powder contained in the toner particle is generally 1.0 µm or smaller, and preferably in the range between 0.05 and 1.0 µm.
The content of the magnetic powder in the toner particle is in the range of 0.1 to 5 parts by weight, more preferably 0.5 to 4 parts by weight, and most preferably 0.5 to 3 parts by weight per 100 parts by weight of the binder resin. When the content is too small, the toner can be scattered during the development and the transfer efficiency of the toner can be decreased as described above.

(Inner additives in the toner particles)

The toner particle contains, as described above, the binder resin and the magnetic powder as indispensable components, and can optionally include some inner additive generally used for a toner, if necessary.
Examples of such additives include a coloring agent and a release agent.
As the coloring agent, the following pigments can be used:

Black pigment:
carbon black, acetylene black, lampblack, aniline black;
Extender:
barite powder, barium carbonate, clay, silica, white carbon, talc, alumina white.

Such a pigment is contained in the toner particle in the range of 2 to 20 parts by weight, and preferably 5 to 15 parts by weight per 100 parts by weight of the binder resin.
As the release agent, various wax and olefin resins can be used as in a conventional toner. Examples of the olefin resin include polypropylene, polyethylene, and propylene-ethylene copolymers, and polypropylene is particularly preferred.

(Preparation of the toner)

The toner particles in the toner included in a two-component type developer of the present inventioncan be produced by any ordinary method for toner particles such as crushing and classification, fusing granulation, spray granulation and polymerization, and are generally produced by the crushing and classification method.
For example, the components for the toner particles are previously mixed in a mixer such as a Henschel mixer, kneaded with a kneader such as a biaxial extruder, and then cooled. The resultant is crushed and classified to give toner particles. The particle diameter of the toner particle is generally in the range between 5 and 15 µm and preferably between 7 and 12 µm in the volume-base averaged particle diameter (a medium size measured with a Coulter counter).
It is possible to improve the fluidity of the toner by attaching, as an outer additive, a fluidity enhancer such as hydrophobic vapor depositioned silica particles onto the surfaces of the toner particles, if necessary. The primary particle diameter of the fluidity enhancer such as the silica particles is generally approximately 0.015 µm, and such a fluidity enhancer is added to the toner in the range of 0.1 to 2.0 percent by weight on the basis of the weight of the entire toner, i.e., the total weight of the toner particles and the fluidity enhancer.
Furthermore, spacer particles having a larger particle diameter than that of the fluidity enhancer are preferably added in the present invention. As the spacer particles, any of organic and inorganic inactive particles with a particle diameter of 0.05 through 1.0 µm, more preferably 0.07 through 0.5 µm can be used. Examples of the material for such inactive particles include silica, alumina, titanium oxide, magnesium carbonate, an acrylic resin, a styrene resin and magnetic materials. The spacer particle can not only work as a fluidity enhancer but also increase the transfer efficiency as described above. As the spacer particle, the same type of magnetic powder as included in the toner particle, in particular, triiron tetroxide (magnetite) in the shape of fine particle is preferably used. The magnetic powder, when used as the spacer particles, effectively suppresses the scattering of the toner as described above. The content of the spacer particles is 10 percent by weight or less, more preferably in the range of 0.1 to 10 percent by weight, and most preferably 0.1 to 5 percent by weight on the basis of the total weight of the toner. When the spacer particles are excessively included in toner, the density of a copied image is insufficient. When the magnetic powder is used as the spacer particles, the total amount of the magnetic powder together with that contained in the toner particles is preferably 10 parts by weight or less per 100 parts by weight of the binder resin. When it is excessively included, the density of a copied image can be decreased.
When the fluidity enhancer and the spacer particles are added to the toner particles, the following production method is preferred. The fluidity enhancer and the spacer particles are first sufficiently mixed with each other, and then the obtained mixture is added to the toner particles, and then is sufficiently unbound. Thus, the spacer particles can be attached onto the surfaces of the toner particles. To "be attached" herein means both to be held in contact with the surface of the toner particle and to be partly embedded in the toner particle. In this manner, the toner of the present invention is produced.

(Carrier particle)

Each particle in the carrier used in the present developer is preferably formed from a particle with a two-layered structure including a core particle and a coating layer covering the core particle. Because of the coating layer, the electric resistance of the carrier particle is stable and varies very little with time or by the environmental change. As a result, chargeability of the particle can be stabilized. Furthermore, since the surface of the carrier particle can be made smooth by the coating layer, the spent is prevented from being caused by the friction between the carrier particles and the toner particles. As a result, the durability of the carrier is improved, thereby elongating the life time of the resultant developer.
The core particle is made of a magnetic material represented by the following Formula (A):

Formula (A): MOFe₂O₃

wherein M indicates at least one metal selected from the group consisting of Cu, Zn, Fe, Ba, Ni, Mg, Mn, Al and Co.
The compound represented by Formula (A) is magnetite (wherein M indicates Fe) or ferrite (wherein M indicates one of the metals other than Fe), and ferrite wherein M indicates Cu, Zn, Mn, Ni or Mg is preferably used. Such magnetite and ferrite have little variation in electrical resistance with time, and can be formed into a soft spicated shape when a magnetic field is applied in the developing device. The core particle comprising such a magnetic material has a particle diameter of 30 through 200 µm, and preferably 50 through 150 µm. The core particles are obtained by granulating the fine particles of the magnetic material by spray granulation and the like, and heating the resultant particles. The core particle has a volume specific resistivity between 10⁵ and 10⁹ Ω·cm, and preferably 10⁶ and 10⁸ Ω·cm. The saturation magnetization of the core particle is in the range between 30 and 70 emu/g, and preferably between 45 and 65 emu/g.
In one aspect of the invention, a resin composition for forming the coating layer on a carrier particle is preferably a resin having a cationic group, and the resin can be a thermoplastic resin or a thermosetting resin. A thermosetting resin or a mixture including a thermosetting resin is preferred in terms of the heat resistance and the durability. Examples of the cationic group include a basic nitrogen containing group such as primary, secondary and tertiary amino groups, a quaternary ammonium group, an amido group, an imino group, an imido group, a hydrazino group, a guanidino group and an amidino group, among which an amino group and a quaternary ammonium group are particularly preferred.
Examples of the thermoplastic resin having a cationic group include thermoplastic acrylic resins, thermoplastic styrene-acrylic resins, polyester resins, polyamide resins and olefin copolymer, each of which includes a cationic group. Examples of the thermosetting resin include modified and unmodified silicone resins, thermosetting acrylic resins, thermosetting styrene-acrylic resins, phenol resins, urethane resins, thermosetting polyester resins, epoxy resins and amino resins, each of which includes a cationic group. Such a resin including a cationic group is obtained by polymerizing a monomer having a cationic group or a mixture of the monomer having a cationic group with other monomers. Alternatively, such a resin is obtained by linking a compound having a cationic group with a resin having no cationic group. Alternatively, a monomer having a cationic group and/or other monomer are (co)polymerized by using a polymerization initiator having a cationic group, thereby introducing the cationic group into the resultant resin.
When a resin prepared from alkoxysilane or alkoxytitanium is used, it is possible to produce the resin having a cationic group by allowing a silane coupling agent having a cationic group to react with the resin during or after the preparation of the resin. Examples of the silane coupling agent include N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane and N-phenyl-3-aminopropyltrimethoxysilane. This type of silane coupling agent can be linked onto the surface of the core particle via a hydroxyl group generally present on the surface of the core particle. Therefore, such a silane coupling agent can form the coating layer by itself. Examples of the polymerization initiator having a cationic group include amidine type compound, e.g., azobis compounds.
The resin having a cationic group for forming the coating layer is used singly or together with any other of the aforementioned resins, or together with another resin having no cationic group.
The content of the cationic group in the resin having a cationic group is generally in the range of 0.1 to 2000 mmole, and preferably 0.5 to 1,500 mmole per 100 g of the resin. When the resin having a cationic group is used with a resin having no cationic group, the cationic group is preferably contained in the entire resins forming the coating layer of the carrier particle at a proportion in the aforementioned range.
The resin composition forming the coating layer of the carrier particle includes at least one of the above-mentioned resins having a cationic group, together with another resin having no cationic group, if necessary. Examples of a mixture of the resin having a cationic group and the resin having no cationic group include a mixture of an alkylated melamine resin and a styrene-acrylic copolymer, and a mixture of an alkylated melamine resin and an acryl-modified silicone resin.
In another aspect of the invention, the resin composition forming the coating layer on a carrier particle preferably includes an alkylated melamine resin, that is, a thermosetting resin having a cationic group, and an acryl-modified silicone resin.
An alkylated melamine resin is obtained from an alkylation of a methylolmelamine through reaction between any of alcohols and part of methylol groups in the methylolmelamine, which is obtained by addition polymerizing any of melamines and formaldehyde.
The melamines include melamine and melamine derivatives such as benzoguanamine and acetoguanamine. A melamine has three amino groups, and a guanamine has two amino groups. In using any of these compounds as the melamine for preparing the above-described resin, 1.0 through 8.0 mole, preferably 2.0 through 7.0 mole of formaldehyde is used per 1 mole of the melamine in the reaction between the melamine and formaldehyde (i.e., in the methylolmelamine forming reaction). This methylolmelamine forming reaction is effected in the presence of a hydroxide such as an alkaline metal or an alkaline earth metal or an alkaline catalyst such as ammonia. During the methylolmelamine forming reaction, a condensation reaction within the methylolmelamine is simultaneously caused to bind the methylolmelamines to each other via a methylene group, resulting in increasing the molecular weight.
When the methylolmelamine and an alcohol is reacted, an ether bond is formed through the condensation. Examples of the usable alcohol include methanol, ethanol, n-propanol, iso-propanol, n-butanol and iso-butanol. Through the usage of such an alcohol, an alkyl group having a desired number of carbon atoms is introduced into the melamine molecule, thereby forming the alkylated melamine resin. The extent of the alkylation, i.e., the ether-bond forming reaction, is in the range of 10 to 85%, preferably 20 to 80% on the basis of the total number of the methylol groups in the melamine.
The acryl-modified silicone resin can be a block copolymer or a graft copolymer having a silicone resin component and an acrylic resin component, or a mixture of these copolymers with a silicone resin and/or an acrylic resin. The term "acryl-modified silicone resin" herein designates both the copolymer and the mixture of the copolymers with a silicone resin and/or an acrylic resin.
As the silicone resin component is used a silicone resin having an organo-polysiloxane unit such as dimethyl polysiloxane, diphenyl polysiloxane and methylphenyl polysiloxane and having reactive functional groups at the end of the molecular chain or in the molecular chain. Examples of such reactive groups include a hydroxyl group, a mono-alkoxysilyl group, a di-alkoxysilyl group, a tri-alkoxysilyl group, an alkoxysiloxy group, a vinyl organosilyl group and a vinyl organosiloxy group.
As the acrylic resin component is used a copolymer obtained from a large amount of a acrylate or methacrylate monomer and a small amount of an ethylenically unsaturated monomer having an alkoxysilyl group. Examples of the acrylate and methacrylate monomer include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 2-aminoethyl acrylate, 2-aminoethyl methacrylate, N-ethyl-2-aminoethyl acrylate and N-ethyl-2-aminoethyl methacrylate. Examples of the ethylenically unsaturated monomer having an alkoxysilyl group include vinyl triethoxysilane, 3-triethoxysilylpropyl acrylate and 3-triethoxysilylpropyl methacrylate.
When the silicone resin component and the acrylic resin component are reacted with each other, a reaction is caused between the reactive functional group in the silicone resin and the reactive alkoxy group in the acrylic resin, thereby producing a copolymer having the silicone resin component and the acrylic resin component. Specifically, the silicone resin is modified with the acrylic resin to give the acryl-modified silicone resin.
The weight ratio of the acrylic resin component and the silicone resin component in the acryl-modified silicone resin is preferably in the range from 80:20 to 20:80, and more preferably 70:30 to 30:70. The acrylic resin component herein includes both the acrylic resin itself and an acrylic resin component contained in the copolymer when the mixture of an acrylic-silicone copolymer and an acrylic resin and/or a silicone resin is used as the acryl-modified silicone resin. Similarly, the silicone resin component includes both the silicone resin itself and a silicone resin component contained in the copolymer.
The acryl-modified silicone resin has a group reactive with a methylol group or an etherified methylol group, such as a hydroxyl group and an alkoxy group. The concentration of the reactive group is generally in the range from 1 to 400 mmole, and preferably 3 to 200 mmole per 100 g of the resin.
The resin composition forming the coating layer on a carrier particle can include the alkylated melamine resin and the acryl-modified silicone resin at an optional proportion. The mixed ratio of the alkylated melamine resin and the acryl-modified silicone resin is preferably in the range from 5:95 to 70:30. When the mixed ratio is within this range, the chargeability of the resultant carrier particles can be further improved, and the smoothness of the surface of the coating layer can be also further improved. In addition, the occurence of the spent can be decreased.
In still another aspect of the invention, the resin composition for forming the coating layer on a carrier particle preferably includes a methyl silicone resin and a methylated melamine resin.
The methyl silicone resin can be prepared from, for example, a methyl chlorosilane. The methyl silicone resin is generally added to the core particles as a methyl silicone resin oligomer, and then cured as described below.
The methylated melamine resin has a weight-average molecular weight of 700 or more. The upper limit of the molecular weight of the methylated melamine resin is not herein specified, but is preferably 2000. The proportions of the methyl silicone resin and the methylated melamine resin are not herein specified, but the proportion of the methylated melamine resin in the coating layer is preferably in the range between 5 and 70 wt%. When the proportion of the methylated melamine resin is within this range, the chargeability of the resultant carrier particles is further stabilized. Moreover, in this case, the extent of the self-crosslinkage of the methylated melamine resin is so appropriate that the reactivity of the methylated melamine resin in the curing process is excellent, and that the film forming property of the methylated melamine resin is satisfactory. Therefore, the adhesion between the resultant coating layer and the core particle is further improved.
In still another aspect of the invention, it is preferable that the resin composition forming the coating layer on a carrier particle includes a methyl silicone resin including a T unit at a proportion of 70 mol% or more. In order to cure the methyl silicone resin so as to include 70 mol% or more of the T unit, a methyl silicone resin oligomer including 70 mol% or more of the T unit is used. In order to adjust the content of the T unit in the oligomer to be 70 mol% or more, the mixed ratio of methyl trichlorosilane (CH₃SiCl₃), that is, a source of the T unit among the methyl chlorosilanes to be used as the material for the methyl silicone resin oligomer, is made to be 70 mol% or more.
In still another aspect of the invention, it is preferable that the resin composition for forming the coating layer on a carrier particle includes a thermosetting resin and a certain type of thermoplastic resin. The thermosetting resin is at least one selected from the group consisting of a modified or unmodified silicone resin, a thermosetting acrylic resin, a thermosetting styrene-acrylic resin, a phenol resin, a urethane resin, a thermosetting polyester resin, an epoxy resin and an amino resin. The usable thermoplastic resin has a quaternary ammonium group, and examples of such a thermoplastic resin include an acrylic resin, a styrene-acrylic resin, a polyester resin, a polyamide resin and an olefin resin, all of which include a quaternary ammonium group. Such a thermoplastic resin having a quaternary ammonium group can be obtained through polymerization of a monomer having a quaternary ammonium group or a mixture having a quaternary ammonium group with other monomers. Alternatively, it can be obtained by linking a thermoplastic resin having no quaternary ammonium group with a compound having a quaternary ammonium group.
The thermoplastic resin having a quaternary ammonium group includes the quaternary ammonium group at a concentration of 0.1 through 20 mmole per 100 g of the resin composition. When such a thermoplastic resin is included in the coating layer, the chargeability of the carrier can be further stabilized.
In still another aspect of the invention, it is preferable that the resin composition for forming the coating layer on a carrier particle includes a thermosetting resin. Examples of the thermosetting resin include a modified or unmodified silicone resin, a thermosetting acrylic resin, a thermosetting styrene-acrylic resin, a phenol resin, a urethane resin, a thermosetting polyester resin, an epoxy resin and an amino resin. Such a thermosetting resin is increased in the molecular weight and becomes unsoluble in a solvent when thermally cured. In order to attain the curing degree of 85% or more as described above, the heating temperature and time are required to be adjusted. In the present invention, a thermoplastic resin can be included in the coating layer as far as it does not degrade the characteristics of the coating layer. Examples of such a thermoplastic resin include an acrylic resin, a styrene-acrylic resin, a polyester resin, a polyamide resin, and an olefin copolymer resin. One or a combination of two or more of them can be used.
The resin composition for forming the coating layer on a carrier particle can further include an additive such as silica, alumina, carbon black, a fatty acid metallic salt, a silane coupling agent and silicone oil, if necessary. These additives work for adjusting the characteristics of the coating layer.

(Preparation of the carrier)

The resin composition including a cationic group is applied to the surface of the core particle by a known method to form the coating layer. For example, the core particle is coated with a solution or a dispersion of the resin composition and dried, thereby forming the coating layer. Alternatively, when a thermosetting resin or a reactive resin oligomer is used, the core particle is coated with an uncured resin, or a solution or a dispersion of the oligomer, and then heated to cure the resin. The coating layer can be formed by any of the generally used methods such as immersion, spray, a fluidized bed method, a moving bed method and a tumbling layer method. As a solvent used to dissolve or disperse the resin composition, any of the ordinary organic solvents can be used. Examples of the solvent include aromatic hydrocarbons such as toluene and xylene; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; cyclic ethers such as tetrahydrofuran and dioxane; alcohols such as ethanol, propanol and butanol; cellosolves such as ethyl cellosolve and butyl cellosolve; esters such as ethyl acetate and butyl acetate; and amide type solvents such as dimethylformamide and dimethylacetoamide. The solvent is appropriately selected in accordance with the chemical properties of the resin such as the solubility.
The particle diameter of the thus obtained carrier particle is in the range of 30 to 200 µm, and preferably 50 to 150 µm. The weight ratio of the coating layer on the carrier particle is in the range of 0.001 to 2.5 parts by weight, and preferably 0.005 to 2.0 parts by weight per 100 parts by weight of the core particle. The obtained carrier particle has a volume specific resistivity in the range between 10⁵ and 10¹³ Ω·cm, and preferably 10⁷ and 10¹ Ω·cm, and a saturation magnetization in the range between 30 and 70 emu/g, and preferably 45 and 65 emu/g.

(Preparation of a developer)

A two-component type developer is prepared by mixing the above-mentioned toner and carrier. The mixing ratio of the carrier and the toner is generally 98:2 through 90:10, and preferably 97:3 through 94:6, by weight.
A copying operation is conducted using the toner included in a two-component type developer of the present inventionby a general electrophotographic method. Specifically, for example, a photoconductive layer on a photosensitive body is uniformly charged, and an image is exposed to form an electrostatic latent image thereon. Then, a magnetic brush made of the two-component magnetic developer is allowed to come in contact with the photosensitive body, thereby developing the electrostatic latent image with ease into a toner image. The thus obtained toner image is transferred onto transfer paper to form a transfer image, which is then applied with heat and pressure by a heat roller to fix the image thereon.

Examples

The present invention will now be described by way of examples. It is noted that the invention is not limited to these examples.

(Example 1.1)

The above listed components were fused and kneaded with a biaxial extruder, and the resultant was crushed with a jet mill, and classified with a pneumatic classifier to give toner particles with an average particle diameter of 10.0 µm.
To the obtained toner particles were added 0.3 part by weight of hydrophobic silica fine powder with an average particle diameter of 0.015 µm as a fluidity enhancer on the basis of 100 parts by weight of the toner particles. The resultant mixture was mixed with a Henschel mixer to give toner.

Spherical ferrite particles with an average particle diameter of 100 µm were used as the magnetic core particles. To 1000 parts by weight of the ferrite particles was added a coating agent with components as listed in Table 1, and the resultant was mixed with a thermal stirrer. The solvent was removed from the resultant mixture, and the residue was subjected to a heat treatment at a temperature of 200°C for 1 hour to give carrier particles each having a coating layer.

The thus obtained toner and carrier were homogeneously mixed to give a two-component type developer having a toner concentration of 3.5 wt%.

(Example 1.2)

The same procedure was repeated as in Example 1.1 except that a coating agent with components as listed in Table 1 was used, thereby preparing a developer.

(Example 1.3)

(Comparative Example 1)

The same procedure was repeated as in Example 1.1 except that a coating layer is not formed on a carrier particle without using any coating agent, thereby preparing a developer.

Table 1

Coating agents of Examples 1.1-1.3.
component	Example 1.1	Example 1.2	Example 1.3
Resin 1	Acryl-modified silicone	Metylphenyl silicone	Styrene-acrylic polymer
(parts by weight)	2.5	4.8	3.5

Resin 2	Metylated melamine	γ-aminopropyltriethoxysilane	Methylated melamine
(parts by weight)	2.5	0.2	1.5

Solvent: toluene (parts by weight)	200	200	200

(Example 2.1)

Spherical ferrite particles with an average particle diameter of 100 µm were used as the magnetic core particles. To 1000 parts by weight of the ferrite particles was added a coating agent with components as listed in Table 2, and the resultant was mixed with a thermal stirrer. The solvent was removed from the resultant mixture, and the residue was subjected to a heat treatment at a temperature of 200°C for 1 hour to give carrier particles each having a coating layer.

The toner prepared in Example 1.1 and the thus obtained carrier were homogeneously mixed to give a two-component type developer having a toner concentration of 3.5 wt%.

Table 2

Coating agent of Example 2.1.
component	Example 2.1
Resin 1	Acryl-modified silicone
(parts by weight)	2.5
Resin 2	Metylated melamine
;molecular weight	700
(parts by weight)	2.5

Solvent: toluene
(parts by weight)	200

(Example 3.1)

Spherical ferrite particles with an average particle diameter of 100 µm were used as the magnetic core particles. To 1000 parts by weight of the ferrite particles was added a coating agent with components as listed in Table 3, and the resultant was mixed with a thermal stirrer. The solvent was removed from the resultant mixture, and the residue was subjected to a heat treatment at a temperature of 200°C for 1 hour to give carrier particles each having a coating layer.

(Example 3.2)

A developer was prepared in the same manner as in Example 3.1 by using a coating agent with components as listed in Table 3.

Table 3

Coating agents of Examples 3.1. and 3.2.
component	Example 3.1	Example 3.2
Resin 1	Methylsilicone	Metylsilicone
;T unit (mol%)	87	87
parts by weight	3.5	5

Resin 2	Metylated melamine	none
;molecular weight	700
parts by weight	1.5

Solvent: toluene	200	200
(parts by weight)

(Example 4.1)

Spherical ferrite particles with an average particle diameter of 100 µm were used as the magnetic core particles. To 1000 parts by weight of the ferrite particles was added a coating agent with components as listed in Table 4, and the resultant was mixed with a thermal stirrer. The solvent was removed from the resultant mixture, and the residue was subjected to a heat treatment at a temperature of 200°C for 1 hour to give carrier particles each having a coating layer.

(Example 4.2)

A developer was prepared in the same manner as in Example 4.1 except that the content of the quaternary ammonium group was as listed in Table 4.

(Example 4.3)

A developer was prepared in the same manner as in Example 4.1 except that the styrene-acrylic resin having a quaternary ammonium group was not used.

(Example 4.4)

(Example 5.1)

Spherical ferrite particles with an average particle diameter of 100 µm were used as the magnetic core particles. To 1000 parts by weight of the ferrite particles was added a coating agent with components as listed in Table 5, and the resultant was mixed with a thermal stirrer. The solvent was removed from the resultant mixture, and the residue was subjected to a heat treatment at a temperature of 180°C for 1 hour to give carrier particles each having a coating layer.
Then, 10 g of the thus obtained carrier was charged in a glass vessel, and toluene was added thereto to dissolve the uncured portion of the resin. Next, the toluene solution was discarded with the carrier attracted with a magnet onto the bottom of the glass vessel. This procedure was repeated several times, and the resultant carrier was dried with an oven. Then, the amount of carbon in the thus toluene treated (washed) carrier was measured with a Carbon Analyzer (manufactured by Horiba Co., Ltd.). Based on the thus measured carbon amount and that measured before the toluene treatment, the curing degree was calculated, which are listed in Table 5.

(Example 5.2)

A developer was prepared in the same manner as in Example 5.1 except that the heat treatment was performed at a temperature of 200°C.

(Example 5.3)

A developer was prepared in the same manner as in Example 5.1 except that the heat treatment was performed at a temperature of 170°C.

[Evaluation of the developers]

The developers obtained in the above described examples and comparative example were evaluated with regard to the following items. An electric copying machine (manufactured by Mita Industrial Co., Ltd.; brand name: DC-4685) was modified so as to make easier evaluation sampling, and the modified copying machine was used in the evaluation.

(a) Transfer efficiency:
The amount of toner in a toner hopper in the copying machine was measured at first, and a predetermined number of copies were made. Then, the amount of the toner left in the toner hopper was measured. From a difference between the amounts of the toner before and after the copying operation, a consumed amount of the toner was calculated. At the same time, the amount of the toner collected in a cleaning process during the copying operation was also measured as a collected amount. Based on these amounts, the transfer efficiency of the toner was calculated by using Equation (i) as below. An original used in the copying operation bore characters with a black area ratio of 8%. This evaluation was conducted to perform various evaluation tests described in the following items (b) through (i). $Equation (i): Transfer efficienty (%) = \frac{(Consumed amount)-(Collected amount)}{(Consumed amount)}$

With regard to the developers of Examples 4.1 through 4.4 and 5.1 through 5.3, 50,000 copies were made, and the results obtained from these developers are listed in Tables 9 and 10.
(b) Image density (I.D.):
A copying operation was continued by using an original bearing characters with a black area ratio of 8% until 50,000 copies were made with regard to the developers of Examples 4.1 through 4.4 and 5.1 through 5.3 and until the transfer efficiency became less than 70% with regard to the other developers. The density of a black portion in a copied image on every 5000 copies was measured by a reflection densitometer (manufactured by Tokyo Denshoku Co., Ltd.; TC-6D), and the average density was taken as an image density (I.D.). An original used for sampling every 5000 copies had a black area ratio of 15% including a black solid portion. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, those of Examples 3.1 and 3.2 in Table 8, those of Examples 4.1 through 4.4 in Table 9 and those of Examples 5.1 through 5.3 in Table 10.
(c) Fog density (F.D.):
A copying operation was continued by using an original bearing characters with a black area ratio of 8% until 50,000 copies were made with regard to the developers of Examples 4.1 through 4.4 and 5.1 through 5.3 and until the transfer efficiency became less than 70% with regard to the other developers. The density of a white portion in a copied image on every 5000 copies was measured by the reflection densitometer (manufactured by Tokyo Denshoku Co., Ltd.; TC-6D). A difference between the thus measured density and the density of paper to be used for the copying operation (base paper) measured by the reflection densitometer was calculated, and the maximum difference was taken as a fog density (F.D.). An original used for sampling every 5000 copies had a black area ratio of 15% including a black solid portion. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, those of Examples 3.1 and 3.2 in Table 8, those of Examples 4.1 through 4.4 in Table 9 and those of Examples 5.1 through 5.3 in Table 10.
(d) Resolution:
A copying operation was conducted by using an original bearing characters with a black area ratio of 8%. When 50,000 copies were made (in the case where the transfer efficiency became less than 70% before making 50,000 copies, at that time), a normal chart original (an original bearing a plurality of patterns in each of which a predetermined number of parallel lines are drawn per 1 mm) was copied, and the obtained copied image was visually evaluated. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, those of Examples 3.1 and 3.2 in Table 8, those of Examples 4.1 through 4.4 in Table 9 and those of Examples 5.1 through 5.3 in Table 10.
(e) Charge amount:
A copying operation was continued by using an original bearing characters with a black area ratio of 8% until 50,000 copies were made with regard to the developers of Examples 4.1 through 4.4 and 5.1 through 5.3 and until the transfer efficiency became less than 70% with regard to the other developers. During this copying operation, after making every 5,000 copies, the charge amount of 200 mg of the developer was measured by a blowoff type powder charge amount measuring device (manufactured by Toshiba Chemical Co., Ltd.), and the average of the charge amount per 1 g of the toner was calculated based on the measured value. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, those of Examples 3.1 and 3.2 in Table 8, those of Examples 4.1 through 4.4 in Table 9 and those of Examples 5.1 through 5.3 in Table 10.
(f) Toner scattering:
A copying operation was continued by using an original bearing characters with a black area ratio of 8% until 50,000 copies were made with regard to the developers of Examples 4.1 through 4.4 and 5.1 through 5.3 and until the transfer efficiency became less than 70% with regard to the other developers. Then, the toner scattering state in the copying machine was visually observed and evaluated. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, those of Examples 3.1 and 3.2 in Table 8, those of Examples 4.1 through 4.4 in Table 9 and those of Examples 5.1 through 5.3 in Table 10. In these tables, ○ indicates that the toner was not scattered; and × indicates that the toner was scattered.
(g) Durability:
After making every 10,000 copies, the transfer efficiency was calculated based on the consumed amount and the collected amount of the toner to find the number of copies that had been made before the transfer efficiency became less than 70%. The number was taken as an indicator for the durability of the developer. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, and those of Examples 3.1 and 3.2 in Table 8.
(h) Amount of attachment on the surface of the carrier particle due to the spent:
A copying operation was conducted by using an original bearing characters with a black area ratio of 8%. After making 50,000 copies (in the case where the transfer efficiency became less than 70% before making 50,000 copies, at that time), the developer was tested as follows: The developer was placed on a screen of 400 mesh, and sucked from the below with a blower, thereby separating the toner and the carrier. Five g of the carrier remained on the screen was charged in a beaker, to which toluene was added. Thus, the toner component attached onto the surfaces of the carrier particles due to the spent was dissolved. Then, the toluene solvent was discarded with the carrier attracted upon the bottom of the beaker with a magnet. This procedure was repeated several times until the resultant toluene solution became transparent. Then, the resultant carrier was heated with an oven to evaporate the toluene attached thereto, and the weight of the obtained residue was measured. A difference between the weight of the carrier charged in the beaker at first (i.e., 5 g in this case) and the weight of the residue after evaporating the toluene was taken as the amount of the toner components attached onto the surfaces of the carrier particles due to the spent (i.e., the spent amount). The spent amount is indicated as the weight in mg of the toner components attached to 1 g of the carrier. The results obtained from the developers of Examples 1.1 through 1.3 and Comparative Example 1 are listed in Table 6, those of Example 2.1 in Table 7, and those of Examples 3.1 and 3.2 in Table 8.
(i) Filming:
A copying operation was performed by using an original bearing characters with a black area ratio of 8%. After making 50,000 copies, the state of the photosensitive drum in the copying machine was visually observed. The results are listed in Table 10, wherein ○ indicates that no filming was observed; and × indicates that a filming was observed.

Table 6

Evaluation of Examples 1.1-1.3 and Comparative Example 1.
	Example 1.1	Example 1.2	Example 1.3	Comparative Example 1
I.D.	1.340	1.275	1.342	1.306
F.D.	0.004	0.003	0.004	0.006
Resolution	5	5	5	5
Charge amount (µC/g)	-22.3	-24.2	-23.8	-19.4
Toner scattering	○	○	○	○
Durability (copies)	100, 000	100, 000	90, 000	60, 000
Spent amount (mg) at 50, 000 copies	0.45	0.43	0.51	0.67

Table 7

Evaluation of Examples 2.1 and Comparative Example 1.
	Example 2.1	Comparative Example 1
I.D.	1.352	1.306
F.D.	0.003	0.006
Resolution	5	5
Charge amount (µC/g)	-22.8	-19.4
Toner scattering	○	○
Durability (copies)	110, 000	60, 000
Spent amount (mg) at 50, 000 copies	0.42	0.67

Table 8

Evaluation of Examples 3.1 and 3.2, and Comparative Example 1.
	Example 3.1	Example 3.2	Comparative Example 1
I.D.	1.334	1.288	1.306
F.D.	0.004	0.003	0.006
Resolution	5	5	5
Charge amount (µC/g)	-23.4	-25.8	-19.4
Toner scattering	○	○	○
Durability (copies)	110, 000	110, 000	60, 000
Spent amount (mg) at 50, 000 copies	0.41	0.42	0.67

Table 9

Evaluation of Examples 4.1-4.4.
	Example 4.1	Example 4.2	Example 4.3	Example 4.4
I.D.	1.362	1.354	1.315	1.372
F.D.	0.003	0.004	0.005	0.009
Resolution	5	5	5	5
Charge amount (µC/g)	-23.0	-22.1	-26.4	-14.5
Toner scattering	○	○	○	×
Transfer efficiency (%)	78.4	77.6	73.1	71.0
Spent amount (mg) at 50, 000 copies	0.40	0.44	0.45	0.57

Table 10

Evaluation of Examples 5.1-5.3.
	Example 5.1	Example 5.2	Example 5.3
I.D.	1.351	1.358	1.328
F.D.	0.004	0.003	0.005
Resolution	5	5	5
Charge amount (µC/g)	-22.5	-23.4	-20.8
Toner scattering	○	○	○
Transfer efficiency (%)	77.4	79.2	72.1
Spent amount (mg) at 50, 000 copies	0.41	0.38	0.64
Filming	○	○	×

[Review of the evaluation]

The developers produced in Examples 1.1 through 1.3 were excellently stable in the image density, the fog density, the resolution and the charge amount. Further, when these developers were used, no toner scattering was observed. The developers of Examples 1.1 through 1.3 had a smaller spent amount and improved durability than the developer of Comparative Example 1 containing the carrier having no coating layer.
The developer produced in Example 2.1 was excellently stable in the image density, the fog density, the resolution and the charge amount. Further, when the developer was used, no toner scattering was observed. In particular, this developer had a smaller spent amount and improved durability as compared with the developer of Comparative Example 1 containing the carrier having no coating layer.
The developers produced in Examples 3.1 and 3.2 were excellently stable in the image density, the fog density, the resolution and the charge amount. Further, when these developers were used, no toner scattering was observed. In particular, the developers of Examples 3.1 and 3.2 had a smaller spent amount and improved durability as compared with the developer of Comparative Example 1 containing the carrier having no coating layer.
The developers produced in Examples 4.1 through 4.4 were excellently stable in the image density and the resolution and had a small spent amount. Further, the developers of Examples 4.1 and 4.2 containing the carrier having a predetermined concentration of a quaternary ammonium group in the coating layer had a smaller amount of attachment onto the surfaces of the carrier particles, a lower fog density and improved transfer efficiency as compared with the developer of Example 4.3 containing the carrier having no quaternary ammonium group in the coating layer and that of Example 4.4 containing the carrier having an excessive concentration of the quaternary ammonium group in the coating layer.
The developers produced in Examples 5.1 through 5.3 were excellently stable in the image density, the fog density, the resolution and the charge amount. When these developers were used, no toner scattering was observed. Further, the developers of Examples 5.1 and 5.2 containing the carrier including the coating layer having the curing degree of 85% or more had further improved transfer efficiency and a further smaller spent amount as compared with the developer of Example 5.3 containing the carrier including the coating layer having the curing degree of 80%. Further, in the developers of Example 5.1 and 5.2, the filming of the photosensitive body was effectively suppressed.
Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be broadly construed.

Claims

A two-component type developer comprising toner and a carrier,
wherein said toner includes toner particles,
said toner particles include a binder resin and magnetic powder dispersed in said binder resin,
said binder resin is made of a composition including a resin having an anionic group,
said magnetic powder is included in said toner particles in the range of 0.1 to 5 parts by weight per 100 parts by weight of said binder resin,
each carrier particle in said carrier has a core particle and a coating layer covering said core particle, and
said core particle is made of a magnetic material represented by the following Formula (A):

Formula (A): MOFe₂O₃

wherein M indicates at least one metal selected from the group consisting of Cu, Zn, Fe, Ba, Ni, Mg, Mn, Al and Co.
A two-component type developer according to claim 1,
wherein an extracted solution obtained by extracting said toner with methanol has substantially no absorption peak in the range of 280 to 350 nm, and has a substantially zero absorbance in the range of 400 to 700 nm.
A two-component type developer according to claim 1,
wherein said magnetic powder is contained in the range of 0.5 to 3 parts by weight per 100 parts by weight of said binder resin.
A two-component type developer according to claim 1,
wherein said toner particles have a volume-based average particle diameter of 5 through 15 µm, and spacer particles having a volume-based average particle diameter of 0.05 through 1.0 µm are attached onto surfaces of said toner particles.
A two-component type developer according to claim 1,
wherein said coating layer is made of a resin composition including a resin having a cationic group.
A two-component type developer according to claim 1,
wherein said coating layer is made of a resin composition including an alkylated melamine resin and an acryl-modified silicone resin, and
said alkylated melamine resin has a weight-average molecular weight M represented by the following Formula (B): $Formula (B): M ≧ 1100C - 400$
wherein C indicates the number of carbon atoms included in an alkyl group contained in said alkylated melamine resin.
A two-component type developer according to claim 1,
wherein said coating layer is made of a resin composition including a methyl silicone resin and a methylated melamine resin, and said methylated melamine resin has a weight-average molecular weight of 700 or more.
A two-component type developer according to claim 1,
wherein said coating layer is made of a resin composition including a methyl silicone resin containing a T unit at a proportion of 70 mol% or more.
A two-component type developer according to claim 1,
wherein said coating layer is made of a resin composition including a thermosetting resin and a thermoplastic resin, said thermoplastic resin including a quaternary ammonium group at a concentration of 0.1 through 20 mmole per 100 g of said resin composition.
A two-component type developer according to claim 1,
wherein said coating layer includes a thermosetting resin and has a curing degree of 85% or more.
A two-component type developer according to claim 1,
wherein said coating layer has a content of 0.001 through 2.5 parts by weight per 100 parts by weight of said core particle.
A two-component type developer according to any of claims 9 and 10,
wherein said thermosetting resin is at least one selected from the group consisting of a modified or unmodified silicone resin, a thermosetting acrylic resin, a thermosetting styrene-acrylic resin, a phenol resin, a urethane resin, a thermosetting polyester resin, an epoxy resin and an amino resin.
A two-component type developer according to claim 5,
wherein said resin having a cationic group is a resin having a basic nitrogen containing group.
A two-component type developer according to claim 1,
wherein said core particle has a particle diameter of 50 through 150 µm.