Positively chargeable toner and two component developer containing positively chargeable toner

Abstract

To provide a positively chargeable toner capable of achieving all of charge stability, charge rising property and fluidity, and a two component developer containing the positively chargeable toner.SOLUTION: The positively chargeable toner includes toner particles each including a toner mother particle and an external additive adhering to a surface of the toner mother particle. The external additive includes a silica particle whose surface is positively charged, a strontium titanate particle whose surface is not positively charged, and a resin fine particle formed of an acrylic resin. The silica particles and the strontium titanate particles each have a number-average primary-particle size of at least 10nm and no greater than 40nm. The coverage ratio of the silica particles with respect to the surface region of the toner base particle is from 30% to 40%, and the coverage ratio of the strontium titanate particles with respect to the surface region of the toner base particle is from 5% to 10%.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positively charging toner and a two-component developer containing the positively charging toner. [Background technology]

[0002] In general, in electrophotography, the surface of an electrostatic latent image carrier is charged by corona discharge or other methods, and then exposed to light such as a laser to form an electrostatic latent image. The formed electrostatic latent image is then developed with toner to form a toner image. The formed toner image is then transferred to a recording medium to obtain a high-quality image. Toners used in electrophotography are typically prepared by mixing a binder resin such as a thermoplastic resin with a colorant, a charge control agent, a release agent, a magnetic material, and other components, followed by kneading, pulverization, and classification to form toner particles (toner base particles) with an average particle size of 5 μm to 10 μm. Inorganic fine powders such as silica and titanium oxide are added to the toner base particles to provide fluidity, favorable charging properties, and improved cleaning performance of the toner from the photoreceptor drum.

[0003] External additives on the toner surface undergo changes such as embedding and detachment due to mechanical stress caused by agitation in the developer. A commonly known method for suppressing changes such as embedding and detachment of external additives is to use large-diameter external additives as spacer particles. Silica is sometimes used as spacer particles, but when used for positively charged toners, the silica particles themselves must be treated to be positively charged, which can cause charging problems (such as fogging).

[0004] Patent Document 1 discloses a toner containing toner particles containing a binder resin, and strontium titanate particles and hydrotalcite particles on the surfaces of the toner particles, wherein the hydrotalcite particles contain fluorine, and when the area ratio of the strontium titanate particles to the toner particles is T1 (%) and the area ratio of the hydrotalcite particles to the toner particles is H1 (%), T1 / H1 is 0.15 to 9.00. Patent Document 1 proposes that by specifying the area ratio of the strontium titanate particles and hydrotalcite particles to the toner particles, it is possible to achieve both charge stabilization and suppression of charge difference when toner is replenished.

[0005] Patent Document 2 discloses a toner for developing electrostatic latent images, in which holes are formed extending from the surface of a toner base particle toward the inside of the toner base particle, and a plurality of particles made of a metal oxide are accommodated inside the holes and adjacent to each other along the longitudinal direction of the holes, and each of the plurality of particles made of a metal oxide has a diameter of 20 nm to 100 nm, and the metal oxide is zinc oxide or strontium titanate. Patent Document 2 proposes that by specifying the particle diameter of the zinc oxide or strontium titanate contained in the toner base particle, charging stability of the toner can be achieved in low-humidity environments, high-humidity environments, and after long-term storage. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-159905 [Patent Document 2] Japanese Patent Application Publication No. 2019-56808 Summary of the Invention [Problem to be solved by the invention]

[0007] Increasing the coverage of resin particles is one method for improving the charge stability of positively charged toner against mechanical stress. However, when resin particles are used as an external additive, the adhesive properties of the resin particles can impair the fluidity of the toner, potentially causing toner clogging when the toner is discharged from a toner container. Patent Documents 1 and 2 discuss the charge stability of the toner, but do not discuss how to achieve both charge stability, charge buildup, and fluidity of the toner.

[0008] In view of the above problems, an object of the present invention is to provide a positively chargeable toner and a two-component developer containing the positively chargeable toner that can achieve both charge stability, charge rise property and fluidity. [Means for solving the problem]

[0009] In order to achieve the above object, a first aspect of the present invention is a positively charged toner comprising toner particles including toner base particles and an external additive attached to the surface of the toner base particles. The external additive includes silica particles with a positively charged surface, strontium titanate particles with a non-positively charged surface, and resin microparticles formed of an acrylic resin. The number-average primary particle diameter of the silica particles and the number-average primary particle diameter of the strontium titanate particles are each 10 nm or more and 40 nm or less. The coverage of the surface area of ​​the toner base particles with the silica particles is 30% or more and 40% or less, and the coverage of the surface area of ​​the toner base particles with the strontium titanate particles is 5% or more and 10% or less. [Effects of the Invention]

[0010] According to the first aspect of the present invention, a positively charged toner having excellent charge stability, charge rise property, and fluidity is obtained. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing an example of a cross-sectional structure of a toner 101 for developing electrostatic latent images according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described in detail. Unless otherwise specified, evaluation results (values ​​indicating shape or physical properties, etc.) for powders (more specifically, toner core particles, toner base particles, external additives, toner, etc.) are the number averages of values ​​measured for a considerable number of average particles selected from the powder. Furthermore, unless otherwise specified, the number average particle diameter of a powder is the number average of the equivalent circle diameters (diameters of a circle having the same area as the projected area of ​​a particle) of primary particles measured using a microscope. Unless otherwise specified, the measured volume median diameter (D50) of a powder is a value measured using a laser diffraction / scattering particle size distribution analyzer ("LA-750" manufactured by Horiba, Ltd.). Unless otherwise specified, the measured acid value and hydroxyl value are values ​​measured in accordance with "JIS (Japanese Industrial Standards) K0070-1992." Furthermore, unless otherwise specified, the measured values ​​of number average molecular weight (Mn) and weight average molecular weight (Mw) are values ​​measured using gel permeation chromatography.

[0013] Hereinafter, the compound and its derivatives may be collectively referred to by adding "based" after the compound name. When the polymer name is expressed by adding "based" after the compound name, it means that the repeating unit of the polymer is derived from the compound or its derivative. Also, acrylic and methacrylic may be collectively referred to as "(meth)acrylic". Also, acryloyl (CH 2 =CH-CO-) and methacryloyl (CH 2 =C(CH 3 )-CO-) are sometimes collectively referred to as "(meth)acryloyl".

[0014] The toner according to this embodiment can be suitably used, for example, as a positively charged toner for developing electrostatic latent images. The toner according to this embodiment is a powder containing a plurality of toner particles (particles each having the configuration described below). The toner may be used as a single-component developer. Alternatively, a two-component developer may be prepared by mixing the toner and a carrier using a mixing device (for example, a ball mill). In order to form high-quality images, it is preferable to use a ferrite carrier as the carrier.

[0015] Furthermore, in order to form high-quality images over a long period of time, it is preferable to use magnetic carrier particles having a carrier core and a resin layer covering the carrier core. To produce magnetic 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, the magnetic particles may be dispersed in the resin layer covering the carrier core. In order to form high-quality images, the amount of toner in the two-component developer is preferably 5 to 15 parts by mass per 100 parts by mass of carrier. Note that positively charged toner becomes positively charged due to friction with the carrier.

[0016] The toner particles contained in the toner according to this embodiment include toner base particles and external additives attached to the surfaces of the toner base particles. That is, the toner particles before the external additives are attached are referred to as toner base particles. When the toner base particles have a shell layer, the particles before the shell layer is formed are referred to as toner core particles. When the toner base particles do not have a shell layer, the toner base particles are also referred to as toner core particles.

[0017] The toner according to this embodiment can be used to form an image in, for example, an electrophotographic apparatus (image forming apparatus). An example of an image forming method using an electrophotographic apparatus will be described below.

[0018] First, an electrostatic latent image is formed on a photoreceptor (e.g., the surface of a photoreceptor drum) based on image data. Next, the formed electrostatic latent image is developed using a developer containing toner. In the development process, toner (e.g., toner charged by friction with a carrier or blade) on a development sleeve (e.g., the surface of a development roller in a developing unit) located near the photoreceptor is attached to 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, after primary transfer to an intermediate transfer medium (e.g., a transfer belt), the toner image on the intermediate transfer medium is further secondary transferred to the recording medium. The toner is then 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 overlapping four color toner images: black, yellow, magenta, and cyan.

[0019] [1. Basic composition of toner] Fig. 1 is a diagram showing an example of the cross-sectional structure of a positively charged toner 101 of the present invention. As shown in Fig. 1, the positively charged toner (hereinafter also simply referred to as toner) 101 of the present invention comprises toner base particles 102 and external additives 103 attached to the surfaces of the toner base particles 102. The external additives 103 include silica particles 104, strontium titanate particles 105, and resin fine particles 106.

[0020] The silica particles 104 and the strontium titanate particles 105 are added to adjust the chargeability of the toner 101. The silica particles 104 have been subjected to a positive charging treatment. Specifically, the surface of the silica particles 104 is modified with a coupling agent to increase the positive chargeability (ease of being positively charged) of the silica particles 104. The strontium titanate particles 105 have not been subjected to a positive charging treatment. Therefore, the strontium titanate particles 105 have a negative chargeability.

[0021] The resin particles 106 function as spacer particles, reducing the adhesive force between the toner particles 101 or between the toner 101 and the carrier, thereby improving charging stability. They also reduce the adhesive force between the toner 101 and the photosensitive drum or cleaning blade, thereby improving cleaning performance.

[0022] The resin particles 106 are made of an acrylic resin. The resin particles 106 are preferably made of a silicone-modified acrylic resin. By making the resin particles 106 from a silicone-modified acrylic resin, the silicone contained in the particles reduces the adhesive force of the resin particles 106. This makes it possible to prevent the resin particles 106 from forming aggregates. As a result, it is possible to effectively prevent the external additive from slipping through the edge portion of the cleaning blade.

[0023] The number average primary particle diameter of the resin particles 106 is preferably 30 nm to 150 nm, and more preferably 50 nm to 120 nm. The coverage of the surface of the toner base particle 102 by the resin particles 106 (the area ratio of the surface area of ​​the toner base particle 102 that is covered with the resin particles 106) is preferably 10 to 40%, and more preferably 15 to 30%.

[0024] When the toner 101 is subjected to mechanical stress, the external additives 103 are embedded in the toner base particles 102 and gradually lose their positive charge. In the case of the silica particles 104 that have been treated to be positively charged, the more mechanical stress they are subjected to, the more embedded the silica particles 104 become in the toner base particles 102, causing the toner 101 to lose its positive charge and decreasing the charge amount of the entire toner 101.

[0025] On the other hand, in the case of strontium titanate particles 105 that have not been subjected to a positive charging treatment, the more mechanical stress they are subjected to, the more they become embedded in the toner base particles 102, causing the strontium titanate particles 105 to lose their negative chargeability and increasing the charge of the entire toner.

[0026] It has been found that this change in charging performance in response to mechanical stress is closely related to the particle diameters of the silica particles 104 and the strontium titanate particles 105. Therefore, in the present invention, by specifying the particle diameters of the silica particles 104 and the strontium titanate particles 105, it is possible to control the change in the charge amount of the toner 101 in response to mechanical stress.

[0027] Specifically, the number average primary particle diameter of the positively charged silica particles 104 is set to 10 nm or more and 40 nm or less, and the number average primary particle diameter of the non-positively charged strontium titanate particles 105 is set to 10 nm or more and 40 nm or less.

[0028] Furthermore, the change in charging performance in response to mechanical stress also varies depending on the coverage (area ratio) of the silica particles 104 and the strontium titanate particles 105 with respect to the toner base particles 102. Therefore, in the present invention, by specifying the coverage of the silica particles 104 and the strontium titanate particles 105 with respect to the toner base particles 102 as well as the particle diameters of the silica particles 104 and the strontium titanate particles 105, it becomes possible to control the change in the charge amount of the toner 101 in response to mechanical stress.

[0029] Specifically, the coverage of the positively charged silica particles 104 is set to 30% or more and 40% or less. If the coverage of the silica particles 104 is less than 30%, the initial charge amount decreases, causing toner scattering. The fluidity of the toner also decreases. On the other hand, if the coverage of the silica particles 104 exceeds 40%, external additives contaminate the carrier, degrading the carrier's charge-imparting performance.

[0030] Furthermore, the coverage of the strontium titanate particles 105 that have not been subjected to a positive charging treatment is set to 5% or more and 40% or less. If the coverage of the strontium titanate particles 105 is less than 5%, the contribution to charge stabilization is reduced. On the other hand, if the coverage of the strontium titanate particles 105 is more than 10%, the charge buildup is poor.

[0031] By setting the particle size and coverage of the silica particles 104 and the strontium titanate particles 105 within the above ranges, the change in charge amount of the toner 101 in response to mechanical stress can be appropriately controlled, and the charge stability and charge rise property of the toner 101 can be achieved at the same time as fluidity.

[0032] [2. Toner Materials] Next, essential and optional components constituting the toner of the present invention will be described. The toner core particles contain at least a binder resin. If necessary, the toner core particles may contain a release agent, a colorant, a charge control agent, a magnetic powder, etc. in the binder resin.

[0033] The binder resin, release agent, charge control agent, colorant, magnetic powder, and resin fine particles that form the shell layer, as well as the first resin fine particles, second resin fine particles, and cleaning aid particles that constitute the external additives, which in turn constitute the toner core particles, will be described below, along with the method for producing the toner of the present invention.

[0034] (binder resin) The toner core particles constituting the toner of the present invention contain a binder resin. The binder resin that can be contained in the toner 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-based resins, acrylic-based resins, styrene-acrylic resins, polyethylene-based resins, polypropylene-based resins, vinyl chloride-based resins, polyester resins, polyamide resins, polyurethane resins, polyvinyl alcohol-based resins, vinyl ether-based resins, N-vinyl-based resins, and styrene-butadiene resins. Among these resins, it is preferable to contain at least one of polyester resins and styrene-acrylic acid-based resins, with polyester resins being more preferable, in terms of the dispersibility of colorants in the binder resin, the chargeability of the toner, and the fixability to paper. The polyester resin will be described below.

[0035] The polyester resin can be obtained by condensation polymerization or co-condensation polymerization of a divalent or trivalent or higher alcohol component and a divalent or trivalent or higher carboxylic acid component. The components used in synthesizing the polyester resin include the following alcohol components and carboxylic acid components.

[0036] Specific examples of the dihydric or trihydric or higher alcohol component 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 bisphenols such as hydroxypropylated bisphenol A and polyoxypropylated bisphenol A; and trihydric 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.

[0037] Specific examples of the divalent or trivalent or higher carboxylic acid component include divalent carboxylic acids such as maleic acid, fumaric acid, citraconic acid, itaconic acid, glutaconic acid, phthalic acid, isophthalic acid, terephthalic acid, cyclohexanedicarboxylic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, malonic acid, or 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-naphthalenetricarboxylic acid, 1,2,4-naphthalenetricarboxylic 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 trimer acid. These divalent 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" refers to an alkyl group having 1 to 6 carbon atoms.

[0038] When the binder resin is a polyester resin, the softening point of the polyester resin is preferably 70°C or higher and 130°C or lower, more preferably 80°C or higher and 120°C or lower. In order to improve the strength of the toner core and the fixability of the toner, the number average molecular weight (Mn) of the polyester resin is preferably 1000 or higher and 2000 or lower. The molecular weight distribution of the polyester resin (the ratio Mw / Mn of the mass average molecular weight (Mw) to the number average molecular weight (Mn)) is preferably 9 or higher and 21 or lower.

[0039] As the binder resin, it is preferable to use a thermoplastic resin because it has good fixability to paper. However, in addition to using a thermoplastic resin alone, a crosslinking agent or a thermosetting resin can be added to the thermoplastic resin. By adding a crosslinking agent or a thermosetting resin to introduce a partial crosslinked structure into the binder resin, it is possible to improve the heat-resistant storage stability and durability of the toner without reducing the fixability of the toner. When a thermosetting resin is used, the amount of crosslinked portions (gel amount) of the binder resin extracted using a Soxhlet extractor is preferably 10% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less, based on the mass of the binder resin.

[0040] Thermosetting resins that can be used together with thermoplastic resins are preferably epoxy resins or cyanate resins. Specific examples of suitable thermosetting resins include bisphenol A epoxy resins, hydrogenated bisphenol A epoxy resins, novolac epoxy resins, polyalkylene ether epoxy resins, cycloaliphatic epoxy resins, and cyanate resins. These thermosetting resins can be used in combination of two or more.

[0041] The glass transition point (Tg) of the binder resin is preferably 40° C. or higher and 70° C. or lower. If the glass transition point is too high, the low-temperature fixability of the toner tends to decrease. If the glass transition point is too low, the heat-resistant storage stability of the toner tends to decrease.

[0042] The glass transition point of the binder resin can be determined from the change point 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 Inc. as the measuring device. 10 mg of a measurement 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 of the binder resin obtained by measuring at room temperature and normal humidity in a measurement temperature range of 25°C to 200°C at a heating rate of 10°C / min.

[0043] The weight average molecular weight (Mw) of the binder resin is not particularly limited as long as it does not impair the object of the present invention. Typically, the weight average molecular weight (Mw) of the binder resin is preferably 20,000 or more and 300,000 or less, and more preferably 30,000 or more and 2,000,000 or less. The weight 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 resins.

[0044] (mold release agent) The toner core particles may contain a release agent for the purpose of improving fixability and offset resistance. The type of release agent that can be contained in the toner core particles is not particularly limited as long as it does not impair the object of the present invention. Wax is preferred as the release agent, and examples of wax 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. Adding such a release agent to the toner core particles 102 can more efficiently suppress the occurrence of offset and image smearing (staining around the image when the image is rubbed).

[0045] When a 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. When a 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.

[0046] Fischer-Tropsch wax is a linear hydrocarbon compound with few isostructural molecules and few side chains, produced by utilizing the Fischer-Tropsch reaction, which is a catalytic hydrogenation reaction of carbon monoxide.

[0047] Among Fischer-Tropsch waxes, those having a mass average molecular weight of 1,000 or more and having an endothermic peak bottom temperature observed by DSC measurement in the range of 100° C. to 120° C. are more preferred. Examples of such Fischer-Tropsch waxes include Sasolwax C1 (endothermic peak bottom temperature: 106.5° C.), Sasolwax C105 (endothermic peak bottom temperature: 102.1° C.), and Sasolwax SPRAY (endothermic peak bottom temperature: 102.1° C.), all of which are available from Sasol.

[0048] The amount of release agent used is not particularly limited as long as it does not impair the object of the present invention. Specifically, the amount of release agent used is preferably 1% by mass or more and 10% by mass or less, based on the total mass of the toner core particles 102. If the amount of release agent used is too small, the desired effect of suppressing offset and image smearing in the formed image may not be achieved. If the amount of release agent used is too large, the toner particles may fuse together, resulting in a decrease in the heat-resistant storage stability of the toner.

[0049] (coloring agent) The toner core particles may contain a colorant. The colorant that can be contained in the toner core particles can be a known pigment or dye, depending on the color of the toner. Specific examples of suitable colorants that can be added to the toner include black pigments such as carbon black, acetylene black, lamp black, and aniline black; yellow pigments such as yellow lead, 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 yellow lead, molybdenum orange, permanent orange GTR, pyrazolone orange, vulcan 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 suitable colorants include red pigments such as Ron 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 Purple, Fast Violet B, and Methyl Violet Lake; blue pigments such as Prussian Blue, Cobalt Blue, Alkali Blue Lake, Victoria Blue (partially chlorinated), Fast Sky Blue, Indanthrene 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 oxide, antimony white, and zinc sulfide; and extender pigments such as baryte powder, barium carbonate, clay, silica, white carbon, talc, and alumina white. These colorants can also be used in combination of two or more types to adjust the toner to a desired hue.

[0050] The amount of the colorant used is not particularly limited as long as it does not impair the object of the present invention. Specifically, the amount of the 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, based on the total mass of the toner core particles.

[0051] The colorant may also be used as a masterbatch in which the colorant is dispersed in advance in a resin material such as a thermoplastic resin. When the colorant is used as a masterbatch, the resin contained in the masterbatch is preferably the same type of resin as the binder resin.

[0052] (charge control agent) The toner core particles may contain a charge control agent for the purpose of improving the charge level of the toner and the charge rise property, which is an index of whether the toner can be charged to a predetermined charge level in a short time, and obtaining a toner with excellent durability and stability. Since the toner of the present invention is a positively charged toner that is positively charged for development, a positively charged charge control agent is used.

[0053] The type of charge control agent that can be contained in the toner core particles is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected from charge control agents that have been used in toners. Specific examples of positively chargeable charge control agents include pyridazine, pyrimidine, pyrazine, orthooxazine, metaoxazine, paraoxazine, orthothiazine, metathiazine, parathiazine, 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, azine compounds such as quinazoline and quinoxaline; azine pharmacophores; Examples of suitable positively charged charge control agents include direct dyes composed of azine compounds such as Streed Red FC, Azin Fast Red 12BK, Azin Violet BO, Azin Brown 3G, Azin Light Brown GR, Azin Dark Green BH / C, Azin Deep Black EW, and Azin Deep Black 3RL; nigrosine compounds such as nigrosine, nigrosine salts, and nigrosine derivatives; acid dyes composed 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 more rapid charge buildup. These positively charged charge control agents can be used in combination of two or more.

[0054] Resins having a quaternary ammonium salt, a carboxylate, or a carboxyl group as a functional group can also be used as positively charged charge control agents. More specifically, examples include styrene-based resins having a quaternary ammonium salt, acrylic-based resins having a quaternary ammonium salt, styrene-acrylic resins having a quaternary ammonium salt, polyester resins having a quaternary ammonium salt, styrene-based resins having a carboxylate, acrylic resins having a carboxylate, styrene-acrylic resins having a carboxylate, polyester resins having a carboxylate, styrene-based resins having a carboxyl group, acrylic resins having a carboxyl group, styrene-acrylic resins having a carboxyl group, and polyester resins having a carboxyl group. The molecular weight of these resins is not particularly limited as long as it does not impair the object 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 a quaternary ammonium salt as a functional group are more preferred because the charge amount can be easily adjusted to a value within a desired range. Specific examples of preferred acrylic comonomers to be copolymerized with styrene units in styrene-acrylic resins having a quaternary ammonium salt as a functional group include (meth)acrylic acid alkyl esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, and isobutyl methacrylate.

[0056] The quaternary ammonium salt may be a unit derived from a dialkylaminoalkyl (meth)acrylate, a dialkyl (meth)acrylamide, or a dialkylaminoalkyl (meth)acrylamide via a quaternization process. 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 dimethylaminopropyl methacrylamide. Hydroxy-containing polymerizable monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, and N-methylol (meth)acrylamide may also be used in combination during polymerization.

[0057] The amount of charge control agent used is not particularly limited as long as it does not impair the objectives of the present invention. The amount of charge control agent used is typically preferably 0.1% by mass or more and 10% by mass or less, based on the total mass of the toner core particles. If the amount of charge control agent used is too small, 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. Furthermore, it is difficult for the charge control agent to be uniformly dispersed, which may result in fogging of the formed image or contamination of the latent image bearing portion by toner components. If the amount of charge control agent used is too large, deterioration of environmental resistance may result in poor charging under high temperature and high humidity conditions, which may lead to image defects in the formed image, or contamination of the latent image bearing portion by toner components.

[0058] (magnetic powder) The toner core particles may contain magnetic powder. Suitable materials for the magnetic powder include, for example, ferromagnetic metals (more specifically, iron, cobalt, nickel, or alloys containing one or more of these metals), ferromagnetic metal oxides (more specifically, ferrite, magnetite, chromium dioxide, etc.), or materials that have been subjected to ferromagnetic treatment (more specifically, carbon materials that have been given ferromagnetism by heat treatment, etc.). In order to prevent metal ions (e.g., iron ions) from eluting from the magnetic powder, it is preferable to use surface-treated magnetic particles as the magnetic powder. One type of magnetic powder may be used alone, or multiple types of magnetic powders may be used in combination.

[0059] The toner core particles may have their surfaces coated with a shell layer, if desired. When a shell layer is formed on the toner core particles, the shell layer is formed from resin fine particles. In order to provide the shell layer with an appropriate surface adsorption force, it is particularly preferred that the shell layer includes a resin film mainly composed of an aggregate of resin particles having a glass transition temperature of 50°C to 100°C, the heat-resistant particles constituting the resin film have a number-average circularity of 0.55 to 0.75, the heat-resistant particles contain a resin including one or more repeating units derived from a styrene-based monomer, a repeating unit having an alcoholic hydroxyl group, and a repeating unit derived from a nitrogen-containing vinyl compound, and the repeating unit having the highest mass ratio among the repeating units contained in the resin contained in the heat-resistant particles is a repeating unit derived from a styrene-based monomer.

[0060] The shell layer constituting the toner of the present invention contains vinyl-based resin fine particles (first resin fine particles) having a relatively small average particle diameter and vinyl-based resin fine particles (second resin fine particles) having a relatively large average particle diameter. The first resin fine particles form the sea-like region of the shell layer. The second resin fine particles form the convex portions of the shell layer. The average particle diameter of the first resin fine particles is preferably about 10 nm to 40 nm. The average particle diameter of the second resin fine particles is preferably about 70 nm to 150 nm.

[0061] Regarding the shell layer (i.e., a resin film primarily composed of an aggregate of heat-resistant particles), the thickness of the shell layer is preferably 10 nm to 35 nm to ensure sufficient heat-resistant storage stability, fixability, and chargeability of the toner. The shell layer thickness can be measured by analyzing a TEM (transmission electron microscope) image of the cross section of a toner particle using commercially available image analysis software (e.g., "WinROOF" manufactured by Mitani Shoji Co., Ltd.). If the shell layer thickness of a single toner particle is not uniform, the shell layer thickness is measured at four evenly spaced locations (specifically, by drawing two perpendicular lines at approximately the center of the cross section of the toner particle and measuring the shell layer thickness at each of the four locations where these two lines intersect), and the arithmetic mean of the four measured values ​​is used as the evaluation value (shell layer thickness) of the toner particle. The boundary between the toner core particle and the shell layer can be confirmed, for example, by selectively dyeing only the shell layer of the toner core particle and the shell layer. If the boundary between the toner core particle and the shell layer is unclear in the TEM image, the boundary between the toner core particle and the shell layer can be clarified by combining TEM with electron energy loss spectroscopy (EELS) to map the characteristic elements contained in the shell layer in the TEM image.

[0062] With regard to the shell layer (i.e., a resin film mainly composed of an aggregate of heat-resistant particles), in order to ensure sufficient heat-resistant storage stability, fixability, and chargeability of the toner, it is preferable that the shell layer covers 50% to 80% of the surface area of ​​the toner core particle. The area ratio of the surface area of ​​the toner core particle covered by the shell layer can be measured by photographing the surface of the toner particle (e.g., a pre-dyed toner particle) with an electron microscope and analyzing the photographed image using commercially available image analysis software.

[0063] (external additives) The toner of the present invention is treated with an external additive after forming a shell layer on the surface of the toner core particle. Hereinafter, the toner core particle before being treated with the external additive will also be referred to as a toner base particle. The toner of the present invention contains silica particles, strontium titanate particles, and resin fine particles as external additives.

[0064] (silica particles) The silica particles used in the toner of the present invention are surface-modified (positively charged) with a coupling agent to enhance the positive chargeability (ease of positive charging) of the silica particles. Examples of the coupling agent include silane coupling agents such as dimethylpolysiloxane and 3-aminopropyltrimethoxysilane. The number-average primary particle diameter of the silica particles is 10 nm or more and 40 nm or less. The surface coverage of the toner base particles by the silica particles is 30% or more and 40% or less.

[0065] (strontium titanate particles) The strontium titanate particles used in the toner of the present invention have not been subjected to a positive charging treatment. The strontium titanate particles may be surface-modified (hydrophobized) with a silane coupling agent. The number-average primary particle diameter of the strontium titanate particles is 10 nm or more and 40 nm or less. The surface coverage of the toner base particles with the strontium titanate particles is 5% or more and 10% or less.

[0066] (Resin fine particles) The resin microparticles are formed of an acrylic resin. The resin microparticles are preferably formed of a silicone-modified acrylic resin. The silicone-modified acrylic resin has a structure in which silicone side chains are attached to an acrylic main chain skeleton, and is a resin that is endowed with the releasability and lubricity that are characteristic of silicone. The silicone-modified acrylic resin is a copolymer of a polydiorganosiloxane macromer having an acrylic functional group and a radically polymerizable organic monomer.

[0067] Furthermore, other monomers can be copolymerized with the above-mentioned monomers. Examples of the other monomers to be copolymerized include styrene-based monomers such as styrene, methylstyrene, methoxystyrene, ethylstyrene, propylstyrene, butylstyrene, phenylstyrene, and chlorostyrene; and acrylic acid ester or methacrylic acid ester-based monomers such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, dodecyl acrylate, stearyl acrylate, ethylhexyl acrylate, lauryl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, pentyl methacrylate, dodecyl methacrylate, stearyl methacrylate, ethylhexyl methacrylate, and lauryl methacrylate.

[0068] Among the above, styrene-acrylic acid resins containing a styrene monomer and one or more acrylic acid monomers are preferred. Styrene-acrylic acid resins have strong hydrophobicity and tend to be easily positively charged.

[0069] The number average primary particle diameter of the resin particles is 80 nm or more and 120 nm or less, and the amount of resin particles added is 0.3 to 2.0 parts by mass with respect to 100 parts by mass of the toner core particles.

[0070] In addition to the silica particles, strontium titanate particles, and resin microparticles described above, other external additives may be added as long as they do not impair the object of the present invention. The type of external additive that can be added is not particularly limited and can be appropriately selected from external additives that have traditionally been used for toners. Specific examples of suitable external additives include metal oxides such as alumina, titanium oxide, magnesium oxide, zinc oxide, and barium titanate. These external additives can be used in combination of two or more types.

[0071] Furthermore, when the toner of the present invention is mixed with a carrier and used as a two-component developer, using a silicone-coated carrier coated with a silicone resin as the carrier can reduce carrier contamination due to adhesion of external additives to the carrier. This is because the silicone resin in the coating layer has low adhesiveness, and the first and second resin particles (spacer particles) contained in the toner are made of a silicone-modified acrylic resin, which can suppress adhesion of the first and second resin particles to the carrier. Furthermore, because both the carrier coating layer and the spacer particles are made of silicone-based materials, even if the spacer particles adhere to the carrier, changes in the charge amount of the carrier can be reduced.

[0072] [Toner manufacturing method] Next, a method for producing the toner of the present invention will be described. The toner production method includes a method for producing toner core particles and an external additive treatment method in which an external additive is attached to the surface of toner base particles. The method for producing toner core particles is not particularly limited as long as the toner core particles are formed to have a predetermined structure. If necessary, toner core particles coated with a shell layer may be used as the toner base particles. As suitable methods for producing the positively charged toner described above, the method for producing toner core particles, the method for forming the shell layer, and the external additive treatment method will be described below in this order.

[0073] (Method of manufacturing toner core particles) The method for producing the toner core particles is not particularly limited as long as it can satisfactorily disperse optional components such as a colorant, a release agent, a charge control agent, and a magnetic powder in a binder resin. Suitable methods for producing the toner core particles include, for example, a pulverization method or an aggregation method.

[0074] In the pulverization method, a binder resin is mixed with components such as a colorant, a release agent, a charge control agent, and a magnetic powder in a mixer or the like, and then the binder resin and the components to be blended in the binder resin are melt-kneaded in a kneader such as a single-screw or twin-screw extruder, and the cooled kneaded product is pulverized and classified. The average particle size of the toner core particles is not particularly limited as long as it does not impede the object of the present invention, but is generally preferably 5 μm or more and 10 μm or less.

[0075] In the aggregation method, fine particles of a binder resin, a release agent, a charge control agent, and a colorant are aggregated in an aqueous medium containing these fine particles until they reach a desired particle size. This results in aggregated particles containing the binder resin, the release agent, the charge control agent, and the colorant. The resulting aggregated particles are then heated to unify the components contained in the aggregated particles. This results in toner core particles having a desired particle size.

[0076] (Method of forming shell layer) When the surface of the toner core particle is coated with a shell layer, the shell layer is formed by adhering resin particles to the surface of the toner core particle.

[0077] A more specific method will be described below. First, hydrochloric acid is added to ion-exchanged water in a mixer to prepare a weakly acidic aqueous medium (for example, a pH selected from 3 to 5). Next, a resin dispersion (suspension) as a shell material and toner core particles are added to the aqueous medium with the adjusted pH.

[0078] Next, while stirring the mixture containing the shell material and toner core particles, the temperature of the mixture is raised to a predetermined holding temperature (e.g., a temperature selected from the range of 50°C to 90°C) at a predetermined rate (e.g., a rate selected from the range of 0.1°C / min to 3°C / min). Furthermore, while stirring the mixture, the temperature of the mixture is maintained at the holding temperature for a predetermined time (e.g., a time selected from the range of 30 minutes to 4 hours). While the temperature of the mixture is maintained at a high temperature, a reaction (solidification of the shell layer) is thought to occur between the toner core particles and the shell material. The shell material bonds with the toner core particles, forming a shell layer. The shell layer is formed on the surface of the toner core particles in the mixture, resulting in a dispersion of toner base particles.

[0079] (External addition treatment method) The method for treating the toner base particles with the external additive is not particularly limited, and the toner base particles can be treated according to a conventionally known method. Specifically, the treatment conditions are adjusted so that the particles of the external additive are not embedded in the toner base particles, and the toner base particles are treated with the external additive using a mixer such as a Henschel mixer or a Nauta mixer.

[0080] The toner of the present invention described above has excellent fixing properties and heat-resistant storage properties, and when forming images over a long period of time under various environments such as high-temperature, high-humidity environments and low-temperature, low-humidity environments, the toner can be charged to a desired charge amount, thereby forming images of a desired density. Therefore, the toner of the present invention can be suitably used in various image forming apparatuses. The effects of the present invention will be explained more specifically below using examples. However, the present invention is not limited to these examples in any way. [Example]

[0081] [Manufacturing Example 1] (Production of amorphous polyester resin) A reaction vessel equipped with a thermometer (thermocouple), dehydration tube, nitrogen inlet tube, and stirrer (stirring blade) was placed on a mantle heater. 150 g of BPA-EO (bisphenol A ethylene oxide 2 mol adduct), 50 g of BPA-PO (bisphenol A propylene oxide 2 mol adduct), 30 g of adipic acid, and 54 g of catalyst (tin(II) 2-ethylhexanoate) were added to the reaction vessel. After replacing the atmosphere with nitrogen, the temperature inside the reaction vessel was raised to 235°C while stirring the contents, and polymerization reaction was carried out until all the monomers were dissolved. The pressure inside the reaction vessel was reduced to 8 kPa (absolute pressure), and the contents were reacted until the specified acid value was reached. The reaction product was then removed from the reaction vessel and cooled to obtain an amorphous polyester resin.

[0082] [Manufacturing Example 2] (Production of crystalline polyester resin) A reaction vessel equipped with a thermometer (thermocouple), dehydration tube, nitrogen inlet tube, and stirrer (stirring blade) was placed on a mantle heater. 69 g of ethylene glycol, 214 g of sebacic acid, and 54 g of catalyst (tin(II) 2-ethylhexanoate) were placed in the reaction vessel and heated to 235°C over 2 hours under a nitrogen atmosphere. After confirming that the reaction rate had reached 95% or higher at 235°C, the vessel was cooled to 160°C, and a mixed solution of 156 g of styrene, 195 g of butyl methacrylate, and 0.5 g of dibutyl peroxide was added dropwise over 1 hour. The mixture was then maintained at 160°C for 30 minutes (aging), heated to 200°C, and further reacted under a reduced pressure of 8 kPa (absolute pressure) for 1 hour, after which it was cooled to 180°C. 4-t-butylcatechol was added as a radical polymerization inhibitor, and the mixture was heated to 210°C over 2 hours. Thereafter, the reaction was carried out at 210° C. for 1 hour, and then at 40 kPa to obtain a crystalline polyester resin.

[0083] [Manufacturing Example 3] (Production of toner base particles) (3-1. Production of toner core particles) 35 parts by weight of the amorphous polyester resin obtained in Production Example 1 as the binder resin, 12 parts by weight of the crystalline polyester resin obtained in Production Example 2, 9 parts by weight of an ester wax (Nissan Electol WEP-8, NOF Corporation) as a release agent, and 9 parts by weight of carbon black (MA-100, Mitsubishi Chemical Corporation) as a colorant were mixed in an FM mixer (FM-10B, Nippon Coke & Engineering Co., Ltd.) to obtain a mixture. The mixture was then melt-kneaded in a twin-screw extruder (PCM-30, Ikegai Corporation) to obtain a kneaded product. The melt-kneading was performed under conditions of a cylinder temperature of 100 °C, a rotation speed of 150 rpm, and a material feed rate of 100 g / min. The kneaded product was cooled and coarsely pulverized using a pulverizer (Rotoplex 16 / 8, Hosokawa Micron Corporation) to a set particle size of 2 mm. The resulting coarsely pulverized product was then finely pulverized using a mechanical pulverizer (Turbo Mill, Freund-Turbo Corporation). The finely pulverized material was classified using a classifier (Elbow Jet, manufactured by Nittetsu Mining Co., Ltd.) to obtain toner core particles having a volume average particle diameter (D50) of 6.7 μm. The volume average particle diameter of the toner core particles was measured using a Coulter Counter Multisizer 3 (manufactured by Beckman Coulter, Inc.).

[0084] (3-2. Shell layer formation) A 1-L three-neck flask equipped with a thermometer and a stirring blade was charged with 100 mL of ion-exchanged water, and the internal temperature of the flask was maintained at 30°C using a water bath. Next, 10 g of an oxazoline-containing polymer aqueous solution (Epocross WS-300, manufactured by Nippon Shokubai Co., Ltd., solids concentration 10% by weight) was added as the shell layer raw material and thoroughly stirred. After that, 100 g of the toner core particles obtained in 3-1 were added, and the contents of the flask were stirred at 200 rpm for 1 hour. Next, 100 mL of ion-exchanged water was added to the flask. After adding 4 mL of 1% aqueous ammonia, the internal temperature of the flask was increased to 60°C at a rate of 0.5°C / min while stirring the contents of the flask at 150 rpm. After the temperature increase, the contents of the flask were stirred for 1 hour at the same temperature and a stirring speed of 100 rpm. After completion of the stirring, the pH of the contents of the flask was adjusted to 7 by adding 1% aqueous ammonia to the flask, and the contents were cooled to room temperature to obtain a dispersion containing toner base particles.

[0085] (3-3. Cleaning process) The washing method is not particularly limited, and for example, a wet cake of toner base particles is collected by filtration from a dispersion containing the toner base particles using a Buchner funnel. This wet cake is dispersed again in ion-exchanged water, and the toner base particles are washed. The same washing operation of the toner base particles with ion-exchanged water is repeated five times.

[0086] (3-4. Drying process) The drying method is not particularly limited, and examples thereof include a method in which the wet cake of toner base particles obtained in 3-3 is supplied to a continuous surface modification device (Coatmizer, manufactured by Freund Corporation) and the wet cake is dried to obtain toner base particles. The drying conditions using the Coatmizer are a hot air temperature of 45°C and a blower air volume of 2 m 3 / min.

[0087] [Manufacturing Example 4] (Production of Silica Particles) 100 g of dimethylpolysiloxane and 100 g of 3-aminopropyltrimethoxysilane (Shin-Etsu Chemical Co., Ltd.) were dissolved in 200 g of toluene. This solution was diluted 10-fold, and the resulting diluted solution was gradually added dropwise to 200 g of fumed silica corresponding to each particle size while stirring to obtain a mixture. The mixture was then ultrasonically irradiated for 30 minutes while stirring. The ultrasonically irradiated mixture was heated to 150°C using a thermostatic chamber. Subsequently, the toluene was removed from the mixture using a rotary evaporator to obtain a solid. The resulting solid was dried using a vacuum dryer at a set temperature of 50°C until no weight loss occurred. The dried solid was heated in an electric furnace in a nitrogen gas stream at a set temperature of 200°C for 3 hours. The heated solid was then crushed using a jet mill and collected using a bag filter to obtain positively charged silica particles.

[0088] [Manufacturing Example 5] (Production of strontium titanate particles) Strontium titanate particles were synthesized by mixing a mineral acid peptized product of a titanium compound hydrolyzate with a water-soluble compound containing strontium. Subsequently, the surface of the particles was treated with a silane coupling agent to produce non-positively charged strontium titanate particles of the respective particle sizes.

[0089] [Manufacturing Example 6] (Toner manufacturing) 100 parts by mass of the toner base particles obtained in Production Example 3, the silica particles obtained in Production Example 4, the strontium titanate particles obtained in Production Example 5, and 0.2 parts by mass of acrylic resin microparticles were mixed for 5 minutes using a 10 L FM mixer (FM-10B, manufactured by Nippon Coke Corporation) to adhere external additives (silica particles and strontium titanate particles) to the surfaces of the toner base particles. The amounts of the silica particles and strontium titanate particles added were adjusted so that the coverage of the toner base particles was a predetermined value. The resulting powder was sieved using a 200 mesh (75 μm opening) sieve. By appropriately changing the type and amount of silica particles and strontium titanate particles added, toners according to Inventions 1 to 8 and Comparative Examples 1 to 24 were obtained.

[0090] [Measurement of average particle size and coverage of silica particles and strontium titanate particles] Using a scanning electron microscope (Regulus8200, Hitachi High-Tech) equipped with an energy dispersive X-ray analyzer (EDXAztec4.1, Oxford Instruments), 10 images of the toner particles to be measured and element mapping images (magnification: 10,000 times) were created, and the average particle size of the silica particles and strontium titanate particles and their coverage relative to the toner base particles were calculated from the average values ​​of the 10 images.

[0091] [Evaluation of toner charge stability, fluidity, and charge rise] The toners of the present invention 1 to 8 and the toners of the comparative examples 1 to 24 were evaluated for charge stability, fluidity, and charge rise property according to the following methods. (Charging stability) A Cu-Zn ferrite carrier (F-80, manufactured by Powder Tech Co., Ltd.) was added to the toners of Inventions 1 to 9 and Comparative Examples 1 to 24 to prepare two-component developers with a toner concentration of 10% by mass. The prepared developers were left standing overnight in an environment of normal temperature and humidity (temperature 20°C, relative humidity 65% ​​RH). The left-standing developers were mixed for 5 minutes and 30 minutes using a mixer (Turbler Mixer, manufactured by WAB Co., Ltd.). The charge amount of the toner in the developer after mixing was measured using a Q / m meter (MODEL 210HS-2A, manufactured by Trek Co., Ltd.). The charge amount Q after 30 minutes of mixing was calculated as the charge amount Q after 5 minutes of mixing relative to the charge amount Q after 30 minutes of mixing. 30 The ratio (=Q 30 The charge stability was evaluated based on the following criteria: ○ (Good): Charge amount Q relative to charge amount Q5 30 The percentage is over 80%. × (Failure): Charge amount Q relative to charge amount Q5 30 The percentage is less than 80%.

[0092] (Liquidity) Using a powder measuring device (powder tester, manufactured by Hosokawa Micron Corporation), 10.0 g of the toners of Inventions 1 to 9 and Comparative Examples 1 to 24, which had been left for 12 hours in an environment of normal temperature and humidity (temperature 23°C, relative humidity 60% RH), was placed on a sieve with 60 μm openings, and vibration was applied for 10 seconds at an amplitude of 1 mm and a frequency of 60 Hz, and the fluidity was calculated using the following formula: The higher the calculated value, the better the fluidity. Fluidity (%) = {(10.0 (g) - mass of toner remaining on sieve (g)) / 10.0 (g)} × 100 The liquidity assessment criteria are as follows: ○ (Good): Calculated value is 90% or more × (bad): Calculated value is less than 90%

[0093] (Charge rise time) A Cu-Zn ferrite carrier (F-80, manufactured by Powder Tech Co., Ltd.) was added to the toners of Inventions 1 to 8 and Comparative Examples 1 to 24 to prepare two-component developers with a toner concentration of 10% by mass. The prepared developers were left standing overnight under normal temperature and humidity conditions (temperature 20°C, relative humidity 65% ​​RH). The left-standing developers were mixed for 1 minute and 5 minutes using a mixer (Turbler Mixer, manufactured by WAB). The charge amount of the toner in the developer after mixing was measured using a Q / m meter (MODEL 210HS-2A, manufactured by Trek). The ratio of the charge amount Q1 after 1 minute of mixing to the charge amount Q5 after 5 minutes of mixing (= Q1 / Q5 × 100) was calculated as an index of charge rise property. The evaluation criteria for charge rise property are shown below. ○ (Good): The ratio of the charge amount Q1 to the charge amount Q5 is 80% or more. × (bad): The ratio of the charge amount Q1 to the charge amount Q5 is less than 80%.

[0094] The evaluation results of the charge stability, fluidity, and charge rise property of the toners of the present invention 1 to 8 and comparative examples 1 to 24 are shown in Table 1, along with the particle size, addition amount, and coverage of the silica particles and strontium titanate particles used in the production of the toner.

[0095] [Table 1]

[0096] As is clear from Table 1, in inventions 1 to 8, in which silica particles and strontium titanate particles with an average particle size of 10 to 40 nm were used as external additives added to the surface of the toner base particles, and the coverage of the silica particles with respect to the toner base particles was 30 to 40%, and the coverage of the strontium titanate particles was 5 to 10%, all of the charging stability, fluidity, and charging start-up properties were good.

[0097] In contrast, in Comparative Examples 1 and 2, where the silica particle coverage was 25%, the silica particle coverage was too low, resulting in poor charge buildup and toner scattering. Furthermore, the total coverage of the external additives on the toner base particles was also low, resulting in poor fluidity. Meanwhile, in Comparative Examples 5 and 6, where the silica particle coverage was 45%, the silica particle coverage was too high, resulting in detachment of the silica particles from the toner base particles and poor charge stability.

[0098] In Comparative Examples 3 and 4, where the coverage of the strontium titanate particles was 3%, the coverage was too low, so the contribution to charge stability was small and the charge stability deteriorated. On the other hand, in Comparative Examples 7 and 8, where the coverage of the strontium titanate particles was 15%, the coverage was too high, so the charge buildup was poor and toner scattering occurred.

[0099] In Comparative Examples 9 to 12, in which the average particle diameter of the silica particles was 5 nm, the average particle diameter of the silica particles was too small, resulting in large fluctuations in the charge amount due to mechanical stress and poor charge stability.On the other hand, in Comparative Examples 13 to 16, in which the average particle diameter of the silica particles was 45 nm, the average particle diameter of the silica particles was too large, resulting in detachment of the silica particles from the toner base particles and poor charge stability.

[0100] In Comparative Examples 17, 18, 23, and 24, in which the average particle diameter of the strontium titanate particles was 5 nm, the average particle diameter of the strontium titanate particles was too small, resulting in large fluctuations in the charge amount due to mechanical stress and poor charge stability.On the other hand, in Comparative Examples 19 to 22, in which the average particle diameter of the strontium titanate particles was 45 nm, the average particle diameter of the strontium titanate particles was too large, resulting in detachment of the strontium titanate particles from the toner base particles and poor charge stability.

[0101] From the above results, it was confirmed that using positively charged silica particles and non-positively charged strontium titanate particles as external additives, and adjusting the particle size and coverage of the silica particles and strontium titanate particles to an appropriate range, contributes to improving the toner's charging stability, fluidity, and charge rise characteristics. [Industrial Applicability]

[0102] The present invention can be applied to a positively charged toner used in an electrophotographic system, and by using the present invention, it is possible to provide a positively charged toner and a two-component developer containing the positively charged toner that can achieve both charge stability, charge rise property, and fluidity. [Explanation of symbols]

[0103] 1 Photosensitive drum 2 cleaning blades 2a Edge part 101 Toner 102 Toner base particles 103 External additives 104 Silica particles 105 Strontium titanate particles 106 Resin fine particles

Claims

1. toner base particles; an external additive attached to the surface of the toner base particles; A positively charged toner comprising toner particles comprising: The external additive is Silica particles whose surfaces have been positively charged; Strontium titanate particles whose surfaces have not been subjected to a positive charging treatment; Resin particles formed from an acrylic resin; Including, the number average primary particle diameter of the silica particles and the number average primary particle diameter of the strontium titanate particles are each 10 nm or more and 40 nm or less; a coverage of the silica particles with respect to the surface area of ​​the toner base particles is 30% or more and 40% or less; A positively charged toner, wherein the strontium titanate particles have a coverage of 5% to 10% of the surface area of ​​the toner base particles.

2. The toner base particles are toner core particles; a shell layer covering the surface of the toner core particle; 2. The positively charged toner according to claim 1, which has a core-shell structure having the formula:

3. 2. The positively charged toner according to claim 1, wherein the resin particles are formed from a silicone-modified acrylic resin.

4. The positively charged toner according to any one of claims 1 to 3, a carrier capable of positively charging the positively chargeable toner by friction; A two-component developer comprising:

Images