Magnetic one-component toner and image forming apparatus using the same

Abstract

The present invention provides a magnetic single-component toner that improves the blackness of the toner and stabilizes the toner's chargeability over a long period of time, and an image forming apparatus that uses the same. [Solution] The magnetic one-component toner consists of toner particles comprising toner matrix particles containing at least a binder resin and magnetic powder, and an external additive attached to the surface of the toner matrix particles. The toner matrix particles contain nigrosine dye. The external additive contains strontium titanate particles whose surface has been hydrophobized. The nitrogen content of the toner particles is 0.5% by mass or more and 5% by mass or less, the Abs value, which is the amount of nigrosine dye that dissolves when the toner particles are immersed in methanol and expressed as spectrophotometric value, is 0.2 or more and 2.0 or less, and the volume resistivity of the strontium titanate is 1.0E+7 [Ω·cm] or more and 1.0E+10 [Ω·cm] or less.

Description

Technical Field

[0001] The present invention relates to a magnetic one-component toner and an image forming apparatus using a magnetic one-component development method using the magnetic one-component toner.

Background Art

[0002] Generally, in the electrophotographic method, 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.

[0003] As methods for developing an electrostatic latent image with toner, a magnetic one-component development method using only a magnetic toner and a two-component development method using a two-component developer containing a non-magnetic toner and a magnetic carrier are known. Since the magnetic toner is less expensive than the two-component developer, the magnetic one-component development method is widely used in monochrome printers.

[0004] In the magnetic one-component development method, the requirement for the coloring power of the magnetic toner is increasing so that printing with a high black density can be performed even with a small amount of magnetic toner. If the content of magnetic iron oxide in the toner is increased, the coloring power of the toner will increase, but the influence on the image quality is large, and problems such as poor charging, deterioration of fixing properties, and wear of members (such as a developing roller and a cleaning blade) that the toner contacts will occur.

[0005] As a means for increasing the coloring power, as described in Patent Document 1, a method of increasing the hiding power of the toner by reducing the particle size of magnetic iron oxide and increasing the number of magnetic iron oxide contained in the magnetic toner is known. In this method, disadvantages such as a decrease in the black density of the magnetic iron oxide itself (reddish tint) due to the miniaturization of the magnetic iron oxide and an increase in charging defects and wear of members that the toner contacts due to an increase in the exposure of magnetic powder on the toner surface occur.

[0006] Patent Document 2 discloses a toner using a crystalline polyester resin, an amorphous resin, and needle-shaped magnetic particles surface-treated with nigrosine as a magnetic powder. Patent Document 3 discloses a toner in which the toner particles contain at least a crystalline polyester resin and nigrosine, and the degree of compatibility between the crystalline polyester resin and nigrosine is defined.

[0007] Patent Document 4 discloses a toner containing a crystalline resin and magnetic powder, with strontium titanate doped with metallic elements other than titanium and strontium added as an external additive. In Patent Document 4, strontium titanate suppresses the exposure of magnetic powder to the toner surface, and the magnetic powder present on the toner surface is effective in suppressing the embedding of strontium titanate. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 6440141 [Patent Document 2] Japanese Patent Publication No. 2017-211446 [Patent Document 3] Japanese Patent Publication No. 2017-215377 [Patent Document 4] Japanese Patent Publication No. 2022-171125 [Overview of the project] [Problems that the invention aims to solve]

[0009] The toner described in Patent Document 2 has an issue where the magnetic powder portion exposed on the surface becomes excessively charged. As a result, in low-humidity environments, the electrostatic adhesion force causes poor release from the toner carrier, leading to a decrease in developability (image density).

[0010] Furthermore, in the toner described in Patent Document 3, the compatibility of the crystalline polyester resin and the nigrosine dye increased the resistance of the compatible nigrosine, which is the charged portion, and it tended to become excessively charged, especially in low-humidity environments. In the toner described in Patent Document 4, the strontium titanate itself has low charge-imparting ability, and if toner components migrate to the toner carrier or toner additives become embedded in the toner particles after prolonged use, the toner's chargeability may become insufficient.

[0011] In view of the above problems, the present invention aims to provide a magnetic single-component toner that can improve the blackness of the toner and stabilize the toner's chargeability over a long period of time, and an image forming apparatus using the same. [Means for solving the problem]

[0012] To achieve the above objective, the first configuration of the present invention is a magnetic one-component toner comprising toner particles comprising toner matrix particles containing at least a binder resin and magnetic powder, and an external additive attached to the surface of the toner matrix particles. The toner matrix particles contain nigrosine dye. The external additive contains strontium titanate particles whose surface has been hydrophobized. The nitrogen element content of the toner particles is 0.5% by mass or more and 5% by mass or less, the Abs value, which is the amount of nigrosine dye that dissolves when the toner particles are immersed in methanol and expressed as spectrophotometric value, is 0.2 or more and 2.0 or less, and the volume resistivity of the strontium titanate is 1.0E+7 [Ω·cm] or more and 1.0E+10 [Ω·cm] or less. [Effects of the Invention]

[0013] According to the first configuration of the present invention, a magnetic single-component toner is obtained that improves the blackness of the toner and stabilizes the toner's chargeability over a long period of time, thereby suppressing the decrease in image density and the occurrence of toner layer irregularities after durable printing. [Brief explanation of the drawing]

[0014] [Figure 1] Schematic cross-sectional view of an image forming apparatus 100 using the magnetic one-component toner of the present invention. [Modes for carrying out the invention]

[0015] [1. Overall configuration of the image forming apparatus] Embodiments of the present invention will be described below with reference to the drawings. Figure 1 is a schematic cross-sectional view of an image forming apparatus 100 in which the magnetic one-component toner of the present invention is used. In the image forming apparatus (e.g., a monochrome printer) 100, when printing is performed, an electrostatic latent image is formed in the image forming unit 9 within the image forming apparatus 100 based on original image data transmitted from a higher-level device (not shown), such as a personal computer, and toner is attached to the electrostatic latent image by the developing device 4 to form a toner image. Toner is supplied to the developing device 4 from the toner container 5. In the image forming apparatus 100, the image forming process on the photoreceptor drum 1 is performed while the photoreceptor drum 1 is rotated clockwise in Figure 1.

[0016] The image forming unit 9 is equipped with a charging device 2, an exposure unit 3, a developing device 4, a transfer roller 6, a cleaning device 7, and a static elimination device (not shown) along the rotation direction (clockwise) of the photoreceptor drum 1. The photoreceptor drum 1 is, for example, an aluminum drum with a photosensitive layer laminated on its surface (outer surface). The surface (outer surface) of the photoreceptor drum 1 is uniformly charged by the charging device 2. Then, an electrostatic latent image is formed on the surface that receives a light beam from the exposure unit 3, which will be described later, with the charge attenuated. The photosensitive layer is not particularly limited, but amorphous silicon (a-Si), which has excellent durability, is preferred.

[0017] The charging device 2 uniformly charges the surface of the photoreceptor drum 1. The charging device 2 uses, for example, a corona discharge device that discharges by applying a high voltage with a thin wire or the like as an electrode. Instead of the corona discharge device, a contact charging device that applies a voltage in a state where a charging member typified by a charging roller is brought into contact with the surface of the photoreceptor drum 1 may be used. The exposure unit 3 irradiates the photoreceptor drum 1 with a light beam (for example, a laser beam) based on image data to form an electrostatic latent image on the surface of the photoreceptor drum 1.

[0018] The developing device 4 attaches toner to the electrostatic latent image on the photoreceptor drum 1 to form a toner image. In this embodiment, a magnetic single-component toner (magnetic single-component developer) is accommodated in the developing device 4. The developing device 4 is a magnetic single-component jumping development method having a mechanism for charging the toner via a developing roller 4a. The cleaning device 7 includes a cleaning blade 7a that makes line contact in the longitudinal direction of the photoreceptor drum 1 (the direction perpendicular to the plane of FIG. 1), and after the toner image is transferred (transferred) to the paper, the cleaning blade 7a removes the toner remaining on the surface of the photoreceptor drum 1.

[0019] Toward the photoreceptor drum 1 on which the toner image is formed as described above, the paper is conveyed from the paper storage unit 10 to the image forming unit 9 at a predetermined timing via the paper conveyance path 11 and the registration roller pair 13. The transfer roller 6 contacts the photoreceptor drum 1 to form a nip portion (transfer nip portion), and transfers (transfers) the toner image formed on the surface of the photoreceptor drum 1 to the paper passing through the transfer nip portion without disturbing it. Thereafter, in preparation for the formation of a new electrostatic latent image that is subsequently performed, the cleaning device 7 removes the remaining toner on the surface of the photoreceptor drum 1, and the discharging device removes the remaining charge.

[0020] The paper onto which the toner image has been transferred is separated from the photoreceptor drum 1, conveyed to the fixing device 8, heated and pressurized, and the toner image is fixed on the paper. The paper that has passed through the fixing device 8 passes through the discharge roller pair 14 and is discharged to the paper discharge unit 15.

[0021] [2. Basic Toner Configuration] The following describes in detail the magnetic one-component toner of the present invention (hereinafter also simply referred to as toner) used in the image forming apparatus 100. Unless otherwise specified, the evaluation results (values ​​indicating shape or physical properties, etc.) for the powder (more specifically, toner core particles, toner matrix 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 (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 volume median 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.

[0022] In the following, the compound name may be followed by "system" to comprehensively refer to the compound and its derivatives. When "system" is followed by a compound name to represent a polymer name, it means that the repeating unit of the polymer originates from the compound or its derivative. Also, acrylic and methacrylic may be comprehensively referred to as "(meth)acrylic". Furthermore, acryloyl (CH) 2 =CH-CO-) and methacryloyl (CH 2 =C(CH 3 The terms )-CO-) are sometimes collectively referred to as "(meth)acryloyl".

[0023] The toner according to this embodiment can be suitably used as a positively charged toner for developing electrostatic latent images. The toner of this embodiment is a powder containing a plurality of toner particles (each having a configuration described later). The toner contains magnetic powder and is used as a one-component developer.

[0024] The toner particles contained in this embodiment comprise toner matrix particles and an external additive attached to the surface of the toner matrix particles. The toner matrix particles contain at least a binder resin, magnetic powder, and a nigrosine dye as a coloring agent. The toner matrix particles may also optionally contain a release agent, a charge control agent, etc., in addition to the binder resin. Furthermore, in the toner of the present invention, strontium titanate particles are externally added to the surface of the toner matrix particles as an external additive.

[0025] The toner of the present invention has a nigrosine dye dispersed in the binder resin of the toner matrix particles in order to improve the blackness and positive charge properties of the toner. It is said that a larger amount of nigrosine dye is better for increasing the blackness of the toner, but this negatively affects the charging characteristics of the toner, such as deterioration of dispersibility in the binder resin and excessive charging of the toner. As a result, image defects such as toner scattering (fogging) into non-image areas (white areas) and a decrease in image density due to insufficient development amount (amount of toner attached) occur.

[0026] This overcharging of the toner can be suppressed by adding strontium titanate particles as an external additive. Furthermore, in magnetic one-component jumping development, a high charge level of the toner matrix particles contributes to stabilizing the toner's charge state. In addition, the presence of highly dielectric strontium titanate on the surface of the toner matrix particles acts to adjust the charge between toner particles to a uniform state. Therefore, by combining strontium titanate particles as an external additive with toner matrix particles containing nigrosine dye, extremely stable image quality can be obtained.

[0027] The toner of the present invention improves the blackness and positive charge of the toner by adding nigrosine dye to the toner matrix particles, eliminating the need to reduce the particle size of magnetic powder or increase the amount of magnetic powder added. Therefore, the exposure of magnetic powder on the toner surface can be reduced, and wear of developing rollers, cleaning blades, etc. that come into contact with the toner can be suppressed. Accordingly, it is particularly suitable for use in an image forming apparatus 100 using a magnetic one-component jumping development method and a blade cleaning method, as shown in Figure 1.

[0028] [3. Toner materials] The binder resin, magnetic powder, colorant, release agent, charge control agent, and external additives added to the toner matrix particles, as well as the method for manufacturing the toner of the present invention, will be described below in order.

[0029] (Binding resin) The toner matrix particles constituting the toner of the present invention include a binder resin. The binder resin that can be contained in the toner matrix 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, it is preferable to contain at least one of polyester resin and styrene-acrylic acid resin, with polyester resin being more preferable, from the viewpoint of dispersibility of the colorant in the binder resin, the electrostatic properties of the toner, and the fixation to paper. Polyester resin will be described below.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 matrix particles 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] (magnetic powder) The toner matrix particles contain magnetic powder in the binder resin. Examples of magnetic powder materials include magnetic iron oxides such as magnetite, maghemite, and ferrite, or compounds of divalent metals and iron oxides, powders of metals such as iron, cobalt, and nickel, or alloys of these metals such as aluminum, cobalt, copper, lead, magnesium, tin, zinc, antimony, beryllium, bismuth, cadmium, calcium, manganese, selenium, titanium, tungsten, and vanadium, and mixtures of these powders.

[0040] The particle size of the magnetic powder is not limited as long as it does not hinder the objective of the present invention. Specifically, the particle size of the magnetic powder is preferably 0.05 to 2.0 μm, and more preferably 0.1 μm or more and 1.0 μm or less. When using magnetic powder with such particle size, it is easy to uniformly disperse the magnetic powder in the binder resin.

[0041] Magnetic powders can be surface-treated using surface treatment agents such as titanium-based coupling agents or silane-based coupling agents, for purposes such as improving the dispersibility of magnetic powders in the binder resin.

[0042] The amount of magnetic powder used is not particularly limited as long as it does not hinder the objective of the present invention. Specifically, the amount of magnetic powder used is preferably about 10 to 150 parts by weight, and more preferably 30 to 60 parts by weight, per 100 parts by weight of binder resin. If the amount of magnetic powder is excessive, it may become difficult to form an image with the desired image density over a long period, or the toner's adhesion to the paper may be severely reduced. If the amount of magnetic powder is insufficient, image fogging may occur more easily, or it may become difficult to form an image with the desired image density over a long period.

[0043] (Coloring agent) The toner matrix particles contain nigrosine dye as a colorant in the binder resin. Nigrosine dye can improve the blackness of the toner by producing a deep black or bluish-black color. Furthermore, nigrosine dye has high positive charge properties and acts as a charge control agent, contributing to the adjustment of the toner's positive charge and its charge stability.

[0044] From the viewpoint of blackness, the nigrosine dye content is preferably 2 parts by mass or more and 20 parts by mass or less per 100 parts by mass of binder resin. The nigrosine dye content can be measured by the amount of nitrogen element (mass%) obtained by CHN analysis of the toner particles. The toner of the present invention has a nitrogen element amount of 0.5% by mass or more and 5% by mass obtained by CHN analysis. The nitrogen element amount is the amount of nitrogen element (mass%) derived from the nigrosine dye. If the nitrogen element amount is less than 0.5% by mass, the coloring power is insufficient, and if it exceeds 5.0% by mass, the image density decreases due to poor charging.

[0045] Furthermore, in order to adjust the positive charge properties and charge stability of the toner to a desired range, it is necessary to control the nigrosine concentration on the surface of the toner particles. As a method for controlling the nigrosine concentration on the surface of the toner particles, it is desirable to control the dispersion state of the nigrosine dye, and it is desirable that the amount of nigrosine dye that dissolves when the toner particles are immersed in methanol, expressed by a spectrophotometer, i.e., the absorbance (Abs value), be between 0.2 and 2.0. As a method for controlling the dispersion state of the nigrosine dye, it is possible to adjust it by changing the particle size of the nigrosine dye, and the desired dispersion state can be obtained by melt-mixing nigrosine dye particles with other toner materials such as binder resin, with a particle size of 1 to 20 μm.

[0046] Furthermore, the toner of the present invention may contain other colorants other than nigrosine dye, as long as they do not impair the effects of the present invention. As the colorant, dyes, pigments, etc. used as colorants for toners can be used, but since those containing magnetic powder and nigrosine dye tend to exhibit black color, the toner of the present invention is preferably a black toner, and it is preferable that the colorant is black. Examples of colorants include carbon black, aniline black, and titanium-based black pigments.

[0047] (Release agent) Toner matrix particles may contain a release agent to improve adhesion and offset resistance. The type of release agent that can be added to the toner matrix particles is not particularly limited. Wax is preferred as such a 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 the toner matrix particles, the occurrence of offset and image smearing (smudges around the image when the image is rubbed) can be suppressed more efficiently.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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 toner matrix 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.

[0052] (Charge control agent) In the toner of the present invention, the nigrosine dye contained as a colorant in the toner matrix particles functions as a positively charged charge control agent. However, other positively charged charge control agents other than the nigrosine dye may be included, as long as they do not impair the effects of the present invention.

[0053] Specific examples of positively charged charge control agents that can be incorporated into toner matrix particles include 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, and 1,2,3 Examples include azine compounds such as ,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; metal salts of naphthenic acids or higher fatty acids; triphenylmethane-based dyes; alkoxylated amines; alkylamides; and quaternary ammonium salts such as benzylmethylhexyldecylammonium and decyltrimethylammonium chloride. Two or more of these positively charged charge control agents can be used in combination.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] The toner matrix particles may be toner matrix particles without a shell layer (non-capsulated toner matrix particles) or toner matrix particles with a shell layer (capsulated toner matrix particles). Capsulated toner matrix particles can be manufactured by forming a shell layer on the surface of non-capsulated toner matrix particles (toner core particles). The shell layer may consist substantially of thermosetting resin only, substantially of thermoplastic resin only, or may contain both thermoplastic resin and thermosetting resin.

[0058] (External additive) The toner of the present invention has its toner matrix particles treated with an external additive. The toner of the present invention contains strontium titanate particles as an external additive. When nigrosine dye is added as a coloring agent, the positive charge of the toner becomes too strong. Therefore, by adding strontium titanate particles as an external additive, the excessive charging of the toner can be suppressed.

[0059] Strontium titanate may be used with added metallic elements to alter its properties, such as crystal structure, resistance, and shape. Examples of metallic elements include lanthanides, silicon, aluminum, calcium, magnesium, barium, phosphorus, sulfur, calcium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, yttrium, zinc, niobium, molybdenum, ruthenium, rhodium, palladium, silver, indium, tin, antimony, barium, tantalum, tungsten, rhenium, osmium, iridium, platinum, bismuth, yttrium, zirconium, niobium, silver, and tin.

[0060] The strontium titanate particles used in the toner of the present invention are surface-modified (hydrophobic treated). By hydrophobicizing the surface of the strontium titanate particles, the surface electrical resistance can be increased, thereby suppressing the leakage of charge from the charged toner.

[0061] Examples of hydrophobic treatment agents for strontium titanate particles include silicon-containing organic compounds such as silane coupling agents (alkoxysilane compounds, silazane compounds, silicone oils, etc.), titanate-based coupling agents such as isopropyltriisostearoyl titanate, organofluorine compounds, and organic fatty acids.

[0062] The volume resistivity of the strontium titanate particles used in the toner of the present invention shall be between 10E+7 [Ω·cm] and 10E+10 [Ω·cm]. The number-average primary particle diameter of the strontium titanate particles is preferably between 30 nm and 100 nm.

[0063] The strontium titanate content is preferably 0.1 to 1% by mass relative to the total mass of the toner particles (toner matrix particles and external additives). The coverage rate of the toner matrix particles by strontium titanate particles (the area ratio of the surface area of ​​the toner matrix particles covered by strontium titanate particles) is preferably 2% to 30%.

[0064] In addition to the strontium titanate particles described above, other external additives may be added as long as they do not hinder the objectives of the present invention. The types of external additives that can be added are not particularly limited and can be appropriately selected from external additives conventionally used for toner. Specific examples of suitable external additives include metal oxides such as silica, alumina, titanium oxide, magnesium oxide, zinc oxide, and barium titanate, as well as resin particles. These external additives are added from the viewpoint of improving the charging performance, fluidity, and cleaning properties of the toner particles and adhere to the surface of the toner matrix particles. Two or more of these external additives can be used in combination.

[0065] [Toner manufacturing method] Next, the method for manufacturing the toner of the present invention will be described. The method for manufacturing the toner includes a method for manufacturing toner matrix particles and an external additive treatment method for attaching an external additive to the surface of the toner matrix particles. The method for manufacturing the toner matrix particles is not particularly limited as long as the toner matrix particles are formed to have a predetermined structure. In addition, toner matrix particles coated with a shell layer may be used as needed. As a preferred method for manufacturing the positively charged toner described above, the method for manufacturing the toner matrix particles and the external additive treatment method will be described in order below.

[0066] (Method for manufacturing toner matrix particles) The method for producing toner matrix particles is not particularly limited, as long as magnetic powder, a colorant (nigrosine dye), and any other components such as a mold release agent and a charge control agent can be well dispersed in the binder resin. Suitable methods for producing toner matrix particles include, for example, pulverization or agglomeration.

[0067] The pulverization method involves mixing the binder resin with components such as magnetic powder, colorants, release agents, and charge control agents using a mixer, then melt-kneading the binder resin and the components to be incorporated into the binder resin using a kneader such as a single-screw or twin-screw extruder, and finally pulverizing and classifying the cooled kneaded material. The average particle size of the toner mother particles is not particularly limited as long as it does not hinder the objectives of the present invention, but is generally preferably 5 μm to 10 μm.

[0068] The agglutination method involves agglutinating fine particles of various components, such as a binder resin, magnetic powder, colorant, release agent, and charge control agent, in an aqueous medium until the particles reach a desired size. This forms agglutinated particles containing the binder resin, release agent, charge control agent, and colorant. Subsequently, the resulting agglutinated particles are heated to unify the components contained within them. This yields toner matrix particles with the desired particle size.

[0069] (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 external additives using a mixer such as a Turbuler mixer, Henschel mixer, Nauter mixer, or V-type mixer.

[0070] The toner of the present invention, as described above, can stabilize the charge amount of the toner when forming images over a long period of time under various environmental conditions such as high temperature and high humidity environments, and low temperature and low humidity environments, thereby enabling the formation of images of the desired density. Furthermore, it can effectively suppress the occurrence of image fogging after durable printing. For this reason, the toner of the present invention can be suitably used in various image forming apparatuses. The effects of the present invention will be described in more detail below with reference to examples. However, the present invention is not limited in any way by these examples. [Examples]

[0071] [Manufacturing Example 1] (Manufacturing of Digrosine Dye A) Crude nigrosine was obtained by adding nitrobenzene to aniline and aniline hydrochloride in the presence of iron chloride, iron, and hydrochloric acid, and carrying out an oxidation reaction at 160-180°C. After neutralizing the obtained crude nigrosine, a base treatment was performed by adding aniline and sodium hydroxide solution, and the nigrosine and iron hydroxide precipitate were separated using a screw decanter centrifuge. After removing the iron hydroxide precipitate, the obtained liquid was washed with water. After washing with water, methanol was added to remove any remaining nitrobenzene and aniline, and the mixture was heated to 60°C while stirring and washing. The reaction solution was filtered, and after drying the solid, it was ground to a volume-average particle size of 10 μm using a jet mill (PJM-100NP, manufactured by Nippon Pneumatic Mfg. Co., Ltd.) to obtain nigrosine dye A.

[0072] (Manufacturing of Digrosine Dye B) 200 g of nigrosine dye A and 1000 g of methanol were weighed into a beaker, thoroughly stirred and mixed to disperse the nigrosine dye in the methanol solution. This slurry containing dispersed nigrosine dye A was wet-milled using a media-stirring wet mill (DinoMill MultiLab, manufactured by Shinmaru Enterprises, 1.4 L capacity). The milling conditions were: peripheral speed 10 m / s, media (material: zirconia) diameter 1.25 mm, solution flow rate 45 kg / h, cooling water flow rate 5 L / min, and pressure 0.1 kg / cm². 2 The following steps were taken: A circulating cycle was performed for 15 minutes, followed by wet grinding for 15 minutes to obtain a slurry of finely ground nigrosine dye. The obtained slurry was filtered, washed, and dried to obtain nigrosine dye B. Upon checking the particle size distribution, the D50 (cumulative 50 percent diameter) was found to be 1 μm.

[0073] (Manufacturing of Digrosine Dye C) After synthesis under the same conditions as for nigrosine dye A, the dye C was obtained by grinding it to a volume-average particle size of 20 μm using a jet mill (PJM-100NP, manufactured by Nippon Pneumatic Mfg. Co., Ltd.).

[0074] (Manufacturing of Digrosine Dye D) 200 g of nigrosine dye A and 1000 g of methanol were weighed into a beaker, thoroughly stirred and mixed to disperse the nigrosine dye in the methanol solution. This slurry containing dispersed nigrosine dye A was wet-milled using a media-stirring wet mill (DinoMill MultiLab, manufactured by Shinmaru Enterprises, capacity 1.4 L). The milling conditions were: peripheral speed 50 m / s, media (material: zirconia) diameter 1.25 mm, solution flow rate 45 kg / h, cooling water flow rate 5 L / min, and pressure 0.1 kg / cm². 2 The following steps were taken: Circulation was performed for 15 minutes, followed by wet grinding for 15 minutes to obtain a slurry of finely ground nigrosine dye. The obtained slurry was filtered, washed, and dried to obtain nigrosine dye D. Upon checking the particle size distribution, the D50 (cumulative 50 percent diameter) was found to be 0.5 μm.

[0075] (Manufacturing of Digrosine Dye E) After synthesis under the same conditions as for nigrosine dye A, the dye E was obtained by grinding it to a volume-average particle size of 50 μm using a jet mill (PJM-100NP, manufactured by Nippon Pneumatic Mfg. Co., Ltd.).

[0076] [Manufacturing Example 2] (Manufacturing of strontium titanate particles T-1) After de-ironizing and bleaching the metatitanic acid obtained by the sulfuric acid method, sodium hydroxide aqueous solution was added to adjust the pH to 9.0 for desulfurization. Subsequently, the pH was neutralized to 5.8 with hydrochloric acid and then filtered and washed. Water was added to the washed cake to make a slurry with a TiO2 concentration of 2.00 mol / L, and then hydrochloric acid was added to adjust the pH to 3.0 for gelatinization. 0.5 mol of this metatitanic acid was taken as TiO2 and added to a 3 L reaction vessel. Next, the mixture was heated to 90°C while stirring, and then 300 mL of 5 mol / L sodium hydroxide aqueous solution was added over 20 hours. After that, stirring was continued at 100°C for 1 hour to complete the reaction.

[0077] After the reaction, the slurry was cooled to 50°C, hydrochloric acid was added until the pH reached 5.0, and stirring was continued for 1 hour. The resulting precipitate was washed, and hydrochloric acid was added to the slurry containing the precipitate to adjust the pH to 6.5. 10% by mass of isobutyltrimethoxysilane relative to the solid content was added, and stirring was continued for 1 hour. Next, the mixture was filtered and washed, and the resulting solid content was dried in air at 120°C for 8 hours to obtain strontium titanate particles T-1 (average particle size 40 nm, volume resistivity 10E+08 Ω·cm).

[0078] (Manufacturing of strontium titanate particles T-2) Strontium titanate particles T-2 (average particle size 40 nm, volume resistivity 10E+08 Ω·cm) were obtained using the same method as for strontium titanate particles T-1, except that the amount of isobutyltrimethoxysilane added was changed to 5% by mass.

[0079] (Manufacturing of strontium titanate particles T-3) Strontium titanate particles T-3 (average particle size 40 nm, volume resistivity 10E+10 Ω·cm) were obtained using the same method as for strontium titanate particles T-1, except that the amount of isobutyltrimethoxysilane added was changed to 15% by mass.

[0080] (Manufacturing of strontium titanate particles T-4) After de-ironizing and bleaching the metatitanic acid obtained by the sulfuric acid method, sodium hydroxide aqueous solution was added to adjust the pH to 9.0 for desulfurization. Subsequently, the pH was neutralized to 5.8 with hydrochloric acid and then filtered and washed. Water was added to the washed cake to make a slurry with a TiO2 concentration of 2.00 mol / L, and then hydrochloric acid was added to adjust the pH to 3.0 for gelatinization. 0.5 mol of this metatitanic acid was taken as TiO2 and added to a 3 L reaction vessel. Next, the mixture was heated to 80°C while stirring, and then 300 mL of 5 mol / L sodium hydroxide aqueous solution was added over 20 hours. After that, stirring was continued at 80°C for 1 hour to complete the reaction.

[0081] After the reaction, the slurry was cooled to 50°C, and hydrochloric acid was added until the pH reached 5.0, followed by stirring for 1 hour. The resulting precipitate was washed, and hydrochloric acid was added to the slurry containing the precipitate to adjust the pH to 6.5. 13% by mass of isobutyltrimethoxysilane relative to the solid content was added, and stirring was continued for 1 hour. The mixture was then filtered and washed, and the resulting solid content was dried in air at 120°C for 8 hours to obtain strontium titanate particles T-4 (average particle size 30 nm, volume resistivity 10E+08 Ω·cm).

[0082] (Manufacturing of strontium titanate particles T-5) After de-ironizing and bleaching the metatitanic acid obtained by the sulfuric acid method, sodium hydroxide aqueous solution was added to adjust the pH to 9.0 for desulfurization. Subsequently, the pH was neutralized to 5.8 with hydrochloric acid and then filtered and washed. Water was added to the washed cake to make a slurry with a TiO2 concentration of 2.00 mol / L, and then hydrochloric acid was added to adjust the pH to 3.0 for papillation. 0.5 mol of this metatitanic acid was taken as TiO2 and added to a 3 L reaction vessel. Next, the mixture was heated to 100°C while stirring, and then 300 mL of 5 mol / L sodium hydroxide aqueous solution was added over 10 hours. After that, stirring was continued at 100°C for 1 hour to complete the reaction.

[0083] After the reaction, the slurry was cooled to 50°C, and hydrochloric acid was added until the pH reached 5.0, followed by stirring for 1 hour. The resulting precipitate was washed, and hydrochloric acid was added to the slurry containing the precipitate to adjust the pH to 6.5. 5% by mass of isobutyltrimethoxysilane relative to the solid content was added, and stirring was continued for 1 hour. The mixture was then filtered and washed, and the resulting solid content was dried in air at 120°C for 8 hours to obtain strontium titanate T-5 (average particle size 100 nm, volume resistivity 10E+08 Ω·cm).

[0084] (Manufacturing of strontium titanate particles T-6) Strontium titanate T-6 (average particle size 40 nm, volume resistivity 10E+11 Ω·cm) was obtained using the same method as for strontium titanate particles T-1, except that the amount of isobutyltrimethoxysilane added was changed to 20% by mass.

[0085] (Manufacturing of strontium titanate particles T-7) Strontium titanate T-7 (average particle size 40 nm, volume resistivity 10E+06 Ω·cm) was obtained using the same method as for strontium titanate T-1, except that the amount of isobutyltrimethoxysilane added was changed to 2% by mass.

[0086] [Manufacturing Example 3] (Manufacturing of toner matrix particles) A mixture was obtained by mixing 100 parts by mass of polyester resin (HP-313, manufactured by Nippon Synthetic Chemical Co., Ltd.) as a binder resin, 50 parts by mass of magnetic powder (Magnetite MG1306, manufactured by Mitsui Mining & Smelting Co., Ltd.), 5 parts by mass of nigrosine dye A obtained in Production Example 1 as a coloring agent, and 10 parts by mass of paraffin wax (HNP-9, manufactured by Nippon Seiro Co., Ltd.) as a release agent using an FM mixer (FM-20B, manufactured by Nippon Coke Industries Co., Ltd.).

[0087] The obtained mixture was melt-kneaded at 150°C using a twin-screw extruder (TEM-45, manufactured by Toshiba Machine Co., Ltd.) to obtain a kneaded product. After the kneaded product cooled, it was coarsely ground using a pulverizer (Feather Mill 350 x 600 type, manufactured by Hosokawa Micron Co., Ltd.). The obtained coarsely ground product was ground using an air-jet pulverizer (Jet Mill IDS-2 type, manufactured by Nippon Pneumatic Mfg. Co., Ltd.) to obtain finely ground product. The finely ground product was classified using an air-powered classifier (Elbow Jet EJ-LABO type, manufactured by Nippon Steel Mining Co., Ltd.) to obtain toner matrix particles.

[0088] [Manufacturing Example 4] (Toner manufacturing) To 100 parts by mass of toner matrix particles obtained in Production Example 3, 1.5 parts by mass of silica particles (AEROSIL REA90, manufactured by Nippon Aerosil Co., Ltd.) and 0.2 parts by mass of strontium titanate particles T-1 obtained in Production Example 2 were added as external additives. The mixture was then mixed using an FM mixer (FM-10, manufactured by Nippon Coke Industries Co., Ltd.) to adhere (add) the silica particles and strontium titanate particles T-1 to the toner matrix particles. Subsequently, the mixture was sieved using a vibrating electric sieve (ANF-30, manufactured by Nittokagaku Co., Ltd.) to a 200-mesh (75 μm opening) sieve to obtain the toner of the present invention 1.

[0089] Except for changing the nigrosine dye and other additives added to the toner matrix particles, and the type and amount of strontium titanate particles added externally to the toner matrix particles, toners of Inventions 2 to 13 and Comparative Examples 1 to 11 were obtained by the same method as the toner of Invention 1. The types and amounts of nigrosine dye, strontium titanate particles, and other additives in the toners of Inventions 1 to 13 and Comparative Examples 1 to 11 are shown in Table 1.

[0090] [Table 1]

[0091] (Measurement of nitrogen content in toner) The amount of nitrogen in the toner was measured using a CHN analyzer (2400II, PerkinElmer). The electric furnace temperature was 800°C for the pyrolysis section and 900°C for the catalyst section. The measurement conditions were 300 mL / min main O2 flow rate, 300 mL / min O2 flow rate, and 400 mL / min air flow rate. Quantitative analysis was performed based on a calibration curve created using standard samples such as indomethacin.

[0092] (Measurement of Abs value of nigrosine dye) The Abs value of the nigrosine dye contained in the toner matrix particles was measured by the following method. First, a solution containing the methanol-soluble components of the toner matrix particles was prepared. 0.5 g of toner was weighed into a container, and 5.00 g of methanol was added. The contents of the container were mixed for 3 minutes at a rotation speed of 100 rpm using a ball mill at 25°C. Subsequently, the container was allowed to stand for 3 minutes, and the supernatant liquid was taken from the contents of the container. Solid components in the supernatant liquid were precipitated using a centrifuge, and the methanol solution was taken from the supernatant liquid. This yielded a methanol solution containing the methanol-soluble components of the toner.

[0093] The absorbance (Abs value) of the methanol solution containing the obtained methanol-soluble components was measured using a spectrophotometer (U-3900, Hitachi). Since nigrosine characteristically absorbs ultraviolet light at a wavelength of 516 nm, the absorbance was measured by irradiating the methanol solution with ultraviolet light at wavelengths between 300 nm and 800 nm.

[0094] (Measurement of volume resistivity of strontium titanate particles) The volume resistivity of strontium titanate particles was measured by the following method: 100 kg / cm³ of strontium titanate powder. 2After applying pressure to compress and mold the material into a 10mm diameter tablet, the electrical resistance was measured using a digital multimeter (DM7560, Yokogawa Electric Corporation), and the resulting resistivity was converted to the volume resistivity of strontium titanate.

[0095] (Measurement of the number-average primary particle diameter of strontium titanate particles) The number-average primary particle diameter of strontium titanate was measured using the following method: After adding (dispersing) strontium titanate to toner matrix particles, 100 primary strontium titanate particles were observed at 40,000x magnification using a scanning electron microscope (JSM-7401F, JEOL Ltd.). The longest and shortest diameters of each particle were measured by image analysis of the primary particles, and the equivalent circle diameter was calculated from the midpoint of these two values. The average of the measured primary particle diameters of the 100 particles was then defined as the number-average primary particle diameter.

[0096] (Measurement of the coverage rate of the toner surface by strontium titanate particles) The coverage rate of the toner matrix particle surface by strontium titanate particles was measured using the following method. First, a 30,000x magnification image of the toner particle surface was taken using a scanning electron microscope (SEM). EDX elemental analysis was also performed in the same field of view to obtain a Sr element distribution image. The acquired SEM image and Sr element distribution image were analyzed using an image processing and analysis system (WINROOF, manufactured by Mitani Corporation). The area of ​​strontium titanate was determined from the brightness distribution image and the Sr element distribution image, and the coverage rate on the toner matrix particle surface was calculated as an area ratio from the image area of ​​strontium titanate / image area of ​​toner matrix particles.

[0097] Table 2 shows the nitrogen content of the toners for Inventions 1-13 and Comparative Examples 1-11, the Abs value of the nigrosine dye, the volume resistivity of the strontium titanate particles added to the toner matrix particles, the number-average primary particle diameter, and the coverage rate of the toner surface by the strontium titanate particles.

[0098] [Table 2]

[0099] [Evaluation of image density, color intensity, and toner thinness] The image density, coloring power, and toner thin layer state were evaluated using the toners of Invention 1 to 13 and Comparative Examples 1 to 11 according to the method described below. (Image density) A monochrome printer (ECOSYS FS-3060DN, manufactured by Kyocera Document Solutions Corporation) was used as the evaluation machine. The toners of Invention 1-13 and Comparative Examples 1-11 obtained in Manufacturing Example 4 were installed in the developer unit of the evaluation machine. In addition, replenishment toner (the same toner as the toner installed in the developer unit) was installed in the toner container of the evaluation machine. After toner installation, a text document with a print density of 1% was printed on 1,000 sheets of printing paper (Multi Paper Super Economy A4, manufactured by ASKUL Corporation) in double-sided mode under normal temperature and humidity conditions (temperature 23°C, humidity 50%RH). Subsequently, an evaluation image including a solid image (evaluation image 1) was printed on one sheet of printing paper.

[0100] After printing evaluation image 1, the evaluation machine was taken to a high-temperature, high-humidity environment (temperature 28°C, humidity 80%RH) and exposed to this environment for 24 hours. Then, a text original with a print density of 1% was printed on 4,000 sheets of paper in double-sided mode. Subsequently, an evaluation image including a solid image (evaluation image 2) was printed on a single sheet of paper.

[0101] The image density (evaluation value 1) of evaluation image 1 and the image density (evaluation value 2) of evaluation image 2 were measured using a reflectance densitometer (TC-60, manufactured by Tokyo Denshoku Co., Ltd.). The evaluation criteria for image density (ID) are shown below. ○ (Good): The ID for evaluation value 1 is 1.2 or higher, and the ID for evaluation value 2 is 1.0 or higher. × (Defective); IDs with a value of 1 are less than 1.2, or IDs with an evaluation value of 2 are less than 1.0.

[0102] (Colorability) The brightness L*, chromaticity a*, and b* of evaluation image 1, obtained by evaluating image density using a fluorescence spectrophotometer (FD-5, manufactured by Konica Minolta), were measured. The evaluation criteria for colorability are shown below. ○ (Good): L* ≤ 20, a* ≤ 0, and b* ≤ 0.5 × (Defective): L*>20, or a*>0, or b*>0.5

[0103] (Thin toner layer) Using the evaluation machine, a text document with a print density of 1% was printed on 1,000 sheets of printing paper in double-sided mode under low temperature and low humidity conditions (temperature 10°C, humidity 10%RH). The printed document was visually inspected for any toner spatter (toner spatter onto non-printed areas) or abnormal images. After image evaluation, the developing unit was removed from the evaluation machine, and the toner thin layer formed on the toner carrier (developing sleeve) was visually inspected for any abnormalities in toner thin layer formation, such as toner clogging, streaks, or deposits. The evaluation criteria for the toner thin layer state are shown below. ○ (Good): No abnormal images were generated, and there were no problems with the toner layer on the toner carrier. × (Defective): Abnormal images are generated, or there is a defect in the formation of the toner layer on the toner carrier.

[0104] Table 3 shows the evaluation results for image density, coloring power, and toner thin layer state when using toners according to Inventions 1 to 13 and Comparative Examples 1 to 11.

[0105] [Table 3]

[0106] As is clear from Tables 2 and 3, in toners according to Invention 1 to 13, where the nitrogen element content of the toner particles is 0.5% by mass or more and 5% by mass or less, the absorbance (Abs value) of the toner particles relative to methanol measured by a spectrophotometer is 0.2 or more and 2.0 or less, and the volume resistivity of the strontium titanate particles added externally to the toner matrix particles is 1.0E+7 [Ω·cm] or more and 1.0E+10 [Ω·cm] or less, the image density, colorability, and toner thin layer state were all evaluated favorably.

[0107] In contrast, the toners in Comparative Examples 1-3, which had carbon black, titanium black, and quaternary ammonium salt added instead of nigrosine dye, and the toner in Comparative Example 4, which had no added colorants, all showed poor colorability. Furthermore, a decrease in image density was observed in the toners of Comparative Examples 1, 2, and 4, and abnormalities occurred in the toner thin layer in the toners of Comparative Examples 2 and 4. In addition, the toner in Comparative Example 5, which did not have strontium titanate particles added externally, became overcharged, resulting in abnormalities in the toner thin layer.

[0108] Furthermore, in Comparative Example 6, the toner, which had a nitrogen content of 0.3% by mass in the toner particles, contained insufficient nigrosine dye, resulting in a poor evaluation of its coloring power. On the other hand, in Comparative Example 7, the toner, which had a nitrogen content of 6.5% by mass and an Abs value of 2.5, contained excessive nigrosine dye, resulting in a decrease in image density due to poor toner charging.

[0109] Furthermore, in the toner of Comparative Example 8, which had an Abs value of 0.1, the nigrosine dye particle size was small at 0.5 μm, and the nigrosine dye was not dispersed on the surface of the toner matrix particles (low Abs value), resulting in a decrease in image density due to poor toner charging. In addition, in the toner of Comparative Example 9, which had a large nigrosine dye particle size of 30 μm, detachment of the nigrosine dye from the surface of the toner matrix particles occurred, resulting in a decrease in image density due to poor toner charging.

[0110] Furthermore, in the toner of Comparative Example 10, where the volume resistivity of the strontium titanate particles was 1.0E+11 [Ω·cm], the toner became overcharged due to the excessively high volume resistivity of the strontium titanate particles, resulting in an abnormality in the toner layer. On the other hand, in the toner of Comparative Example 11, where the volume resistivity of the strontium titanate particles was 1.0E+6 [Ω·cm], the image density decreased due to insufficient toner charging because the volume resistivity of the strontium titanate particles was too low.

[0111] Based on the above results, it was confirmed that by adding nigrosine dye to the toner matrix particles, externally attaching strontium titanate particles to the surface of the toner matrix particles, and setting the nitrogen element content of the toner particles to 0.5% by mass or more and 5% by mass or less, the absorbance (Abs value) of the toner particles relative to methanol measured by a spectrophotometer to 0.2 or more and 2.0 or less, and the volume resistivity of the strontium titanate particles to 1.0E+7 [Ω·cm] or more and 1.0E+10 [Ω·cm] or less, a magnetic single-component toner is produced that improves image density and colorability and suppresses abnormalities in the toner thin layer state. [Industrial applicability]

[0112] The present invention is applicable to positively charged, single-component magnetic toners used in electrophotographic systems. By utilizing the present invention, it is possible to provide a single-component magnetic toner that improves the blackness of the toner and stabilizes its charge over a long period of time.

Claims

1. Toner matrix particles containing at least a binder resin and magnetic powder, The external additive adhering to the surface of the toner matrix particles, A magnetic one-component toner consisting of toner particles containing, The toner matrix particles contain nigrosine dye, The aforementioned external additive contains strontium titanate particles whose surface has been hydrophobized. The nitrogen content of the toner particles is 0.5% by mass or more and 5% by mass or less. The Abs value, expressed as the amount of nigrosine dye that dissolves when the toner particles are immersed in methanol, is 0.2 or more and 2.0 or less. A magnetic single-component toner characterized in that the volume resistivity of the strontium titanate is 1.0E+7 [Ω・cm] or more and 1.0E+10 [Ω・cm] or less.

2. The magnetic one-component toner according to claim 1, characterized in that the number-average primary particle diameter of the strontium titanate is 30 nm or more and 100 nm or less.

3. The magnetic one-component toner according to claim 1, characterized in that the coverage rate of the surface of the toner matrix particles by the strontium titanate particles is 2% or more and 30% or less.

4. The magnetic one-component toner according to claim 1, characterized in that the toner matrix particles are formed by a pulverization method in which at least the binder resin, the magnetic powder, and the nigrosine dye are melt-kneaded in a melt-kneading step, and then the cooled kneaded mixture is pulverized.

5. A developing apparatus for developing an electrostatic latent image formed on an image carrier into a toner image using a magnetic one-component toner according to any one of claims 1 to 4, A transfer device for transferring the toner image developed by the developing device onto a recording medium, A cleaning device for removing the toner remaining on the image carrier, Equipped with, The developing apparatus is a magnetic single-component jumping developing method that charges the magnetic single-component toner via a toner carrier that carries the magnetic single-component toner, The cleaning device is an image forming apparatus that uses a cleaning blade to remove the toner remaining on the image carrier.

Images