Magnetic carriers and two-component developers containing magnetic carriers

Abstract

This invention provides an electrophotographic magnetic carrier and a two-component developer that exhibit excellent charge imparting and charge retention capabilities even in high-temperature and high-humidity environments, offering high durability and enabling high-quality development. [Solution] The magnetic carrier comprises a carrier core and a resin coating layer covering the surface of the carrier core, and can positively charge the toner by friction. The resin coating layer contains polyimide silicone resin and silica particles.

Description

Technical Field

[0001] The present invention relates to a magnetic carrier that charges toner by friction and a two-component developer containing 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 obtained by coating the surface of magnetic particles with a resin are used as carrier particles.

[0004] Patent Documents 1 and 2 disclose resin-coated magnetic particles (carrier particles) in which the resin coating layer of magnetic particles is made of a silicone resin and contains a conductive substance in the resin coating layer, and a developer composed of a mixture of resin-coated magnetic particles in which the resin coating layer of magnetic particles is made of a silicone resin and contains a coupling agent in the resin coating layer and toner particles. Patent Document 3 discloses a developer composed of a mixture of resin-coated magnetic particles in which the resin coating layer of magnetic particles is made of a polyimide silicone resin and contains a conductive substance in the resin coating layer and toner particles.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-118725 [Patent Document 2] Japanese Patent Application Publication No. 5-341581 [Patent Document 3] Japanese Patent Publication No. 2022-34879 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the carrier particles described in Patent Documents 1 and 2 did not exhibit sufficient charge imparting and charge retention capabilities in high-temperature, high-humidity environments due to the influence of the water vapor permeability of the silicone resin.

[0007] Furthermore, when electrostatic latent image formation is repeatedly performed in a high-humidity environment, ozone generated during the charging process that charges the photoreceptor reacts with nitrogen in the air to produce nitrogen oxides. These nitrogen oxides then react with moisture in the air to form nitric acid, which adheres to the surface of the photoreceptor, causing image blurring on the photoreceptor. Therefore, even when using the carrier described in Patent Document 3, the image quality in high-temperature, high-humidity environments was not sufficient.

[0008] 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, as well as a two-component developer containing the magnetic carrier. [Means for solving the problem]

[0009] 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 positively charge toner by friction. The resin coating layer contains a polyimide silicone resin and silica particles. [Effects of the Invention]

[0010] 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. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described in detail below. Unless otherwise specified, the evaluation results (values ​​indicating shape or physical properties, etc.) for the powder (more specifically, toner core particles, toner mother particles, external additives, or toner, etc.) are the number average values ​​of the values ​​measured for each of the average particles selected from the powder. Unless otherwise specified, the number average particle diameter of the powder is the number average value of the equivalent circle diameter (the diameter of a circle having the same area as the projected area of ​​the particle) of the primary particles measured using a microscope. Unless otherwise specified, the measured value of the median volume diameter (D50) of the powder is the value measured using a laser diffraction / scattering particle size distribution analyzer ("LA-750" manufactured by Horiba, Ltd.). Unless otherwise specified, the measured values ​​of the acid value and hydroxyl value are the values ​​measured according to "JIS (Japanese Industrial Standards) K0070-1992". Furthermore, unless otherwise specified, the measured values ​​for number-average molecular weight (Mn) and mass-average molecular weight (Mw) are those obtained using gel permeation chromatography.

[0012] In the following, the compound name may be followed by "system" to refer to the compound and its derivatives collectively. When "system" is followed by a compound name to represent a polymer name, it means that the repeating units of the polymer originate from the compound or its derivative. Additionally, acrylic and methacrylic may be collectively referred to as "(meth)acrylic." Furthermore, acryloyl (CH2=CH-CO-) and methacryloyl (CH2=C(CH3)-CO-) may be collectively referred to as "(meth)acryloyl."

[0013] The magnetic carrier of the present invention is a powder containing a plurality of carrier particles (each having a configuration described later). The carrier particles of the present invention also have a carrier core and a resin layer covering the carrier core. To produce the carrier particles, the carrier core may be formed from a magnetic material (e.g., ferrite), or from a resin in which magnetic particles are dispersed. Alternatively, magnetic particles may be dispersed in the resin layer covering the carrier core.

[0014] The magnetic carrier of the present invention positively charges positively charged toner through friction, and together with the positively charged toner, constitutes a two-component developer that can be suitably used for developing electrostatic latent images. To form high-quality images, the amount of toner in the two-component developer is preferably 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of carrier. The positively charged toner becomes positively charged through friction with the carrier.

[0015] The magnetic carrier according to this embodiment can be used, for example, to form images in an electrophotographic apparatus (image forming apparatus). An example of an image forming method using an electrophotographic apparatus will be described below.

[0016] First, an electrostatic latent image is formed on the photoreceptor (e.g., the surface layer 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 layer 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.

[0017] [1. Basic Structure of Magnetic Carrier] The magnetic carrier according to this embodiment has the following basic structure. The magnetic carrier includes a plurality of carrier particles each comprising a magnetic carrier core and a resin layer. The resin layer covers the surface of the magnetic carrier core. The resin layer may cover the entire surface of the magnetic carrier core or may partially cover the surface of the magnetic carrier core. The resin layer contains a polyimide silicone resin and silica particles.

[0018] The polyimide silicone resin has properties of excellent toner filming property, durability, and low water vapor permeability. The silica particles have properties of excellent charging ability to positively charged toner and excellent polishing ability on the surface of the photoreceptor. Therefore, by forming an image using a two-component developer containing the magnetic carrier having the above basic structure, it is possible to have excellent charging ability and charge holding ability even in a high temperature and high humidity environment, and to form high-quality images over a long period of time.

[0019] [2. Basic Structure of Toner] The magnetic carrier according to this embodiment is used as a two-component developer by being mixed with toner. As the toner used together with the magnetic carrier, those known as positively charged toner for two-component developers can be adopted.

[0020] The toner includes a plurality of toner particles each containing toner core particles. The toner core particles contain at least a binder resin, a release agent, and a colorant. The surface of the toner core particles may be coated with a shell layer. The shell layer preferably covers an area of 50% or more and 99% or less of the surface area of the toner core particles. Note that the entire surface of the toner core particles may be coated with the shell layer. When the toner core particles are coated with the shell layer, the particles containing the toner core particles and the shell layer are toner mother particles. When the toner core particles are not coated with the shell layer, the toner core particles are toner mother particles.

[0021] The thickness of the shell layer can be measured by observing the cross-section of the toner using a transmission electron microscope (TEM) and analyzing the TEM image with commercially available image analysis software. As the commercially available image analysis software, WinROOF (manufactured by Mitani Corporation) etc. can be used. The coating state by the shell layer on the surface of the toner can be confirmed using a scanning electron microscope (SEM). Further, the formation state of the shell layer and the inside of the shell layer can be confirmed by observing the cross-section of the toner using a transmission electron microscope (TEM).

[0022] External additives may adhere to the surface of the toner mother particles. As the external additives, inorganic fine particles or resin fine particles can be used. By attaching resin fine particles as external additives to the toner mother particles, the cleaning property of the toner (for example, the adhesion resistance to the photoreceptor drum) and the developing property (for example, the transfer efficiency) tend to be improved. This is presumably because the resin fine particles function as spacers, making it difficult for the toner to adhere to the photoreceptor drum, the intermediate transfer belt, etc.

[0023] [3. Material and Manufacturing Method of Magnetic Carrier] Next, essential or optional components constituting the magnetic carrier of the present invention will be described. The magnetic carrier of the present invention includes at least a carrier core and a resin coating layer. Further, if necessary, a conductive agent may be included in the resin coating layer. Hereinafter, the carrier core forming the magnetic carrier particles, the resin material forming the resin coating layer, the conductive agent, and the manufacturing method of the magnetic carrier of the present invention will be described in order.

[0024] (Carrier 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.

[0025] The volume-average particle size of the carrier core is preferably 20 to 70 μm. This allows for good developability. The volume-average particle size can be measured using a laser diffraction scattering particle size analyzer. Examples of laser diffraction scattering particle size analyzers include the LA-700 (manufactured by Horiba, Ltd.).

[0026] (Resin materials) The resin used to constitute the resin coating layer is a mixture of polyimide silicone resin, which has excellent toner filming and durability and low water vapor permeability, and silica particles, which have excellent ability to impart charge to positively charged toner and polish the surface of the photoreceptor drum.

[0027] Polyimide silicone resin is a copolymer having 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.

[0028] The average particle size (volume median diameter) of the silica particles is preferably 15 nm to 150 nm, and more preferably 30 nm to 100 nm.

[0029] The mixing ratio of polyimide silicone resin to silica particles is preferably 1:9 to 9:1 by mass, and more preferably 2:8 to 8:2.

[0030] (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 their ability to impart charge to the toner can be adjusted.

[0031] 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.

[0032] 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.

[0033] (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.

[0034] 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.

[0035] (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 coating step of applying a resin coating layer forming solution to a carrier core, and a heating step of heating the carrier core after the coating step. The resin coating layer forming solution contains a polyimide silicone resin, silica particles, a solvent, and other components added as needed (e.g., conductive agents and additives). The polyimide silicone resin may be a thermosetting polyimide silicone resin.

[0036] Examples of solvents for the resin coating layer forming solution include 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.

[0037] The solid content concentration of the resin coating layer forming solution is preferably 3% by mass or more and 20% by mass or less.

[0038] (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 tends to be 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. In contrast, in the method of spraying the resin coating layer forming solution onto the carrier core in a fluidized bed, the resin coating layer forming solution tends to be applied uniformly to both the convex and concave parts of the carrier core's surface. For these reasons, the method of spraying the resin coating layer forming solution onto the carrier core in a fluidized bed is preferred.

[0039] (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.

[0040] [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 may be treated with an external additive as desired.

[0041] The binder resin, release agent, charge control agent, colorant, magnetic powder, shell material (when forming a shell layer), and external additives used to form the toner core particles, along with the toner manufacturing method, will be described in order below.

[0042] (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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] (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.

[0053] 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.

[0054] 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.

[0055] (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 a release agent to the toner core particles 102, the occurrence of offset and image smearing (smudges around the image when the image is rubbed) can be suppressed more efficiently.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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 102. 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.

[0060] (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.

[0061] 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 conventionally 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 Stread 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.

[0062] 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.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] (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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] (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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] (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.

[0077] (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.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] (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.

[0084] The two-component developer comprising a magnetic carrier and toner as described above exhibits excellent fixability and heat resistance for storage. It can charge the toner to a desired level of charge when forming images over long periods in various environments, such as high-temperature, high-humidity environments or low-temperature, low-humidity environments, thereby enabling the formation of images of desired density. For this reason, the two-component developer of the present invention can be suitably used in various 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]

[0085] [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.).

[0086] [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.), which had been 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.

[0087] [Manufacturing Example 3] (Manufacturing of magnetic carriers) 40 parts by mass of manganese oxide (MnO), 9 parts by mass of magnesium oxide (MgO), 50 parts by mass of iron(III) oxide (Fe2O3), and 1 part by mass of strontium oxide (SrO) were mixed and ground in a ball mill for 2 hours, and then calcined at 1000°C for 5 hours to obtain manganese-based ferrite particles. The resulting magnetic particles had a particle size of 40 μm and a saturation magnetization of 65 Am. 2 / kg(3000×10 3 The pressure was / 4π·A / m (when applied). A coating resin was obtained by dispersing polyimide silicone resin (KJR-651, manufactured by Shin-Etsu Chemical Co., Ltd.) and silica particles (AEROSIL90G, manufactured by Nippon Aerosil Co., Ltd.) in a mass ratio of 5:5 in N-methyl-2-pyrrolidone. 3 parts by mass of the coating resin was spray-coated to 100 parts by mass of magnetic particles (carrier core) using a fluid coating apparatus. Subsequently, the resin was cured by heat treatment at 230°C for 1 hour in a fluid bed to obtain the carrier of the present invention 1.

[0088] Except for dispersing polyimide silicone resin (KJR-651, manufactured by Shin-Etsu Chemical Co., Ltd.) and silica particles (AEROSIL90G, manufactured by Nippon Aerosil Co., Ltd.) in a mass ratio of 2:8 in N-methyl-2-pyrrolidone to form a coating resin, the process was carried out in the same manner as in Invention 1 to obtain the carrier of Invention 2.

[0089] Except for dispersing polyimide silicone resin (KJR-651, manufactured by Shin-Etsu Chemical Co., Ltd.) and silica particles (AEROSIL90G, manufactured by Nippon Aerosil Co., Ltd.) in a mass ratio of 8:2 in N-methyl-2-pyrrolidone to form a coating resin, the same procedure as in Invention 1 was used to obtain the carrier of Invention 3.

[0090] A carrier according to the present invention 4 was obtained by the same procedure as in the present invention 1, except that 4 parts by mass of carbon black (#3230B, manufactured by Mitsubishi Carbon Black Co., Ltd.) was added to 100 parts by mass of coating resin, and 4 parts by mass of coating resin was spray-coated to 100 parts by mass of magnetic particles (carrier core) using a fluid coating apparatus.

[0091] A carrier according to the present invention 5 was obtained by the same procedure as in the present invention 1, except that 6 parts by mass of titanium oxide (EC100, manufactured by Titanium Industries Co., Ltd.) was added to 100 parts by mass of coating resin, and 1 part by mass of coating resin was spray-coated to 100 parts by mass of magnetic particles (carrier core) using a fluid coating apparatus.

[0092] The carrier of the present invention 6 was obtained by processing in the same manner as in the present invention 1, except that silica particles that had been hydrophobized by exposing them to a solution of HMDS (hexamethyldisilazane) diluted to 5% with toluene in an atmosphere heated to 80°C were used.

[0093] A carrier according to the present invention 7 was obtained by dispersing polyimide silicone resin (KJR-651, manufactured by Shin-Etsu Chemical Co., Ltd.) and tetrafluoroethylene-perfluoroalkyl vinyl ether (PFA) resin fine particles in N-methyl-2-pyrrolidone in a mass ratio of 5:5 to 100 parts by mass of a coating resin, to which 4 parts by mass of carbon black (#3230B, manufactured by Mitsubishi Carbon Black Co., Ltd.) was added, and 3 parts by mass of the coating resin was spray-coated using a fluid coating apparatus with 100 parts by mass of magnetic particles (carrier core) in the same manner as in the present invention 1.

[0094] The carrier for Comparative Example 1 was obtained by processing in the same manner as in Invention 1, except that a polyimide silicone resin (KJR-651, manufactured by Shin-Etsu Chemical Co., Ltd.) was dispersed in N-methyl-2-pyrrolidone to form a coating resin.

[0095] The carrier for Comparative Example 2 was obtained in the same manner as in Comparative Example 1, except that 4 parts by mass of carbon black (#3230B, manufactured by Mitsubishi Carbon Black Co., Ltd.) was added to 100 parts by mass of coating resin, and 4 parts by mass of coating resin was spray-coated to 100 parts by mass of magnetic particles (carrier core) using a fluid coating apparatus.

[0096] The carrier for Comparative Example 3 was obtained by processing in the same manner as in Invention 1, except that a silicone resin (SR2400, manufactured by Toray Dow Corning) was dispersed in N-methyl-2-pyrrolidone to form a coating resin.

[0097] The carrier for Comparative Example 4 was obtained by processing in the same manner as for Comparative Example 3, except that 4 parts by mass of carbon black (#3230B, manufactured by Mitsubishi Carbon Black Co., Ltd.) was added to 100 parts by mass of the coating resin.

[0098] Table 1 shows the resin coating layer, resin ratio, coating amount, and conductive agent of the magnetic carriers of Invention 1-6 and Comparative Examples 1-4 obtained in Manufacturing Example 3.

[0099] [Table 1] *Hydrophobic treated silica particles.

[0100] [Manufacturing Example 4] (Manufacturing of two-component developers) The toner obtained in Production Example 2 and the carriers of Invention 1-6 and Comparative Examples 1-4 obtained in Production Example 3 were mixed together to achieve a toner concentration of 8% by mass (i.e., the amount of toner added was 8 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.

[0101] [Evaluation of image density and image haze of two-component developers] An image forming apparatus (TASKalfa500ci manufactured by Kyocera Document Solutions) was loaded with a two-component developer containing the carriers of the first to fifth embodiments and Comparative Examples 1 to 4 of the present invention, and continuous printing was performed at a printing rate of 2% for 5,000 sheets and at a printing rate of 20% for 5,000 sheets under normal temperature environment (temperature: 23°C, humidity: 50%). Then, continuous printing was performed at a printing rate of 5% for 5,000 sheets under high temperature and high humidity environment (32.5°C, humidity: 80%). Then, the environment was returned to normal temperature and normal humidity (temperature: 23°C, humidity: 50%), and continuous printing was performed at a printing rate of 5% for 100,000 sheets.

[0102] (Image density) The solid image density (ID) of the printed matter at the start of printing, after printing 5,000 sheets at a printing rate of 2%, after printing 5,000 sheets at a printing rate of 20%, after printing 5,000 sheets at a printing rate of 5% under high temperature and high humidity environment, and after returning to normal temperature and normal humidity environment and printing 100,000 sheets at a printing rate of 5% was measured with a Macbeth reflection densitometer (RD914, manufactured by Gretag Macbeth). The evaluation criteria for image density are shown below. ◎: ID ≥ 1.3 (The image density is very dark and extremely good) ○: 1.0 ≤ ID < 1.3 (The image density is dark and good) ×: ID < 1.0 (The image density is very light and the image quality is poor)

[0103] (Image fogging) The fogging density (FD) of the white background portion of the image of the printed matter at the start of printing, after printing 5,000 sheets at a printing rate of 2%, after printing 5,000 sheets at a printing rate of 20%, after printing 5,000 sheets at a printing rate of 5% under high temperature and high humidity environment, and after returning to normal temperature and normal humidity environment and printing 100,000 sheets at a printing rate of 5% was measured with a reflection densitometer (R710, manufactured by IHARA). The fogging density (FD) was calculated by the following formula (1). FD = (Reflection density of the blank portion 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 particularly 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)

[0104] (Image sequence) After printing 5,000 sheets at 5% coverage in a high-temperature, high-humidity environment, and then leaving them for another 24 hours, a halftone image was output and an image flow test was performed. The evaluation criteria for image flow are shown below. ◎: No image blurring occurred at all. ○: No image blurring occurred, but slight variations in image density were observed in the halftone image. ×: Image blurring occurred.

[0105] Table 2 shows the evaluation results for image density, image fringing, and image streaking of two-component developers using carriers according to Inventions 1-6 and Comparative Examples 1-4.

[0106] [Table 2]

[0107] As is clear from Table 2, in the carriers of the present invention 1 to 5, in which the carrier core was coated with a resin coating liquid containing silica particles mixed with polyimide silicone resin, the image density and image fringing were all extremely good or good after durable printing in a normal temperature and humidity environment, after durable printing in a high temperature and high humidity environment, and after durable printing after returning to a normal temperature and humidity environment. Furthermore, no image flow occurred at all in the high temperature and high humidity environment.

[0108] In particular, in Invention 4 and 5, where a resin coating liquid was prepared by mixing silica particles with polyimide silicone resin and a conductive agent was added to the resin coating layer, the image density and image fringe were extremely good after printing 5,000 images with print density of 2% and 20% under normal temperature and humidity conditions. Furthermore, in Invention 6, which uses hydrophobized silica particles, the ability to impart charge under high temperature and high humidity conditions is stable, resulting in extremely good image density and image fringe under high temperature and high humidity conditions.

[0109] In contrast, in Comparative Examples 1 and 2, where the carrier core was coated using only polyimide silicone resin, image streaking occurred in high-temperature, high-humidity environments. Furthermore, in Comparative Examples 3 and 4, where the carrier core was coated using only silicone resin, the image density slightly decreased after printing 5,000 images at 20% print density in normal temperature and humidity environments. Additionally, the image density decreased and image streaking occurred after durable printing in high-temperature, high-humidity environments.

[0110] Based on these results, it was confirmed that using a magnetic carrier in which the surface of the carrier core is coated with polyimide silicone resin and silica particles results in a two-component developer that exhibits good image density at the start of printing, after durable printing, and in high-temperature and high-humidity environments, while also suppressing the occurrence of image fogging and image streaking. [Industrial applicability]

[0111] 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.

Claims

1. Carrier core and A resin coating layer covering the surface of the carrier core, A magnetic carrier having the ability to positively charge toner by friction, The aforementioned resin coating layer is characterized by containing a polyimide silicone resin and silica particles, thereby providing a magnetic carrier.

2. The magnetic carrier according to claim 1, characterized in that the resin coating layer contains a conductive agent.

3. The magnetic carrier according to claim 2, characterized in that the conductive agent is carbon black or titanium oxide.

4. The magnetic carrier according to claim 1, characterized in that the resin coating layer has a mixing ratio of the polyimide silicone resin and the silica particles of 2:8 to 8:2 by mass.

5. The magnetic carrier according to claim 1, characterized in that the volume median diameter of the silica particles is 30 nm or more and 100 nm or less.

6. The magnetic carrier according to claim 1, characterized in that the silica particles are subjected to hydrophobic treatment.

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

Images