Magnetic carrier, two-component developer containing magnetic carrier, and method for producing a magnetic carrier

Abstract

The present invention provides an electrophotographic magnetic carrier that exhibits excellent charge imparting and charge retention capabilities even in high-temperature and high-humidity environments, as well as high durability and high-quality development capabilities, a two-component developer, and a method for manufacturing the magnetic carrier. [Solution] The magnetic carrier comprises a carrier core and a resin coating layer covering the surface of the carrier core, and can charge toner by friction. The carrier core has fine particles with a volume-average primary particle diameter of 1 μm or less in the recesses on its surface. The amount of fine particles present on the surface of the carrier core is 2% to 20% based on the number of particles when the particle size distribution of the carrier core is measured.

Description

Technical Field

[0001] The present invention relates to a magnetic carrier for charging toner by friction, a two-component developer containing the magnetic carrier, and a method for manufacturing the magnetic carrier.

Background Art

[0002] Generally, in electrophotography, after the surface of an electrostatic latent image carrier is charged by corona discharge or the like, it is exposed by a laser or the like to form an electrostatic latent image. The formed electrostatic latent image is developed with toner to form a toner image. Further, the formed toner image is transferred onto a recording medium to obtain a high-quality image. As a method for developing an electrostatic latent image, a two-component development method using a two-component developer composed of toner and a magnetic carrier, a one-component development method using a one-component developer that does not use a magnetic carrier, and the like are known. However, when higher image quality and higher speed are required, the two-component development method is preferably used.

[0003] The two-component developer used in the two-component development method consists of a magnetic carrier and toner. In development, only the toner is consumed, and the carrier is repeatedly used while being agitated in the developing device. In order to meet the requirements for high durability in recent years, resin-coated magnetic particles (coating carriers) in which the surface of magnetic particles (carrier cores) is coated with a resin (coat layer) are used as carrier particles.

[0004] In the conventional method for manufacturing a coating carrier, the fixation of the coat layer to the surface of the carrier core is not sufficient, so that the coat layer may peel off due to durable printing. In addition, there are partially thin portions of the coat layer, and the amount of resin that can be coated is small, so the durability (life) of the carrier is low. Furthermore, since the roundness of the carrier is low and the fluidity is insufficient, the mixing property with toner is not sufficient, and the toner cannot be charged well.

[0005] Patent Document 1 discloses an electrostatic latent image developing carrier having core particles and a resin coating layer covering the surface of the core particles, wherein the average spacing Sm of surface irregularities Sm of the core particles is Sm ≤ 2.0 μm, the surface roughness Ra is Ra ≥ 0.1 μm, the surface roughness Ra of the carrier is Ra ≤ 0.5 μm, and the circularity of the carrier is 0.975 or higher. Patent Document 2 discloses a carrier having magnetic particles having irregularities on their surface, a conductive layer provided on the surface of the magnetic particles and composed of conductive metal nanoparticles, with irregularities on its surface that follow the irregularities of the surface of the magnetic particles, and a resin layer provided on the conductive layer. The carriers in Patent Documents 1 and 2 are said to have high durability by suppressing peeling of the resin layer from the magnetic particles.

[0006] Patent Document 3 discloses a carrier core material in which the average maximum height Rz of the particle group constituting the carrier core material is 2.00 μm or more and 3.50 μm or less, and the average root mean square slope angle RΔq of the particle group constituting the carrier core material is 0.70 or more and 1.00 or less. Patent Document 4 discloses ferrite particles having Mn ferrite as the main phase and containing Sr ferrite, in which the degree of roughness of the particle surface is in the range of 2.5 μm to 4.5 μm, and the standard deviation of the size of the grains appearing on the particle surface is in the range of 1.5 μm to 3.5 μm. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2007-286092 [Patent Document 2] Japanese Patent Publication No. 2011-141542 [Patent Document 3] Japanese Patent Publication No. 2017-197134 [Patent Document 4] Japanese Patent Publication No. 2018-189482 [Overview of the project] [Problems that the invention aims to solve]

[0008] However, although the carriers described in Patent Documents 1 to 4 all specify the average spacing Sm of the surface irregularities of the core particles and the surface roughness Ra, their ability to impart charge to the toner in high-temperature and high-humidity environments was not sufficient.

[0009] In view of the above problems, the present invention aims to provide an electrophotographic magnetic carrier that exhibits excellent charge imparting and charge retention capabilities even in high-temperature and high-humidity environments, is highly durable, and enables high-quality development; a two-component developer containing the magnetic carrier; and a method for manufacturing the magnetic carrier. [Means for solving the problem]

[0010] To achieve the above objective, the first configuration of the present invention is a magnetic carrier comprising a carrier core and a resin coating layer covering the surface of the carrier core, which can charge toner by friction. The carrier core has fine particles with a volume-average primary particle diameter of 1 μm or less in the recesses on its surface. The amount of fine particles present on the surface of the carrier core is 2% to 20% based on the number of particles when the particle size distribution of the carrier core is measured. [Effects of the Invention]

[0011] According to the first configuration of the present invention, a magnetic carrier for electrophotography is obtained that exhibits excellent charge imparting and charge retention capabilities even in high-temperature and high-humidity environments, resulting in high durability and enabling high-quality development. Therefore, a two-component developer can be manufactured that exhibits good image density in high-temperature and high-humidity environments after durable printing, and that also suppresses the occurrence of image fringing. [Brief explanation of the drawing]

[0012] [Figure 1] Cross-sectional view showing carrier core 1 before smoothing. [Figure 2] Cross-sectional view showing carrier core 1 after smoothing. [Figure 3] Figure 2 shows a magnified cross-sectional view of the surface area of ​​the carrier core 1 after smoothing. [Figure 4]Cross-sectional enlarged view near the surface of the carrier core 1 showing a state where a resin coating layer 5 is formed on the smoothed carrier core 1 [Figure 5] Electron micrograph of the carrier core C-1 used in the example magnified 500 times [Figure 6] Electron micrograph of the carrier core C-1 used in the example magnified 1000 times [Figure 7] Electron micrograph of the carrier core C-4 used in the example magnified 500 times [Figure 8] Electron micrograph of the carrier core C-4 used in the example magnified 1000 times [Figure 9] Electron micrograph of the carrier core C-6 used in the example magnified 500 times [Figure 10] Electron micrograph of the carrier core C-6 used in the example magnified 1000 times

Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described in detail. The evaluation results (values indicating shape or physical properties, etc.) regarding powders (more specifically, toner core particles, toner mother particles, external additives, or toner, etc.) are, if not otherwise specified, the number average of the values measured for each of a considerable number of average particles selected from the powder. Further, the number average particle diameter of the powder is, if not otherwise specified, the number average value of the equivalent circle diameter of the primary particles (the diameter of a circle having the same area as the projected area of the particle) measured using a microscope. Further, the measured value of the volume median diameter (D50) of the powder is, if not otherwise specified, the value measured using a laser diffraction / scattering type particle size distribution measuring device ("LA-750" manufactured by Horiba, Ltd.). Further, the measured value of each of the acid value and the hydroxyl value is, if not otherwise specified, the value measured in accordance with "JIS (Japanese Industrial Standard) K0070-1992". Further, the measured value of each of the number average molecular weight (Mn) and the mass average molecular weight (Mw) is, if not otherwise specified, the value measured using gel permeation chromatography.

[0014] Hereinafter, the compound and its derivatives may be collectively referred to by adding "system" after the compound name. When representing the polymer name by adding "system" after the compound name, it means that the repeating unit of the polymer is derived from the compound or its derivative. In addition, acrylic and methacrylic may be collectively referred to as "(meth)acrylic". Also, acryloyl (CH 2 =CH-CO-) and methacryloyl (CH 2 =C(CH 3 )-CO-) may be collectively referred to as "(meth)acryloyl".

[0015] The magnetic carrier of the present invention is a powder containing a plurality of carrier particles (particles having the configurations described below). Further, the carrier particles of the present invention have a carrier core and a resin layer covering the carrier core. In order to produce the carrier particles, a carrier core is formed of a magnetic material (for example, ferrite).

[0016] The magnetic carrier of the present invention, for example, positively charges a positively charged toner by friction, constitutes a two-component developer together with the positive charge property, and can be suitably used for developing an electrostatic latent image. In order to form a high-quality image, the amount of toner in the two-component developer is preferably 5 parts by mass or more and 15 parts by mass or less with respect to 100 parts by mass of the carrier. Note that the positively charged toner is positively charged by friction with the carrier.

[0017] The magnetic carrier according to the present embodiment can be used, for example, for forming an image in an electrophotographic apparatus (image forming apparatus). Hereinafter, an example of an image forming method by an electrophotographic apparatus will be described.

[0018] First, an electrostatic latent image is formed on the photoreceptor (e.g., the surface of the photoreceptor drum) based on the image data. Next, the formed electrostatic latent image is developed using a two-component developer containing toner and a carrier. In the development process, toner (e.g., positively charged toner due to friction with the carrier) on a developing sleeve (e.g., the surface of the developing roller in the developing unit) placed near the photoreceptor is deposited onto the electrostatic latent image, forming a toner image on the photoreceptor. Then, in the subsequent transfer process, the toner image on the photoreceptor is directly transferred to a recording medium (e.g., paper). Alternatively, it is first transferred to an intermediate transfer medium (e.g., a transfer belt), and then the toner image on the intermediate transfer medium is secondarily transferred to the recording medium. After that, the toner is heated to fix it to the recording medium. As a result, an image is formed on the recording medium. For example, a full-color image can be formed by superimposing toner images of four colors: black, yellow, magenta, and cyan.

[0019] [1. Basic Configuration of Magnetic Carriers] The magnetic carrier according to this embodiment has the following basic configuration. The magnetic carrier includes a plurality of carrier particles, each comprising a magnetic carrier core and a resin coating layer. The resin coating layer covers the surface of the magnetic carrier core. The resin coating layer may cover the entire surface of the magnetic carrier core or it may partially cover the surface of the magnetic carrier core.

[0020] The recesses on the surface of the carrier core contain fine particles with a volume-average primary particle diameter (D50) of 1 μm or less. The presence of these fine particles in the recesses of the carrier core allows the resin forming the coating layer to penetrate the particles and enter the recesses, resulting in stronger fixation of the coating layer to the carrier core. Furthermore, the presence of these fine particles increases the surface area of ​​the carrier, which increases the amount of resin that forms the coating layer, thus improving the durability of the carrier.

[0021] Furthermore, the surface irregularities of the carrier core are smoothed by the filling of the recesses of the carrier core with fine particles, improving the circularity of the carrier core. By forming a resin coating layer on this carrier core, the circularity is further improved and the fluidity of the carrier is enhanced. Therefore, by forming an image using a magnetic carrier having the above basic configuration, it becomes possible to form high-quality images with excellent charge imparting and charge retention capabilities even in high-temperature and high-humidity environments, and over a long period of time.

[0022] [2. Basic Toner Configuration] The magnetic carrier according to this embodiment is used as a two-component developer by mixing it with toner. As the toner used with the magnetic carrier, any known positively charged toner for two-component developers can be used.

[0023] The toner contains multiple toner particles, each containing a toner core particle. The toner core particle contains at least a binder resin, a release agent, and a colorant. The surface of the toner core particle may be covered with a shell layer. Preferably, the shell layer covers 50% to 99% of the surface area of ​​the toner core particle. Alternatively, the entire surface of the toner core particle may be covered with a shell layer. When the toner core particle is covered with a shell layer, the particle containing both the toner core particle and the shell layer is the toner matrix particle. When the toner core particle is not covered with a shell layer, the toner core particle is the toner matrix particle.

[0024] The thickness of the shell layer can be measured by observing a cross-section of the toner using a transmission electron microscope (TEM) and analyzing the TEM image with commercially available image analysis software. Examples of commercially available image analysis software include WinROOF (manufactured by Mitani Corporation). The coating state of the shell layer on the surface of the toner can be confirmed using a scanning electron microscope (SEM). Furthermore, the formation state of the shell layer and its interior can be confirmed by observing a cross-section of the toner using a transmission electron microscope (TEM).

[0025] External additives may be attached to the surface of the toner matrix particles. Inorganic microparticles or resin microparticles can be used as external additives. Attaching resin microparticles to the toner matrix particles as an external additive tends to improve the toner's cleaning properties (e.g., resistance to adhesion to the photoreceptor drum) and developability (e.g., transfer efficiency). This is thought to be because the resin microparticles function as spacers, making it less likely for toner to adhere to the photoreceptor drum, intermediate transfer belt, etc.

[0026] [3. Materials and manufacturing methods for magnetic carriers] Next, the essential or optional components constituting the magnetic carrier of the present invention will be described. The magnetic carrier of the present invention comprises at least a carrier core and a resin layer. The resin layer may also optionally contain a conductive agent. The carrier core forming the magnetic carrier particles, the resin material forming the resin layer, the conductive agent, and the method for manufacturing the magnetic carrier of the present invention will be described in order below.

[0027] (Career Core) The carrier core is not particularly limited, and any known binary carrier for electrophotography can be used. Examples include ferrite, magnetite, and metals such as iron, nickel, and cobalt; alloys or mixtures of the aforementioned metals with metals such as copper, zinc, antimony, aluminum, lead, tin, bismuth, beryllium, manganese, magnesium, selenium, tungsten, zirconium, and vanadium; mixtures of the aforementioned ferrite with metal oxides such as iron oxide, titanium oxide, and magnesium oxide; nitrides such as chromium nitride and vanadium nitride; carbides such as silicon carbide and tungsten carbide; and ferromagnetic ferrite. Ferrite and magnetite are particularly preferred.

[0028] The volume-average particle diameter of the carrier core is preferably 20 μm or more and 100 μm or less, and particularly preferably 30 μm or more and 65 μm or less. If the average particle size of the carrier core is less than 20 μm, the magnetization per particle becomes low, resulting in carrier development. If the average particle size of the carrier core exceeds 100 μm, the specific surface area for contact with the toner decreases, and the ability to impart charge to the toner also decreases, resulting in toner scattering. Furthermore, it is undesirable because sufficient developability cannot be obtained since the toner concentration in the developer (mass ratio of toner to carrier, T / C) cannot be set high. The volume-average particle diameter can be measured using a laser diffraction scattering particle size analyzer. An example of a laser diffraction scattering particle size analyzer is the LA-700 (manufactured by Horiba, Ltd.).

[0029] (Resin materials) A silicone resin is used as the resin constituting the resin coating layer. The silicone resin is a resin having a polysiloxane structure (for example, an alkylpolysiloxane structure). Examples of silicone resins include silicone resins having methyl groups and epoxy resin-modified silicone resins. Examples of silicone resins having methyl groups include silicone resins having methyl groups but no phenyl groups, and silicone resins having both methyl and phenyl groups (hereinafter sometimes referred to as "methylphenyl silicone resin"). Methylphenyl silicone resin or epoxy resin-modified silicone resin is preferred as the silicone resin. An example of a silicone resin is "SR-2411" (manufactured by Toray Dow Corning).

[0030] Instead of silicone-based resins, polyimide silicone resins, which offer excellent toner-filming properties, durability, and low water vapor permeability, can also be used. Polyimide silicone resins have both a polyimide structure and a polysiloxane structure (e.g., an alkylpolysiloxane structure). Examples of polyimide silicone resins include "KJR-651," "KJR-655," "KJR-657," and "KJR-663" (all manufactured by Shin-Etsu Chemical Co., Ltd.). The polyimide silicone resin may also be a resin obtained by curing a thermosetting polyimide silicone resin.

[0031] Furthermore, a mixture of the above-mentioned polyimide silicone resin and a fluorine-containing resin with excellent toner filming and charge imparting capabilities to positively charged toner can also be used. The fluorine-containing resin should preferably be one or more resins selected from the group consisting of polyvinyl fluoride, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polytrifluoroethylene (more specifically, polychlorotrifluoroethylene, etc.), polyhexafluoropropylene, and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) and polytetrafluoroethylene (PTFE), with FEP, PFA, and PTFE being particularly preferred. The mixing ratio of the polyimide silicone resin to the fluorine resin is preferably 1:9 to 9:1 by mass, and more preferably 2:8 to 8:2.

[0032] (Conductive agent) The resin coating layer preferably further contains a conductive agent. By further containing a conductive agent in the coating layer, the electrical resistance of the carrier particles and the ability to impart charge to the toner can be adjusted.

[0033] Examples of conductive agents include carbon black (especially conductive carbon black), metal oxide particles (e.g., titanium oxide particles and tin oxide particles), and organic conductive agents. Carbon black or titanium oxide particles are preferred as the conductive agent.

[0034] If the resin coating layer contains carbon black, the carbon black content is preferably 1.0 part by mass or more and 10.0 parts by mass or less, and more preferably 2.0 parts by mass or more and 6.0 parts by mass or less, per 100 parts by mass of the coating resin.

[0035] (Additives) The resin coating layer may contain at least one of the following additives: a charge control agent, an adhesion enhancer, and a crosslinking agent. A silane coupling agent (or a component derived from a silane coupling agent) is preferred as the additive, and an aminosilane coupling agent (or a component derived from an aminosilane coupling agent) is more preferred. The aminosilane coupling agent (or a component derived from an aminosilane coupling agent) functions as a charge control agent, an adhesion enhancer, and a crosslinking agent.

[0036] If the resin coating layer contains additives, the additive content is preferably 4.0 parts by mass or more and 20.0 parts by mass or less, and more preferably 8.0 parts by mass or more and 15.0 parts by mass or less, per 100 parts by mass of the coating resin.

[0037] (Method for manufacturing magnetic carriers) An example of a carrier manufacturing method of the present invention will be described. The carrier manufacturing method comprises a smoothing step of scraping the surface of the carrier core and filling the depressions on the surface of the carrier core with fine powder, a coating step of applying a resin coating layer forming solution to the carrier core, and a heating step of heating the carrier core after the coating step. The resin coating layer forming solution contains a coating resin such as a silicone resin, a solvent, and other components added as needed (e.g., a conductive agent and additives).

[0038] Suitable solvents for the resin coating layer forming solution include, for example, lactam compounds (e.g., 2-pyrrolidone and N-methyl-2-pyrrolidone), ketone compounds (e.g., methyl ethyl ketone and methyl isobutyl ketone), cyclic ether compounds (e.g., tetrahydrofuran and tetrahydropyran), alcohol compounds (e.g., n-butanol and isobutanol), ester solvents (e.g., ethyl acetate and isobutyl acetate), and aromatic hydrocarbon compounds (e.g., toluene and xylene). N-methyl-2-pyrrolidone is preferred as the solvent for the coating layer forming solution. The solid content concentration of the resin coating layer forming solution is preferably 3% by mass or more and 20% by mass or less.

[0039] (Smoothing process) In this process, the carrier core is stirred using a mixer to remove and smooth the protrusions on its surface. Figures 1 and 2 are cross-sectional views of the carrier core 1 before and after smoothing, respectively. As shown in Figure 1, the carrier core 1 before smoothing has protrusions 2 and recesses 3 on its surface. The tips of the protrusions 2 are sharp. On the other hand, as shown in Figure 2, the tips of the protrusions 2 in the carrier core 1 after smoothing have been removed and become rounded.

[0040] Figure 3 is a magnified cross-sectional view of the area near the surface of the carrier core 1 after smoothing, as shown in Figure 2. During the smoothing process, the protrusions are removed, generating fine particles 4 with the same composition as the carrier core 1. The fine particles 4 have a volume-average primary particle diameter of 1 μm or less and, as shown in Figure 3, are concentrated and deposited in the recesses 3 on the surface of the carrier core 1.

[0041] Specifically, the smoothing process rounds off the protrusions 2 of the carrier core 1, and the fine particles 4 generated by the removal of the protrusions 2 fill the recesses 3 of the carrier core 1. As a result, the circularity of the carrier core 1 increases. Furthermore, since the fine particles 4 are generated by the removal of the protrusions 2 of the carrier core 1, they do not adversely affect the physical properties of the carrier core 1. In addition, there is no need to separately add fine particles to fill the recesses 3, which contributes to reducing manufacturing costs.

[0042] Preferably, the amount of fine particles 4 present on the surface of the carrier core 1 is 2% to 20% based on the number of particles 1 μm or smaller.

[0043] Examples of mixers used in the smoothing process include multi-purpose mixers (manufactured by Nippon Coke Co., Ltd.). The amount of fine particles 4 relative to the carrier core 1 can be adjusted by the rotation speed and stirring time of the mixer in the smoothing process.

[0044] (Coating process) Methods for applying the resin coating layer forming solution to a carrier core include, for example, immersing the carrier core in the resin coating layer forming solution and spraying the resin coating layer forming solution onto the carrier core in a fluidized bed. In the method of immersing the carrier core in the resin coating layer forming solution, a small amount of the resin coating layer forming solution is applied to the convex parts of the carrier core's surface, while a large amount is applied to the concave parts of the carrier core's surface, resulting in an uneven application of the resin coating layer forming solution.

[0045] In contrast, the method of spraying a resin coating layer forming solution onto a carrier core in a fluidized bed tends to allow for uniform application of the resin coating layer forming solution to both the convex and concave areas of the carrier core's surface. For these reasons, the method of spraying a resin coating layer forming solution onto a carrier core in a fluidized bed is preferred.

[0046] In this invention, the carrier core 1 is subjected to a smoothing process, thereby increasing its circularity. As a result, the resin coating layer forming solution can be applied more uniformly to the surface of the carrier core 1 compared to when the smoothing process is not performed.

[0047] (Heating process) In this process, the carrier core is heated after the coating process to remove the solvent contained in the resin coating layer forming solution. Furthermore, if the resin coating layer forming solution contains uncured polyimide silicone resin, the uncured polyimide silicone resin is heat-cured. As a result, a resin coating layer is formed from the resin coating layer forming solution. For example, the heating conditions can be set to a temperature of 200°C to 300°C and a heating time of 30 minutes to 90 minutes.

[0048] Figure 4 is an enlarged cross-sectional view of the area near the surface of the carrier core 1, showing the state after the smoothing process and the formation of a resin coating layer 5 on the carrier core 1. Because the carrier core 1 that has undergone the smoothing process has a high degree of circularity, the thickness of the resin coating layer 5 can be made uniform in the convex portions 2 and concave portions 3 of the carrier core 1 using the resin coating layer forming solution. In addition, since the resin coating layer forming solution is coated so as to penetrate the fine particles 4 deposited in the concave portions 3, the fixation of the resin coating layer 5 to the carrier core 1 can be made stronger by the anchoring effect.

[0049] [4. Toner materials and manufacturing method] Next, the essential or optional components constituting the toner mixed with the magnetic carrier of the present invention will be described. The toner core particles contain at least a release agent and a colorant in the binder resin. They may also contain a charge control agent, magnetic powder, etc., as needed. Furthermore, the surface of the toner of the present invention may be treated with an external additive as desired.

[0050] The binder resin, release agent, charge control agent, colorant, magnetic powder, shell material for forming the toner core particles, and external additives, along with the method for manufacturing the toner of the present invention, will be described below in order.

[0051] (Binding resin) The toner core particles that make up the toner contain a binder resin. The binder resin that can be contained in the toner core particles is not particularly limited as long as it is a resin that has been conventionally used as a binder resin for toner. Specific examples of binder resins include thermoplastic resins such as styrene resins, acrylic resins, styrene-acrylic resins, polyethylene resins, polypropylene resins, vinyl chloride resins, polyester resins, polyamide resins, polyurethane resins, polyvinyl alcohol resins, vinyl ether resins, N-vinyl resins, and styrene-butadiene resins. Among these resins, polyester resins are preferred in terms of the dispersibility of the colorant in the binder resin, the electrostatic properties of the toner, and the fixation to the paper. Polyester resins will be described below.

[0052] Polyester resins can be obtained by condensation polymerization or copolymerization of a divalent or trivalent or higher alcohol component with a divalent or trivalent or higher carboxylic acid component. The following alcohol and carboxylic acid components are examples of components used in the synthesis of polyester resins.

[0053] Specific examples of divalent or trivalent or higher alcohol components include diols such as ethylene glycol, diethylene glycol, triethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, neopentyl glycol, 1,4-butenediol, 1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, dipropylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; bisphenol A, hydrogenated bisphenol A, and polyoxyethylene Examples include bisphenols such as bisphenol A and polyoxypropylene bisphenol A; and trivalent or higher alcohols such as sorbitol, 1,2,3,6-hexanetetrol, 1,4-sorbitan, pentaerythritol, dipentaerythritol, tripentaerythritol, 1,2,4-butanetriol, 1,2,5-pentanetriol, glycerol, diglycerol, 2-methylpropanetriol, 2-methyl-1,2,4-butanetriol, trimethylolethane, trimethylolpropane, and 1,3,5-trihydroxymethylbenzene.

[0054] Specific examples of divalent or trivalent or higher carboxylic acid components include maleic acid, fumaric acid, citraconic acid, itaconic acid, glutaconic acid, phthalic acid, isophthalic acid, terephthalic acid, cyclohexanedicarboxylic acid, succinic acid, adipic acid, sebatic acid, azelaic acid, malonic acid, or divalent alkyl or alkenyl succinic acids such as n-butylsuccinic acid, n-butenylsuccinic acid, isobutylsuccinic acid, isobutenylsuccinic acid, n-octylsuccinic acid, n-octenylsuccinic acid, n-dodecylsuccinic acid, n-dodecenylsuccinic acid, isododecylsuccinic acid, and isododecenylsuccinic acid. Carboxylic acids include trivalent or higher carboxylic acids such as 1,2,4-benzenetricarboxylic acid (trimellitic acid), 1,2,5-benzenetricarboxylic acid, 2,5,7-naphthalentricarboxylic acid, 1,2,4-naphthalentricarboxylic acid, 1,2,4-butanetricarboxylic acid, 1,2,5-hexanetricarboxylic acid, 1,3-dicarboxyl-2-methyl-2-methylenecarboxypropane, 1,2,4-cyclohexanetricarboxylic acid, tetra(methylenecarboxyl)methane, 1,2,7,8-octanetetracarboxylic acid, pyromellitic acid, and empol trimeric acid. These divalent or trivalent or higher carboxylic acid components may be used as ester-forming derivatives such as acid halides, acid anhydrides, and lower alkyl esters. Here, "lower alkyl" means an alkyl group having 1 to 6 carbon atoms.

[0055] When the binder resin is a polyester resin, the softening point of the polyester resin is preferably 70°C to 130°C, and more preferably 80°C to 120°C. To improve the strength of the toner core and the fixation of the toner, the number-average molecular weight (Mn) of the polyester resin is preferably 1000 to 2000. The molecular weight distribution of the polyester resin (ratio of mass-average molecular weight (Mw) to number-average molecular weight (Mn) Mw / Mn) is preferably 9 to 21.

[0056] As the binder resin, a thermoplastic resin is preferable because it has good adhesion to paper. However, thermoplastic resins can be used alone, or crosslinking agents or thermosetting resins can be added to them. By adding crosslinking agents or thermosetting resins and introducing a partially crosslinked structure into the binder resin, the heat resistance, storage properties, and durability of the toner can be improved without reducing the toner's adhesion. When using a thermosetting resin, the amount of crosslinked portion (gel amount) of the binder resin extracted using a Soxhlet extractor is preferably 10% by mass or less, and more preferably 0.1% by mass or more and 10% by mass or less, relative to the mass of the binder resin.

[0057] Epoxy resins and cyanate resins are preferred thermosetting resins that can be used with thermoplastic resins. Specific examples of suitable thermosetting resins include bisphenol A type epoxy resins, hydrogenated bisphenol A type epoxy resins, novolac type epoxy resins, polyalkylene ether type epoxy resins, cyclic aliphatic type epoxy resins, and cyanate resins. Two or more of these thermosetting resins can be used in combination.

[0058] The glass transition temperature (Tg) of the binder resin is preferably between 40°C and 70°C. If the glass transition temperature is too high, the low-temperature fixability of the toner tends to decrease. If the glass transition temperature is too low, the heat resistance of the toner tends to decrease.

[0059] The glass transition point of a binder resin can be determined from the point of change in the specific heat of the binder resin using a differential scanning calorimeter (DSC). More specifically, the glass transition point of the binder resin can be determined by measuring the endothermic curve of the binder resin using a differential scanning calorimeter (DSC-6200, manufactured by Seiko Instruments Corporation) as the measuring device. A 10 mg sample is placed in an aluminum pan, and an empty aluminum pan is used as a reference. The glass transition point of the binder resin can be determined from the endothermic curve obtained by measuring the binder resin at room temperature and humidity with a temperature range of 25°C to 200°C and a heating rate of 10°C / min.

[0060] The mass-average molecular weight (Mw) of the binder resin is not particularly limited as long as it does not hinder the objectives of the present invention. Typically, the mass-average molecular weight (Mw) of the binder resin is preferably 20,000 to 300,000, and more preferably 30,000 to 2,000,000. The mass-average molecular weight of the binder resin can be determined by gel permeation chromatography (GPC) using a calibration curve prepared in advance using standard polystyrene resin.

[0061] (Coloring agent) Toner core particles contain a colorant. The colorant that can be included in the toner core particles can be any known pigment or dye, depending on the color of the toner. Specific examples of suitable colorants that can be added to toner include: black pigments such as carbon black, acetylene black, lamp black, and aniline black; yellow pigments such as lead yellow, zinc yellow, cadmium yellow, yellow iron oxide, mineral fast yellow, nickel titanium yellow, navel yellow, naphthol yellow S, Hansa yellow G, Hansa yellow 10G, benzidine yellow G, benzidine yellow GR, quinoline yellow lake, permanent yellow NCG, tartrazine lake, monoazo yellow, and diazo yellow; orange pigments such as red lead yellow, molybdenum orange, permanent orange GTR, pyrazolone orange, balkan orange, and induthrene brilliant orange GK; red iron oxide, cadmium red, red lead, mercury cadmium sulfide, permanent red 4R, lithol red, and pyrazolone. Examples of colorants include red pigments such as Lon Red, Watching Red Calcium Salt, Lake Red D, Brilliant Carmine 6B, Eosin Lake, Rhodamine Lake B, Alizarin Lake, Brilliant Carmine 3B, and Monoazo Red; purple pigments such as Manganese Violet, Fast Violet B, and Methyl Violet Lake; blue pigments such as Prussian Blue, Cobalt Blue, Alkali Blue Lake, Victoria Blue Partial Chloride, Fast Sky Blue, Induthlene Blue BC, and Phthalocyanine Blue; green pigments such as Chrome Green, Chromium Oxide, Pigment Green B, Malachite Green Lake, and Final Yellow Green G; white pigments such as Zinc Oxide, Titanium Dioxide, Antimony White, and Zinc Sulfide; and extender pigments such as Barite Powder, Barium Carbonate, Clay, Silica, White Carbon, Talc, and Alumina White. Two or more of these colorants can also be used in combination to adjust the toner to a desired hue.

[0062] The amount of colorant used is not particularly limited as long as it does not hinder the objective of the present invention. Specifically, the amount of colorant used is preferably 1% by mass or more and 10% by mass or less, and more preferably 2% by mass or more and 7% by mass or less, relative to the total mass of the toner core particles.

[0063] Furthermore, a colorant can also be used as a masterbatch in which the colorant is pre-dispersed in a resin material such as a thermoplastic resin. When using a colorant as a masterbatch, it is preferable that the resin contained in the masterbatch is the same type of resin as the binder resin.

[0064] (Release agent) Toner core particles may contain a release agent to improve adhesion and offset resistance. The type of release agent that can be included in the toner core particles is not particularly limited as long as it does not hinder the objectives of the present invention. Wax is preferred as the release agent, and examples of waxes include carnauba wax, synthetic ester wax, polyethylene wax, polypropylene wax, fluororesin wax, Fischer-Tropsch wax, paraffin wax, montan wax, and rice wax. Two or more of these release agents can be used in combination. By adding such release agents to toner core particles, the occurrence of offset and image smearing (smudges around the image when the image is rubbed) can be suppressed more efficiently.

[0065] When polyester resin is used as the binder resin, from the viewpoint of compatibility, one or more release agents selected from the group consisting of carnauba wax, synthetic ester wax, and polyethylene wax are preferably used as the release agent. Similarly, when polystyrene resin is used as the binder resin, from the viewpoint of compatibility, Fischer-Tropsch wax and / or paraffin wax are preferably used as the release agent.

[0066] Fischer-Tropsch wax is a straight-chain hydrocarbon compound with few iso-structure molecules or side chains, produced using the Fischer-Tropsch reaction, which is a catalytic hydrogenation reaction of carbon monoxide.

[0067] Among Fischer-Tropsch waxes, those with a mass-average molecular weight of 1,000 or more and whose endothermic peak bottom temperature observed by DSC measurement is in the range of 100°C to 120°C are more preferable. Examples of such Fischer-Tropsch waxes include Sazol wax C1 (endothermic peak bottom temperature: 106.5°C), Sazol wax C105 (endothermic peak bottom temperature: 102.1°C), and Sazol wax SPRAY (endothermic peak bottom temperature: 102.1°C), all available from Sazol.

[0068] The amount of release agent used is not particularly limited as long as it does not hinder the objectives of the present invention. Preferably, the amount of release agent used is 1% by mass or more and 10% by mass or less, relative to the total mass of the toner core particles. If the amount of release agent used is too little, the desired effect of suppressing offset and image smearing in the formed image may not be obtained, and if the amount of release agent used is too much, the heat resistance of the toner may decrease due to fusion of toners.

[0069] (Charge control agent) The toner core particles may contain a charge control agent to improve the charge level of the toner and the charge rise characteristics, which are indicators of whether or not it can be charged to a predetermined charge level in a short time, thereby obtaining a toner with excellent durability and stability. In this embodiment, a positively charged charge control agent is used because the magnetic carrier positively charges the toner for development.

[0070] The types of charge control agents that can be contained in toner core particles are not particularly limited as long as they do not hinder the objectives of the present invention, and can be appropriately selected from charge control agents that have been used in toners. Specific examples of positively charged charge control agents include azine compounds such as pyridazine, pyrimidine, pyrazine, orthoxazine, metaoxazine, paraoxazine, orthothiaidine, metathiaidine, parathiaidine, 1,2,3-triazine, 1,2,4-triazine, 1,3,5-triazine, 1,2,4-oxadiazine, 1,3,4-oxadiazine, 1,2,6-oxadiazine, 1,3,4-thiadiazine, 1,3,5-thiadiazine, 1,2,3,4-tetrazine, 1,2,4,5-tetrazine, 1,2,3,5-tetrazine, 1,2,4,6-oxatriazine, 1,3,4,5-oxatriazine, phthalazine, quinazoline, and quinoxaline; azine compounds Examples include direct dyes consisting of azine compounds such as Fast Red FC, Azin Fast Red 12BK, Azin Violet BO, Azin Brown 3G, Azin Light Brown GR, Azin Dark Green BH / C, Azin Deep Black EW, and Azin Deep Black 3RL; nigrosine compounds such as nigrosine, nigrosine salts, and nigrosine derivatives; acid dyes consisting of nigrosine compounds such as nigrosine BK, nigrosine NB, and nigrosine Z; metal salts of naphthenic acid or higher fatty acids; alkoxylated amines; alkylamides; and quaternary ammonium salts such as benzylmethylhexyldecylammonium and decyltrimethylammonium chloride. Among these positively charged charge control agents, nigrosine compounds are particularly preferred because they provide a faster charge rise. Two or more of these positively charged charge control agents can be used in combination.

[0071] Resins having quaternary ammonium salts, carboxylates, or carboxyl groups as functional groups can also be used as positively charged charge control agents. More specifically, examples include styrene resins having quaternary ammonium salts, acrylic resins having quaternary ammonium salts, styrene-acrylic resins having quaternary ammonium salts, polyester resins having quaternary ammonium salts, styrene resins having carboxylates, acrylic resins having carboxylates, styrene-acrylic resins having carboxylates, polyester resins having carboxylates, styrene resins having carboxyl groups, acrylic resins having carboxyl groups, styrene-acrylic resins having carboxyl groups, and polyester resins having carboxyl groups. The molecular weight of these resins is not particularly limited as long as it does not hinder the objectives of the present invention, and they may be oligomers or polymers.

[0072] Among resins that can be used as positively charged charge control agents, styrene-acrylic resins having quaternary ammonium salts as functional groups are more preferred because the amount of charge can be easily adjusted to a value within a desired range. Specific examples of preferred acrylic comonomers copolymerized with styrene units in styrene-acrylic resins having quaternary ammonium salts as functional groups include alkyl (meth)acrylates such as methyl acrylate, ethyl acrylate, n-propyl acrylate, iso-propyl acrylate, n-butyl acrylate, iso-butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, and iso-butyl methacrylate.

[0073] Furthermore, as quaternary ammonium salts, dialkylaminoalkyl(meth)acrylates, dialkyl(meth)acrylamides, or units derived from dialkylaminoalkyl(meth)acrylamides through a quaternization process can be used. Specific examples of dialkylaminoalkyl(meth)acrylates include dimethylaminoethyl(meth)acrylate, diethylaminoethyl(meth)acrylate, dipropylaminoethyl(meth)acrylate, and dibutylaminoethyl(meth)acrylate. Specific examples of dialkyl(meth)acrylamides include dimethylmethacrylamide, and specific examples of dialkylaminoalkyl(meth)acrylamides include dimethylaminopropylmethacrylamide. In addition, hydroxyl group-containing polymerizable monomers such as hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate, and N-methylol(meth)acrylamide can be used in combination during polymerization.

[0074] The amount of charge control agent used is not particularly limited as long as it does not hinder the objectives of the present invention. Typically, the amount of charge control agent used is preferably 0.1% by mass or more and 10% by mass or less, relative to the total mass of the toner core particles. If the amount of charge control agent used is insufficient, it is difficult to stably charge the toner to a predetermined polarity, which may result in the image density of the formed image falling below the desired value or making it difficult to maintain the image density over a long period of time. In addition, because the charge control agent is difficult to disperse uniformly, the formed image is more prone to blurring, and contamination of the latent image-carrying area by toner components is more likely to occur. If the amount of charge control agent used is excessive, the environmental resistance deteriorates, making it easier for image defects in the formed image due to poor charging under high temperature and high humidity conditions, and contamination of the latent image-carrying area by toner components to occur.

[0075] (Shell material) When a shell layer is formed on the surface of toner core particles, the shell layer is formed, for example, from a vinyl-based resin. Furthermore, the vinyl-based resin used for forming the shell layer includes a charge-controlling resin. Because the shell layer is made of a charge-controlling resin, the toner can be charged to a desired level of charge over long periods in various environments, such as high-temperature, high-humidity or low-temperature, low-humidity environments, thereby enabling the formation of images of the desired density.

[0076] The vinyl resin is preferably a styrene-acrylic acid resin containing a styrene monomer and one or more acrylic acid monomers. Styrene-acrylic acid resins have strong hydrophobicity and tend to be positively charged. Furthermore, it is believed that forming the shell layer with a styrene-acrylic acid resin increases its affinity with resin fine particles formed from silicone-modified acrylic resin, which is attached to the toner matrix particles as an external additive, thereby suppressing the detachment of resin fine particles from the shell layer.

[0077] The amount of vinyl resin used is not particularly limited as long as it does not hinder the objectives of the present invention. Typically, the amount of vinyl resin used is preferably 1 to 20 parts by mass, and more preferably 3 to 15 parts by mass, per 100 parts by mass of toner core particles. If the amount of vinyl resin used is insufficient, the entire surface of the toner core particles may not be covered by the shell layer. If the entire surface of the toner core particles is not covered by the shell layer, the toner is prone to agglomeration during storage at high temperatures, and its heat-resistant storage properties tend to decrease. On the other hand, if the amount of vinyl resin used is excessive, the shell layer tends to become thick. In this case, it is difficult to obtain toner with excellent fixation properties.

[0078] The mass-average molecular weight (Mw) of the vinyl resin used to form the shell layer is not particularly limited as long as it does not hinder the objectives of the present invention. Typically, the mass-average molecular weight is preferably 20,000 to 1,500,000, and more preferably 200,000 to 400,000. The mass-average molecular weight (Mw) of the vinyl resin can be measured by gel permeation chromatography according to conventionally known methods.

[0079] The polymerization method of the above-mentioned monomer is not limited to the extent that it does not hinder the objective of the present invention, and any method such as solution polymerization, bulk polymerization, emulsion polymerization, suspension polymerization, etc. can be selected.

[0080] When additive polymerization of monomers having unsaturated bonds is carried out using an aqueous medium, such as emulsion polymerization or suspension polymerization, surfactants can be used. The surfactant is not limited to the extent that it does not hinder the objective of the present invention, and can be appropriately selected from the group consisting of anionic surfactants, cationic surfactants, and nonionic surfactants. Examples of anionic surfactants include sulfate ester salt type surfactants, sulfonate type surfactants, phosphate ester salt type surfactants, and soaps. Examples of cationic surfactants include amine salt type surfactants and quaternary ammonium salt type surfactants. Examples of nonionic surfactants include polyethylene glycol type surfactants, alkylphenol ethylene oxide adduct type surfactants, and polyhydric alcohol type surfactants which are derivatives of polyhydric alcohols such as glycerin, sorbitol, and sorbitan. Among these surfactants, it is preferable to use at least one of anionic surfactants and nonionic surfactants. These surfactants may be used individually or in combination of two or more.

[0081] (External additive) The surface of the toner matrix particles can be treated with an external additive as desired. The type of external additive is not particularly limited as long as it does not hinder the objective of the present invention, and can be appropriately selected from external additives conventionally used for toner. Specific examples of suitable external additives include inorganic fine particles made of silica, alumina, titanium oxide, magnesium oxide, zinc oxide, strontium titanate, barium titanate, and other metal oxides, as well as resin fine particles formed from acrylic resin or silicone-modified acrylic resin. Two or more of these external additives can be used in combination.

[0082] The particle size of the external additive is not particularly limited as long as it does not hinder the objective of the present invention, but is typically preferably 0.01 μm or more and 1.0 μm or less.

[0083] The amount of external additive used is not particularly limited as long as it does not hinder the objectives of the present invention. Typically, the amount of external additive used is preferably 0.1% to 10% by mass, and more preferably 0.2% to 5% by mass, relative to the total mass of toner matrix particles manufactured by forming a shell layer on the surface of toner core particles. If the amount of external additive used is insufficient, the hydrophobicity of the toner tends to decrease. As a result, it becomes more susceptible to the influence of water molecules in the air under high temperature and high humidity environments, and problems such as a decrease in image density of the formed image due to an extreme decrease in the charge amount of the toner, and a decrease in toner fluidity are likely to occur. On the other hand, if the amount of external additive used is excessive, there is a risk of a decrease in image density due to excessive toner charge buildup.

[0084] Next, a method for manufacturing toner will be described. The method for manufacturing toner is not particularly limited as long as the toner core particles and the shell layer are formed to have a predetermined structure. In addition, if necessary, toner core particles coated with a shell layer may be used as toner matrix particles, and an external additive treatment may be performed to attach an external additive to the surface of the toner matrix particles. As a suitable method for manufacturing electrostatic latent image developing toner described above, the method for manufacturing toner core particles, the method for forming the shell layer, and the external additive treatment method will be described in order below.

[0085] (Method for manufacturing toner core particles) The method for producing toner core particles is not particularly limited as long as any component such as a colorant, release agent, charge control agent, and magnetic powder can be well dispersed in the binder resin. A specific example of a preferred method for producing toner core particles is to mix the binder resin with components such as a colorant, release agent, charge control agent, and magnetic powder using a mixer, then melt-knead the binder resin and the components to be blended into the binder resin using a kneader such as a single-screw or twin-screw extruder, and finally crush and classify the cooled kneaded product. The average particle size of the toner core particles is not particularly limited as long as it does not hinder the objective of the present invention, but is generally preferably 5 μm or more and 10 μm or less.

[0086] (Method for forming the shell layer) The shell layer is formed by attaching vinyl resin microparticles to the surface of the toner core particles, creating a shell layer that covers the surface of the toner core particles.

[0087] Let me explain the method in more detail. First, in a mixing device, hydrochloric acid is added to deionized water to prepare a weakly acidic aqueous medium (for example, a pH selected from 3 to 5). Next, a dispersion of vinyl resin fine particles (suspendion) as a shell material and toner core particles are added to the pH-adjusted aqueous medium.

[0088] Next, while stirring the mixture containing the shell material and toner core particles, the temperature of the mixture is raised at a predetermined rate (for example, a rate selected from 0.1°C / min to 3°C / min) to a predetermined holding temperature (for example, a temperature selected from 50°C to 90°C). Furthermore, while stirring the mixture, the temperature of the liquid is maintained at the above holding temperature for a predetermined time (for example, a time selected from 30 minutes to 4 hours). It is believed that a reaction (immobilization of the shell layer) proceeds between the toner core particles and the shell material while the temperature of the mixture is maintained at a high temperature. A shell layer is formed when the shell material binds to the toner core particles. A dispersion of toner matrix particles is obtained when a shell layer is formed on the surface of the toner core particles in the mixture.

[0089] As described above, by attaching hydrophobic vinyl resin microparticles to the surface of toner core particles in a mixed solution and heating the mixed solution, the vinyl resin microparticles can be dissolved and formed into a film. However, the film formation of the vinyl resin microparticles may also proceed by heating during the drying process or by physical impact force during the external addition process.

[0090] After forming the shell layer as described above, the dispersion of toner matrix particles is neutralized using, for example, sodium hydroxide. Next, the dispersion of toner matrix particles is cooled to, for example, room temperature (approximately 25°C). Subsequently, the dispersion of toner matrix particles is filtered using, for example, a Buchner funnel. This separates the toner matrix particles from the liquid (solid-liquid separation), yielding wet cake-like toner matrix particles. Next, the obtained wet cake-like toner matrix particles are washed. Subsequently, the washed toner matrix particles are dried. After that, if necessary, the toner matrix particles and external additives may be mixed using a mixer (for example, an FM mixer manufactured by Nippon Coke Industries, Ltd.) to adhere the external additives to the surface of the toner matrix particles. When using a spray dryer in the drying process, the drying process and the external additive process can be performed simultaneously by spraying a dispersion of external additives (for example, silica particles) onto the toner matrix particles. In this way, toner containing a large number of toner particles is produced.

[0091] The contents and sequence of the toner manufacturing method described above can be arbitrarily changed according to the required toner composition or characteristics. Furthermore, the toner may be sieved after the external additive step. Unnecessary steps may also be omitted. For example, if a commercially available product can be used as is, the step of preparing that product can be omitted. Also, if the reaction for forming the shell layer proceeds well without adjusting the pH of the mixture, the pH adjustment step may be omitted. If the external additive is not attached to the surface of the toner mother particles (the external additive step is omitted), the toner mother particles correspond to the toner particles. To efficiently manufacture toner, it is preferable to form a large number of toner particles simultaneously. Toner particles manufactured simultaneously are considered to have substantially the same composition.

[0092] (External processing method) The method for treating toner matrix particles with external additives is not particularly limited, and the toner matrix particles can be treated according to conventionally known methods. Specifically, the treatment conditions are adjusted so that the particles of the external additive do not become embedded in the toner matrix particles, and the toner matrix particles are treated with the external additive using a mixer such as a Henschel mixer or a Nauter mixer.

[0093] The two-component developer using the toner described above and the magnetic carrier of the present invention exhibits excellent fixability and heat resistance for long-term image formation in various environments, such as high-temperature, high-humidity environments and low-temperature, low-humidity environments. This allows the toner to be charged to a desired level, thus enabling the formation of images of desired density. Therefore, the magnetic carrier of the present invention can be suitably used in various two-component developing image forming apparatuses. The effects of the present invention will be further described in detail below with reference to examples. However, the present invention is not limited in any way by these examples. [Examples]

[0094] [Manufacturing Example 1] (Manufacturing of toner core particles) As a binder resin, 100 parts by mass of polyester resin (XPE258, manufactured by Mitsui Chemicals, Inc.) was weighed, 5 parts by mass of polypropylene wax (660P, manufactured by Sanyo Chemical Co., Ltd.), 5 parts by mass of carbon black (REGAL330R, manufactured by Cabot Corporation), and 1 part by mass of charge control agent (Bontron P-51, manufactured by Orient Chemical Co., Ltd.) were weighed and mixed in a Henschel mixer (FM-10B, manufactured by Nippon Coke Industries, Ltd.), and then melt-kneaded in a twin-screw extruder. After the resulting kneaded material was cooled, it was pulverized and classified to obtain toner core particles with a volume-average particle size of 7 μm. The volume-average particle size was measured using a Coulter Counter Multisizer 3 (manufactured by Beckman Coulter, Inc.).

[0095] [Manufacturing Example 2] (Toner manufacturing) In the toner core particles obtained in Production Example 1, 1.0% by mass of titanium oxide (EC-100, manufactured by Titanium Industries Co., Ltd.) and 0.7% by mass of hydrophobic silica (RA-200H, manufactured by Nippon Aerosil Co., Ltd.), both treated with aminosilane, were added. The mixture was then mixed for 5 minutes at a rotation speed of 3500 rpm using a Henschel mixer (FM-10B, manufactured by Nippon Coke Industries Co., Ltd.) to obtain toner for electrostatic latent image development.

[0096] [Manufacturing Example 3] (Manufacturing of magnetic carriers) (3-1. Manufacturing of carrier cores) Uncoated ferrite particles (F-350, manufactured by Powdertech, volume-average primary particle diameter 35 μm) were used as magnetic particles and stirred (smoothed) using a multi-purpose mixer (manufactured by Nippon Coke Co., Ltd., tank: CP tank, stirring blades: for CP tank). By adjusting the rotation speed and stirring time, carrier cores C-1 to C-8 with different amounts of fine particles were obtained. Carrier core C-1 was left untreated (without smoothing treatment by the mixer).

[0097] The obtained carrier cores C-1 to C-8 were measured (wet measurement) using a laser diffraction particle size distribution analyzer (SYNC, Microtrac-Bell) to determine the amount of fine particles relative to the carrier core. The amount of fine particles in carrier cores C-1 to C-8, along with the mixer rotation speed and stirring time, are shown in Table 1. Scanning electron microscope (SEM) images (500x and 1000x) of carrier cores C-1, C-4, and C-6 are shown in Figures 5 to 10.

[0098] [Table 1]

[0099] (3-2. Formation of the resin coating layer) A silicone resin (SR-2411, manufactured by Toray Dow Corning Silicone Co., Ltd., solids content 20% by mass) was mixed with 15% by mass of aminosilane relative to its solids content and dissolved in toluene to obtain a coating resin layer forming solution. The obtained coating resin layer forming solution was spray-coated onto the carrier core C-4 obtained in 3-1 above using a fluidized bed coating apparatus (manufactured by Powrec Co., Ltd.). Subsequently, the resin was cured by heat treatment at 250°C for 3 hours in a fluidized bed to obtain the carrier of the present invention 1.

[0100] The carrier of the present invention 2 was obtained by processing in the same manner as in the present invention 1, except that carrier core C-5 was used instead of carrier core C-4.

[0101] Except for using carrier core C-6 instead of carrier core C-4, the process was carried out in the same manner as in Invention 1 to obtain the carrier of Invention 3.

[0102] Except for using carrier core C-7 instead of carrier core C-4, the process was carried out in the same manner as in Invention 1 to obtain the carrier of Invention 4.

[0103] The carriers of Comparative Example 1 were obtained by processing in the same manner as in Invention 1, except that carrier core C-1 was used instead of carrier core C-4.

[0104] The carriers of Comparative Example 2 were obtained by processing in the same manner as in Invention 1, except that carrier core C-2 was used instead of carrier core C-4.

[0105] The carriers of Comparative Example 3 were obtained by processing in the same manner as in Invention 1, except that carrier core C-3 was used instead of carrier core C-4.

[0106] The carriers of Comparative Example 4 were obtained by processing in the same manner as in Invention 1, except that carrier core C-8 was used instead of carrier core C-4.

[0107] [Evaluation of image haze in two-component developers] The toner obtained in Production Example 2 and the carriers of Invention 1-4 and Comparative Examples 1-4 obtained in Production Example 3 were mixed to achieve a toner concentration of 7% by mass (i.e., the amount of toner added was 7 parts by mass per 100 parts by mass of the total amount of toner and carrier), and the mixture was stirred and mixed in a rocking mixer for 30 minutes to obtain a two-component developer containing the carriers of Invention 1-4 and Comparative Examples 1-4.

[0108] The two-component developer obtained above was installed in the developing unit of the evaluation machine (TASKalfa2553ci, manufactured by Kyocera Document Solutions Corporation), and 100,000 test images with a print density of 5% were printed continuously under normal temperature and humidity conditions (NN environment, temperature 25°C, humidity 50%). After that, the evaluation machine was moved to a high temperature and high humidity environment (HH environment, 32.5°C, humidity 80%) and left for one day. Then, initial images were acquired under the HH environment conditions.

[0109] At the initial stage of the NN environment (at the start of printing), after 100,000 sheets of durable printing, the fog density (FD) of the white background part of the printed matter image at the initial stage of the HH environment was measured using a reflection densitometer (SpectroEye, manufactured by X-Rite). The fog density (FD) was calculated by the following formula (1). FD = (reflection density of the white paper part of the printing paper) - (reflection density of the paper without printing) The evaluation criteria for image fogging are shown below. ◎: FD ≤ 0.005 (FD is extremely low and extremely good) ○: 0.005 < FD ≤ 0.010 (FD is low and good) ×: FD > 0.010 (FD is very high and the image quality is poor)

[0110] Table 2 shows the evaluation results of the image fogging of the two-component developer using the carriers of the first to fourth aspects of the present invention and Comparative Examples 1 to 4.

[0111]

Table 2

[0112] As is clear from Table 2, in the carriers of the first to fourth aspects of the present invention using carrier cores C-4 to C-7 in which the abundance of fine particles on the surface of the carrier core is 2% or more and 20% or less, the image fogging after being left for one day under high-temperature and high-humidity environment was extremely good or good. In particular, in the carriers of the third and fourth aspects of the present invention using carrier core C-6 with an abundance of fine particles on the surface of the carrier core of 10% and carrier core C-7 with an abundance of 20%, the image fogging was extremely good.

[0113] In contrast, in Comparative Examples 1-3, the carriers using carrier cores C-1-C-3, which had a surface particle content of less than 2%, showed worsening image fringing after being left for one day in a high-temperature, high-humidity environment. Furthermore, in Comparative Example 4, the carrier using carrier core C-8, which had a surface particle content of 25%, exhibited numerous cracks in the carrier core, inducing carrier development and resulting in quality problems. Therefore, based on an overall quality assessment, it was judged to be defective.

[0114] Based on the above results, it was confirmed that a two-component developer can be obtained that suppresses image fogging in high-temperature and high-humidity environments by using a magnetic carrier in which the carrier core is smoothed, generating fine particles by removing the protrusions on the carrier core, and then ensuring that the amount of fine particles on the surface of the carrier core is between 2% and 20%, and then coating the surface of the carrier core with a resin coating layer. [Industrial applicability]

[0115] This invention can be used in magnetic carriers that charge toner by friction. By using this invention, it is possible to provide an electrophotographic magnetic carrier and a two-component developer that have excellent charge imparting and charge retention capabilities even in high-temperature and high-humidity environments, are highly durable, and enable high-quality development. [Explanation of Symbols]

[0116] 1 Carrier Core 2. Convex part 3 recesses 4 Fine particles 5. Resin coating layer

Claims

1. Carrier core and A resin coating layer covering the surface of the carrier core, A magnetic carrier having the ability to charge toner by friction, The magnetic carrier is characterized in that the carrier core has fine particles with a volume-average primary particle diameter of 1 μm or less in recesses on its surface, and the amount of the fine particles on the surface of the carrier core is 2% or more and 20% or less based on the number of particles when the particle size distribution of the carrier core is measured.

2. The magnetic carrier according to claim 1, characterized in that the carrier core has a volume-average primary particle diameter of 30 μm or more and 65 μm or less.

3. The magnetic carrier according to claim 1, characterized in that the fine particles have the same composition as the carrier core.

4. The magnetic carrier according to claim 3, characterized in that the fine particles are generated by applying an external force to the carrier core.

5. The magnetic carrier according to claim 1, characterized in that the resin coating layer is formed of a silicone resin.

6. A magnetic carrier according to any one of claims 1 to 5, The toner, which becomes positively charged due to friction with the carrier, A two-component developer containing the following:

7. Carrier core and A resin coating layer covering the surface of the carrier core, A method for manufacturing a magnetic carrier that is equipped with and capable of charging toner by friction, A smoothing step is performed by stirring the carrier core with a mixer to remove protrusions on the surface of the carrier and generate fine particles with a volume-average primary particle diameter of 1 μm or less. After the smoothing step is performed, a coating step is performed in which a resin coating layer forming solution is applied to the surface of the carrier core, After the coating step is performed, a heating step is performed in which the carrier core is heated to form the resin coating layer on the surface of the carrier core, A method for manufacturing magnetic carriers containing magnetic carriers.

8. The method for manufacturing a magnetic carrier according to claim 7, characterized in that by adjusting the rotation speed and stirring time of the mixer in the smoothing step, the amount of fine particles present on the surface of the carrier core is set to 2% or more and 20% or less based on the number of particles when the carrier core is measured for particle size distribution.

Images