Two-component developer

Abstract

To provide a two-component developer which can suppress rise of a carrier and can provide an image with a stable concentration.SOLUTION: The two-component developer includes: a carrier having a carrier core material and a resin coating layer covering the carrier core material; and a toner on which an external additive is attached to the surface of toner particles. The resin coating layer is made of at least two layers including an external layer and an internal layer located closer to the carrier core material than the external layer. The internal layer is less conductive than the external layer. The mass ratio of the internal layer to the external layer is in the range of 1.2 to 11, both inclusive. The external additive includes titanium oxide.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] This disclosure relates to a two-component developer. [Background technology]

[0002] Two-component developers containing toner (toner particles with an external additive attached to the surface) and a carrier are widely used as developers in image forming devices such as photocopiers, multifunction printers, and facsimile machines that utilize the electrophotographic method.

[0003] In two-component developers, resin-coated carriers have been conventionally used as carriers, in which the surface of a magnetic carrier core material such as ferrite is coated with a resin coating layer to provide the function of stably charging the toner to a desired charge level. Furthermore, as resin-coated carriers, carriers with a two-layer resin coating structure (two-layer coated carriers) are known (for example, Patent Documents 1-3).

[0004] Patent Document 1 discloses a two-component developer containing a carrier, wherein the resin coating layer contains a nitrogen atom-containing acrylic resin and a conductive material, and the resin coating layer has a laminated structure of an inner layer on the carrier core side and an outer layer laminated on top of it, the inner layer being the layer with the highest concentration of nitrogen atoms and the lowest concentration of the conductive material, the concentration of nitrogen atoms and the concentration of the conductive material in the inner layer being constant, and the outer layer having a higher concentration of nitrogen atoms on the inner layer side than on the surface side in the film thickness direction, the concentration of the conductive material on the surface side being higher than on the inner layer side, and the film thickness of the inner layer being in the range of 32% to 49% of the total film thickness of the resin coating layer. The carrier disclosed in Patent Document 1 aims to mitigate changes in carrier performance in a carrier in which a region allowing wear is provided in the resin coating layer to give the carrier spend resistance.

[0005] Patent Document 2 discloses a carrier in which a resin coating layer consisting of two layers is applied around a carrier core material: an inner layer made of synthetic resin without conductive material and an outer layer made of synthetic resin in which conductive material is dispersed. The carrier disclosed in Patent Document 2 aims to maintain the carrier's resistance stably.

[0006] Patent Document 3 discloses a carrier in which an oxidation treatment layer is formed on the surface of a carrier core material, and two or more resin coating layers with different resistance values ​​are formed on the oxidation treatment layer such that the resistance value decreases from the inner layer side to the outer layer side. The carrier disclosed in Patent Document 3 also aims to maintain the resistance of the carrier stably. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2014-48455 [Patent Document 2] Japanese Patent Publication No. 2005-345676 [Patent Document 3] Japanese Patent Publication No. 2007-272165 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, when the charging performance is enhanced by introducing nitrogen atoms into the resin coating layer of the carrier, as in the two-component developer disclosed in Patent Document 1, the nitrogen atoms become charge carriers, which reduces the insulating properties of the resin coating layer. Therefore, once an electric field is applied and a conductive path is created in the resin coating layer, the effect of preventing carrier rise is significantly reduced, and there is a problem that the resin coating layer may not be able to perform its function. Carrier rise refers to the phenomenon in which carriers adhere to the surface of the photoreceptor during development due to the Coulomb attraction between the electric field on the surface of the photoreceptor and the carriers.

[0009] The two-component developer disclosed herein was discovered in view of these circumstances, and its primary purpose is to provide a two-component developer that can suppress carrier rise and has excellent image density stability. [Means for solving the problem]

[0010] To solve the above problems, the two-component developer of the present disclosure is a two-component developer comprising a carrier having a carrier core material and a resin coating layer covering the carrier core material, and toner having an external additive attached to the surface of toner particles, wherein the resin coating layer is composed of two or more layers including an outer layer and an inner layer located closer to the carrier core material than the outer layer, the inner layer has lower conductivity than the outer layer, the mass ratio of the inner layer to the outer layer is 1.2 or more and 11 or less, and the external additive contains titanium oxide.

[0011] The above-mentioned two-component developer makes it possible to suppress carrier rise and realize a two-component developer with excellent image density stability.

[0012] In the case of the above-mentioned two-component developer, it is preferable that, as development is repeated, the titanium oxide that was attached to the surface of the toner particles as an external additive migrates to the carrier surface. Specifically, the amount of titanium oxide that migrates is preferably 0.02% to 2% per 1,000 printed sheets when a two-component developer is set in a copier with a two-component developer and 400,000 sheets of A4 paper are printed at a print density of 5%. The amount of titanium oxide that migrates to the carrier surface by printing is expressed as the ratio of the amount of titanium oxide contained in the toner before printing (initial toner) to the amount of titanium oxide that migrates to the carrier surface by printing.

[0013] The above two-component developer contains a two-layer coated carrier with excellent performance in suppressing carrier rise, and is designed to tolerate changes in carrier resistance when the two-component developer is used for a long period of time. The inventors have found that with the above two-component developer, even if the carrier resistance changes after long-term use of the developer, the overall performance of the developer can be suppressed. With the above two-component developer, titanium oxide, which is an external additive for toner, migrates to the carrier side, which suppresses the occurrence of fogging due to continuous printing, especially in low-humidity environments, and changes in fogging when left unattended in low-humidity environments, thereby realizing a two-component developer with minimal changes in image quality. The specific mechanism of action will be described later. [Effects of the Invention]

[0014] According to the two-component developer of the present disclosure, it is possible to suppress carrier rise and has excellent effects such as excellent stability of image density.

Brief Description of the Drawings

[0015] [Figure 1] It is a cross-sectional view schematically showing the state at the initial stage of use of the carrier according to this embodiment. [Figure 2] It is a cross-sectional view schematically showing the state after long-term use of the carrier according to this embodiment. [Figure 3] It is a cross-sectional view schematically showing the state of charge transfer at the initial stage of use of the carrier according to this embodiment. [Figure 4] It is a cross-sectional view schematically showing the state of charges after long-term use of the carrier according to this embodiment. [Figure 5] It is a cross-sectional view schematically showing the state of charge transfer during development of the carrier according to this embodiment after long-term use. [Figure 6] It is a schematic diagram showing a measuring jig used for measuring the bridge resistance of the carrier. [Figure 7] It is a graph showing the resistance characteristics at the initial stage of use of the carrier according to the example.

Modes for Carrying Out the Invention

[0016] Hereinafter, embodiments of the two-component developer of the present disclosure will be described in detail.

[0017] The two-component developer according to this embodiment is a two-component developer containing a carrier including a carrier core material and a resin coating layer covering the carrier core material, and toner with an external additive attached to the surface of the toner particles.

[0018] 1. Carrier Figure 1 is a schematic cross-sectional view showing the initial state of the carrier according to this embodiment. The carrier in the two-component developer of this disclosure comprises a resin coating layer composed of two or more layers, including an inner layer and an outer layer. In carrier C of Figure 1, the resin coating layers are laminated on the surface of the carrier core material 1 in the order of inner layer 2 and outer layer 3.

[0019] The volume-average particle size of the carrier is not particularly limited, but is preferably between 20 μm and 100 μm, and more preferably between 30 μm and 60 μm. If the volume-average particle size of the carrier is too small, the carrier may move from the developing roller to the photosensitive drum during development, causing white spots in the resulting image. If the volume-average particle size of the carrier is too large, dot reproducibility will be poor, and the image may become coarse.

[0020] The volume-average particle size of the carriers refers to the value measured using a laser diffraction particle size distribution analyzer HELOS (SYMPATEC) and a dry dispersion analyzer RODOS (SYMPATEC) under conditions of a dispersion pressure of 3.0 bar.

[0021] As carrier core materials, those commonly used in this field can be used, such as magnetic metals such as iron, copper, nickel, and cobalt, and magnetic metal oxides such as ferrite and magnetite. Among these, particles containing a ferrite component (ferrite-based particles) are preferred. Ferrite-based particles have high saturation magnetization and can produce carriers with low density. Therefore, carrier adhesion to the photoreceptor is less likely to occur, and images with high dot reproduction due to soft dot formation can be obtained. Known ferrite-based particles can be used, such as zinc-based ferrite, nickel-based ferrite, copper-based ferrite, nickel-zinc-based ferrite, manganese-magnesium-based ferrite, copper-magnesium-based ferrite, manganese-zinc-based ferrite, and manganese-copper-zinc-based ferrite.

[0022] The resin constituting the resin coating layer is not particularly limited, and known resins can be used, but it is preferable that it contains a silicone resin or an acrylic-modified silicone resin. This improves the release properties of the toner from the carrier during development, resulting in good developability. Furthermore, the resin coating layer can be made to the desired hardness, and the adhesion to the carrier core material can be improved, allowing the toner to be stably charged over a long period of time.

[0023] Among the silicone resins mentioned above, crosslinkable silicone resins are more preferred. The inclusion of crosslinkable silicone resin further improves the release properties of the toner from the carrier during development, resulting in better developability. Furthermore, the resin coating layer can be made to the desired hardness, and the adhesion to the carrier core material can be further improved. Crosslinkable silicone resin is a silicone resin that has been cured by crosslinking hydroxyl groups bonded to Si atoms, or hydroxyl groups and -OX groups, through a heat dehydration reaction, a room-temperature curing reaction, etc., as shown in the formula below. In the formula below, R represents a monovalent organic group, and multiple Rs may be the same or different. The -OX group is an acetoxy group, aminooxy group, alkoxy group, oxime group, etc.

[0024] [ka]

[0025] There are no particular restrictions on the crosslinking silicone resin; either heat-curing silicone resin or room-temperature curing silicone resin can be used. To crosslink a heat-curing silicone resin, it is necessary to heat the resin to approximately 200°C to 250°C. Heating is not necessary to cure a room-temperature curing silicone resin, but it is preferable to heat it to 150°C to 280°C to shorten the curing time.

[0026] Among cross-linked silicone resins, those in which the monovalent organic group represented by R is a methyl group are preferred. Since cross-linked silicone resins in which R is a methyl group have a dense cross-linked structure, when a resin coating layer of a carrier core material is formed using a cross-linked silicone resin, a carrier with good water repellency and moisture resistance can be obtained. However, if the cross-linked structure becomes too dense, the resin coating layer tends to become brittle, so the molecular weight of the cross-linked silicone resin should be appropriately selected.

[0027] In this embodiment, it is preferable that the inner and outer layers of the resin coating layer contain the same resin. By containing the same resin, the interface between the inner and outer layers is less likely to peel off, and the adhesion between the two layers is improved.

[0028] A conductive material can be added to the resin coating layer. In the two-component developer of this embodiment, since the inner layer has lower conductivity than the outer layer, it is preferable that the outer layer contains a conductive material. By containing a conductive material in the outer layer, a significant increase in toner charge during the initial stages of use can be suppressed. Furthermore, if both layers contain the same conductive material, for example, it is preferable that the conductive material content is higher in the outer layer than in the inner layer.

[0029] Examples of conductive materials include silicon dioxide, alumina, carbon black, graphite, zinc oxide, titanium black, iron oxide, titanium oxide, tin oxide, potassium titanate, calcium titanate, aluminum borate, magnesium oxide, barium sulfate, and calcium carbonate. One conductive material may be used alone, or two or more may be used in combination.

[0030] Among these conductive materials, carbon black is preferred from the viewpoint of manufacturing stability, cost, and low electrical resistance. The type of carbon black is not particularly limited, but those with a DBP (dibutyl phthalate) oil absorption rate in the range of 90 ml / 100g to 170 ml / 100g are preferred for their excellent manufacturing stability. Furthermore, those with a primary particle size of 50 nm or less are preferred for their excellent dispersibility.

[0031] The content of conductive material in the outer layer is preferably 0.1 parts by mass to 20 parts by mass, and preferably 5 parts by mass to 15 parts by mass, per 100 parts by mass of resin constituting the resin coating layer. If the content of conductive material exceeds the above upper limit, fogging may occur. If the content of conductive material is below the above lower limit, the effect of adding conductive material may not be realized.

[0032] In this embodiment, the resin coating layer of the carrier is formed such that the mass ratio of the inner layer to the outer layer is 1.2 or more and 11 or less. More preferably, this mass ratio is 4 or more and 11 or less. By forming a thicker inner layer, which has lower conductivity than the outer layer, the carrier maintains its ability to prevent carrier rise over a long period of time. In addition, the two-component developer in this embodiment has an outer layer in which the carrier has conductivity, and the toner contains titanium oxide as an external additive, resulting in excellent stability of image density.

[0033] On the other hand, as the outer layer wears away with prolonged use and the inner layer is exposed, the charge transfer performance of the carrier deteriorates. However, in the two-component developer according to this embodiment, as will be described later, titanium oxide, which is an external additive for the toner, migrates to the carrier surface (the surface of the exposed inner layer) and helps in charge transfer at the carrier surface, thereby maintaining the charging performance.

[0034] The proportion of the inner layer in the carrier is preferably 1.5 parts by mass or more and 12 parts by mass or less per 100 parts by mass of the carrier core material, and the proportion of the outer layer in the carrier is preferably 1 part by mass or more and 1.5 parts by mass or less per 100 parts by mass of the carrier core material.

[0035] Known methods can be used to form the resin coating layer. For example, there is an immersion method in which the raw materials for the resin coating layer are dissolved in a solvent (e.g., an organic solvent such as toluene or acetone) and the carrier core material is immersed in the resulting solution; a spray method in which the raw material solution for the resin coating layer is sprayed onto the carrier core material; a fluidized bed method in which the raw material solution for the resin coating layer is sprayed onto the carrier core material while the carrier core material is suspended by fluidized air; and a kneader coater method in which the carrier core material and the raw material solution for the resin coating layer are mixed in a kneader coater and the solvent is removed. If the resin coating layer contains a conductive material, it can be formed by adding the conductive material together with the resin to the raw material solution for the resin coating layer.

[0036] By forming an inner layer using the method described above, and then forming an outer layer using the same method, an inner and outer layer can be formed (a two-layer coating can be achieved). By advancing the curing reaction during the coating process, independent coating layers can be formed, and by suppressing the progress of the curing reaction, the changes between layers can be made continuous. The curing reaction can be controlled by the temperature during the coating process; for example, the coating process can be performed at around 50°C to 250°C.

[0037] In this embodiment, the resistance characteristics of the carrier preferably have a voltage of 25V to 300V at which a current of 0.1μA is obtained in the bridge resistance measurement method, and more preferably 70V to 280V. Having the voltage within the above range allows for a two-component developer that is more suitable for preventing carrier rise.

[0038] Furthermore, the resistance characteristics of the carrier according to this embodiment are preferably such that the voltage at which a current of 10 μA is obtained in the bridge resistance measurement method is 250 V to 900 V, and more preferably 400 V to 850 V. By having the voltage within the above range, a two-component developer with less change in image density can be obtained.

[0039] The carrier bridge resistance can be measured using a measuring jig 90 as shown in Figure 6, and the specific measurement method is described in the embodiment below.

[0040] 2. Toner The toner according to this embodiment is a toner in which an external additive is attached to the surface of toner particles. Furthermore, optional components may be included as needed, to the extent that they do not impair the effects relating to this disclosure. The volume average particle size of the primary particles of the toner can be appropriately selected depending on the purpose, for example, 5 μm or more and 8 μm or less. In this embodiment, the toner particles contain a binder resin and internal additives such as a colorant and a release agent, and the internal additives are dispersed in the binder resin.

[0041] <Binding resin> The binder resin included in the toner particles according to this embodiment is not particularly limited, but a polyester resin can be suitably used.

[0042] Polyester resins used as binder resins are typically obtained by polycondensation reactions via esterification or transesterification reactions using known methods, involving one or more components selected from divalent alcohol components and trivalent or higher polyvalent alcohol components, and one or more components selected from divalent carboxylic acids and trivalent or higher polyvalent carboxylic acids.

[0043] The conditions for the condensation polymerization reaction can be appropriately set depending on the reactivity of the monomer components, and the reaction should be terminated when the polymer achieves desirable physical properties. For example, the reaction temperature is approximately 170°C to 250°C, and the reaction pressure is approximately 5 mmHg to atmospheric pressure.

[0044] Examples of divalent alcohol components include alkylene oxide adducts of bisphenol A such as polyoxypropylene(2.2)-2,2-bis(4-hydroxyphenyl)propane, polyoxypropylene(3.3)-2,2-bis(4-hydroxyphenyl)propane, polyoxypropylene(2.0)-2,2-bis(4-hydroxyphenyl)propane, polyoxypropylene(2.0)-polyoxyethylene(2.0)-2,2-bis(4-hydroxyphenyl)propane, and polyoxypropylene(6)-2,2-bis(4-hydroxyphenyl)propane; ethylene Examples include diols such as lycopropyl 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; propylene adducts of bisphenol A; ethylene adducts of bisphenol A; and hydrogenated bisphenol A.

[0045] Examples of polyhydric alcohol components with a valency of 3 or higher include sorbitol, 1,2,3,6-hexanetetrol, 1,4-sorbitan, pentaerythritol, dipentaerythritol, tripentaerythritol, sucrose, 1,2,4-butanetriol, 1,2,5-pentanetriol, glycerol, 2-methylpropanetriol, 2-methyl-1,2,4-butanetriol, trimethylolethane, trimethylolpropane, and 1,3,5-trihydroxymethylbenzene.

[0046] In the toner according to this embodiment, one of the above-mentioned divalent alcohol component and trivalent or higher polyvalent alcohol component may be used alone, or two or more may be used in combination.

[0047] Examples of divalent carboxylic acids include 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, n-dodecenylsuccinic acid, n-dodecylsuccinic acid, n-octylsuccinic acid, isooctenylsuccinic acid, isooctylsuccinic acid, and their acid anhydrides, lower alkyl esters, etc.

[0048] Examples of polycarboxylic acids with three or more valent values ​​include 1,2,4-benzenetricarboxylic 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, empoletrimeric acid, and their acid anhydrides and lower alkyl esters.

[0049] In the toner according to this embodiment, one of the above-mentioned divalent carboxylic acids and trivalent or higher polyvalent carboxylic acids may be used alone, or two or more may be used in combination.

[0050] The weight-average molecular weight (Mw) of the polyester resin used as the binder resin can be appropriately selected depending on the purpose, but it is preferably between 5,000 and 50,000. In this disclosure, the weight-average molecular weight is the value measured by gel permeation chromatography (GPC), with tetrahydrofuran (THF) used as the mobile phase and polystyrene used as the standard substance.

[0051] The binder resin preferably has a glass transition temperature of 30°C to 80°C. If the glass transition temperature of the binder resin is below the lower limit, toner is more likely to undergo thermal agglomeration and blocking inside the image forming apparatus, which may reduce storage stability. If the glass transition temperature of the binder resin exceeds the upper limit, the toner's ability to adhere to the recording medium will decrease, which may result in poor adhesion.

[0052] Furthermore, the binder resin preferably has a softening temperature of 80°C to 150°C. In addition, the binder resin preferably has an acid value of 0 KOH mg / g to 30 KOH mg / g.

[0053] The binder resin content in the toner particles is preferably 60% by mass or more and 98% by mass or less, and more preferably 70% by mass or more and 95% by mass or less.

[0054] <Coloring agent> In the toner according to this embodiment, the toner particles may contain a colorant. Various types and colors of organic and inorganic pigments and dyes commonly used in the field of electrophotography can be used as the colorant, for example, black, white, yellow, orange, red, purple, blue, and green colorants.

[0055] Examples of black colorants include carbon black, copper oxide, manganese dioxide, aniline black, activated carbon, non-magnetic ferrite, magnetic ferrite, and magnetite.

[0056] Examples of white colorants include zinc oxide, titanium dioxide, antimony white, and zinc sulfide.

[0057] Examples of yellow colorants include lead yellow, zinc yellow, cadmium yellow, yellow iron oxide, mineral fast yellow, nickel titanium yellow, navel yellow, naphthol yellow S, Hansa yellow G, Hansa yellow 10G, benzidine yellow G, benzidine yellow GR, quinoline yellow lake, permanent yellow NCG, tartrazine lake, CI pigment yellow 12, CI pigment yellow 13, CI pigment yellow 14, CI pigment yellow 15, CI pigment yellow 17, CI pigment yellow 93, CI pigment yellow 94, and CI pigment yellow 138.

[0058] Examples of orange colorants include red lead, molybdenum orange, permanent orange GTR, pyrazolone orange, balkan orange, induthrene brilliant orange RK, benzidine orange G, induthrene brilliant orange GK, CI pigment orange 31, and CI pigment orange 43.

[0059] Examples of red colorants include red iron oxide, cadmium red, red lead, mercury sulfide, cadmium, permanent red 4R, lysol red, pyrazolone red, watching red, calcium salt, lake red C, lake red D, brilliant carmine 6B, eosin lake, rhodamine lake B, alizarin lake, brilliant carmine 3B, CI pigment red 2, CI pigment red 3, CI pigment red 5, CI pigment red 6, CI pigment Examples include CI Pigment Red 7, CI Pigment Red 15, CI Pigment Red 16, CI Pigment Red 48:1, CI Pigment Red 53:1, CI Pigment Red 57:1, CI Pigment Red 122, CI Pigment Red 123, CI Pigment Red 139, CI Pigment Red 144, CI Pigment Red 149, CI Pigment Red 166, CI Pigment Red 177, CI Pigment Red 178, and CI Pigment Red 222.

[0060] Examples of purple colorants include manganese purple, fast violet B, and methyl violet lake.

[0061] Examples of blue colorants include Prussian blue, cobalt blue, alkali blue lake, Victoria blue lake, phthalocyanine blue, metal-free phthalocyanine blue, partially chlorinated phthalocyanine blue, First Sky Blue, Induthlene Blue BC, CI Pigment Blue 15, CI Pigment Blue 15:2, CI Pigment Blue 15:3, CI Pigment Blue 16, and CI Pigment Blue 60.

[0062] Examples of green colorants include chromium green, chromium oxide, pigment green B, micalite green lake, final yellow green G, and CI pigment green 7.

[0063] In the toner according to this embodiment, one of the above-mentioned colorants may be used alone or in combination of two or more, and the combination may be of different colors or the same color. The amount of colorant in the toner particles is preferably 0.1 parts by mass or more and 20 parts by mass or less, and more preferably 3 parts by mass or more and 15 parts by mass or less, per 100 parts by mass of the binder resin.

[0064] <Release agent> The toner particles according to this embodiment may contain a release agent. As the release agent, waxes commonly used in the field of electrophotography can be used. Examples include petroleum-based waxes such as paraffin wax and its derivatives, microcrystalline wax and its derivatives; hydrocarbon-based synthetic waxes such as Fischer-Tropsch wax and its derivatives, polyolefin wax and its derivatives, polypropylene wax and its derivatives, polyolefin polymer waxes (such as low molecular weight polyethylene wax) and its derivatives; plant-based waxes such as carnauba wax and its derivatives, rice wax and its derivatives, candelilla wax and its derivatives, and wood wax; animal-based waxes such as beeswax and whale wax; oil-based synthetic waxes such as fatty acid amides, phenolic fatty acid esters and their derivatives; silicone polymers, higher fatty acids, etc. One of these may be used alone, or two or more may be used in combination. Derivatives include oxides, block copolymers of vinyl monomers and waxes, graft-modified vinyl monomers and waxes, etc.

[0065] In this embodiment, the content of the release agent in the toner particles is preferably 0.5 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the binder resin.

[0066] <Other oral additives> In the toner according to this embodiment, other internal additives may be included as needed. Examples of other internal additives include charge control agents. Charge control agents are added to impart desirable charge properties to the toner. The charge control agent is not particularly limited, and charge control agents used in the field of electrophotography for controlling positive and negative charges can be used.

[0067] <External additives> The toner additive according to this embodiment contains titanium oxide. The two-component developer according to this embodiment contains a carrier having the above-described configuration and titanium oxide as the toner additive, thereby suppressing carrier rise and providing excellent stability of image density. The toner according to this embodiment may also contain additives other than titanium oxide; for example, in the later-described embodiment, small-particle silica (hereinafter also simply referred to as "small silica") is added as an additive.

[0068] As described above, in the two-component developer according to this embodiment, titanium oxide, which acts as an external additive for the toner, migrates to the carrier surface as development is repeated. Therefore, the effects exhibited by the migration of titanium oxide to the carrier surface in the two-component developer according to this embodiment will be explained.

[0069] Figure 1 is a schematic cross-sectional view showing the state of the carrier in the initial stages of use according to this embodiment, and Figure 2 is a schematic cross-sectional view showing the state of the carrier after long-term use according to this embodiment. As shown in these figures, when the carrier is used with toner as a two-component developer, the resin coating layer (coating layer) is scraped off, and components from the toner migrate to the carrier surface. Since this scraping off of a portion of the resin coating layer has a refreshing effect on the carrier surface, the carrier is designed to tolerate scraping. In the two-component developer according to this embodiment, titanium oxide is included as an external additive to the toner, so as shown in Figure 2, titanium oxide adheres to the inner layer that is exposed when the outer layer of the resin coating layer is scraped off.

[0070] Figures 3 to 5 schematically illustrate the charge transfer in the two-component developer according to this embodiment. Figure 3 is a schematic cross-sectional view showing the charge transfer in the carrier during the initial period of use according to this embodiment. Figure 4 is a schematic cross-sectional view showing the charge state of the carrier after long-term use according to this embodiment. Figure 5 is a schematic cross-sectional view showing the charge transfer during development of the carrier according to this embodiment after long-term use.

[0071] As shown in Figure 3, in the initial stages of use of the two-component developer according to this embodiment, the charge supplied from charge sources such as the developing electrode and other carriers mainly moves through the outer layer, which is a conductive layer. As printing is repeated, the outer layer is worn away, exposing the inner layer, and the toner component adheres to the carriers. As shown in Figure 4, the overall charge of the two-component developer is basically balanced, with the carriers being positively charged and the toner being negatively charged.

[0072] When toner is developed and moves, a difference in charge is created, and internal charge needs to move to fill this gap. However, charge has difficulty moving to the non-conductive resin layer. In other words, when printing continuously, there is a tendency for the supply of charge to the exposed areas of the non-conductive resin layer to not keep up. As a result, some of the undeveloped toner becomes increasingly charged, while the ability of the carriers to charge the newly supplied toner decreases, which is thought to cause fogging and changes in image density.

[0073] In the two-component developer according to this embodiment, as shown in Figure 5, charge is supplied from adjacent carriers, etc., even through titanium oxide present on the surface of the inner layer, which is a non-conductive resin layer. In other words, the titanium oxide, which is an external additive of the toner, migrates to the carrier surface, thereby assisting in the supply of charge to the exposed portion of the inner layer. Therefore, the two-component developer according to this embodiment can suppress disturbances in the charge distribution on the carrier after long-term use. Consequently, when left in low-humidity and high-humidity environments after long-term use, it is possible to suppress the occurrence of fogging in printing immediately after leaving it, the occurrence of fogging in continuous printing thereafter, and changes in fogging between the two.

[0074] Examples of titanium oxides used as external additives include titanium dioxide and strontium titanate, but the titanium oxide contained as an external additive in the toner according to this embodiment is preferably strontium titanate. By using strontium titanate as the titanium oxide, the amount transferred to the carrier becomes more appropriate, and fluctuations in fogging in low-humidity environments can be further suppressed.

[0075] The external additive for the toner according to this embodiment preferably contains strontium titanate as a silica-modified fine powder. In other words, it is preferable that the strontium titanate used as an external additive is silica-modified. By using silica-modified strontium titanate, fogging in high-humidity environments can be suppressed even when a large amount of strontium titanate is transferred to the carrier.

[0076] A fine powder of strontium titanate modified with silica can be manufactured, for example, by the following procedure (1) to (5). (1) After de-ironizing and bleaching the metatitanic acid obtained by the sulfuric acid method, a sodium hydroxide aqueous solution is added to perform desulfurization, then it is neutralized with hydrochloric acid, and after filtration and washing, a washed cake is obtained. (2) After adding water to the washed cake to make a slurry, hydrochloric acid is added to perform a gelatinization treatment. This is called Solution 1, and Solution 2, which is an aqueous solution of strontium chloride, and Solution 3, which is an aqueous solution of sodium silicate are mixed together. The mixing ratio of Solution 1, Solution 2, and Solution 3 should be such that the molar ratio of (Sr+Si) / Ti is 1.2. (3) Heat the mixed solution to 90°C under a nitrogen gas atmosphere, and stir for 2 hours while adding aqueous sodium hydroxide solution to complete the reaction. (4) Cool the slurry after the reaction to 50°C, add hydrochloric acid and stir for 2 hours. Wash the resulting precipitate, separate it by filtration, and then dry it. (5) The obtained dried material is ground in a blender for 1 minute, and after removing the coarse powder with a sieve with a mesh size of 32 μm, the obtained fine powder substrate is surface coated with a silane coupling agent. Methods for surface coating with a silane coupling agent include surface treatments commonly used in the art, such as hexamethyldisilazane (HMDS), dimethyl-dichlorosilane (DDS), octylsilane (OTAS), and polydimethylsiloxane (PDMS).

[0077] In the two-component developer according to this embodiment, the amount of titanium oxide transferred to the carrier surface by printing, which represents the ratio of the amount of titanium oxide contained in the toner (initial toner) as an external additive, is preferably 0.02% to 2% per 1000 printed pages, and more preferably 0.08% to 1.45%. When a fine powder of strontium titanate modified with silica is used as the titanium oxide, the amount of transfer is particularly preferably 0.17% to 0.82%. By keeping the amount of transfer within the above range, it is possible to suppress the occurrence of fogging due to continuous printing, especially in low-humidity environments, and the change in fogging when left standing in a low-humidity environment, thereby realizing a two-component developer with minimal change in image quality. Furthermore, by using strontium titanate as the titanium oxide and setting the amount of transfer within a more appropriate range, a two-component developer with minimal change in image quality can be obtained in both low-humidity and high-humidity environments. The specific method for measuring the amount of titanium oxide transferred is as shown in the example below. Fifty images with a print density of 5% are printed continuously on A4 paper, and after a few seconds, the printing is repeated (intermittent printing) to print a total of 400,000 sheets. The amount of titanium oxide transferred to the carrier is then measured after printing these 400,000 sheets.

[0078] In this embodiment, the titanium oxide content is preferably 0.01 parts by mass or more and 1.5 parts by mass or less per 100 parts by mass of toner particles, and more preferably 0.03 parts by mass or more and 1 part by mass or less. In particular, when using fine powder obtained by silica modification of strontium titanate as titanium oxide, it is preferable that the content of said fine powder be 0.1 parts by mass or more and 0.8 parts by mass or less. By keeping the titanium oxide content within the above range, the amount transferred to the carrier can be kept within an appropriate range, and a two-component developer can be made in which the image quality changes little in both low-humidity and high-humidity environments.

[0079] <Toner manufacturing method> The toner manufacturing method according to this embodiment includes a kneading step S1 in which toner raw materials containing a binder resin, a release agent, and a colorant are kneaded to produce a kneaded product, and a pulverizing step S2 in which the kneaded product produced in the kneading step S1 is pulverized to produce toner particles. Furthermore, the toner manufacturing method according to this embodiment includes a classification step S3 in which the toner particles produced in the pulverizing step S2 are classified, and an external additive step S4 in which an external additive is added to the toner particles after classification in the classification step S3.

[0080] In the kneading step S1, the binder resin, release agent, colorant, etc., which are the raw materials for toner particles, are mixed in a mixer such as a Henschel mixer, and then kneaded using a kneader to obtain a kneaded product. The kneading is carried out by heating to a temperature above the softening temperature of the binder resin and below the thermal decomposition temperature. This melts or softens the binder resin, and allows the release agent, colorant, etc. to be dispersed in the binder resin. The specific heating temperature during kneading is preferably, for example, 80°C to 200°C, and more preferably 100°C to 150°C. Mixing machines can include kneaders, twin-screw extruders, two-roll mills, three-roll mills, and lab blast mills. Examples of such mixers include single-screw or twin-screw extruders such as the TEM-100B (product name, manufactured by Toshiba Machine Co., Ltd.), PCM-65, PCM-65 / 87, and PCM-30 (all product names, manufactured by Ikegai Co., Ltd.), and open-roll type mixers such as the Nidex (product name, manufactured by Mitsui Mining Co., Ltd.). Furthermore, the mixing process may be carried out using multiple mixers.

[0081] In the grinding step S2, the kneaded material obtained in the kneading step S1 is solidified by cooling, and the solidified material is coarsely ground to obtain coarse material. For coarse grinding, a speed mill, hammer mill, or cutter mill can be used as a grinder. After that, the coarse material is finely ground (fine grinding step). In the fine grinding step, for example, a jet grinder that grinds using a supersonic jet stream, or an impact grinder that grinds the coarse material by introducing it into the space formed between a high-speed rotating rotor and stator can be used. Alternatively, the classification step S3 below may be omitted, and the group of finely ground particles with a volume average particle size of 5 μm or more and 8 μm or less obtained in the grinding step S2 may be recovered as toner particles.

[0082] In the classification step S3, the finely ground particle group obtained in the grinding step S2 is classified using a classifier to obtain toner particles having a volume-average particle size of 5 μm to 8 μm. As the classifier, for example, a rotary wind classifier (rotary wind classifier) ​​can be used.

[0083] In the external additive step S4, the toner particles obtained in the classification step S3 and the external additive are mixed in a powder mixer such as a Henschel mixer to adhere the external additive to the toner particles. In the external additive step S4, the adhesion strength of the external additive to the toner particles can be adjusted by appropriately changing the mixing conditions. [Examples]

[0084] The two-component developer of this disclosure will be described in detail below based on examples and comparative examples.

[0085] <Career Development> A silicone resin solution (product name: KR251, manufactured by Shin-Etsu Chemical Co., Ltd.) was diluted with toluene to prepare a solution with a solid content concentration of 10% by mass. A curing catalyst (product name: D-20, manufactured by Shin-Etsu Chemical Co., Ltd.) was added to this solution at a concentration of 0.5% by mass relative to the silicone resin to prepare coating solution 1. In addition, coating solution 2 (a carbon dispersion with a solid content concentration of 2% by mass) was prepared by dispersing a conductive material (carbon black, product name: MA100, manufactured by Mitsubishi Chemical Corporation) in toluene.

[0086] The carrier core material (MnMg ferrite, volume-average particle size: 41 μm) was weighed into the container of the mixing and stirring machine, and the coating solution was also weighed to the specified mass parts. Here, the specified mass parts are the number determined for each example and comparative example shown in Table 1 below. The coating solution was added to the stirring machine while stirring at room temperature, and once homogenized, the temperature was raised to 80°C. By continuing stirring while degassing and removing the solvent from the coating solution, the first coating layer (inner layer) was formed around the carrier core material. After the solvent had evaporated and about 10 minutes had passed, the second layer (outer layer) of coating solution was added, and the same procedure as for the first layer was performed to obtain a multi-layered coated carrier.

[0087] To harden and stabilize the resulting carrier coating layer, it was cured in a 180°C oven for 1 hour. After removal and allowing it to cool to room temperature, the coated carrier was sieved to remove coarse material, thereby obtaining the carrier for evaluation.

[0088] <Preparation of strontium titanate fine powder> (1) Preparation of fine powder A in which strontium titanate is silica-modified. After desulfurization and bleaching of the metatitanic acid obtained by the sulfuric acid method, sodium hydroxide aqueous solution was added to adjust the pH to 9.0, and desulfurization treatment was carried out. After desulfurization treatment, the pH was neutralized with hydrochloric acid to 5.8, and the mixture was filtered and washed to obtain a washed cake. Water was added to the washed cake to form a slurry, and hydrochloric acid was added to adjust the pH to 1.4, and the mixture was dispacted. This metatitanic acid was placed in a reaction vessel, and strontium chloride solution and sodium silicate were added. Next, the mixture was heated to 90°C while stirring, and then 10N sodium hydroxide aqueous solution was added over 2 hours. After that, stirring was continued at 95°C for 1 hour to complete the reaction.

[0089] The reaction 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 decanted and washed, then adjusted to 50°C, hydrochloric acid was added to adjust the pH to 2.5, and hydrophobic treatment was performed. Next, sodium hydroxide solution was added to adjust the pH to 6.5, and stirring was continued for 1 hour. After filtration washing, the resulting cake was dried in 120°C air for 10 hours to obtain a fine powder (hereinafter referred to as fine powder A) in which the surface of the composition containing strontium titanate and silica was hydrophobicized with a silane compound. The number-average particle size of the primary particles was 40 nm.

[0090] (2) Preparation of fine powder B in which strontium titanate is not silica-modified. After desulfurization and bleaching of the metatitanic acid obtained by the sulfuric acid method, sodium hydroxide aqueous solution was added to adjust the pH to 9.0, and desulfurization treatment was carried out. After desulfurization treatment, the pH was neutralized with hydrochloric acid to 5.8, and the mixture was filtered and washed to obtain a washed cake. Water was added to the washed cake to form a slurry, and then hydrochloric acid was added to adjust the pH to 1.4, and the mixture was dispacted. This metatitanic acid was placed in a reaction vessel, and strontium chloride solution was added. Next, the mixture was heated to 90°C while stirring, and then 10N sodium hydroxide aqueous solution was added over 2 hours. After that, stirring was continued at 95°C for 1 hour to complete the reaction.

[0091] The reaction 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 decanted and washed, then adjusted to 50°C, hydrochloric acid was added to adjust the pH to 2.5, and hydrophobic treatment was performed. Next, sodium hydroxide solution was added to adjust the pH to 6.5, and stirring was maintained for 1 hour. After filtration washing, the resulting cake was dried in 120°C air for 10 hours to obtain a fine powder (hereinafter referred to as fine powder B) containing strontium titanate in the core and free of silica. The number-average particle size of the primary particles was 40 nm.

[0092] <Toner production> Toner particles (toner cores) were produced using the following raw materials by melt-pulverization. (Binding resin) Resin A: Amorphous polyester resin / 2000g mass molecular weight 1×10 3 ~1 × 10 4 Distribution of the range: 76% Viscosity at 80°C measured with a flow tester: 8.8 × 10⁻⁶ 4 Pa·s Resin B: Amorphous polyester resin / 500g mass molecular weight 1×10 4 ~1 × 10 5 Distribution of the range: 28% The main peak's molecular weight is 6.6 × 10⁻⁶. 3 Resin C: Amorphous polyester resin / 2500g mass molecular weight 1×10 5 The above distribution is 6% Gel content 18% (Coloring agent) Carbon Black (manufactured by Mitsubishi Chemical Corporation, product name: #44) / 500g (Release agent) Release agent A: Paraffin wax (melting point 90℃, manufactured by Nippon Seiro Co., Ltd., product name: Fischer-Tropschwax FNP0090) / 86g Release agent B: Polypropylene wax (melting point 140℃, manufactured by Mitsui Chemicals, Inc., product name: NP-505) / 57g (Static control agent) Potassium salt, bis[benzilat(2-)-k(2)O,O]borate(1-)potassium, solubility in water 4.382 g / L (20℃), manufactured by Nippon Carlit Co., Ltd., product name: Ion conductive material LR-147 / 50g

[0093] The raw materials for the toner particles described above were pre-mixed for 5 minutes at a rotation speed of 1500 rpm using a high-performance fluidized bed mixer (Henschel mixer, total capacity: 20L, manufactured by Mitsui Mining Co., Ltd. (now Nippon Coke Industries Co., Ltd.), model: FM20C).

[0094] The obtained mixture was melt-kneaded using a twin-screw extruder (manufactured by Ikegai Co., Ltd., model: PCM-30) under the following conditions: cylinder temperature set to 100°C, barrel rotation speed to 250 rpm, and raw material supply speed to 10 kg / hour, to obtain a molten kneaded product.

[0095] The resulting molten mixture was cooled and solidified using a cooling belt. The solidified material was then finely ground using a fluidized bed counter-jet mill (manufactured by Hosokawa Micron Corporation, model: Counter Jet Mill AFG), and then classified (particle size adjusted) using a rotary (centrifugal airflow) classifier (manufactured by Hosokawa Micron Corporation, model: TSP Separator) to produce toner particles with a volume average particle size of 6.3 μm.

[0096] The toner particles, along with strontium titanate (fine powder A or B prepared as described above) or titanium oxide (manufactured by Titanium Industry Co., Ltd., product name: STT-65ASC) and small particle size silica (product name: R976s, manufactured by Nippon Aerosil Co., Ltd.), were added to a Henschel mixer and stirred to obtain the toner after external additions. The amount of each external additive added per 100 parts by mass of toner particles is shown in Table 2 below. Here, "○" in the silica modification column of Table 2 indicates that fine powder A was used, and "×" indicates that fine powder B was used.

[0097] The physical properties of the polyester resin were measured using the method described below. (Softening point Tm (°C) of polyester resin) Using a flow property evaluation device (manufactured by Shimadzu Corporation, Flow Tester, model number: CFT-100C), while heating 1 g of the sample at a heating rate of 6 °C / min, a load of 20 kgf / cm 2 (9.8×10 5 Pa) is applied, and the sample is allowed to flow out from a die (nozzle diameter 1 mm, length 1 mm). The temperature at which half of the sample has flowed out is defined as the softening point (Tm).

[0098] (Viscosity of polyester resin: Pa·s) Measurement is carried out using the same measuring device and conditions as the above softening point, and the viscosity (Pa·s) is automatically calculated from the plunger sedimentation curve.

[0099] (Glass transition temperature Tg (°C) of polyester resin) Using a differential scanning calorimeter (manufactured by Seiko Instruments Inc. (currently Hitachi High-Tech Corporation), model number: DSC220), in accordance with Japanese Industrial Standard (JIS) K7121-1987, 1 g of the sample is heated at a heating rate of 10 °C / min to measure the DSC curve. In the obtained DSC curve, the temperature of the intersection of the straight line obtained by extending the baseline on the high-temperature side of the endothermic peak corresponding to the glass transition to the low-temperature side and the tangent line drawn at the point where the gradient becomes maximum with respect to the curve from the rising part to the apex of the peak is defined as the glass transition temperature (Tg).

[0100] (Molecular weight distribution of polyester resin) The polyester resin is dissolved in tetrahydrofuran (THF) to a concentration of 0.25% by mass, 200 μL of the sample is injected into a GPC device (manufactured by Tosoh Corporation, model: HLC-8220GPC), and the molecular weight distribution curve is obtained at a temperature of 40 °C. The molecular weight distribution is determined from the obtained molecular weight distribution curve. The molecular weight calibration curve is prepared using standard polystyrene.

[0101] (Gel content (%) of polyester resin) Approximately 3.0 g of polyester resin, which has been weighed in advance, is dissolved in 1000 mL of tetrahydrofuran (THF) at 45°C for 15 minutes. The insoluble portion is filtered off using an Omnipore membrane filter (Merck, product name: JAWP04700), and the remaining insoluble portion on the membrane filter is dried at 85°C for 10 hours. The mass of the resulting dried product is measured, and the percentage of THF-insoluble material is calculated as the percentage of gel.

[0102] (Melting point of release agent (wax): °C) Using a differential scanning calorimeter (PerkinElmer Japan Co., Ltd., Model: Diamond DSC), 0.01 g of wax was heated from 20°C to 200°C at a heating rate of 10°C / min, and then rapidly cooled from 200°C to 20°C. This process was repeated twice, and the DSC curve was measured. The temperature of the endothermic peak corresponding to melting in the DSC curve measured in the second operation was defined as the melting point (°C) of the wax.

[0103] (Volume-average particle size of toner particles: μm) 20 mg of the sample and 1 mL of alkyl ether sulfate sodium were added to 50 mL of electrolyte (Beckman Coulter, Inc., product name: ISOTON-II), and the mixture was dispersed for 3 minutes at a frequency of 20 kHz using an ultrasonic disperser (AS ONE Corporation, model: benchtop dual-frequency ultrasonic cleaner VS-D100) to prepare the sample for measurement. The obtained sample was measured using a particle size distribution analyzer (Beckman Coulter, Inc., model: Multisizer3) under conditions of aperture diameter: 100 μm and number of particles measured: 50,000 counts, and the volume-average particle size (μm) was determined from the volume particle size distribution of the sample particles.

[0104] <Preparation of two-component developer> The carrier and toner prepared as described above were weighed to achieve a toner concentration (T / D, where D is the mass of the developer and T is the mass of the toner) of 7%, placed in a cylindrical resin container, and then mixed and stirred at 200 rpm for 1 hour on a dual-axis driven poly bottle rotating stand to produce the two-component developers of the examples and comparative examples.

[0105] <Method for evaluating the initial characteristics of two-component developers> The two-component developer prepared as described above, along with the replenishment toner used in its preparation, were filled into a cartridge, and the initial characteristics were evaluated using a copier equipped with a two-component developer (print speed: color 50 ppm, monochrome 62 ppm, product name: MX-6201N, manufactured by Sharp Corporation).

[0106] (1) Evaluation methods for career advancement First, the cleaning field was set to 175V, and the machine was run idle under non-printing conditions (print rate 0%) to check for carrier adhesion. The number of carrier particles attached to a 2nm x A4 length tape sample was counted, and based on that number, the carrier rise as an initial characteristic was evaluated according to the following criteria.

[0107] ◎: Excellent (less than 10 items) ○: Good (Number of items is 10 or more but less than 20) △: Acceptable (Quantity is 20 or more but less than 40) ×: Not allowed (quantity of 40 or more)

[0108] (2) Method for evaluating changes in image density A patch for density measurement was prepared for an image with a print density of 5%, and the development bias was adjusted to select a bias that resulted in an initial density ID=1.4. The surface potential of the photoreceptor drum was set to match the development bias so that the cleaning field remained at 175V. Under these conditions, toner was continuously replenished according to the toner consumption, and 50 sheets of A4 paper were printed continuously while maintaining a toner density that did not change significantly. Based on the difference (ID difference) between the image density after this continuous printing and the initially set image density, the change in image density as an initial characteristic was evaluated according to the following criteria.

[0109] ◎: Excellent (ID difference less than 0.05) ○: Good (ID difference is between 0.05 and less than 0.10) △: Acceptable (ID difference is 0.10 or more but less than 0.20) ×: Not allowed (ID difference is 0.20 or more)

[0110] (3) Method for evaluating paint cover Using a whiteness meter (model: ZE6000, manufactured by Nippon Denshoku Industries Co., Ltd.), the whiteness of the non-image-forming area was measured after printing 50 sheets. The initial characteristic of fogging was evaluated based on the difference between this measurement and the whiteness of the paper before printing, which had been measured in advance, according to the following criteria.

[0111] ◎: Excellent (Difference in whiteness is less than 1.0) ○: Good (Difference in whiteness is between 1.0 and less than 2.0) △: Acceptable (Difference in whiteness is 2.0 or greater but less than 3.0) ×: Not allowed (difference in whiteness is 3.0 or greater)

[0112] (4) Comprehensive evaluation method for the initial characteristics of two-component developers Based on the evaluation results of the three evaluation items above (carrier rise, image density change, and haze), an overall evaluation was performed according to the following criteria.

[0113] ◎: Excellent (All three evaluation criteria are marked with ◎.) ○: Good (The lowest rating among the three evaluation items is ○.) △: Acceptable (△ is the lowest rating among the three evaluation items.) ×: Fail (The lowest rating among the three evaluation items is ×.)

[0114] <Method for measuring the resistance characteristics of initial carriers> The bridge resistance of the target carrier was measured. The bridge resistance was measured using a measuring jig 90 as shown in Figure 6. The detailed configuration of the measuring jig 90 is as follows: The electrodes 92 are parallel flat plates measuring 10 mm × 40 mm, and two of them are arranged on a substrate 93. The distance between these two electrodes 92 is 2 mm. The magnets 91 arranged on the electrodes 92 are positioned so that the north pole and south pole face each other. The surface magnetic flux density of the magnets 91 is 1500 gauss, and the magnetic area of ​​the opposing parts is 10 mm × 30 mm.

[0115] First, 200 mg of the carrier to be measured was inserted between electrodes 92. While holding this carrier between electrodes 92, the current value was measured when a DC voltage was applied to electrodes 92 in 1V steps up to 1000V. Furthermore, the resistance value was calculated based on this current value and defined as the bridge resistance value.

[0116] <Preparation of two-component developer after long-term use> Using the two-component developer and toner used in the initial characteristic evaluation described above, a two-component developer after long-term use was prepared. Similar to the initial characteristic evaluation, 50 images with a print density of 5% were continuously printed on A4 paper, and then the printing was repeated with a few seconds' interval (intermittent printing). This resulted in 400,000 prints, and a two-component developer after long-term use was prepared.

[0117] Similar to the evaluation of initial characteristics, the prepared two-component developer (after printing 400,000 sheets) was set in a copier equipped with a two-component developer to evaluate the developer. In addition, a very small sample of the prepared two-component developer (after printing 400,000 sheets) was taken, and the carrier was separated from it to be used as an analysis sample for the carrier.

[0118] <Evaluation method for two-component developers after long-term use> (1) Evaluation methods for career advancement Except for measuring the carrier separated from the two-component developer after printing 400,000 sheets as described above, the carrier rise was evaluated in the same manner as the initial characteristics evaluation. The evaluation criteria were also the same.

[0119] (2) Method for evaluating fog after being left in a normal temperature and low humidity environment After leaving the film in a two-component developer overnight in a normal temperature, low humidity environment (temperature 25°C, relative humidity 5%), printing was performed, and the fogging after the initial print was evaluated. Next, a total of 5,000 prints were made using intermittent printing, similar to the 400,000 print run described above, and the fogging after continuous printing was evaluated. The evaluation criteria for fogging were the same as those for the initial characteristics.

[0120] By comparing the evaluation of fogging immediately after standing with the evaluation of fogging after continuous printing following standing, it is possible to determine whether or not a two-component developer can solve the problem of good fogging immediately after standing deteriorating with continued development (deterioration of the developer's responsiveness). Therefore, the difference δ between the evaluation of fogging immediately after standing and the evaluation of fogging after continuous printing following standing was evaluated according to the following criteria.

[0121] ◎: Excellent (No change in evaluation.) ○: Good (The rating is one level different.) △: Acceptable (The rating differs by two levels.) ×: Not acceptable (The rating differs by three levels.)

[0122] (3) Method for evaluating fog after being left in a normal temperature and high humidity environment After leaving the film in a high-humidity environment (temperature 25°C, relative humidity 80%) with a two-component developer overnight, printing was performed, and the fogging after the initial print was evaluated. Next, a total of 5,000 prints were made using intermittent printing, similar to the 400,000 print run described above, and the fogging after continuous printing was evaluated. The evaluation criteria for fogging were the same as those for fogging after leaving the film in a normal temperature, low-humidity environment.

[0123] <Method for measuring the amount of titanium oxide transferred> The amount of titanium oxide transferred, which is the ratio of the amount of titanium oxide transferred to the carrier surface after printing 400,000 pages to the amount of titanium oxide contained in the toner before printing 400,000 pages, was measured by the following procedure (i) to (iii).

[0124] (i) Toner in a two-component developer after printing 400,000 sheets was separated from the carrier by sieving. A sieve with an opening of approximately 16 μm was used, which allows only toner to pass through and not the carrier. The carrier for measurement was obtained by repeatedly aspirating the toner until there was no change in mass.

[0125] (ii) As measurement samples, disk samples were prepared so as to fit into a circular sample holder. For the carrier, a 2cm x 2cm square tape was attached to the center of a resin disk, and approximately 10mg of carrier was uniformly dispersed on the tape, with the excess blown off to prepare the measurement sample. For the toner, 1g of toner was uniformly placed in a circular ring of approximately 3cm and compressed at a pressure of approximately 30MPa to prepare the measurement sample.

[0126] (iii) The X-ray intensity of each sample was measured using an X-ray fluorescence analyzer (model: ZSX Primus II, manufactured by Rigaku Corporation).

[0127] Using these measurement results, the amount (transfer rate) of titanium oxide was calculated using the X-ray intensity ratio based on the following formula. By converting this to a value per 1000 sheets, the transfer amount I (% / k sheets) per 1000 printed sheets was calculated. X-ray intensity ratio: {KC (after long-term use) - KC (initial)} / KT

[0128] Here, KC is the X-ray intensity per gram of carrier, obtained by dividing the X-ray intensity of Ti obtained by measuring the carrier by the amount of carrier used in the measurement. In the above formula, KC(initial) represents the X-ray intensity of the carrier separated from the two-component developer in the initial stages of use, and KC(after long-term use) represents the X-ray intensity of the carrier separated from the two-component developer after 400,000 prints. Also, KT in the above formula represents the X-ray intensity of Ti in the toner, that is, the X-ray intensity of Ti obtained by performing the above measurement on the toner.

[0129] [Examples 1-10, Comparative Examples 1-5] Table 1 below shows the mass ratio of the inner layer to the outer layer, and the amount of coating liquid added per 100 parts by mass of carrier core material in the examples and comparative examples. Comparative Example 1 is an example in which no conductive material is added to the outer layer of the two-layer coating, in other words, there is no conductive layer. Comparative Example 2 is an example in which conductive material is added to both the inner and outer layers of the two-layer coating, in other words, there is no non-conductive layer.

[0130] Table 2 below shows the mass ratio of the non-conductive layer (in the example) to the conductive layer (outer layer in the example), and the amount of external additive added per 100 parts by mass of toner particles. Examples 5', 6', and 7' are examples in which the fine powder of strontium titanate with added silica in Examples 5, 6, and 7 is changed to one without added silica. Example 10 is an example in which titanium oxide is used as the titanium oxide external additive.

[0131] By using the additive quantities shown in Tables 1 and 2 in the carrier and toner preparation procedures described above, two-component developers for Examples 1 to 10 and Comparative Examples 1 to 5 were obtained.

[0132] [Table 1]

[0133] [Table 2]

[0134] [Table 3]

[0135] [Table 4]

[0136] Tables 3 and 4 show the evaluation results for the examples and comparative examples. Table 3 shows the evaluation results for the initial characteristics of the two-component developer and carrier. Table 4 shows the evaluation results for the characteristics (life characteristics) of the two-component developer after 400,000 prints. The life characteristic evaluation shown in Table 4 was performed on Examples 1 to 10 and Comparative Example 4, which satisfy the following two conditions: (i) the inner layer has lower conductivity than the outer layer (the inner layer corresponds to the non-conductive layer in Table 2, and the outer layer corresponds to the conductive layer in Table 2), and (ii) the mass ratio of the inner layer to the outer layer is between 1.2 and 11. Figure 7 is a graph of the resistance characteristics of the carrier at the initial stage of use, as shown in Table 4.

[0137] As is clear from the evaluation results shown in Table 3, the two-component developer of Examples 1 to 10 contained a carrier comprising a carrier core material and a resin coating layer covering the carrier core material, and toner with an external additive attached to the surface of toner particles, wherein the resin coating layer is composed of two or more layers including an outer layer and an inner layer located closer to the carrier core material than the outer layer, the inner layer has lower conductivity than the outer layer, the mass ratio of the inner layer to the outer layer is 1.2 or more and 11 or less, and the external additive contains titanium oxide. The two-component developer of Examples 1 to 10 was able to suppress carrier rise and had excellent stability of image density. It also performed well in evaluating fogging in the initial stages of use.

[0138] In contrast, Comparative Examples 1-5, which did not meet these requirements, performed worse than the examples in terms of carrier rise or image density change evaluation.

[0139] Regarding the resistance characteristics of the carriers measured by the bridge resistance measurement method, it can be seen that the two-component developers of Examples 1 to 10, which have carriers with a voltage of 25V to 300V at which a current of 0.1μA occurs, effectively prevent carrier rise. In particular, it can be seen that the two-component developers of Examples 3 to 10, which have carriers with a voltage of 95V to 280V at which a current of 0.1μA occurs, suppress carrier rise even more effectively. Furthermore, it can be seen that the two-component developers of Examples 1 to 10, which have carriers with a voltage of 250V to 900V at which a current of 10μA occurs, exhibit less change in image density. According to the overall evaluation of the initial characteristics of the two-component developers, it is more preferable that the voltage at which a current of 10μA occurs is between 400V and 850V.

[0140] Furthermore, according to the evaluation results shown in Table 4, for two-component developers after long-term use (after 400,000 prints), the two-component developers of Examples 1 to 10, in which the amount of titanium oxide transferred to the carrier surface was 0.02% to 2% per 1,000 prints, were able to suppress the occurrence of fogging in prints immediately after being left in low-humidity and high-humidity environments, the occurrence of fogging in subsequent continuous printing, and the change in fogging between the two, resulting in less change in image quality. In addition, the example in which the amount of titanium oxide transferred was 0.08% to 1.45% per 1,000 prints showed superior evaluation of fogging after long-term use, and the example in which it was 0.17% to 0.82% showed even greater superior evaluation of fogging after long-term use.

[0141] Comparing Example 7, in which 0.06 parts by mass of strontium titanate was added as the titanium oxide, with Example 10, in which 0.06 parts by mass of titanium oxide was added as the titanium oxide, it can be seen that in Example 7, where the titanium oxide is strontium titanate, the amount transferred to the carrier surface is more appropriate, and the variation in fogging in low humidity environments can be further suppressed.

[0142] Comparing Examples 5-7, in which strontium titanate is silica-modified, with Examples 5'-7', which are not silica-modified, it can be seen that using silica-modified strontium titanate further suppresses fogging in high-humidity environments, even when the amount of strontium titanate transferred to the carrier surface is large.

[0143] Examples 5-9, in which the strontium titanate content was varied, show that in Examples 6-7, where the content was 0.1 parts by mass to 0.8 parts by mass per 100 parts by mass of toner particles, the amount of strontium titanate transferred to the carrier surface could be kept within an appropriate range, resulting in a two-component developer with minimal change in image quality in both low-humidity and high-humidity environments.

[0144] The embodiments disclosed herein are illustrative in all respects and are not intended to be restrictive. Therefore, the technical scope of this disclosure is not construed solely by the embodiments described above, but is defined by the claims. This includes all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0145] C Career C' Adjacent carriers T Toner 1. Carrier core material 2. Inner layer (non-conductive layer) 3 Outer layer (conductive layer) 4 Titanium oxide A. Charge source (developed charge, other carriers)

Claims

1. A two-component developer comprising a carrier having a carrier core material and a resin coating layer covering the carrier core material, and toner having an external additive attached to the surface of toner particles, The resin coating layer is composed of two or more layers, including an outer layer and an inner layer located closer to the carrier core material than the outer layer. The inner layer has lower conductivity than the outer layer. The mass ratio of the inner layer to the outer layer is 1.2 or more and 11 or less. The aforementioned external additive contains titanium oxide, The resistance characteristics of the carrier measured by the bridge resistance measurement method are such that the voltage at which a current of 0.1 μA is obtained is between 70 V and 162 V, and the voltage at which a current of 10 μA is between 499 V and 900 V. The titanium oxide contained in the aforementioned external additive is a fine powder of strontium titanate modified with silica. A two-component developer characterized in that the content of the fine powder is 0.1 parts by mass or more and 0.6 parts by mass or less per 100 parts by mass of the toner particles.

2. A two-component developer according to claim 1, The amount of titanium oxide transferred, which represents the ratio of the amount of titanium oxide transferred to the carrier surface by printing to the amount of titanium oxide contained in the toner, is characterized in that when the two-component developer is set in a copier having a two-component developing device and 400,000 sheets of A4 paper are printed at a print density of 5%, the amount of titanium oxide transferred is 0.02% or more and 2% or less per 1,000 sheets.

3. A two-component developer according to claim 1 or claim 2, A two-component developer characterized in that the amount of titanium oxide transferred to the carrier surface by printing, which represents the ratio of the amount of titanium oxide transferred to the carrier surface by printing to the amount of titanium oxide contained in the toner, is 0.08% or more and 1.45% or less per 1,000 sheets when the two-component developer is set in a copier having a two-component developing device and 400,000 sheets are printed on A4 paper at a print density of 5%.

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