Two-component developer and image forming apparatus

Abstract

To provide a two-component developer and an image forming apparatus that offer excellent developability while suppressing carrier rise and fogging. [Solution] A two-component developer comprising a carrier having a resin coating layer on the surface of a carrier core material and a toner having toner particles has the following configuration: The resin coating layer includes an outer layer and an inner layer having a higher electrical resistance than the outer layer. The outer layer and the inner layer contain a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights, and the weight-average molecular weights of the two or more silicone resins satisfy specific conditions. In the carrier, the content of the inner layer per 100 parts by mass of the carrier core material is 0.2 parts by mass or more, the content of the outer layer per 100 parts by mass of the carrier core material is 0.1 parts by mass or more, and the total content of both layers per 100 parts by mass of the carrier core material is 1.0 part by mass or less. The toner contains a magnetic material in the toner particles.

Description

[Technical Field]

[0001] This disclosure relates to a two-component developer and an image forming apparatus. [Background technology]

[0002] A two-component developer is known for use in image forming devices such as photocopiers, multifunction printers, printers, and facsimile machines that utilize the electrophotographic method. This developer includes a toner having toner particles (toner core) and a carrier having a resin coating layer on the surface of a carrier core material.

[0003] For example, Patent Document 1 discloses a two-component developer comprising toner with an external additive attached to the surface of toner particles and a carrier having a resin coating layer on the surface of a carrier core material. In this two-component developer, the resin coating layer contains a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights, and the weight-average molecular weight and the volume resistivity of the two-component developer are configured to satisfy specific conditions. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2010-169836 [Overview of the project] [Problems that the invention aims to solve]

[0005] As printing continues in an image forming apparatus, the carrier in the two-component developer deteriorates due to toner components adhering to the carrier surface, leading to a decrease in the carrier's charging capacity. If this trend continues, fogging and toner scattering will become more likely as the usage time of the two-component developer increases. Therefore, to compensate for the decrease in the carrier's charging capacity, a design that increases the charging capacity inside the carrier can be considered. The inside of the carrier includes the carrier core material and the inside of the resin coating layer of the carrier (for example, the inner layer if the resin coating layer is a two-layer coating with an inner and outer layer). As the use of the two-component developer continues, the number of carriers with highly charged interiors exposed on the surface increases, thus compensating for the decrease in the carrier's charging capacity.

[0006] This design maintains the carrier's charging ability even in the latter half of the two-component developer's lifespan. However, it also presents a problem: in low-humidity environments, the toner's charge increases, leading to poor developability. Some image forming machines are equipped with a control mechanism that, upon detecting poor developability (i.e., a decrease in the formed image density), increases the toner concentration of the two-component developer in the developing tank to reduce the toner's charge and ensure developability. While this control mechanism ensures developability even in low-humidity environments, it makes handling due to electrostatic force difficult, leading to increased carrier buildup and fogging. Carrier buildup refers to the phenomenon where carriers adhere to the image carrier surface during development due to the electrostatic force between the electric field on the image carrier surface and the carriers. Fogging refers to the phenomenon where toner is developed in non-image areas where it should not be developed.

[0007] The contents of this disclosure were discovered in view of the above circumstances, and the main purpose is to provide a two-component developer and an image forming apparatus that have excellent developability while suppressing carrier rise and fogging. [Means for solving the problem]

[0008] To solve the above problems, the two-component developer of this disclosure is provided. A two-component developer comprising a carrier having a resin coating layer on the surface of a carrier core material and toner having toner particles, The resin coating layer includes an outer layer and an inner layer located closer to the carrier core material side than the outer layer and having a higher electrical resistance than the outer layer, The outer layer and the inner layer include a resin obtained by crosslinking and curing two or more types of silicone resins having different weight average molecular weights, Among the two or more types of silicone resins before crosslinking and curing, when the weight average molecular weight of the silicone resin having the smallest weight average molecular weight is Ma and the weight average molecular weight of the silicone resin having the largest weight average molecular weight is Mb, Ma and Mb satisfy the following formula (1), 3.1≧log(Mb / Ma)≧1.9 ···(1) In the carrier, the content of the inner layer with respect to 100 parts by mass of the carrier core material is 0.2 parts by mass or more, the content of the outer layer with respect to 100 parts by mass of the carrier core material is 0.1 parts by mass or more, and the total content of the two layers with respect to 100 parts by mass of the carrier core material is 1.0 parts by mass or less, The toner is characterized in that a magnetic material is contained in the toner particles.

[0009] In the above two-component developer, the saturation magnetization of the toner is 0.4 Am

[0011] , , , , 2 , 2 , 2 ,

[0010] , 2 , , , , ,

[0012] / kg or more and 3.35 Am 2 / kg or less is preferable.

[0010] Also, in the above two-component developer, the content of the magnetic material in the toner particles is preferably 0.7% by mass or more and 4% by mass or less.

[0011] Also, in the above two-component developer, the magnetic material in the toner particles is a soft magnetic material, and the saturation magnetization of the magnetic material is 55 Am 2 / kg or more and 84 Am 2 / kg or less is preferable.

[0012] In addition, in the above two-component developer, when the carrier core material and the resin coating layer are charged under the same conditions, it is preferable that the charge amount of the carrier core material is higher than the charge amount of the resin coating layer.

[0013] In order to solve the above problems, an image forming apparatus of the present disclosure is an image forming apparatus having a control mechanism for increasing the toner concentration of a two-component developer in a developing tank in order to correct the image density when the formed image density is low, and is characterized by using the above two-component developer.

Advantages of the Invention

[0014] According to the two-component developer and the image forming apparatus of the present disclosure, excellent effects such as excellent developability and suppression of carrier rise and fog generation can be achieved.

Brief Description of the Drawings

[0015] [Figure 1] It is a cross-sectional view schematically showing the configuration of a carrier included in the two-component developer according to the present embodiment. [Figure 2] It is a cross-sectional view schematically showing the configuration of a carrier as a reference example. [Figure 3] It is a cross-sectional view schematically showing the configuration of a carrier as a reference example. [Figure 4] It is a cross-sectional view schematically showing the configuration of a carrier as a reference example. [Figure 5] It is a cross-sectional view schematically showing a measuring jig used for measuring the V-I characteristics of a carrier.

Embodiments for Carrying Out the Invention

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

[0017] 1. Two-Component Developer The two-component developer according to the present embodiment is a two-component developer including a carrier having a resin coating layer on the surface of a carrier core material and toner having toner particles, and has a configuration that satisfies the following requirements (A) to (E). (A) The resin coating layer includes an outer layer and an inner layer located closer to the carrier core material than the outer layer and having a higher electrical resistance than the outer layer. (B) The outer layer and the inner layer contain a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights. (C) Of the two or more silicone resins before crosslinking and curing, let Ma be the weight-average molecular weight of the silicone resin with the smallest weight-average molecular weight, and let Mb be the weight-average molecular weight of the silicone resin with the largest weight-average molecular weight. Then Ma and Mb satisfy the following formula (1). 3.1≧log(Mb / Ma)≧1.9 (1) (D) In ​​the carrier, the content of the inner layer per 100 parts by mass of the carrier core material is 0.2 parts by mass or more, the content of the outer layer per 100 parts by mass of the carrier core material is 0.1 parts by mass or more, and the total content of both layers per 100 parts by mass of the carrier core material is 1.0 part by mass or less. (E) The toner contains a magnetic material in the toner particles.

[0018] With this configuration, when manufacturing the carrier in this two-component developer, a mixed solution of solutions with different viscosities is used as the material solution (silicone resin solution) for the resin coating layer. Therefore, a uniform coating film can be formed with a small amount of coating material. Furthermore, by forming the resin coating layer with a two-layer coating of an inner and outer layer, it is possible to create a carrier that suppresses carrier uplift without excessively increasing the overall electrical resistance of the carrier. In addition, because the amount of coating material is small, the resin coating layer is formed thinly, so the distance between the carrier core material and the magnetic material in the toner particles is reduced, improving the magnetic interaction between the carrier core material and the magnetic material. This improved magnetic interaction improves the handling of toner compared to when the carrier holds the toner by electrostatic force alone. This improved handling of toner can suppress the occurrence of fogging, especially when using the two-component developer at high toner concentrations.

[0019] In other words, the two-component developer according to this embodiment can suppress the occurrence of carrier rise over a long period of time, thereby extending its service life. Furthermore, even when the usage environment changes, image defects such as fogging are minimized, and high-quality images can be stably formed.

[0020] The mixing ratio of carrier and toner in the two-component developer according to this embodiment is not particularly limited, but considering its use in a high-speed image forming apparatus capable of printing 40 or more A4-sized images per minute, it is preferable that the ratio of the volume-average particle diameter of the carrier to the volume-average particle diameter of the toner is 5 or more, and that the coverage θ is approximately 40% to 80%. This ensures that good contact between the toner and carrier is stably maintained, and that high-quality images can be stably formed over a long period of time even in a high-speed image forming apparatus. In this disclosure, coverage θ means the ratio of the total projected area of ​​the toner (sum of the projected areas of all toner particles) to the total surface area of ​​the carrier (sum of the surface areas of all carriers), and is calculated by the following formula. Coverage θ[%] = (Total projected area of ​​toner / Total surface area of ​​carrier) × 100

[0021] In low-humidity environments, the toner's charge level increases, resulting in insufficient developability. In such cases, it is possible to increase the toner-to-carrier ratio to further increase coverage θ. By increasing the toner concentration of the two-component developer, the charge level per toner particle can be reduced, ensuring developability. On the other hand, since more toner particles are held per carrier, the handling of the toner deteriorates. The opportunities for contact between toner and carrier also decrease, and low-charge toner particles are generated, making it difficult to handle the toner using electrostatic force alone.

[0022] Regarding the relationship between coverage θ and toner concentration, specifically, if the volume-average particle size of the toner is 6.5 μm and the volume-average particle size of the carrier is 40 μm, setting the coverage θ to 50% to 75% means that the two-component developer contains approximately 6.9 to 10.4 parts by mass of toner per 100 parts by mass of carrier. When high-speed development is performed with such a two-component developer, both the toner consumption and the toner supply amount to the developing tank of the developing device in accordance with toner consumption are maximized, and even then, the supply-demand balance is not disrupted. If the amount of toner in the two-component developer is greater than 10.4 parts by mass per 100 parts by mass of carrier, the charge tends to be lower, and the desired development characteristics may not be obtained. Also, if the toner consumption exceeds the toner supply amount, the toner may not be sufficiently charged, leading to a deterioration in image quality. Conversely, if the amount of toner is less than 6.9 parts by mass, the charge tends to be higher, making it difficult for the toner to separate from the carrier by the electric field, which may ultimately lead to a deterioration in image quality.

[0023] In this embodiment, the total projected area of ​​the toner is calculated as follows. The specific gravity of the toner is assumed to be 1.0, and the calculation is based on the volume-average particle size obtained with a Coulter counter (product name: Coulter Counter Multisizer II, manufactured by Beckman Coulter). That is, the number of toner particles in the weight of toner to be mixed is calculated, and the total projected area of ​​the toner is calculated as the number of toner particles × the toner area (calculated assuming a circle). Similarly, the sum of the surface areas of the carriers is calculated from the weight of carriers to be mixed, based on the particle size obtained with a Microtrac (product name: MT3000, manufactured by Nikkiso Co., Ltd.), with the carrier specific gravity being 4.7.

[0024] 2. Career Figure 1 is a schematic cross-sectional view showing the configuration of a carrier contained in a two-component developer according to this embodiment. The carrier 10 shown in Figure 1 comprises a resin coating layer 12 arranged in the order of an inner layer 12a and an outer layer 12b on the surface of a carrier core material 11.

[0025] In this embodiment, it is preferable that the carrier core material has a higher charge level than the resin coating layer when the carrier core material and the resin coating layer are charged under the same conditions. As the usage time of the two-component developer increases, the carrier core material is exposed to the surface of the carrier, which can suppress a decrease in the carrier's charging ability. Consequently, it becomes easier to maintain the handling properties of the toner. Such a carrier can be manufactured using the carrier core material and resin coating layer materials exemplified later.

[0026] The volume-average particle size of the carrier is preferably 20 μm to 100 μm, and more preferably 35 μm to 55 μm. If the volume-average particle size of the carrier core material is less than 35 μm, carrier uplift may increase. If the volume-average particle size of the carrier core material exceeds 55 μm, images with poor dot reproducibility may be produced.

[0027] 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 particles) are preferred. Ferrite particles have high saturation magnetization and can produce carriers with low density. Therefore, carrier rise is less likely to occur, and images with high dot reproducibility due to soft dot formation can be obtained. Known ferrite particles can be used, such as zinc ferrite, nickel ferrite, copper ferrite, nickel-zinc ferrite, manganese-magnesium ferrite, copper-magnesium ferrite, manganese-zinc ferrite, and manganese-copper-zinc ferrite.

[0028] The resin coating layer can be formed by coating the surface of the carrier core material with a resin coating layer composition. The resin coating layer composition contains a silicone resin and may optionally contain additives such as a conductive material, a silane coupling agent, a resin other than silicone resin, or a bifunctional silicone oil. These additives may be used individually or in combination of two or more.

[0029] The inclusion of silicone resin in the resin coating layer 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 its adhesion to the carrier core material can be improved, allowing the toner to be stably charged over a long period of time.

[0030] A cross-linked silicone resin is more preferable as the silicone resin used to form the resin coating layer. The inclusion of a cross-linked silicone resin 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 its adhesion to the carrier core material can be improved.

[0031] Crosslinked silicone resins are silicone resins that harden through crosslinking of hydroxyl groups bonded to Si atoms, or between hydroxyl groups and -OX groups, by heat dehydration reactions, room-temperature curing reactions, 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.

[0032] [ka]

[0033] 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 room-temperature curing silicone resin, but it may be heated to 150°C to 280°C to shorten the curing time.

[0034] 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 weight-average molecular weight of the cross-linked silicone resin should be selected to satisfy the above formula (1).

[0035] Commercially available cross-linked silicone resins can be used, such as SR2400, SR2410, SR2411, SR2510, SR2405, 840RESIN, 804RESIN (all product names, manufactured by Dow Toray Industries, Inc.), KR271, KR272, KR274, KR216, KR280, KR282, KR261, KR260, KR255, KR266, KR251, KR155, KR152, KR214, KR220, X-4040-171, KR201, KR5202, KR3093, KR240, KR350, KR400 (all product names, manufactured by Shin-Etsu Chemical Co., Ltd.), and TSR127B (product name, manufactured by Momentive). For example, it is advisable to select at least two types of resins from this group such that the above formula (1) is satisfied.

[0036] One form of the resin coating layer composition is a solution obtained by dissolving or dispersing the materials of the resin coating layer composition in an organic solvent. The organic solvent is not particularly limited as long as it can dissolve the silicone resin, and examples include aromatic hydrocarbons such as toluene and xylene, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran and dioxane, and higher alcohols. These may be used individually or as a mixed solvent combining two or more. By using this solution form of the resin coating layer composition (hereinafter also referred to as the coating resin solution), a resin coating layer can be easily formed on the surface of the carrier core material.

[0037] One method for forming a resin coating layer using a coating resin solution is to apply the coating resin solution to the surface of a carrier core material to form a coating layer, remove the organic solvent from the coating layer by heating, and then heat-cur or simply cure the coating layer during or after drying to form the resin coating layer.

[0038] Methods for applying the coating resin solution to the surface of the carrier core material include, for example, an immersion method in which the carrier core material is impregnated with the coating resin solution, a spray method in which the coating resin solution is sprayed onto the carrier core material, and a fluidized bed method in which the coating resin solution is sprayed onto the carrier core material which is suspended by a flowing air current. Among these, the immersion method is preferred because it allows for the easy formation of a resin coating layer.

[0039] The curing of the coating layer is performed by selecting the heating temperature according to the type of cross-linked silicone resin. For example, heating temperatures of around 150°C to 280°C are used. If the cross-linked silicone resin is a type that cures at room temperature, heating is not essential, but it may be done to improve the mechanical strength of the formed resin coating layer, shorten the curing time, etc.

[0040] The resin coating layer according to this embodiment, i.e., the inner layer and outer layer, contains a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights. If Ma is the weight-average molecular weight of the silicone resin with the smallest weight-average molecular weight among the two or more silicone resins before crosslinking and curing, and Mb is the weight-average molecular weight of the silicone resin with the largest weight-average molecular weight, then Ma and Mb satisfy the following formula (1). It is more preferable that log(Mb / Ma) is between 2.0 and 3.0. 3.1≧log(Mb / Ma)≧1.9 (1)

[0041] When Ma and Mb satisfy equation (1) above, a resin coating layer that uniformly covers the surface of the carrier core material can be formed. That is, a structure like the carrier 10 shown in Figure 1 can be realized. The reason for this is presumed to be as follows.

[0042] As shown in Figure 1, the carrier core material 11 has a structure in which crystal grains (microcrystals) 11a are aggregated, or in other words, it has a structure with crystal grain boundaries as boundaries between crystal grains 11a. The surface of such a carrier core material 11 has many irregularities. When Ma and Mb satisfy the above formula (1), the coating resin solution becomes a mixed solution of a high viscosity solution and a low viscosity solution. The low viscosity solution adheres along the irregularities on the surface of the carrier core material 11, and the high viscosity solution adheres to cover the depressions on the surface of the carrier core material 11, making it possible to uniformly adhere the coating resin solution to both the depressions and protrusions on the surface of the carrier core material 11. Consequently, a resin coating layer 12 that uniformly covers the surface of the carrier core material 11 can be formed.

[0043] By adopting a structure like carrier 10, it is possible to realize a carrier with high resistance while keeping the resin coating layer thin. This strengthens the magnetic interaction with the magnetic material contained in the toner particles, improving the handling of the toner. The carrier handles the toner by electrostatic and magnetic forces, but since the magnetic force does not weaken even in environments with high toner concentrations where the electrostatic force weakens, it is possible to suppress the occurrence of fogging even when the toner concentration of the two-component developer increases.

[0044] In contrast, Figures 2 to 4 are schematic cross-sectional views showing the configuration of a carrier as a reference example. The reference example in Figure 2 is an example where the silicone resin in the coating resin solution is only a silicone resin with a large weight-average molecular weight. The carrier 110 shown in Figure 2 has a resin coating layer 112 composed of an inner layer 112a and an outer layer 112b on the surface of a carrier core material 111 made up of crystal grains 111a. However, the inner layer 112a does not uniformly cover the carrier core material 111, and the outer layer 112b does not uniformly cover both the carrier core material 111 and the inner layer 112a. The reason for this is that when the silicone resin in the coating resin solution is only a silicone resin with a large weight-average molecular weight, it is difficult to adhere the coating resin solution along the irregularities on the surface of the carrier core material 111. In addition, to cover the entire carrier core material 111, it is necessary to use a large amount of resin, making it difficult to form a thin resin coating layer 112.

[0045] The reference example in Figure 3 is an example where the silicone resin in the coating resin solution consists only of silicone resin with a low weight-average molecular weight. The carrier 210 shown in Figure 3 has a resin coating layer 212 composed of an inner layer 212a and an outer layer 212b on the surface of a carrier core material 211 made up of aggregated crystal grains 211a. However, the resin coating layer 212 does not completely cover the entire carrier core material 211, and the carrier core material 211 is exposed on the surface of the carrier 210. The reason for this is that when the silicone resin in the coating resin solution consists only of silicone resin with a low weight-average molecular weight, the coating resin solution accumulates in depressions on the surface of the carrier core material 211 when the coating resin solution dries. In addition, a large amount of coating resin solution is required to cover the entire carrier core material 211, making it difficult to form a thin resin coating layer 212.

[0046] The reference example in Figure 4 shows an example where the resin coating layer is a single layer. The carrier 310 shown in Figure 4 has a single-layer resin coating layer 312 on the surface of a carrier core material 311 made up of crystal grains 311a. However, when using a single-layer coating, it is necessary to make the resin coating layer thicker and to lower the content of conductive material in the resin coating layer. Therefore, compared to the carrier 10 shown in Figure 1, the degree of freedom in controlling the carrier's characteristics is lower.

[0047] For the reasons stated above, in this embodiment, the inner and outer layers of the resin coating layer contain a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights, and the two or more silicone resins satisfy formula (1) above.

[0048] In the resin coating layer according to this embodiment, it is preferable that the resin components of the inner layer and the outer layer are the same. This makes it difficult for the interface between the inner and outer layers to peel off, and improves the adhesion between the two layers.

[0049] In the carrier according to this embodiment, the content of the inner layer per 100 parts by mass of the carrier core material is 0.2 parts by mass or more, the content of the outer layer per 100 parts by mass of the carrier core material is 0.1 parts by mass or more, and the total content of the inner and outer layers per 100 parts by mass of the carrier core material is 1.0 part by mass or less. Preferably, the content of the inner layer per 100 parts by mass of the carrier core material is 0.3 parts by mass or more and 0.5 parts by mass or less, the content of the outer layer per 100 parts by mass of the carrier core material is 0.2 parts by mass or more and 0.4 parts by mass or less, and the total content of the inner and outer layers per 100 parts by mass of the carrier core material is 0.7 parts by mass or more and 1.0 part by mass or less.

[0050] In the carrier according to this embodiment, by satisfying the conditions of formula (1) above, a coating film that becomes a uniform resin coating layer can be formed with a small amount of coating. That is, the inner layer and outer layer can be formed so that they are within the above content range. By keeping the content of the inner and outer layers below the above upper limit (i.e., both layers are formed thinly), the distance between the carrier core material and the magnetic material in the toner particles is reduced, and the magnetic interaction between the carrier core material and the magnetic material is improved. The improved magnetic interaction improves the handling of the toner compared to when the carrier holds the toner by electrostatic force alone. Consequently, it is possible to suppress the occurrence of fogging, especially when using a two-component developer at high toner concentrations. Keeping the content of the inner and outer layers above the above lower limit prevents variations in the performance of the carrier, and the effects of this disclosure are more reliably achieved.

[0051] In the two-component developer of this embodiment, the inner layer is configured to have a higher electrical resistance than the outer layer. Therefore, it is preferable that the outer layer contains a conductive material. If both layers contain the same conductive material, it is preferable that the conductive material content is higher in the outer layer than in the inner layer. By including a conductive material in the resin coating layer, the ability to impart charge to the carrier toner is enhanced, and the toner can be charged more stably over a long period of time.

[0052] Examples of conductive materials include conductive carbon black and metal oxides. Examples of metal oxides include conductive titanium oxide and tin oxide. Carbon black is suitable for achieving conductivity with a small amount of additive. These conductive materials may be used individually or in combination of two or more.

[0053] The volume-average particle size of the conductive material is not particularly limited, but is preferably 0.02 μm or more and 2 μm or less, and more preferably 0.02 μm or more and 1 μm or less. The volume-average particle size of the conductive material is measured using a laser diffraction / scattering type particle size analyzer (for example, product name: LA-920 manufactured by Horiba, Ltd.).

[0054] The content of the conductive material in the resin coating layer is preferably 30 parts by mass or less per 100 parts by mass of silicone resin, and more preferably 1 part by mass or more and 30 parts by mass or less. By keeping the content of the conductive material below the above upper limit, the detachment of the conductive material from the resin coating layer can be prevented, thereby suppressing its impact on the image. Furthermore, the mechanical strength of the resin coating layer and its adhesion to the carrier core material can be improved, allowing the toner to be charged stably over a long period of time. Consequently, high-quality images can be formed more stably. In addition, by keeping the content of the conductive material above the above lower limit, the effect of adding the conductive material can be more reliably realized.

[0055] The content of conductive material in the outer layer is preferably 2% to 20% by mass relative to the silicone resin contained in the outer layer. If the content of conductive material exceeds the above upper limit, the conductive material may detach from the outer layer, potentially affecting the image. If the content of conductive material is below the above lower limit, the effect of adding the conductive material may not be observed.

[0056] The resin coating layer may contain a silane coupling agent to facilitate adjustment of the toner's charge level. Among silane coupling agents, those having electron-donating functional groups are preferred. Examples of such silane coupling agents include the amino group-containing silane coupling agent represented by the following formula (2). (Y) n Si(R) m (Z) q ...(2) (In the formula, m R and q Z represent the same or different alkyl groups, alkoxy groups, or chlorine atoms, and n Y represent the same or different hydrocarbon groups containing an amino group. m, n, and q are integers, and m+n+q=4.)

[0057] In formula (2) above, examples of alkyl groups represented by R and Z include linear or branched alkyl groups having 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, and tert-butyl groups, and among these, the methyl group is preferred.

[0058] In formula (2) above, examples of alkoxy groups represented by R and Z include linear or branched alkoxy groups having 1 to 4 carbon atoms, such as methoxy, ethoxy, propoxy, isopropoxy, butoxy, isobutoxy, and tert-butoxy groups. Among these, methoxy and ethoxy groups are preferred.

[0059] In the above formula (2), the hydrocarbon group containing the amino group represented by Y is, for example, -(CH2) a Examples include -X (wherein X represents an amino group, an aminocarbonylamino group, an aminoalkylamino group, a phenylamino group, or a dialkylamino group, and a represents an integer from 1 to 4), -Ph-X (wherein X is the same as above, and -Ph- represents a phenylene group), etc.

[0060] Specific examples of amino group-containing silane coupling agents include the following: H2N(H2C)3Si(OCH3)3 H2N(H2C)3Si(OC2H5)3 H2N(H3C)3Si(CH3)(OCH3)2 H2N(H2C)2HN(H2C)3Si(CH3)(OCH3)2 H2NOCHN(H2C)3Si(OC2H5)3 H2N(H2C)2HN(H2C)3Si(OCH3)3 H2N-Ph-Si(OCH3)3 (wherein -Ph- represents a p-phenylene group) Ph-HN(H2C)3Si(OCH3)3 (wherein Ph- represents a phenyl group) (H9C4)3N(H2C)3Si(OCH3)3

[0061] The coupling agents described above may be used individually or in combination of two or more. The amount of coupling agent to be blended is appropriately selected from a range that provides sufficient charge to the toner without significantly reducing the mechanical strength of the resin coating layer. For example, it is preferably 10 parts by mass or less per 100 parts by mass of resin used to form the resin coating layer, and more preferably 0.01 parts by mass or more and 10 parts by mass or less.

[0062] Furthermore, from the viewpoint of controlling the charging characteristics over the life of the two-component developer, it is possible to pre-treat the carrier core material with a coupling agent or the like. As a coupling agent for treating the carrier core material, for example, a silane coupling agent similar to the one described above, which can be contained in the resin coating layer, can be used. By diluting the silane coupling agent with the solvent used for dilution in the coating resin solution described above, and then adding the carrier core material to the solvent and mixing without adding the coating resin solution, the charging properties of the carrier core material can be controlled. After this mixing (treatment of the carrier core material with the silane coupling agent), the solvent can be evaporated to form a layer of more concentrated silane coupling agent on the surface of the carrier core material. In this way, by concentrating the silane coupling agent and controlling its concentration ratio, or by changing the amount of silane coupling agent added, the carrier core material can be made more highly charged.

[0063] 3. Toner The toner according to this embodiment contains a magnetic material in the toner particles. More specifically, the toner particles according to this embodiment contain a binder resin and internal additives such as a magnetic material, a colorant, and a release agent, and these internal additives are dispersed in the binder resin. Further, if necessary, optional components may be contained within a range that does not impair the effects according to the present disclosure. The volume average particle diameter of the primary particles of the toner particles can be appropriately selected according to the purpose, and for example, it can be 5 μm or more and 8 μm or less.

[0064] The saturation magnetization of the toner is 0.4 Am 2 / kg or more and 3.35 Am 2 / kg or less, preferably 1 Am 2 / kg or more and 3 Am 2 / kg or less, more preferably 1.2 Am 2 / kg or more and 2 Am 2 / kg or less. When the saturation magnetization of the toner is within the above range, when the toner is used in a two-component developer, the toner can be appropriately handled, and both development performance can be maintained and the occurrence of fog can be suppressed.

[0065] Hereinafter, each component constituting the toner will be described. The manufacturing method of the toner particles is not particularly limited, and can be obtained by a known manufacturing method. The toner particles can be manufactured, for example, by a melt-kneading pulverization method.

[0066] <Binder resin> The binder resin contained in the toner particles according to this embodiment is not particularly limited, and examples include polyester resins, styrene resins (polystyrene, styrene-acrylic ester copolymer resins, etc.), acrylic resins (polymethyl methacrylate, etc.), polyolefin resins (polyethylene, etc.), polyurethanes, epoxy resins, etc. These resins may be used alone or in combination of two or more. Among these, a polyester resin can be preferably used.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] The acid value of the polyester resin is preferably between 5 mg KOH / g and 30 mg KOH / g. If the acid value falls below the lower limit, it may lead to a decrease in the charging properties of the resin, and the charging control agent may become difficult to disperse in the polyester resin. This may adversely affect the rise in charge and the stability of the charge when repeated continuous development is performed. If the acid value exceeds the upper limit, the hygroscopicity due to the functional groups in the resin increases, which may lead to a decrease in the charge in high temperature and high humidity environments. The acid value should be measured in accordance with the potentiometric titration method described in Japanese Industrial Standard (JIS) K0070-1992.

[0076] The glass transition temperature (Tg) of the polyester resin can be appropriately selected depending on the purpose, but considering the fixability and storage stability of the resulting toner, it is preferable that it be between 40°C and 80°C. If the glass transition temperature of the polyester resin is below the lower limit, storage stability will be insufficient, making it easier for thermal aggregation of the toner inside the image forming apparatus to occur, which may result in development defects. In addition, the temperature at which the high-temperature offset phenomenon begins to occur will be lowered. If the glass transition temperature of the polyester resin exceeds the upper limit, the fixability will decrease, which may result in fixing defects.

[0077] The softening temperature (Tm) of the polyester resin can be appropriately selected depending on the purpose, but it is preferably 150°C or lower, and more preferably 60°C to 120°C. If the softening temperature of the polyester resin is below 60°C, the storage stability of the toner decreases, making it easier for the toner to agglomerate inside the image forming apparatus, which can prevent the stable supply of toner to the image carrier and may result in development defects. If the softening temperature of the polyester resin exceeds 120°C, the toner becomes difficult to melt or soften when fixing it to the recording medium, which can reduce the toner's ability to fix to the recording medium and may result in fixing defects.

[0078] The weight-average molecular weight (Mw) of the polyester resin can be appropriately selected depending on the purpose, but it is preferably between 5,000 and 500,000. If the weight-average molecular weight of the polyester resin is below the lower limit, the mechanical strength as a binder resin will decrease, making the resulting toner particles more susceptible to pulverization during agitation inside the developing device, which may change the shape of the toner particles and cause variations in their electrostatic performance. If the weight-average molecular weight of the polyester resin exceeds the upper limit, it will become difficult to melt, which may reduce the toner's fixation performance and lead to poor fixation. Here, the weight-average molecular weight is the polystyrene equivalent value measured by gel permeation chromatography (GPC).

[0079] <Coloring agent> In the toner according to this embodiment, the toner particles may contain a colorant. As the colorant, various types and colors of organic and inorganic pigments and dyes commonly used in the field of electrophotography can be used, for example, a colorant for black toner can be used.

[0080] Examples of colorants for black toner include carbon blacks such as channel black, roller black, disc black, gas furnace black, oil furnace black, thermal black, and acetylene black. From these various types of carbon blacks, the appropriate carbon black should be selected according to the design characteristics of the toner to be obtained.

[0081] The colorant may be used in the form of a masterbatch. The masterbatch of the colorant can be manufactured in the same manner as a general masterbatch. For example, it can be manufactured by kneading a molten synthetic resin with the colorant to uniformly disperse the colorant in the synthetic resin, and then granulating the resulting molten mixture. The masterbatch is granulated to a particle size of about 2 mm to 3 mm. The synthetic resin used is of the same type as the binder resin of the toner, or one that has good compatibility with the binder resin of the toner. The ratio of synthetic resin to colorant is not particularly limited, but preferably it is 30 parts by mass or more and 100 parts by mass or less per 100 parts by mass of synthetic resin.

[0082] In the toner according to this embodiment, one of the above-mentioned colorants may be used alone, or two or more may be used in combination. The amount of colorant in the toner particles can be appropriately selected depending on the purpose, but it is preferably 3 parts by mass or more and 15 parts by mass or less per 100 parts by mass of binder resin. This is the amount of colorant itself contained in the masterbatch, not the amount of masterbatch. By keeping the amount of colorant within the above range, it is possible to form high-resolution images with high image density without impairing the various physical properties of the toner.

[0083] In addition to the colorants described above, the colorants in the toner particles according to this embodiment may also include magnetic materials, which will be described later.

[0084] <Magnetic material> The toner according to this embodiment contains a magnetic material in the toner particles. By including a magnetic material in the toner particles, it is possible to maintain developability and suppress the occurrence of fogging even when the toner concentration of the two-component developer increases.

[0085] Examples of metals contained in magnetic materials include ferromagnetic metals, alloys of multiple ferromagnetic metals, alloys that do not contain ferromagnetic metal elements but become ferromagnetic upon heat treatment, and metals obtained by doping iron oxide with chromium dioxide, cobalt, or nickel. Examples of ferromagnetic metals include cobalt, nickel, and iron. Examples of iron include iron oxide used in this form. Examples of iron oxide include triiron tetroxide (specifically magnetite) and ferrite.

[0086] The magnetic material may be one type of magnetic material used alone, or two or more types may be used in combination. To achieve both developmentability and suppression of fogging, the magnetic material preferably contains magnetite.

[0087] The magnetic material content in the toner particles according to this embodiment is preferably 0.7% by mass or more and 4% by mass or less, more preferably 1.5% by mass or more and 3.5% by mass or less, and even more preferably 2% by mass or more and 3% by mass or less. In the two-component developer according to this embodiment, the resin coating layer is a two-layer coating consisting of an inner layer and an outer layer, and since the resin coating layer is formed thinly, the effects of this disclosure can be obtained even if only a small amount of magnetic material is added to the toner particles. If the magnetic material content in the toner particles exceeds the above upper limit, the developability may decrease. If the magnetic material content in the toner particles is below the above lower limit, fogging may occur more easily.

[0088] The magnetic material in the toner particles according to this embodiment is a soft magnetic material, and its saturation magnetization is 55Am. 2 / kg or more84Am 2 Preferably, the saturation magnetization of the magnetic material is 80 Am 2 / kg or more84Am 2The magnetic material is less than or equal to 8Am. Because the magnetic material is a soft magnetic material and its saturation magnetization is within the above range, it is easier to control the strength of the magnetic interaction between the carrier core material and the magnetic material within an appropriate range, thereby improving toner handling. Consequently, even when using a two-component developer at high toner concentrations, fogging can be suppressed. Furthermore, the magnetic material in the toner particles according to this embodiment has a residual magnetization of 8Am. 2 Preferably less than / kg, 7Am 2 It is more preferable that the amount be less than or equal to / kg.

[0089] Examples of soft magnetic materials include ferrite, magnetite, iron, silicon steel, permalloy, Sendust, Permendur, and amorphous magnetic alloys. Among these, magnetite is preferred because it is easier to obtain a soft magnetic material that satisfies the saturation magnetization described above. A soft magnetic material is defined as a material that does not exhibit magnetic behavior when an external magnetic field is removed, and specifically refers to a material with a small maximum energy product BHmax. A guideline for this is BHmax ≤ 5 [kJ / m³] for bulk. 3 ]. If BHmax is unknown, an approximate value can be obtained using the following simplified formula. In the following simplified formula, Br is the remanent magnetic flux density [T] and Hc is the coercivity [A / m]. BHmax ≈ 0.4·Br·Hc

[0090] <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.

[0091] In this embodiment, the amount of release agent in the toner particles is preferably 0.2 parts by mass or more and 20 parts by mass or less per 100 parts by mass of the binder resin.

[0092] <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.

[0093] Examples of charge control agents for controlling negative charge include oil-soluble dyes such as oil black and spiron black, metal-containing azo compounds, azo complex dyes, metal complexes and metal salts of salicylic acid and its derivatives (metals include chromium, zinc, zirconium, etc.), boron compounds, fatty acid soaps, long-chain alkyl carboxylates, and resin acid soaps. Among these, boron compounds are preferred from the viewpoint of not containing heavy metals.

[0094] Examples of charge control agents for controlling positive charge include quaternary ammonium salts, pyrimidine compounds, triphenylmethane derivatives, guanidine salts, and amidine salts.

[0095] These charge control agents may be used individually or in combination of two or more. The content of the charge control agent in the toner particles is preferably 0.5% by mass or more and 3% by mass or less.

[0096] <External additives> The toner according to this embodiment may have an external additive attached to the surface of the toner particles described above. The external additive can be one commonly used in this field, and examples include inorganic particles such as silica, titanium dioxide, silicon carbide, aluminum oxide, and barium titanate, with an average particle diameter of 7 nm to 200 nm. These inorganic particles may be used individually or in combination of two or more. An example of combining two types is the use of a first external additive with an average particle diameter of 50 nm or more and a second external additive with an average particle diameter of 5 nm to 30 nm. These inorganic particles, when surface-treated with a hydrophobic treatment agent such as a silane coupling agent, titanium coupling agent, or silicone oil to impart hydrophobicity, are preferable because they exhibit less decrease in electrical resistance and charge in high-humidity environments.

[0097] Examples of silica particles used as external additives include silica particles commonly used in the relevant field, such as fumed silica particles obtained by burning silicon tetrachloride, dry silica particles such as arc silica which is atomized in the gas phase using high energy such as plasma; wet silica particles such as precipitated silica synthesized under alkaline conditions using an aqueous sodium silicate solution as a raw material, and gel silica synthesized under acidic conditions; colloidal silica particles obtained by polymerizing acidic silicic acid in an alkaline state; and sol-gel silica particles obtained by hydrolysis of organosilane compounds.

[0098] For the silica particles used as an external additive, commercially available hydrophobized silica particles may be used, or unhydrophobized silica particles may be treated before use.

[0099] The titanium dioxide particles used as an external additive may be anatase-type titanium dioxide particles or rutile-type titanium dioxide particles. One method for producing rutile-type titanium dioxide particles is described in Japanese Patent Publication No. 2001-26423, which involves hydrolyzing an aqueous titanium tetrachloride solution to prepare a fine titania sol containing rutile nuclei, separating it, and then heat-treating it to obtain titanium dioxide particles. Another method for producing anatase-type titanium dioxide particles is described in Japanese Patent Publication No. 2000-10335, which involves hydrolyzing a solution of raw materials such as ilmenite ore dissolved in sulfuric acid, granulating it, drying it, and then firing it at a high temperature to obtain titanium dioxide particles.

[0100] The amount of external additive is preferably 1 part by mass or more and 3 parts by mass or less per 100 parts by mass of toner particles. If the amount of external additive is below the lower limit, it becomes difficult to impart the effect of improving fluidity. If the amount of external additive exceeds the upper limit, the fixation performance may decrease.

[0101] One method for adding external additives to toner particles is to mix the toner particles and the external additives using an air-flow mixer such as a Henschel mixer.

[0102] 4. Image forming apparatus For example, there are image forming apparatuses that have a control mechanism to increase the toner concentration of a two-component developer in the developing tank to correct the image density when the formed image density is low, such as the image forming apparatus disclosed in Japanese Patent Application Publication No. 2012-27145. As described above, the two-component developer according to this embodiment aims to solve the problems that arise when conventional two-component developers are used in such image forming apparatuses. For this reason, the two-component developer according to this embodiment is suitable for use in such image forming apparatuses.

[0103] In such an image forming apparatus, when the toner in the two-component developer becomes excessively charged, the toner concentration can be increased to increase the number of toner particles per carrier, thereby keeping the toner's charge level within a range appropriate for development. By including a magnetic material in the toner particles of the two-component developer according to this embodiment, the handling properties of the toner can be maintained, and a balance can be struck between maintaining developability and suppressing the occurrence of fogging. [Examples]

[0104] The two-component developer of this disclosure will be described in detail below based on examples and comparative examples.

[0105] 1.Measurement method <Method for measuring the volume-average particle diameter of carriers> The volume-average particle size of the carrier and carrier core material is measured using Microtrac (product name: MT3000, manufactured by Nikkiso Co., Ltd.). Approximately 10 mg to 15 mg of the sample (carrier or carrier core material) is added to 10 mL of a 5% aqueous solution of Emulgen 109P (product name: polyoxyethylene lauryl ether HLB13.6, manufactured by Kao Corporation), dispersed in an ultrasonic disperser for 1 minute, and then approximately 1 mL of this mixture is added to the designated location on the Microtrac, stirred for 1 minute, and measurement is performed after confirming that the scattered light intensity has stabilized.

[0106] <Method for measuring the weight-average molecular weight of silicone resin> The weight-average molecular weight of each silicone resin used as a material for the resin coating layer is measured by the following method: A solution of only the silicone resin is prepared, and the molecular weight distribution of the silicone resin is measured using GPC (Gel Permeation Chromatography, trade name: HLC-8320GPC, Tosoh Corporation) with toluene as the mobile phase, and the weight-average molecular weight is calculated.

[0107] <Method for measuring carrier VI characteristics> The VI characteristics (resistive characteristics) of the carrier are measured using a measurement jig 90 as shown in Figure 5. The detailed configuration of the measurement jig 90 is as follows: The electrodes 92 are parallel flat plates measuring 10 mm × 40 mm, and two of them are arranged on the substrate 93. The distance between these two electrodes 92 is 1 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.

[0108] First, 200 mg of the carrier to be measured is inserted between the electrodes 92. With this carrier held between the electrodes 92, the current value is measured when a DC voltage is applied to the electrodes 92 in 1V steps up to 1000V, and the current value at 500V is taken as the representative value.

[0109] <Method for measuring the glass transition temperature (Tg) of toner particles and various resins> Using a differential scanning calorimeter (product name: DSC220, manufactured by Hitachi High-Tech Science Co., Ltd.), 1 g of the sample (toner particles or resin to be measured) is heated at a heating rate of 10°C / min and the DSC curve is measured in accordance with Japanese Industrial Standard (JIS) K7121-1987. In the obtained DSC curve, the temperature at the intersection of a straight line extending from the high-temperature baseline of the endothermic peak corresponding to the glass transition to the low-temperature side, and a tangent line drawn at the point where the slope of the curve from the rising part to the apex of the peak is maximum, is defined as the glass transition temperature (Tg).

[0110] <Method for measuring the softening temperature (Tm) of toner particles and various resins> Using a flow characteristics evaluation device (product name: Flow Tester CFT-100C, manufactured by Shimadzu Corporation), 1 g of the sample (toner particles or resin to be measured) was heated at a heating rate of 6°C / min while applying a load of 20 kgf / cm². 2 (9.8 × 10 5 Apply Pa and allow the sample to flow out of the die (nozzle diameter 1 mm, length 1 mm). The temperature at which half of the sample has flowed out is defined as the softening temperature (Tm).

[0111] <Method for measuring the volume-average particle size of toner particles> 50 mL of electrolyte (product name: ISOTON-II, manufactured by Beckman Coulter, Inc.) is mixed with 20 mg of toner particles and 1 mL of alkyl ether sulfate sodium. The mixture is then dispersed for 3 minutes at a frequency of 20 kHz using an ultrasonic disperser (product name: Tabletop Dual-Frequency Ultrasonic Cleaner VS-D100, manufactured by AS ONE Corporation) to prepare the sample for measurement. The obtained sample is then measured using a particle size distribution analyzer (product name: Multisizer3, manufactured by Beckman Coulter, Inc.) under conditions of aperture diameter: 100 μm and number of particles measured: 50,000 counts. The volume-average particle diameter [μm] is determined from the volume particle size distribution.

[0112] <Method for measuring the magnetic properties of toner and magnetic materials> Approximately 100 mg of toner stored at room temperature and humidity (25°C, 50% RH) is taken and placed in a measurement cell. The weight of the toner is determined by calculating the weight difference between the measurement cell before and after placement. Next, the measurement cell is inserted into a vibrating sample magnetometer (product name: VSM-P7, manufactured by Toei Kogyo Co., Ltd.), and measured for 1 minute in a measurement magnetic field of 79.6 kA / m (1 kOe) to obtain a magnetization curve (hysteresis curve). By reading the saturation magnetization and remanent magnetization from various parameters of this magnetization curve, the saturation magnetization per unit weight of toner [Am] can be determined. 2 [kg] and remanent magnetization [Am 2 The value [ / kg] is determined. When measuring the magnetic properties of a magnetic material, the measurement is performed by replacing the toner with the magnetic material as the sample placed in the measurement cell.

[0113] <Method for measuring the charge amount of toner in a two-component developer> The charge of toner in a two-component developer is measured using a charge meter (product name: Model 210HS-2A, manufactured by TREK JAPAN). Specifically, first, about 0.1g of the two-component developer is taken. The weight D of the two-component developer is determined by weighing the taken two-component developer using an electronic balance. Next, the two-component developer is placed on a stand and covered with a mesh. A suction nozzle is placed on the mesh and the toner is sucked up. The charge of the sucked toner is displayed, so the charge per gram of toner [Q / m] is calculated by dividing it by the weight T of the sucked toner. In addition, the toner concentration T / D in the two-component developer is calculated from the weight D of the two-component developer and the weight T of the toner.

[0114] 2. Preparation of a two-component developer <Career Creation> Carriers CC1 to CC16 were prepared as carriers to be used in the examples and comparative examples. First, the procedure for preparing carrier CC8 will be described. When carriers prepared using the materials shown in the following procedure are charged under the same conditions as the carrier core material and resin coating layer, the charge level of the carrier core material is higher than that of the resin coating layer.

[0115] -Creating Carrier CC8- (Preparation of coating resin solution) A silicone resin solution (product name: KR240, 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 0.1% by mass relative to the silicone resin to prepare coating resin solution A. The weight-average molecular weight Mw of this silicone resin was 5.0 × 10⁻⁶. 2 That was the case.

[0116] 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 0.1% by mass relative to the silicone resin to prepare coating resin solution B. The weight-average molecular weight Mw of this silicone resin was 5.0 × 10⁻⁶. 5 That was the case.

[0117] Furthermore, a carbon dispersion (solid content concentration 1% by mass) was prepared by dispersing a conductive material (carbon black, product name: MA100, manufactured by Mitsubishi Chemical Corporation) in toluene.

[0118] The coating resin solution used to form the first coating layer (inner layer) around the carrier core material (hereinafter referred to as the first coating solution) was prepared as follows: Coating resin solutions A and B were measured out and mixed to obtain the specified total amount of resin solids, and a coupling agent (product name: AY43-059, manufactured by Toray Dow Corning Co., Ltd.) in an amount of 3% of the total amount of resin solids was added to prepare the desired solution. Here, the specified total amount of resin solids is the amount determined for each example and comparative example shown in Table 2 below.

[0119] The coating resin solution used to form the second coating layer (outer layer) around the carrier core material (hereinafter referred to as the second coating solution) was prepared as follows: Coating resin solutions A and B were weighed out to the specified total amount of resin solids and mixed together. A coupling agent (product name: AY43-059, manufactured by Toray Dow Corning Co., Ltd.) in an amount of 3% of the total amount of resin solids was added, and then a carbon dispersion was added so that a conductive material (solids in the carbon dispersion) in an amount of 10% of the total amount of resin solids was added. This prepared the desired solution. The specified total amount of resin solids is the amount determined for each example and comparative example shown in Table 2 below.

[0120] (Coating of carrier core material) The carrier core material (MnMg ferrite, volume-average particle size: 40 μm) was weighed into the container of the mixing and stirring machine, and the first and second coating solutions were weighed in the specified mass parts. Here, the specified mass parts are the number determined for each example and comparative example shown in Table 2 below. The first coating solution was added to the stirring machine at room temperature while stirring, and once homogenized, the temperature was raised to 80°C. Stirring was continued while degassing to remove the solvent from the first coating solution, forming the first coating layer (inner layer) around the carrier core material. After the solvent had evaporated and about 10 minutes had passed, the second coating solution was added, and the same procedure as for the first layer was performed to obtain a two-layer coated carrier.

[0121] To harden and stabilize the resulting carrier coating layer, it was cured in an oven at 180°C for 1 hour. After removing it and allowing it to cool to room temperature, the carrier was sieved to remove coarse material, thereby obtaining carrier CC8.

[0122] -Creation of carrier CC1-7 and CC9-19- Table 1 below lists the coating resin solutions used to prepare carriers CC1 to CC19. Coating resin solutions C and D were prepared in the same manner as coating resin solutions A and B, except that the type of silicone resin solution used was changed as shown in Table 1. Coating resin solution B1 was prepared by sealing coating resin solution B and leaving it at room temperature for several days; the weight-average molecular weight Mw of the silicone resin was 6.5 × 10⁻⁶. 5 The coating resin solutions C1 and C2 were prepared by sealing coating resin solution C and leaving it at room temperature for several days, and the weight-average molecular weight Mw of the silicone resin was 7.6 × 10⁻⁶ each. 3 , 6.4×10 3 That was the case.

[0123] [Table 1]

[0124] Carriers CC1-7 and CC9-19 were prepared in the same manner as carrier CC8 described above, except that the types and amounts of components constituting the carrier were changed, as shown in Table 2 below.

[0125] [Table 2]

[0126] Table 3 below shows a list of the physical properties of carriers CC1 to CC19.

[0127] [Table 3]

[0128] <Toner production> Toners TN1 to TN13 were prepared for use in the examples and comparative examples. First, the preparation procedure for toner TN6 will be described.

[0129] - Manufacturing Toner TN6 - (Mixing and kneading process) Toner particles (toner cores) were fabricated using the following toner materials. • Polyester resin (product name: FC1494, manufactured by Mitsubishi Chemical Corporation) 86 parts by mass • Coloring agent (product name: Nipex35, manufactured by Orion Engineered Carbons) 6 parts by mass • Magnetic material B (magnetite, product name: BL-220, manufactured by Titanium Industry Co., Ltd.) 2 parts by mass • Charge control agent (product name: S-34, manufactured by Orient Chemical Industry Co., Ltd.) 2 parts by mass • Release agent (product name: PE-130, manufactured by Clariant) 2 parts by mass • Release agent (product name: 550P, manufactured by Sanyo Chemical Industries, Ltd.) 2 parts by mass

[0130] The above toner materials were mixed for 10 minutes using an air-jet mixer (Henschel mixer, manufactured by Nippon Coke Industries Co., Ltd.), and the resulting mixture was melt-kneaded using a kneading and dispersion processing device (product name: NIDIX MOS140-800, manufactured by Nippon Coke Industries Co., Ltd.).

[0131] Next, the resulting mixture was coarsely ground using a cutting mill, and then finely ground using a jet-type pulverizer (IDS-2 model, manufactured by Nippon Pneumatic Mfg. Co., Ltd.). The resulting finely ground material was classified using an air-powered classifier (MP-250 model, manufactured by Nippon Pneumatic Mfg. Co., Ltd.) to obtain toner particles. The volume-average particle size of the obtained toner particles was 6.8 μm.

[0132] (External addition process) To 99.0 parts by mass of the obtained toner particles, 0.9 parts by mass of silica particles (product name: H-2000, manufactured by Clariant) were added as an external additive, and the mixture was mixed for 1 minute in an air-flow mixer (Henschel mixer, manufactured by Nippon Coke Industries Co., Ltd.) with the tip speed of the stirring blades set to 15 m / sec. Then, 0.1 parts by mass of titanium oxide particles (product name: STT-65-ASC, manufactured by Titanium Industries Co., Ltd.) were added as an external additive, and the mixture was mixed for 1 minute in an air-flow mixer (Henschel mixer, manufactured by Nippon Coke Industries Co., Ltd.) with the tip speed of the stirring blades set to 15 m / sec to obtain toner TN6.

[0133] -Production of toners TN1-5 and TN7-13- Table 4 below shows a list of magnetic materials used as toner materials for toners TN1 to TN13.

[0134] [Table 4]

[0135] Toners TN1-5 and TN7-13 were manufactured in the same manner as toner TN6 described above, except that the type and amount of magnetic material used as toner material were changed, as shown in Table 5 below.

[0136] [Table 5]

[0137] <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.

[0138] 3. Evaluation Method The two-component developers of the examples and comparative examples were evaluated using an evaluation machine modified from an image forming apparatus (digital monochrome multifunction printer, product name: MX-M1055, manufactured by Sharp Corporation) to allow arbitrary changes to the bias and cleaning field. The evaluation tests were conducted under normal temperature and humidity conditions (temperature 25°C, humidity 50%RH). A4 size electrophotographic paper (Multi Receiver, manufactured by Sharp Document Systems Corporation) was used as the test paper.

[0139] <Evaluation Item 1: Evaluation of Career Progress> After running the evaluation machine using the two-component developer of the examples and comparative examples idle for 5 seconds, the evaluation machine was stopped, and the number of carrier particles attached to a certain area (297 mm × 24 mm) in the non-image area of ​​the image carrier was determined, avoiding the area in contact with the magnetic tip when the machine was stopped.

[0140] The number of carrier deposits was determined for each cleaning field setting using the following procedure. The surface of the image carrier was negatively charged to 600V, and the development bias was changed in the order of 400V, 425V, and 450V to generate a cleaning field (CF, an electric field that recovers negatively charged toner) between the developer carrier and the development sleeve. The number of carrier deposits was determined when the CF was set to 200V, 175V, and 150V. Based on the bias that resulted in 50 or fewer carrier deposits, the carrier buildup was evaluated according to the following criteria. When printing on an actual image forming machine, the CF is controlled in the range of approximately 150V to 200V, and if the number of carrier deposits exceeds 50, the developer is depleted quickly, creating a problem with the amount of developer remaining until the end of its lifespan. Therefore, 50 carrier deposits was used as the evaluation criterion.

[0141] ◎(Excellent): CFs with 50 or fewer carrier attachments are up to 200V. ○ (Good): The CF with 50 or fewer carrier attachments is up to 175V. △(OK): CFs with 50 or fewer carrier attachments are up to 150V. × (Not allowed): The CF with 50 or fewer carrier attachments is lower than 150V.

[0142] <Evaluation Item 2: Evaluation of Developability> Using the two-component developers of the examples and comparative examples, test patterns were printed and stabilized while replenishing toner to achieve the toner concentration necessary for the predetermined coverage. The development bias was set to 450V and 600V, and the ID was measured using an X-Rite 938 spectrophotometer (manufactured by X-Rite) on high-density test patches (8mm circles) in the image area. The developability was evaluated according to the following criteria: A higher ID at a lower development bias indicates better developability, and an ID below 1.2 indicates that the image begins to appear faint.

[0143] ◎ (Excellent): With a development bias of up to 450V, the ID is 1.4 or higher. ○ (Good): With a development bias of up to 600V, the ID is between 1.3 and 1.4. △ (Acceptable): With a development bias of up to 600V, the ID is 1.2 or higher and less than 1.3. × (Not acceptable): The development bias is set to 600V and the ID is less than 1.2.

[0144] <Evaluation Item 3: Evaluation of Collision> Using a colorimeter (manufactured by Nippon Denshoku Industries Ltd.), the whiteness W2 of the non-image-forming area of ​​the printed test paper and the whiteness W1 of the test paper before printing (whiteness at the location that becomes a non-image-forming area after printing), which was measured in advance, were measured, and the fogging was evaluated based on the difference in whiteness, W2-W1, according to the following criteria.

[0145] ◎ (Excellent): The difference in whiteness is 0.5 or less. ○ (Good): The difference in whiteness is greater than 0.5 and less than or equal to 1.0. △ (Acceptable): The difference in whiteness is greater than 1.0 and less than or equal to 2.0. × (Not acceptable): The difference in whiteness is greater than 2.0.

[0146] 4. Evaluation (1) Example using toner TN6 with a toner concentration of 7% A print test was conducted by printing 100,000 images (hereinafter also referred to as 100k images in the table) with a print density of 5% under normal temperature and humidity conditions (NN environment, temperature 25°C, humidity 50%RH). First, the three evaluation items above were evaluated for the first print (initial) of the print test. For the examples and comparative examples that were evaluated, those for which the carrier uptake did not receive a "×" evaluation were evaluated for the three evaluation items above for the 100,000th print of the print test. Next, for those for which the carrier uptake did not receive a "×" evaluation after printing 100,000 pages, the evaluation machine was moved to a normal temperature and low humidity environment (NL environment, temperature 25°C, humidity 10%RH) and left overnight, and then the three evaluation items above were evaluated under those conditions. The evaluation results are shown in Table 6 below.

[0147] [Table 6]

[0148] (2) An example in which the toner was replaced in a way that increases the saturation magnetization, with a toner concentration of 9%. Following the evaluation test under the NL environment described in (1) above, toner TN6 was replenished into the developing tank and the evaluation machine was run idle for 1 minute, repeating this process until the toner density reached approximately 9%. Afterward, the three evaluation items described above were evaluated. Next, the toner cartridge was replaced, and 2000 images similar to those in (1) above were printed to replace the toner in the developing tank, and the toner density was reduced to 9% using the same procedure as above. Afterward, the three evaluation items described above were evaluated. This process was repeated to replace the toners with TN7-TN13, and the three evaluation items described above were evaluated for each. The evaluation results are shown in Tables 7 and 8 below. Note that each carrier type is assigned an example number and a comparative example number.

[0149] [Table 7]

[0150] [Table 8] (3) An example where the toner was changed to increase the saturation magnetization, with a toner concentration of 11%. Following the evaluation test under the NL environment described in (1) above, toner TN6 was replenished into the developing tank and the evaluation machine was run idle for 1 minute, repeating this process until the toner density reached approximately 11%. Afterward, the three evaluation items described above were evaluated. Next, the toner cartridge was replaced, and 2000 images similar to those in (1) above were printed to replace the toner in the developing tank, and the toner density was reduced to 11% using the same procedure as above. Afterward, the three evaluation items described above were evaluated. This process was repeated to replace the toners with TN7-TN13, and the three evaluation items described above were evaluated for each. The evaluation results are shown in Tables 9 and 10 below. Note that each carrier type is assigned an example number and a comparative example number.

[0151] [Table 9]

[0152] [Table 10]

[0153] (4) An example in which the toner was replaced in a way that lowers the saturation magnetization, with a toner concentration of 9%. Following the evaluation test under the NL environment described in (1) above, toner TN6 was replenished into the developing tank and the evaluation machine was run idle for 1 minute, repeating this process until the toner density reached approximately 9%. Afterward, the three evaluation items described above were evaluated. Next, the toner cartridge was replaced, and 2000 images similar to those in (1) above were printed to replace the toner in the developing tank, and the toner density was reduced to 9% using the same procedure as above. Afterward, the three evaluation items described above were evaluated. This process was repeated to replace the toners with TN1 to TN5, and the three evaluation items described above were evaluated for each. The evaluation results are shown in Tables 11 and 12 below. Note that for toners TN2 to TN4, example numbers and comparative example numbers are assigned according to the carrier type. Also, since toner TN1 does not contain magnetic material in its toner particles, the results using toner TN1 are designated as comparative example 1. The evaluation results for the case using toner TN6 are the same as in Table 7 and are therefore omitted.

[0154] [Table 11]

[0155] [Table 12]

[0156] (5) Example of replacing toner in a way that lowers the saturation magnetization, with a toner concentration of 11%. Following the evaluation test under the NL environment described in (1) above, toner TN6 was replenished into the developing tank and the evaluation machine was run idle for 1 minute, repeating this process until the toner density reached approximately 11%. Afterward, the three evaluation items described above were evaluated. Next, the toner cartridge was replaced, and 2000 images similar to those in (1) above were printed to replace the toner in the developing tank, and the toner density was reduced to 11% using the same procedure as above. Afterward, the three evaluation items described above were evaluated. This process was repeated to replace the toners with TN1 to TN5, and the three evaluation items described above were evaluated for each. The evaluation results are shown in Tables 13 and 14 below. Note that for toners TN2 to TN4, example numbers and comparative example numbers are assigned according to the carrier type. Also, since toner TN1 does not contain magnetic material in its toner particles, the results using toner TN1 are designated as comparative example 2. The evaluation results for the case using toner TN6 are the same as in Table 9 and are therefore omitted.

[0157] [Table 13]

[0158] [Table 14]

[0159] As is clear from the evaluation results shown in Tables 6 to 14, the two-component developers of Examples 1 to 35, which include a carrier having a resin coating layer on the surface of a carrier core material and a toner having toner particles, and which satisfy the following requirements (A) to (E), exhibited excellent developability while suppressing carrier rise and fogging. (A) The resin coating layer includes an outer layer and an inner layer located closer to the carrier core material than the outer layer and having a higher electrical resistance than the outer layer. (B) The outer layer and the inner layer contain a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights. (C) Of the two or more silicone resins before crosslinking and curing, let Ma be the weight-average molecular weight of the silicone resin with the smallest weight-average molecular weight, and let Mb be the weight-average molecular weight of the silicone resin with the largest weight-average molecular weight. Then Ma and Mb satisfy the following formula (1). 3.1≧log(Mb / Ma)≧1.9 (1) (D) In ​​the carrier, the content of the inner layer per 100 parts by mass of the carrier core material is 0.2 parts by mass or more, the content of the outer layer per 100 parts by mass of the carrier core material is 0.1 parts by mass or more, and the total content of both layers per 100 parts by mass of the carrier core material is 1.0 part by mass or less. (E) The toner contains a magnetic material in the toner particles.

[0160] In contrast, Comparative Examples 1-32 and Toner Comparative Examples 1-2, which did not meet these requirements, showed inferior results in at least one of the three evaluation items compared to the examples.

[0161] Comparative Examples 1-4, 13, 18, 23, and 28 are examples using carriers CC1-4, which have a single-layer resin coating, and therefore do not satisfy the above requirements (A) and (B).

[0162] Comparative Examples 5, 14, 19, 24, and 29 are examples in which carrier CC5 is used in which log(Mb / Ma) in formula (1) above is less than 1.9 and the total content of the inner and outer layers per 100 parts by mass of carrier core material exceeds 1.0 part by mass, that is, examples that do not satisfy the above requirements (C) and (D).

[0163] Comparative Examples 6, 15, 20, 25, and 30 are examples using carrier CC6 in which log(Mb / Ma) in the above formula (1) is less than 1.9, that is, examples that do not satisfy the above requirement (C).

[0164] Comparative Examples 7, 16, 21, 26, and 31 are examples in which carrier CC14 is used in which the total content of the inner and outer layers per 100 parts by mass of carrier core material exceeds 1.0 part by mass, and thus do not satisfy the above requirement (D).

[0165] Comparative Examples 8, 17, 22, 27, and 32 are examples in which carrier CC15 was used in which the total content of the inner and outer layers per 100 parts by mass of carrier core material exceeds 1.0 part by mass, and thus do not satisfy the above requirement (D).

[0166] Comparative Example 9 is an example in which carrier CC16 is used in which log(Mb / Ma) in the above formula (1) exceeds 3.1, that is, an example in which the above requirement (C) is not met.

[0167] Comparative Examples 10 and 11 are examples in which carriers CC17 and 18, in which the outer and inner layers are composed of only one type of silicone resin, are used, and therefore do not satisfy the above requirement (B).

[0168] Comparative Example 12 is an example using carrier CC18 in which the content of the inner layer is 0.1 parts by mass per 100 parts by mass of carrier core material, and the content of the outer layer is 0.1 parts by mass per 100 parts by mass of carrier core material, that is, an example that does not satisfy the above requirement (D).

[0169] Toner Comparative Examples 1 and 2 are examples using toner TN1 that does not contain magnetic material in the toner particles, and therefore do not satisfy the above requirement (E).

[0170] As shown in Tables 7-10, in the example where the toner was switched in a direction that increased the saturation magnetization, the developability (image density) began to decrease as the saturation magnetization increased, but the saturation magnetization of toner TN12 was 3.40 Am. 2 The toner density was sufficient for development up to 3.35 Am. No deterioration in fogging was observed even when the toner density was increased. 2 In the example using toner TN11, which is less than / kg, it can be seen that the evaluation of developability is superior to that using toner TN12.

[0171] As shown in Tables 11-14, in the example where the toner was switched in a direction that lowered the saturation magnetization, the fogging began to worsen as the saturation magnetization decreased, but the saturation magnetization of toner TN2 was 0.30 Am. 2 Up to / kg, fogging was sufficiently suppressed, and good developability was maintained even when the toner density was increased. Saturation magnetization was 0.4Am 2 In the example using toner TN3, which has a weight of 1 kg or more, it can be seen that the evaluation of fogging is superior to that using toner TN2.

[0172] As shown in Tables 7-10, the example using toner TN12, in which the magnetic material content of toner particles is 4% by mass or less, shows superior developability evaluation compared to the example using toner TN13, in which the magnetic material content exceeds 4% by mass.

[0173] As shown in Tables 11-14, the example using toner TN4, which has a magnetic material content of 0.7% by mass or more in the toner particles, shows superior fogging evaluation compared to the example using toner TN3, which has a magnetic material content of less than 0.7% by mass.

[0174] As shown in Tables 7-10, 11, and 13, the saturation magnetization is 84 Am 2 An example using toner TN11 containing magnetite with a saturation magnetization of 84Am / kg or less shows that the saturation magnetization is 84Am. 2 It can be seen that the developability evaluation is superior to that of the example using toner TN12 containing more than 55Am of magnetite. 2 An example using toner TN7 containing ferrite with a saturation magnetization of 55Am / kg or more is shown. 2 It can be seen that the evaluation of fogging is superior to that of the example using toner TN2 which contains ferrite at a concentration of less than / kg.

[0175] 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]

[0176] 10 Carriers 11. Carrier core material 11a Crystal grain (microcrystal) 12 Resin coating layer 12a Inner layer 12b Outer layer

Claims

1. A two-component developer comprising a carrier having a resin coating layer on the surface of a carrier core material, and a toner having toner particles, The resin coating layer includes an outer layer and an inner layer located closer to the carrier core material than the outer layer and having a higher electrical resistance than the outer layer. The outer layer and the inner layer contain a resin obtained by crosslinking and curing two or more silicone resins with different weight-average molecular weights. If, among the two or more silicone resins before crosslinking and curing, the weight-average molecular weight of the silicone resin with the smallest weight-average molecular weight is Ma, and the weight-average molecular weight of the silicone resin with the largest weight-average molecular weight is Mb, then Ma and Mb satisfy the following formula (1), 3.1≧log(Mb / Ma)≧1.9 (1) In the carrier, the content of the inner layer relative to 100 parts by mass of the carrier core material is 0.2 parts by mass or more, the content of the outer layer relative to 100 parts by mass of the carrier core material is 0.1 parts by mass or more, and the total content of both layers relative to 100 parts by mass of the carrier core material is 1.0 part by mass or less. The toner is a two-component developer characterized by containing a magnetic material in the toner particles.

2. A two-component developer according to claim 1, The saturation magnetization of the toner is 0.4 Am ２ / kg or more 3.35Am ２ A two-component developer characterized by having a weight of less than / kg.

3. A two-component developer according to claim 1 or claim 2, A two-component developer characterized in that the content of the magnetic material in the toner particles is 0.7% by mass or more and 4% by mass or less.

4. A two-component developer according to claim 1 or claim 2, The magnetic material in the toner particles is a soft magnetic material. The saturation magnetization of the magnetic material is 55 Am ２ / kg or more 84Am ２ A two-component developer characterized by having a weight of less than / kg.

5. A two-component developer according to claim 1 or claim 2, A two-component developer characterized in that, when the carrier core material and the resin coating layer are charged under the same conditions, the amount of charge of the carrier core material is higher than the amount of charge of the resin coating layer.

6. An image forming apparatus having a control mechanism that increases the toner concentration of a two-component developer in the developing tank to correct the image density when the formed image density is low, An image forming apparatus characterized by using a two-component developer according to claim 1 or claim 2.

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