Charging roller, process cartridge, image forming apparatus, and image forming method

Abstract

To provide a charging roller capable of sufficiently suppressing the occurrence of discharge unevenness and having low rotation resistance.SOLUTION: The charging roller 51 includes a conductive shaft 51a, an elastic layer 51a formed on an outer circumference of the conductive shaft 51b, and a surface layer 51b formed on an outer circumference of the elastic layer 51c. The surface layer 51c contains binder resins 101 and conductive particles 102. The binder resin 101 contains only a thermoplastic resin. The conductive particles 102 include second conductive particles 102b and first conductive particles 102a. The second conductive particles 102b are carbon black particles. The particle size distribution of the conductive particles 102 has at least one peak within a range of 0.01 μm or more and 1 μm or less. The ratio x of the surface area of the 102a parts of the first conductive particles to the surface area of the whole conductive particles 102 satisfies a specific expression.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a charging roller, a process cartridge, an image forming apparatus, and an image forming method. [Background technology]

[0002] Electrophotographic image forming apparatuses use a charging device to charge an image carrier. A charging roller is widely used as the charging device. If the charging roller cannot uniformly charge the surface of the image carrier, image defects called discharge unevenness (for example, uneven density such as spotted unevenness or horizontal stripes) occur.

[0003] For this reason, the charging roller is required to be able to suppress the occurrence of uneven discharge caused by insufficient charging of the surface of the image carrier, and also to have low electrical resistance (rotational resistance) of the charging roller.

[0004] As an example of a charging device that can prevent charging defects and suppress the occurrence of uneven discharge, Patent Document 1 proposes a charging roller that includes a shaft, an elastic layer provided on the outer periphery of the shaft, and a surface layer provided on the outer periphery of the elastic layer, with the surface layer being made of insulating particles and a resin film in which the insulating particles are dispersed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5381028 Summary of the Invention [Problem to be solved by the invention]

[0006] However, even if the charging roller described in Patent Document 1 is used, it is difficult to say that the occurrence of discharge unevenness can be sufficiently suppressed.

[0007] The present invention has been made in view of the above-mentioned problems, and its object is to provide a charging roller that can sufficiently suppress the occurrence of discharge unevenness and has low rotational resistance, as well as a process cartridge, an image forming apparatus, and an image forming method that use the charging roller. [Means for solving the problem]

[0008] The charging roller according to the present invention comprises a conductive shaft, an elastic layer formed on the outer periphery of the conductive shaft, and a surface layer formed on the outer periphery of the elastic layer. The surface layer contains a binder resin and conductive particles. The binder resin contains only a thermoplastic resin. The conductive particles include first conductive particles and second conductive particles. The first conductive particles are carbon black particles. The particle size distribution of the conductive particles has at least one peak in the range of 0.01 μm to 1 μm. The water contact angle of the thermoplastic resin and the ratio of the surface area of ​​the first conductive particles to the surface area of ​​the entire conductive particles satisfy formulas (1) to (3).

[0009] 40≦y<180 (1) y≦(800 / 13)x+(408 / 13) ···(2) y≦-400x+3440 (3)

[0010] In the formulas (1) to (3), y represents the water contact angle of the thermoplastic resin, and x represents the ratio of the surface area of ​​the first conductive particles to the surface area of ​​the entire conductive particles.

[0011] The process cartridge according to the present invention is detachably mounted in an image forming apparatus and includes the charging roller described above.

[0012] The image forming apparatus according to the present invention includes an image carrier, a charging device that charges the surface of the image carrier, an exposure device that exposes the charged surface of the image carrier to light to form an electrostatic latent image on the surface of the image carrier, a developing device that develops the electrostatic latent image into a toner image, and a transfer device that transfers the toner image from the image carrier to a transfer medium. The charging device is the charging roller described above.

[0013] The image forming method according to the present invention includes a charging step of charging the surface of an image bearing member with a charging device, the charging device being the charging roller described above. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a charging roller that can sufficiently suppress the occurrence of discharge unevenness and has low rotational resistance, as well as a process cartridge, an image forming apparatus, and an image forming method that use the charging roller. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view illustrating an example of a charging roller according to a first embodiment of the present invention. [Figure 2] FIG. 4 is a cross-sectional view illustrating an example of an image forming apparatus according to a second embodiment of the present invention. [Figure 3] 3 is a cross-sectional view schematically showing a photosensitive member and its peripheral portion provided in the image forming apparatus shown in FIG. 2. FIG. [Figure 4] 1 is a graph showing the relationship between the CB surface area ratio in the surface layer of each charging roller manufactured in Examples and some Comparative Examples and the water contact angle of the binder resin in the surface layer of each of the charging rollers described above. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments. Various modifications of the present invention are possible within the scope of the object of the present invention, and embodiments obtained by appropriately combining the technical means described in different embodiments are also included in the technical scope of the present invention.

[0017] Hereinafter, the compound name may be followed by "system" to collectively refer to the compound and its derivatives. When the compound name is followed by "system" to refer to a polymer, it means that the repeating unit of the polymer is derived from the compound or its derivative. Unless otherwise specified, evaluation results for powders (values ​​indicating shape, physical properties, etc.) are the number average of values ​​measured for a considerable number of particles of the powder. The particle diameters of various particles (e.g., first conductive particles, second conductive particles, and resin particles) described in the embodiments refer to the circle-equivalent diameter measured using a scanning electron microscope. Furthermore, acrylic and methacrylic may be collectively referred to as "(meth)acrylic." Unless otherwise specified, the "main component" of a material refers to the component that is most abundant in the material by mass. Furthermore, each component described below may be used alone or in combination of two or more. Furthermore, in this specification, "at least one of A and B" means "A and / or B." "A and / or B" means "A or B, or A and B." Furthermore, in this specification, when a numerical range is described as "A to B," this means "A or more and B or less."

[0018] [First embodiment: charging roller] A charging roller according to a first embodiment of the present invention will now be described. The charging roller according to this embodiment includes a conductive shaft, an elastic layer formed on the outer periphery of the conductive shaft, and a surface layer formed on the outer periphery of the elastic layer. The surface layer contains a binder resin and conductive particles. The binder resin contains only a thermoplastic resin. The conductive particles include first conductive particles and second conductive particles. The first conductive particles are carbon black particles. The particle size distribution of the conductive particles has at least one peak within a range of 0.01 μm to 1 μm (in other words, 10 nm to 1000 nm). The water contact angle of the thermoplastic resin and the ratio of the surface area of ​​the first conductive particles to the surface area of ​​all the conductive particles satisfy formulas (1) to (3).

[0019] 40≦y<180 (1) y≦(800 / 13)x+(408 / 13) ···(2) y≦-400x+3440 (3)

[0020] In the formulas (1) to (3), y represents the water contact angle of the thermoplastic resin, and x represents the ratio (percentage) of the surface area of ​​the first conductive particles to the surface area of ​​the entire conductive particles.

[0021] The charging roller according to this embodiment has the above-described configuration, which allows the occurrence of discharge unevenness to be sufficiently suppressed. Furthermore, the charging roller according to this embodiment has the above-described configuration, which allows the rotational resistance to be low. The reason why the charging roller according to this embodiment exhibits the above-described effects is presumed to be as follows.

[0022] The charging roller according to this embodiment contains a thermoplastic resin, first conductive particles, and second conductive particles in its surface layer. A thermosetting resin is typically used as the binder resin. Thermosetting resins have high viscoelasticity and can increase breaking strength by curing. However, there is a trade-off between resin hardness and moisture absorption. Thermosetting resins have a high proportion of polar groups and are highly dependent on the environment. The charge transport capacity of thermosetting resins varies significantly with humidity, making them prone to uneven discharge. According to the inventors' research, even when a thermoplastic resin is used as the binder resin, it is possible to impart sufficient strength to the charging roller to prevent damage. Furthermore, by not using a thermosetting resin, uneven discharge due to environmental changes can be suppressed. Even if the binder resin contains a small amount of thermosetting resin, the binder resin will thermally cure. For this reason, in this embodiment, only a thermoplastic resin is used as the binder resin.

[0023] In particular, thermoplastic resins with a water contact angle of 40° or more and less than 180° have low water absorption and can suppress conduction (ionic conduction by the binder resin) in areas other than the conductive particles, which occurs when the binder resin absorbs water. As a result, uneven discharge can be effectively suppressed. Furthermore, thermoplastic resins with a water contact angle of 40° or more and less than 180° are soluble in alcohols such as methanol, making it possible to form a surface layer by applying a solution. Therefore, such thermoplastic resins have excellent processability and conductive particle dispersibility, and can effectively suppress uneven discharge.

[0024] The conductive particles (first conductive particles and second conductive particles) before mixing are generally agglomerated. When the conductive particles are sufficiently crushed and dispersed throughout the surface layer, the particle size distribution of the conductive particles has a peak within the range of 0.01 μm to 1.00 μm. When both the first conductive particles and the second conductive particles are ideally dispersed throughout the surface layer, the particle size distribution of the conductive particles has two peaks within the range of 0.01 μm to 1.00 μm. However, if the particle size distribution of the conductive particles has at least one peak within the range of 0.01 μm to 1.00 μm, it can be said that the conductive particles are sufficiently crushed and dispersed throughout the surface layer. Therefore, if the particle size distribution of the conductive particles has a peak within the range of 0.01 μm to 1.00 μm, the conductive particles are sufficiently dispersed throughout the surface layer, and uneven discharge can be effectively suppressed.

[0025] The peak in the particle size distribution of the conductive particles can be confirmed by measuring the particle size distribution of the conductive particles in the surface layer-forming liquid used to form the surface layer using a laser diffraction particle size analyzer before forming the surface layer. If the particle size distribution of the conductive particles in the surface layer-forming liquid has a peak in the range of 0.01 μm to 1 μm, a surface layer can be formed in which the particle size distribution of the conductive particles has a peak in the range of 0.01 μm to 1.00 μm. Therefore, if the particle size distribution of the conductive particles in the surface layer-forming liquid has a peak in the range of 0.01 μm to 1.00 μm, it can be assumed that the particle size distribution of the conductive particles in the surface layer has a peak in the range of 0.01 μm to 1.00 μm. Since only a thermoplastic resin is used as the binder resin, the particle size distribution of the conductive particles in the surface layer can be measured by melting the surface layer and dispersing it in, for example, the solvent used in the surface layer-forming liquid. As the laser diffraction particle size distribution analyzer, for example, "SALD-2300" manufactured by Shimadzu Corporation can be used.

[0026] The charging roller according to this embodiment also includes carbon black particles as first conductive particles in the surface layer. Carbon black particles are an electronic conductor (electronic conductive agent). When the binder resin has a low water contact angle and a high water absorption rate, sufficient charge flows through the charging roller even with ionic conductivity. However, when the binder resin has a high water contact angle and a low water absorption rate, it is desirable for the surface layer to have electronic conductivity in order to ensure sufficient charge flow through the charging roller. By including electrically conductive particles with electronic conductivity in the surface layer, the electrical resistance of the surface layer can be reduced, thereby reducing the rotational resistance of the charging roller and achieving good electrical response.

[0027] However, the lower the rotational resistance of the charging roller, the more likely it is that uneven discharge occurs. Also, as mentioned above, when a thermoplastic resin with a water contact angle of 40° or more and less than 180° and low water absorption is used as the binder resin, if the energy gap between the ionic conduction caused by the binder resin absorbing water and the electronic conduction caused by the conductive particles becomes too large, the charge will only flow partially, and the rotational resistance of the charging roller may actually become higher.

[0028] Carbon black particles possess conductivity between ionic and electronic conduction, making them particularly suitable for mediating the transfer of charge between the binder resin and the second conductive particles. Furthermore, the surface area of ​​conductive particles, particularly carbon black particles, significantly affects the surface resistance. Charge flows within the binder resin via ionic conduction, while charge flows within the conductive particles (carbon black particles and metal oxide particles) via electronic conduction. When charge is transferred between the binder resin and the metal oxide particles, an energy gap occurs due to the difference in the conductivity mechanism, resulting in an increase in resistance. Carbon black particles are thought to mitigate the increase in resistance caused by this difference in the conductivity mechanism, and are therefore necessary in conductive systems that combine ionic and electronic conduction. However, the compatibility between carbon black particles and binder resin is poor, and this compatibility deteriorates as the binder resin's water absorption property decreases. Poor compatibility leads to carbon black particles' tendency to aggregate, resulting in uneven discharge (decreased measured values). For this reason, there is an upper limit to the amount of carbon black particles that can be added, and this limit decreases as the binder resin's moisture absorption property decreases.

[0029] Therefore, in the charging roller according to this embodiment, the surface layer contains multiple types of conductive particles, including carbon black particles as first conductive particles and second conductive particles other than carbon black particles, so that the water contact angle of the thermoplastic resin and the ratio of the surface area of ​​the carbon black particles to the surface area of ​​all the conductive particles (i.e., all the conductive particles) satisfy the following formulas (1) to (3).

[0030] This makes it possible to appropriately reduce the resistance of the surface layer, improve electrical response, and sufficiently suppress the occurrence of uneven discharge.

[0031] The charging roller according to this embodiment is suitable as a charging roller for use in an image forming apparatus that uses a one-component developer (specifically, a magnetic one-component developer or a non-magnetic one-component developer), or as a charging roller for use in an image forming apparatus that uses a two-component developer.

[0032] An example of a charging roller according to this embodiment will be described below with reference to FIG. 1. FIG. 1 is a cross-sectional view showing charging roller 51, which is an example of a charging roller according to this embodiment. Charge roller 51 is a cylindrical member. Charge roller 51 includes a conductive shaft 51a, an elastic layer 51b, and a surface layer 51c. Elastic layer 51b covers the outer peripheral surface of conductive shaft 51a. Surface layer 51c covers the outer peripheral surface of elastic layer 51b. The outer diameter of charging roller 51 is, for example, 5 mm or more and 20 mm or less. The structure of charging roller 51 according to this embodiment has been described above with reference to FIG. 1.

[0033] However, the structure of the charge roller 51 according to this embodiment is not limited to that shown in FIG. 1 . For example, the charge roller 51 according to this embodiment may have a laminated structure in which multiple elastic layers are stacked. Furthermore, the charge roller 51 according to this embodiment may have a crowned shape in which the outer diameter decreases from the center toward both ends. When the charge roller 51 has a crowned shape, where the outer diameters at both ends of the charge roller 51 are D1 and D2, and the outer diameter at the center of the charge roller 51 is D3, the crown amount represented by D3-D1 and the crown amount represented by D3-D2 are preferably 50 μm or more and 150 μm or less. The structure of the charge roller 51 according to this embodiment will be described in more detail below.

[0034] [Conductive shaft] The conductive shaft 51a is a cylindrical member made of a conductive metal. The conductive shaft 51a is used as a core bar. Aluminum is a preferable material for the conductive shaft 51a. The diameter of the conductive shaft 51a is, for example, 3 mm or more and 10 mm or less. A load is applied to each of both ends of the conductive shaft 51a, and it is desirable that a load of 200 g or more and 800 g or less is applied to each of both ends of the conductive shaft 51a.

[0035] [Elastic layer] The elastic layer 51b is an elastic layer. The thickness of the elastic layer 51b is, for example, 1.0 mm or more and 5.0 mm or less. The elastic layer 51b contains, for example, rubber (more specifically, vulcanized rubber) as a main component. The rubber content in the elastic layer 51b is preferably 50.0 mass % or more and 80.0 mass % or less, and more preferably 55.0 mass % or more and 65.0 mass % or less.

[0036] Examples of rubber contained in the elastic layer 51b include polyurethane elastomer, hydrin rubber (specifically, epichlorohydrin rubber), styrene-butadiene rubber (SBR), polynorbornene rubber, ethylene-propylene-diene rubber (EPDM), acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), butadiene rubber (BR), isoprene rubber (IR), natural rubber (NR), and silicone rubber. Hydrin rubber is preferred as the rubber contained in the elastic layer 51b.

[0037] The elastic layer 51b preferably further contains at least one of a filler, an electronic conductive agent, and an ionic conductive agent, and may further contain oil.

[0038] Examples of fillers include calcium carbonate and clay. The content of the filler in the elastic layer 51b is preferably 20.0 parts by mass or more and 80.0 parts by mass or less per 100.0 parts by mass of rubber.

[0039] Examples of the electronic conductive agent include carbon black particles, graphite particles, potassium titanate particles, iron oxide particles, titanium oxide particles, zinc oxide particles, and tin oxide particles. Carbon black particles are preferred as the electronic conductive agent. The content of the electronic conductive agent in the elastic layer is preferably 5.0 parts by mass or more and 40.0 parts by mass or less per 100.0 parts by mass of rubber.

[0040] Examples of the ionic conductive agent include organic salts (e.g., sodium trifluoroacetate), inorganic salts (e.g., quaternary ammonium salts), metal complexes, and ionic liquids. Sodium trifluoroacetate is preferred as the ionic conductive agent. The content of the ionic conductive agent in the elastic layer 51b is preferably 0.1 parts by mass or more and 2.0 parts by mass or less per 100.0 parts by mass of rubber.

[0041] [surface] The surface layer 51c is the outermost layer of the charging roller 51. The thickness of the surface layer 51c is preferably 3 μm or more and 80 μm or less, and more preferably 10 μm or more and 20 μm or less. The surface layer 51c contains at least a binder resin 101 and conductive particles 102 as a conductive agent. The surface layer 51c contains at least a binder resin 101 and conductive particles 102 as a conductive agent. By adding a conductive agent to the surface layer 51c, the electrical resistance of the surface layer 51c can be adjusted to a desired value. Preferably, the surface layer 51c further contains resin particles 103.

[0042] (binder resin) A thermoplastic resin is used for the binder resin 101. As described above, the binder resin 101 according to this embodiment contains only a thermoplastic resin. Examples of the binder resin 101 include thermoplastic (meth)acrylic resin, polyamide resin (specifically, polyamide resin containing an aliphatic skeleton (so-called Nylon (registered trademark))), urethane resin, (meth)acrylic fluororesin, and (meth)acrylic silicone resin. The binder resin 101 is preferably a thermoplastic polyamide resin (particularly, a thermoplastic polyamide resin containing an aliphatic skeleton). Examples of such polyamide resins include tetrapolymer polyamide resins such as polyamide 6 (nylon 6), polyamide 12 (nylon 12), polyamide 66 (nylon 66), and polyamide 610 (nylon 610).

[0043] In this embodiment, a thermoplastic resin having a water contact angle of 40° or more and less than 180° is used for the binder resin 101. As described above, a thermoplastic resin having a water contact angle of 40° or more and less than 180° has a low water absorption rate and can suppress conduction (ionic conduction by the binder resin 101) in portions other than the conductive particles 102, which occurs when the binder resin 101 absorbs water. Furthermore, a thermoplastic resin having a water contact angle of 40° or more and less than 180° is soluble in alcohol such as methanol, and therefore the surface layer 51c can be formed by applying a solution. The water contact angle of the thermoplastic resin used as the binder resin 101 is preferably 40° or more and less than 140°, more preferably 60° or more and less than 130°, and even more preferably 80° or more and less than 120°.

[0044] The surface free energy of thermoplastic resin is 20 μJ / m 2 More than 65μJ / m 2 Preferably, it is 20 μJ / m or less. 2 More than 45μJ / m 2 More preferably, it is 20 μJ / m or less. 2 More than 40μJ / m 2 It is more preferable that the surface free energy of the thermoplastic resin is as follows: The surface free energy of the thermoplastic resin can be measured using a contact angle measuring device (for example, "DMs-401" manufactured by Kyowa Interface Science Co., Ltd.).

[0045] The content of binder resin 101 in surface layer 51c is preferably 25.0% by mass or more and 60.0% by mass or less, and more preferably 35.0% by mass or more and 45.0% by mass or less. By setting the content of binder resin 101 in surface layer 51c to 25.0% by mass or more, the strength of surface layer 51c can be sufficiently ensured. By setting the content of binder resin 101 in surface layer 51c to 60.0% by mass or less, it is easier to ensure the amounts of conductive particles 102 and resin particles 103 in surface layer 51c.

[0046] (Conductive particles) The conductive particles 102 impart appropriate conductivity to the surface layer 51c. As described above, multiple types of conductive particles are used as the conductive particles 102. The conductive particles 102 include at least two types of conductive particles, including at least second conductive particles 102b and first conductive particles 102a different from the second conductive particles 102b.

[0047] The total content of the conductive particles 102 in the surface layer 51c (hereinafter sometimes simply referred to as the content of the conductive particles 102) is preferably 15.3 parts by mass or more and 230.0 parts by mass or less, more preferably 50.0 parts by mass or more and 200.0 parts by mass or less, and even more preferably 80.0 parts by mass or more and 150.0 parts by mass or less, relative to 100.0 parts by mass of the binder resin. By setting the content of the conductive particles 102 in the surface layer 51c to 15.3 parts by mass or more relative to 100.0 parts by mass of the binder resin, good conductivity can be imparted to the surface layer 51c, and the resistance of the surface layer 51c can be sufficiently reduced. Furthermore, by setting the content of the conductive particles 102 in the surface layer 51c to 230.0 parts by mass or less relative to 100.0 parts by mass of the binder resin, discharge unevenness due to charge concentration in the surface layer 51c can be suppressed, and the amount of resin particles 103 in the surface layer 51c can be easily secured.

[0048] (first conductive particle) The first conductive particles 102a are carbon black particles. Carbon black particles have conductivity between ionic and electronic conductivity, and are particularly suitable for mediating the exchange of charges between the binder resin and the second conductive particles 102b. Examples of carbon black particles include furnace black. The grade of the carbon black particles is not particularly limited, but a grade with good extrudability is preferred.

[0049] In the charging roller 51 according to this embodiment, the water contact angle of the thermoplastic resin and the ratio of the surface area of ​​the first conductive particles 102a to the surface area of ​​the entire conductive particles 102 measured by the BET method using nitrogen adsorption (sometimes referred to as the "CB surface area ratio") satisfy the above-mentioned formulas (1) to (3).

[0050] This allows the resistance of the surface layer to be appropriately reduced, improving electrical response. This allows the photosensitive member to be sufficiently supplied (transported) with the charge necessary for charging the surface of the photosensitive member. Furthermore, the lower the resistance, the more likely discharge irregularities are to occur. This is because if the resistance is too low, leakage current is more likely to occur. However, as described above, when the water contact angle and C-B surface area ratio of the thermoplastic resin satisfy the above-described formulas (1) to (3), the resistance of the surface layer 51c can be appropriately reduced, thereby sufficiently suppressing discharge irregularities.

[0051] The overall surface area of ​​the conductive particles 102 can be obtained by calculating a weighted average of the BET specific surface areas of each conductive particle 102, with the mass used as the weight. When the conductive particles 102 include only first conductive particles 102a and second conductive particles 102b, the overall surface area of ​​the conductive particles 102 is calculated as a weighted average of the masses of the first conductive particles 102a and the second conductive particles 102b in the conductive particles 102, with the masses being the weights (i.e., the mass-weighted average of the BET specific surface areas of the conductive particles 102).

[0052] Specifically, when the conductive particles 102 include only the first conductive particles 102a and the second conductive particles 102b, the surface area of ​​the entire conductive particles 102 is expressed by the following formula (4). Wa = (α × γ + β × δ) / (γ + δ) (4)

[0053] In formula (4), Wa represents the surface area of ​​the entire conductive particles 102. α represents the BET specific surface area of ​​the first conductive particles 102a. β represents the BET specific surface area of ​​the second conductive particles 102b. γ represents the mass ratio of the first conductive particles 102a when the total mass of the conductive particles 102 is 1 (100.0 mass%) (in other words, the content (content ratio) of the first conductive particles 102a in the conductive particles 102). δ represents the mass ratio of the second conductive particles 102b when the total mass of the conductive particles 102 is 1 (100.0 mass%) (in other words, the content (content ratio) of the second conductive particles 102b in the conductive particles 102).

[0054] The surface area of ​​the first conductive particle 102a is the weighted value of the BET specific surface area of ​​the first conductive particle 102a (in other words, the mass-weighted value of the BET specific surface area of ​​the first conductive particle 102a), where the weight is the mass of the first conductive particle 102a in the conductive particle 102. When the surface area of ​​the first conductive particle 102a is Wv1, Wv1 is expressed by the following formula (5). Wv1=(α×γ) / (γ+δ) ···(5)

[0055] Therefore, the ratio x (percentage) (CB surface area ratio) of the surface area Wv1 of the first conductive particles 102a to the surface area Wa of the entire conductive particles 102 is expressed by the following formula (6). x=(α×γ) / (α×γ+β×δ)×100 ···(6)

[0056] In formulas (5) and (6), α, γ, β, and δ are as described in formula (4). That is, in formulas (5) and (6), α, γ, β, and δ are the same as α, γ, β, and δ in formula (4), and γ+δ=1 (100.0 mass%). In formula (6), x is the same as x in formulas (2) and (3).

[0057] The BET specific surface area of ​​each conductive particle 102 (e.g., the BET specific surface area of ​​the first conductive particle 102a and the second conductive particle 102b) is measured by the BET method using nitrogen adsorption in accordance with "JIS (Japanese Industrial Standards) Z8830:2001 Method for measuring the specific surface area of ​​powders (solids) by gas adsorption." For the measurement, for example, a fully automatic BET specific surface area measuring device ("Macsorb (registered trademark) HM MODEL-1208" manufactured by Mountech Co., Ltd.) can be used.

[0058] As shown in Figure 4 below, x (CB surface area ratio) is in the range of 0.14% or more and 8.5% or less, although it depends on the value of y (water contact angle of the thermoplastic resin) as shown in the above-mentioned formulas (2) and (3).

[0059] The BET specific surface area (α) of the first conductive particles 102a is 20 m 2 / g or more 150m 2 / g or less, and 2 / g or more 135m 2 / g or less is more preferable. By using carbon black particles with a relatively small BET specific surface area (α) for the first conductive particles 102a, it is possible to sufficiently suppress the occurrence of uneven discharge and to provide a charging roller 51 with low rotational resistance.

[0060] The number-average primary particle diameter of the first conductive particles 102a is preferably 10 nm to 200 nm, more preferably 20 nm to 100 nm, and even more preferably 30 nm to 60 nm. By setting the number-average primary particle diameter of the first conductive particles 102a to 10 nm to 200 nm, good conductivity can be imparted to the surface layer 51c, and the surface of the image carrier can be charged more uniformly, thereby further suppressing uneven discharge.

[0061] The content of the first conductive particles 102a in the surface layer 51c is preferably 0.3 parts by mass to 30.0 parts by mass, more preferably 2.0 parts by mass to 10.0 parts by mass, and even more preferably 2.0 parts by mass to 8.0 parts by mass, relative to 100.0 parts by mass of the binder resin. By setting the content of the first conductive particles 102a in the surface layer 51c to 0.3 parts by mass to 30.0 parts by mass relative to 100.0 parts by mass of the binder resin, the surface of the image carrier can be charged more uniformly, and uneven discharge can be further suppressed.

[0062] Furthermore, the content of the first conductive particles 102a in the conductive particles 102 is preferably 0.3% by mass or more and 20.0% by mass or less, more preferably 2.0% by mass or more and 15.0% by mass or less, and even more preferably 2.0% by mass or more and 10.0% by mass or less. By setting the content of the first conductive particles 102a in the conductive particles 102 to 0.3% by mass or more and 30.0% by mass or less, it becomes easy to adjust x (the C-B surface area ratio) so as to satisfy the above-mentioned formulas (2) and (3). Furthermore, good conductivity is imparted to the surface layer 51c, and the resistance of the surface layer 51c can be sufficiently reduced. This allows the photosensitive member to be sufficiently supplied (transported) with the charge necessary for charging the surface of the photosensitive member. Furthermore, it becomes easy to ensure the amount of the second conductive particles 102b in the surface layer 51c.

[0063] (Second conductive particles) The second conductive particles 102b are conductive particles of a different type from the first conductive particles 102a. The second conductive particles 102b are preferably metal oxide particles. Examples of metal oxide particles include potassium titanate particles, iron oxide particles, titanium oxide particles, aluminum oxide particles, zinc oxide particles, and tin oxide particles. Some metal oxide particles exhibit sufficient conductivity by themselves, but others do not. To ensure sufficient conductivity of the material, a dopant may be added to these compounds. For example, antimony, phosphorus, indium, etc. are used as dopants for tin oxide. Therefore, the metal oxide particles may be metal oxide particles to which a dopant has been added, such as phosphorus-doped tin oxide particles, antimony-doped tin oxide particles, or indium-doped tin oxide particles. Among these metal oxide particles, phosphorus-doped tin oxide particles are more preferred.

[0064] The number average primary particle diameter of the second conductive particles 102b is preferably 10 nm to 200 nm, more preferably 10 nm to 40 nm. By setting the number average primary particle diameter of the second conductive particles 102b to 10 nm to 200 nm, good conductivity can be imparted to the surface layer 51c.

[0065] The content of the second conductive particles 102b in the surface layer 51c is preferably 10.0% by mass or more and 55.0% by mass or less. This appropriately reduces the resistance of the surface layer 51c, improves electrical response, and sufficiently suppresses the occurrence of discharge irregularities. Furthermore, the content of the second conductive particles 102b in the surface layer 51c is more preferably 10.0% by mass or more and 45.0% by mass or less, and even more preferably 10.0% by mass or more and 35.0% by mass or less. This further enhances the effect of suppressing the occurrence of discharge irregularities, and makes it easier to ensure the amounts of the first conductive particles 102a and the resin particles 103 in the surface layer 51c.

[0066] The content of the second conductive particles 102b in the surface layer 51c is preferably 15.0 parts by mass or more and 200.0 parts by mass or less, and more preferably 20.0 parts by mass or more and 150.0 parts by mass or less, per 100.0 parts by mass of the binder resin. This appropriately reduces the resistance of the surface layer 51c, improves electrical response, and enhances the effect of suppressing the occurrence of discharge unevenness. It also makes it easier to ensure the amounts of the second conductive particles 102b and the resin particles 103 in the surface layer 51c.

[0067] Furthermore, the content of the second conductive particles 102b in the conductive particles 102 is preferably 80.0 mass% to 99.7 mass%, more preferably 85.0 mass% to 98.0 mass%, and even more preferably 90.0 mass% to 98.0 mass%. By setting the content of the second conductive particles 102b in the conductive particles 102 to 80.0 mass% to 99.7 mass%, good conductivity can be imparted to the surface layer 51c, and the resistance of the surface layer 51c can be sufficiently reduced. Furthermore, by setting the content of the second conductive particles 102b in the conductive particles 102 to 80.0 mass% to 99.7 mass%, it is easier to ensure the amount of the first conductive particles 102a in the surface layer 51c.

[0068] In this embodiment, the first conductive particles 102a and the second conductive particles 102b are crushed and dispersed so that the particle size distribution of the conductive particles 102 has a peak in the range of 0.01 μm to 1.00 μm. As described above, if the particle size distribution of the conductive particles 102 has a peak in the range of 0.01 μm to 1.00 μm, the conductive particles 102 are sufficiently dispersed, and discharge unevenness can be effectively suppressed.

[0069] As described above, when the first conductive particles 102a and the second conductive particles 102b are both ideally dispersed, the particle size distribution of the conductive particles 102 has two peaks in the range of 0.01 μm to 1.00 μm. Therefore, it is particularly desirable that the particle size distribution of the conductive particles 102 have two peaks in the range of 0.01 μm to 1.00 μm. However, if the particle size distribution of the conductive particles 102 has at least one peak in the range of 0.01 μm to 1.00 μm, the conductive particles 102 can be considered to be sufficiently crushed and sufficiently uniformly dispersed in the surface layer 51c. Therefore, while it is sufficient for the particle size distribution of the conductive particles 102 to have at least one peak in the range of 0.01 μm to 1.00 μm, it is more preferable that the metal oxide particles, which are the second conductive particles 102b, have a peak in the range of 0.01 μm to 1.00 μm.

[0070] The particle size distribution of the conductive particles 102 having a peak in the range of 0.01 μm to 1.00 μm means that the peak top is in the range of 0.01 μm to 1.00 μm. The peak refers to the point that gives the maximum value (the locally maximum value) when the particle size distribution curve is viewed.

[0071] It should be noted that, due to cost and other factors, it is difficult and unrealistic to crush and disperse all of the conductive particles 102 so that their particle sizes fall within the range of 0.01 μm to 1.00 μm. Conductive particles 102 that are not sufficiently crushed appear as a large peak (e.g., a maximum peak) at a position where the particle size of the particle size distribution is relatively large. According to the studies of the present inventors, the conductive particles 102 can be considered to be sufficiently crushed when the magnitude (frequency value) of the maximum peak in the particle size distribution of the conductive particles 102 is 5 times or less the magnitude (frequency value) of the minimum peak.

[0072] (resin particles) The resin particles 103 impart appropriate surface roughness to the surface layer 51c. Examples of the resin particles 103 include (meth)acrylic resin particles, urethane resin particles, silicone resin particles, polyester resin particles, polystyrene resin particles, styrene-(meth)acrylic resin particles, and polyolefin resin particles. The resin particles 103 may be crosslinked resin particles. The resin particles 103 are preferably acrylic resin particles, and more preferably crosslinked acrylic resin particles.

[0073] The number-average primary particle diameter of the resin particles 103 is preferably 1 μm or more and 50 μm or less, more preferably 3 μm or more and 20 μm or less, and even more preferably 4 μm or more and 7 μm or less. By setting the number-average primary particle diameter of the resin particles 103 to 1 μm or more, it is possible to impart an appropriate surface roughness to the surface layer 51c. Furthermore, by setting the number-average primary particle diameter of the resin particles 103 to 50 μm or less, it is possible to prevent the surface roughness of the surface layer 51c from becoming excessively rough and the contact area with the image carrier from becoming too small. As a result, it becomes easier to uniformly charge the image carrier.

[0074] The content of resin particles 103 in surface layer 51c is preferably 4.0 parts by mass to 120.0 parts by mass, more preferably 30.0 parts by mass to 90.0 parts by mass, and even more preferably 40.0 parts by mass to 60.0 parts by mass, relative to 100.0 parts by mass of binder resin. By setting the content of resin particles 103 in surface layer 51c to 4.0 parts by mass to 120.0 parts by mass relative to 100.0 parts by mass of binder resin, the surface roughness of surface layer 51c can be optimized.

[0075] The surface roughness of the surface layer 51c is preferably 4.0 μm or more and 30.0 μm or less in terms of ten-point mean roughness (Rz). By making the ten-point mean roughness (Rz) of the surface layer 51c 4.0 μm or more, the contact area between the charging roller 51 and the image carrier can be appropriately reduced, making it difficult for toner components adhering to the image carrier to migrate to the surface of the charging roller 51. Furthermore, by making the ten-point mean roughness (Rz) of the surface layer 51c 30.0 μm or less, the charging roller 51 can more easily uniformly charge the surface of the image carrier. The ten-point mean roughness (Rz) is a value measured in accordance with JIS-B0601:1994.

[0076] The ten-point mean roughness (Rz) of the surface layer of the charging roller 51 can be adjusted by the primary particle size and content of the resin particles 103 contained in the surface layer 51c. Specifically, the ten-point mean roughness (Rz) of the surface layer can be increased by increasing the primary particle size of the resin particles 103 or the content of the resin particles 103.

[0077] The total content of the conductive particles 102 and resin particles 103 in the surface layer 51c is preferably 19.3 parts by mass to 340.0 parts by mass, more preferably 70.0 parts by mass to 240.0 parts by mass, per 100.0 parts by mass of the binder resin.

[0078] The charging roller 51 according to this embodiment has a low rotational resistance of less than 6.5 log Ω when rotated at a rotational speed of 100 mm / sec under a charging voltage (DC voltage) of 500 V after being left in a low temperature and low humidity (LL) environment (specifically, for example, a temperature of 10.0°C and a humidity of 10% RH) for 24 hours.

[0079] For this reason, the charge roller 51 according to this embodiment has sufficient conductivity even in an LL environment. Therefore, the charge roller 51 according to this embodiment has good electrical responsiveness and sufficient charge transport capability even in an LL environment. Furthermore, the charge roller 51 according to this embodiment has low rotational resistance even in an LL environment, and does not require a large force to rotate the charge roller 51. Therefore, the charge roller 51 according to this embodiment can smoothly rotate even in an LL environment. Therefore, the surface layer 51c of the charge roller 51 according to this embodiment is resistant to damage even after long-term use, and is highly durable. Thus, the charge roller 51 according to this embodiment has little environmental dependency, and despite using a thermoplastic resin as the binder resin, has sufficient strength to prevent damage to the charge roller 51.

[0080] [Manufacturing method of charging roller] Next, an example of a method for manufacturing a charging roller according to this embodiment will be described. The method for manufacturing a charging roller according to this embodiment includes, for example, an elastic layer forming step and a surface layer forming step.

[0081] [Elastic layer formation process] In the elastic layer forming step, the elastic layer 51b is formed on the outer circumferential surface of the conductive shaft 51a, thereby obtaining a member (hereinafter sometimes referred to as a first member) including the conductive shaft 51a and the elastic layer 51b covering the outer circumferential surface of the conductive shaft 51a.

[0082] An example of a method for forming the elastic layer 51b on the outer periphery of the conductive shaft 51a is to laminate an elastic layer-forming composition on the outer periphery of the conductive shaft 51a using a molding die or the like, and then heat the elastic layer-forming composition. The elastic layer-forming composition contains, for example, unvulcanized rubber (the unvulcanized rubber described above) and sulfur.

[0083] The elastic layer-forming composition preferably further contains at least one of a filler, an electronic conductive agent, and an ionic conductive agent. The elastic layer-forming composition may further contain oil. The filler, the electronic conductive agent, the ionic conductive agent, and the oil impart desired properties to the elastic layer 51b to be formed. The elastic layer-forming composition preferably further contains at least one of a vulcanizing agent, a vulcanization accelerator, and a vulcanization aid to accelerate vulcanization of unvulcanized rubber. The elastic layer-forming composition may further contain a foaming agent to form a foam structure.

[0084] In the elastic layer-forming composition, the sulfur content is preferably 0.2 parts by mass or more and 3.0 parts by mass or less per 100.0 parts by mass of unvulcanized rubber. In the elastic layer-forming composition, the vulcanization accelerator content is preferably 0.4 parts by mass or more and 4.0 parts by mass or less per 100.0 parts by mass of unvulcanized rubber. In the elastic layer-forming composition, the vulcanization aid content is preferably 2.0 parts by mass or more and 10.0 parts by mass or less per 100.0 parts by mass of unvulcanized rubber.

[0085] In the elastic layer forming step, the heating temperature when heating the elastic layer forming composition is preferably 120° C. or more and 200° C. or less. In the elastic layer forming step, the heating time when heating the elastic layer forming composition is preferably 5 minutes or more and 60 minutes or less.

[0086] [Surface layer formation process] In the surface layer forming step, a surface layer forming liquid is applied to the outer periphery of the elastic layer 51b of the first member to form a surface layer 51c on the outer periphery of the elastic layer 51b of the first member. The surface layer forming liquid contains, for example, binder resin 101, first conductive particles 102a, second conductive particles 102b, resin particles 103, and a solvent. In particle size distribution measurement, the particle size distribution of the surface layer forming liquid shows, for example, at least one peak (a peak derived from the conductive particles 102) located in a particle diameter range of 0.01 μm to 1 μm and at least one peak (a peak derived from the resin particles 103) located in a particle diameter range of 1 μm to 50 μm. In addition, the peaks originating from the conductive particles 102 having particle diameters in the range of 0.01 μm or more and 1 μm or less preferably include a peak originating from the second conductive particles 102b, and more preferably include two peaks, namely, a peak originating from the second conductive particles 102b and a peak originating from the first conductive particles 102a.

[0087] By forming the surface layer 51c using the surface layer forming liquid exhibiting a particle size distribution with a peak attributable to the conductive particles 102 having a particle diameter in the range of 0.01 μm to 1 μm and a peak attributable to the resin particles 103 having a particle diameter in the range of 1 μm to 50 μm, the surface layer 51c can be formed to exhibit a particle size distribution with a peak attributable to the conductive particles 102 having a particle diameter in the range of 0.01 μm to 1 μm and a peak attributable to the resin particles 103 having a particle diameter in the range of 1 μm to 50 μm. Therefore, if the particle size distribution of the conductive particles 102 in the surface layer forming liquid has a peak in the range of 0.01 μm to 1.00 μm, it can be considered that the particle size distribution of the conductive particles 102 in the surface layer 51c has a peak in the range of 0.01 μm to 1.00 μm.

[0088] The solvent for the surface layer forming liquid is not particularly limited as long as it can dissolve or disperse the binder resin 101, the first conductive particles 102a, the second conductive particles 102b, and the resin particles 103. Examples of the solvent include alcohol solvents (e.g., methanol, ethanol, propanol, etc.) and aromatic compound solvents (e.g., benzene, toluene, xylene, etc.). A mixed solvent of methanol and toluene is preferred as the solvent.

[0089] The surface layer forming liquid can be prepared by dispersing binder resin 101, first conductive particles 102a, second conductive particles 102b, resin particles 103, and a solvent using a wet disperser (e.g., a bowl mill).

[0090] Examples of methods for applying the surface layer forming liquid include dip coating and blade coating. After applying the surface layer forming liquid to the elastic layer 51b, it is preferable to heat and dry the surface layer forming liquid. The heating temperature when heating the surface layer forming liquid in the surface layer forming step is, for example, 90°C or higher and 150°C or lower. The heating time when heating the surface layer forming liquid in the surface layer forming step is, for example, 20 minutes or higher and 120 minutes or lower.

[0091] [Second embodiment: image forming apparatus] An image forming apparatus according to a second embodiment of the present invention will now be described. The image forming apparatus according to this embodiment includes an image carrier, a charging device that charges the surface of the image carrier, an exposure device that exposes the surface of the charged image carrier to light to form an electrostatic latent image on the surface of the image carrier, a developing device that develops the electrostatic latent image into a toner image, and a transfer device that transfers the toner image from the image carrier to a transfer medium. The charging device is the charging roller according to the first embodiment.

[0092] The image forming apparatus according to the present embodiment includes the charging roller according to the first embodiment, and therefore can sufficiently suppress the occurrence of discharge unevenness. Furthermore, the image forming apparatus according to the present embodiment includes the charging roller according to the first embodiment, and therefore the rotation resistance of the charging roller is low.

[0093] An example of an image forming apparatus according to this embodiment will be described below with reference to Figures 2 and 3. Figure 2 is a cross-sectional view showing an image forming apparatus 1, which is an example of an image forming apparatus according to this embodiment. Figure 3 is a cross-sectional view schematically showing a photoreceptor 50 and its surroundings provided in the image forming apparatus 1 shown in Figure 2. Note that components having the same functions are given the same reference numerals, and their description will not be repeated. In this embodiment, the X-axis, Y-axis, and Z-axis are perpendicular to one another, the X-axis and Y-axis are parallel to a horizontal plane, and the Z-axis is parallel to a vertical line.

[0094] The image forming apparatus 1 is a full-color printer that uses a two-component developer. As shown in Fig. 2, the image forming apparatus 1 includes a feeding unit 10, a conveying unit 20, an image forming unit 30, a toner supply unit 60, and a discharge unit 70.

[0095] The feeding unit 10 includes a cassette 11 that stores a plurality of sheets P. The feeding unit 10 feeds the sheets P from the cassette 11 to the conveying unit 20. The sheets P are made of, for example, paper or synthetic resin. The conveying unit 20 conveys the sheets P to the image forming unit 30.

[0096] The image forming unit 30 includes an exposure device 31, a magenta unit (hereinafter referred to as M unit) 32M, a cyan unit (hereinafter referred to as C unit) 32C, a yellow unit (hereinafter referred to as Y unit) 32Y, a black unit (hereinafter referred to as BK unit) 32BK, a transfer belt 33, a secondary transfer roller 34, and a fixing device 35. The M unit 32M, the C unit 32C, the Y unit 32Y, and the BK unit 32BK each include a photoconductor 50 (corresponding to an image carrier), a charging roller 51 (corresponding to a charging device), a developing roller 52 (corresponding to a developing device), a primary transfer roller 53 (corresponding to a transfer device), a discharging lamp 54, and a cleaner 55. The M unit 32M, the C unit 32C, the Y unit 32Y, and the BK unit 32BK may each be formed into a cartridge.

[0097] The exposure device 31 irradiates each of the M unit 32M to BK unit 32BK with light based on image data, and forms an electrostatic latent image on the photoconductor 50 of each of the M unit 32M to BK unit 32BK. The M unit 32M forms a magenta toner image on the photoconductor 50 based on the electrostatic latent image. The C unit 32C forms a cyan toner image on the photoconductor 50 based on the electrostatic latent image. The Y unit 32Y forms a yellow toner image on the photoconductor 50 based on the electrostatic latent image. The BK unit 32BK forms a black toner image on the photoconductor 50 based on the electrostatic latent image.

[0098] The toner supply unit 60 includes a cartridge 60M that accommodates magenta toner T (see FIG. 3), a cartridge 60C that accommodates cyan toner T, a cartridge 60Y that accommodates yellow toner T, and a cartridge 60BK that accommodates black toner T. The cartridges 60M, 60C, 60Y, and 60BK supply toner T to the developing rollers 52 of the M unit 32M, the C unit 32C, the Y unit 32Y, and the BK unit 32BK, respectively.

[0099] As shown in FIG. 3, the image forming apparatus 1 includes, in the periphery of the photoreceptor 50, a charging roller 51, a developing roller 52, a primary transfer roller 53, a static elimination lamp 54, and a cleaner 55, in this order from upstream to downstream in the rotation direction R of the photoreceptor 50 (in other words, in the order of the image formation flow). The cleaner 55 has a cleaning blade 81. The photoreceptor 50 is drum-shaped. The photoreceptor 50 may be either a positively charged photoreceptor or a negatively charged photoreceptor. The photoreceptor 50 may also be either a single-layer photoreceptor or a multi-layer photoreceptor. The photoreceptor 50 is preferably a positively charged single-layer photoreceptor.

[0100] The photoreceptor 50 rotates around a rotation axis 50X. The charging roller 51 charges (for example, positively charges) the surface (circumferential surface) of the photoreceptor 50 in a charging step described later.

[0101] The exposure device 31 shown in FIG. 2 exposes the surface (circumferential surface) of the charged photoreceptor 50 to light in an electrostatic latent image forming process described below, thereby forming an electrostatic latent image on the surface (circumferential surface) of the photoreceptor 50.

[0102] As shown in FIG. 3, the developing roller 52 magnetically attracts and carries carrier CA carrying toner T. When a developing bias (developing voltage) is applied to the developing roller 52, a potential difference occurs between the potential of the developing roller 52 and the potential of the circumferential surface of the photoreceptor 50, causing the toner T to move and adhere to the electrostatic latent image formed on the circumferential surface of the photoreceptor 50. In this way, the developing roller 52 supplies toner T to the electrostatic latent image in a developing step described below, and develops the electrostatic latent image into a toner image. As a result, a toner image is formed on the circumferential surface of the photoreceptor 50. The toner image contains toner T.

[0103] The transfer belt 33 abuts against the circumferential surface of the photoreceptor 50. In a primary transfer step described below, the primary transfer roller 53 performs primary transfer of the toner image formed on the circumferential surface of the photoreceptor 50 onto the transfer belt 33 (more specifically, onto the outer surface of the transfer belt 33). Four color toner images are primarily transferred onto the outer surface of the transfer belt 33 in a superimposed manner. The four color toner images are a magenta toner image, a cyan toner image, a yellow toner image, and a black toner image. A color toner image is formed on the outer surface of the transfer belt 33 by the primary transfer.

[0104] A secondary transfer roller 34 shown in FIG. 2 performs a second transfer of the color toner image formed on the outer surface of the transfer belt 33 onto a sheet P in a secondary transfer process described below. A fixing device 35 applies heat and pressure to the sheet P to fix the color toner image onto the sheet P in a fixing process described below. The sheet P with the fixed color toner image is discharged to a discharge section 70. After the primary transfer, a discharge lamp 54 included in each of the M unit 32M to BK unit 32BK discharges electricity from the circumferential surface of the photoreceptor 50. After the primary transfer (more specifically, after the primary transfer and discharge), a cleaner 55 collects toner T remaining on the circumferential surface of the photoreceptor 50.

[0105] [Charging roller] The charging roller 51 is the charging roller according to the first embodiment. The charging roller 51 is disposed so as to be in contact with or in close proximity to the circumferential surface of the photoreceptor 50. The image forming apparatus 1 employs a direct discharge method or a proximity discharge method. The distance between the charging roller 51 and the circumferential surface of the photoreceptor 50 is preferably 50 μm or less, and more preferably 30 μm or less.

[0106] The charging voltage (charging bias) applied to the charging roller 51 is preferably a DC voltage. When the charging voltage is a DC voltage, the amount of discharge from the charging roller 51 to the photoreceptor 50 is smaller than when the charging voltage is a superimposed voltage, and the amount of wear on the photosensitive layer of the photoreceptor 50 can be reduced.

[0107] The image forming apparatus 1 according to this embodiment has been described above using Figures 2 and 3. However, the image forming apparatus of the present invention is not limited to the configuration shown in Figures 2 and 3, and can be modified as appropriate within the scope of the present invention. For example, as long as the image forming apparatus of the present invention includes an image carrier, a charging device, an exposure device, a developing device, and a transfer device, other components (for example, a static eliminator and a cleaning device) may be omitted.

[0108] Furthermore, in the present embodiment, the image forming apparatus 1 has been described as an image forming apparatus that uses a two-component developer containing a carrier CA and a toner T, but the image forming apparatus 1 of the present invention may also be an image forming apparatus that uses a single-component developer. Furthermore, in the present embodiment, the image forming apparatus 1 has been described as an image forming apparatus that employs an intermediate transfer method, but the image forming apparatus of the present invention may also be an image forming apparatus that employs a direct transfer method. Furthermore, in the present embodiment, the image forming apparatus 1 has been described as a full-color printer, but the image forming apparatus of the present invention may also be a monochrome printer or a multifunction device.

[0109] [Third embodiment: process cartridge] The process cartridge according to the third embodiment of the present invention will be described below. The process cartridge according to this embodiment is detachably mounted in an image forming apparatus. The process cartridge according to this embodiment includes the charging roller according to the first embodiment.

[0110] The process cartridge according to this embodiment is a cartridge for image formation that is detachably mounted in an image forming apparatus. The process cartridge according to this embodiment includes the charging roller according to the first embodiment, and therefore can sufficiently suppress the occurrence of discharge unevenness. Furthermore, the process cartridge according to this embodiment includes the charging roller according to the first embodiment, and therefore the rotational resistance of the charging roller is low.

[0111] 2 and 3, a first process cartridge 111, a second process cartridge 112, a third process cartridge 113, and a fourth process cartridge 114, which are examples of process cartridges according to the present embodiment, will be described below. The first process cartridge 111, the second process cartridge 112, the third process cartridge 113, and the fourth process cartridge 114 according to the present embodiment correspond to the M unit 32M, the C unit 32C, the Y unit 32Y, and the BK unit 32BK, respectively. Each of the first process cartridge 111 to the fourth process cartridge 114 is equipped with a charging roller 51, which is the charging roller according to the first embodiment.

[0112] Each of the first to fourth process cartridges 111 to 114 may include a photosensitive member 50 in addition to the charging roller 51. Each of the first to fourth process cartridges 111 to 114 may further include, in addition to the charging roller 51 and the photosensitive member 50, at least one selected from the group consisting of an exposure device 31, a developing roller 52 (developing device), a primary transfer roller 53 (transfer device), a static elimination lamp 54 (static elimination device), and a cleaner 55 (cleaning device).

[0113] Each of the first to fourth process cartridges 111 to 114 is designed to be detachable from the image forming apparatus 1. Therefore, each of the first to fourth process cartridges 111 to 114 is easy to handle. Specifically, each of the first to fourth process cartridges 111 to 114 can be easily and quickly replaced, including the photosensitive member 50, when the sensitivity characteristics of the photosensitive member 50 deteriorate. The process cartridge according to this embodiment has been described above with reference to FIGS. 2 and 3.

[0114] <Fourth embodiment: image forming method> An image forming method according to a fourth embodiment of the present invention will now be described. The image forming method according to this embodiment includes a charging step in which the peripheral surface of an image carrier is charged by a charging device. The charging device is the charging roller according to the first embodiment. The image forming method according to this embodiment further includes, for example, an electrostatic latent image forming step, a developing step, a primary transfer step, a secondary transfer step, and a fixing step.

[0115] The image forming method according to this embodiment uses the charging roller according to the first embodiment, and therefore can sufficiently suppress the occurrence of discharge unevenness. Furthermore, the process cartridge according to this embodiment uses the charging roller according to the first embodiment, and therefore the rotation resistance of the charging roller is low.

[0116] An image forming method according to the present embodiment will be described below, taking as an example an image forming method using the image forming apparatus 1 shown in FIGS. 2 and 3. In the charging step, the circumferential surface of the photoreceptor 50 (image carrier) is charged (e.g., positively charged) by a charging roller 51 (charging device) according to the first embodiment. In the electrostatic latent image forming step, an exposure device 31 exposes the charged circumferential surface of the photoreceptor 50 to light, forming an electrostatic latent image on the circumferential surface of the photoreceptor 50. In the developing step, a development roller 52 supplies toner T to the electrostatic latent image on the circumferential surface of the photoreceptor 50, developing the electrostatic latent image into a toner image. This forms a toner image on the circumferential surface of the photoreceptor 50. In the primary transfer step, a primary transfer roller 53 primarily transfers the toner image formed on the circumferential surface of the photoreceptor 50 to a transfer belt 33 (more specifically, the outer surface of the transfer belt 33). In the secondary transfer step, a secondary transfer roller 34 secondarily transfers the toner image formed on the outer surface of the transfer belt 33 to a sheet P. In the fixing step, the fixing device 35 applies heat and pressure to the sheet P to fix the toner image to the sheet P.

[0117] The image forming method according to this embodiment has been described above using the image forming apparatus 1 shown in Figures 2 and 3 as an example. However, the image forming method according to this embodiment may be a method different from the image forming method using the image forming apparatus 1 described above, as long as it includes a charging step in which the peripheral surface of the image carrier is charged by a charging device. [Example]

[0118] The present invention will be described in more detail below using examples, but the present invention is not limited to the scope of the examples.

[0119] [Manufacturing of charging rollers] Charging rollers of Examples and Comparative Examples were prepared by the following method.

[0120] [Example 1] (Elastic layer formation) 100.0 parts by mass of unvulcanized hydrin rubber (Osaka Soda Co., Ltd., "Epichromer® CG-102"), 5.0 parts by mass of zinc oxide (Mitsui Mining & Smelting Co., Ltd., "Zinc Oxide Type 2") as a vulcanization aid, 1.5 parts by mass of 2-mercaptobenzothiazole (Ouchi Shinko Chemical Industry Co., Ltd., "Noccela® MP") as a vulcanization accelerator, 1.0 part by mass of sulfur (Tsurumi Chemical Industry Co., Ltd., "Sulfax® PS"), 50.0 parts by mass of calcium carbonate (Shiraishi Kogyo Co., Ltd., "Shiraenka® CC") as a filler, 20.0 parts by mass of carbon black particles (Asahi Carbon Co., Ltd., "Asahi #50") as an electronic conductive agent, and 0.5 parts by mass of sodium trifluoroacetate as an ionic conductive agent were mixed. The resulting mixture was thoroughly stirred using a mixer. This produced a composition for forming an elastic layer.

[0121] A conductive shaft (a rod-shaped SUM with a diameter of 6 mm) was set in the molding die. Next, the molding die was filled with the composition for forming an elastic layer. This allowed the composition for forming an elastic layer to be laminated on the outer periphery of the conductive shaft inside the molding die. Next, the molding die was heated at 160°C for 20 minutes. This vulcanized the composition for forming an elastic layer inside the molding die. Next, the molding die was allowed to cool to room temperature, and the contents of the molding die were demolded. This produced a first member comprising a conductive shaft and an elastic layer (thickness 1.8 mm) laminated on the outer periphery of the conductive shaft.

[0122] (Surface layer formation) A mixed solution was obtained by mixing 100.0 parts by mass of polyamide resin ("PA-100A-S" manufactured by T&K TOKA Corporation, polymerized fatty acid polyamide resin) as a binder resin, 80.0 parts by mass of phosphorus-doped tin oxide particles ("EP SP-2" manufactured by Mitsubishi Materials Electronic Chemicals Co., Ltd., number average primary particle diameter 10 nm) as first conductive particles, 2.5 parts by mass of carbon black particles ("SEAST SO" manufactured by Tokai Carbon Co., Ltd., number average primary particle diameter 43 nm) as second conductive particles, 50.0 parts by mass of acrylic resin particles ("GR-800T" manufactured by Negami Chemical Industrial Co., Ltd., number average primary particle diameter 6 μm) as resin particles, and 50 parts by mass of methanol, 50 parts by mass of toluene, and 50 parts by mass of butanol as a solvent.

[0123] The above-mentioned mixed solution and zirconia beads were placed in the vessel of a bowl mill (Yamato Scientific Co., Ltd., "Universal Bowl Mill MODEL UB-32"). Next, the contents of the vessel were stirred for 24 hours using the bowl mill at a rotation speed of 60 rpm. Next, the contents were removed from the vessel of the bowl mill. Next, the above-mentioned contents were filtered to remove the zirconia beads. This resulted in a surface layer forming solution.

[0124] Next, a surface layer forming liquid was applied onto the elastic layer of the first member using a dip coating method. The applied surface layer forming liquid was then heated and dried in an electric furnace at 120°C for 1 hour. This resulted in the production of a charging roller (A-1) of Example 1, which included a conductive shaft, an elastic layer laminated on the outer periphery of the conductive shaft, and a surface layer (thickness 10 μm) laminated on the outer periphery of the elastic layer.

[0125] [Examples 2 to 17 and Comparative Examples 1, 2, 4 to 6, and 8] Charge rollers (A-2) to (A-17) according to Examples 2 to 17 and charge rollers (B-1), (B-2), (B-4) to (B-6), and (B-8) according to Comparative Examples 1, 2, 4 to 6, and 8 were manufactured in the same manner as in Example 1, except that the types and amounts of the binder resin, first conductive particles, second conductive particles, and resin particles used in forming the surface layer were changed as shown in Tables 1 to 7 below.

[0126] [Comparative Examples 3 and 7] Charge rollers (B-3) and (B-7) according to Comparative Examples 3 and 7 were manufactured in the same manner as in Example 1, except that the types and amounts of the binder resin, first conductive particles, second conductive particles, and resin particles used in forming the surface layer were changed as shown in Table 7 below, and the mixing conditions for the mixed solution in the bowl mill in forming the surface layer were changed as shown below. In Comparative Example 3, the mixing time in the bowl mill was changed to 1 hour. In Comparative Example 7, the mixing conditions were the same, but a binder resin with low hygroscopicity was used.

[0127] The binder resins used in Tables 1 to 7 are as follows: "PA-100A-S": "PA-100A-S" manufactured by T&K TOKA Corporation, thermoplastic polyamide resin "CM-8000": "Amilan (registered trademark) CM8000" manufactured by Toray Industries, Inc., a tetrapolymer polyamide resin (thermoplastic polyamide resin) of nylon 6, nylon 12, nylon 66, and nylon 610 "PA-201": "PA-201" manufactured by T&K TOKA Corporation, thermoplastic polyamide resin "X-12-1050": "X-12-1050" manufactured by Shin-Etsu Chemical Co., Ltd., a thermal crosslinking (thermosetting) silicone resin "PA-100": "PA-100" manufactured by T&K TOKA Corporation, thermoplastic polyamide resin "FR-101": Lead City Co., Ltd.'s "Fine Resin FR-101", methoxymethylated polyamide resin (thermoplastic polyamide resin) "FR-104": Lead City Co., Ltd.'s "FR-104", methoxymethylated polyamide resin (thermoplastic polyamide resin)

[0128] The first conductive particles used in Tables 1 to 7 below are as follows. "SP2": Mitsubishi Materials Electronic Chemicals Co., Ltd. "EP SP2", phosphorus-doped tin oxide particles, number-average primary particle diameter 10 nm "SMTA": "SMTA" manufactured by Teika Corporation, titanium oxide particles "S1": "S1" manufactured by Mitsubishi Materials Electronic Chemicals Co., Ltd., tin oxide particles "Al2O3": CIK Nanotech, average primary particle diameter 31μm "T1": "T1" manufactured by Mitsubishi Materials Electronic Chemicals Co., Ltd., antimony-doped tin oxide particles, number-average primary particle diameter 20 nm "E-ITO": Mitsubishi Materials Electronic Chemicals Co., Ltd. "E-ITO", indium-doped tin oxide particles, number-average primary particle diameter 30 nm

[0129] The second conductive particles used in Tables 1 to 7 below are as follows. "SEAST SO": "SEAST SO" manufactured by Tokai Carbon Co., Ltd., carbon black particles, number average primary particle diameter 43 nm "Show Black N330": Cabot Japan Co., Ltd. "Show Black N330", carbon black particles, number average primary particle diameter 30 nm "Asahi #78": Asahi Carbon Co., Ltd. "Asahi #78", carbon black particles, number average primary particle diameter 22 μm

[0130] The resin particles used in Tables 1 to 7 below are as follows. "GR-800T": Negami Chemical Industrial Co., Ltd.'s "Art Pearl (registered trademark) GR-800T", cross-linked acrylic resin particles, number average primary particle diameter 6 μm "MZ-5HN": "MZ-5HN" manufactured by Soken Chemical Co., Ltd., acrylic resin particles (main component: cross-linked acrylic resin), number average primary particle diameter 5 μm

[0131] In Tables 1 to 7 below, "parts" refers to parts by mass. The conductive agent mass ratio "%" of the first conductive particles indicates the content (content ratio) (mass %) of the first conductive particles in the conductive particles. The surface area ratio [%] of the first conductive particles (sometimes referred to as "CB surface area ratio") indicates the ratio of the surface area of ​​the first conductive particles to the surface area of ​​the entire conductive particle. The conductive agent mass ratio "%" of the second conductive particles indicates the content (content ratio) (mass %) of the second conductive particles in the conductive particles. The surface area ratio [%] of the second conductive particles indicates the ratio of the surface area of ​​the second conductive particles to the surface area of ​​the entire conductive particle. The presence or absence of a peak (0.01 to 1.00 μm) indicates whether or not there is a peak in the range of 0.01 μm to 1.00 μm in the particle size distribution of the conductive particles in the surface layer.

[0132] [measurement] [Number average primary particle size] The number-average primary particle diameter of the particles was measured using a scanning electron microscope (field-emission scanning electron microscope, "JSM-7600F" manufactured by JEOL Ltd.) Specifically, the equivalent circle diameter (Heywood diameter: diameter of a circle having the same area as the projected area of ​​a primary particle) of 100 target particles was measured using the scanning electron microscope, and the number-average diameter was calculated.

[0133] Thickness The thicknesses of the elastic layer and the surface layer were measured by observing the cross section with the above-mentioned scanning electron microscope. Specifically, the thickness was measured at 20 randomly selected points of the target layer (elastic layer or surface layer) within the field of view of the electron microscope, and the arithmetic mean value was taken as the thickness of the target layer.

[0134] [Water contact angle] The water contact angle of the thermoplastic resin was measured using a contact angle meter "OCA-40" manufactured by DataPhysics, Inc. In detail, pure water was dropped onto the surface of a thermoplastic resin layer made of the thermoplastic resin to be measured at room temperature (22°C), and the angle formed by the tangent of the droplet drawn from the contact point of the three phases of the solid phase (intermediate transfer body), liquid phase (droplet), and gas phase (atmosphere) and the surface of the thermoplastic resin layer on the droplet side was measured, and this was taken as the water contact angle of the thermoplastic resin.

[0135] [BET specific surface area] The BET specific surface area of ​​the conductive particles was measured by the BET method using nitrogen adsorption in accordance with JIS (Japanese Industrial Standards) Z8830:2001, "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption." For the measurement, a fully automatic BET specific surface area measuring device ("Macsorb (registered trademark) HM MODEL-1208" manufactured by Mountech Co., Ltd.) was used.

[0136] [Particle size distribution measurement] The volumetric particle size distribution of the prepared surface layer forming solution was measured by the laser diffraction scattering method using a laser diffraction particle size distribution analyzer (Shimadzu Corporation, "SALD-2300") to confirm whether there was a peak in frequency (relative particle amount) in the particle diameter range of 0.01 μm to 1.00 μm. In the particle size distribution measurement, the refractive index was set to 2.00 to 0.10i and the absorbance was set to 0.3 to 0.6.

[0137] As a result, for each of the surface layer forming liquids for producing charging rollers (A-1) to (A-17) and (B-1), (B-2), (B-4) to (B-6), and (B-8), at least one peak derived from conductive particles was confirmed in the particle size distribution curve measured by the above-mentioned laser diffraction scattering method in the particle size range of 0.01 μm to 1.00 μm. On the other hand, for the surface layer forming liquids for producing charging rollers (B-3) and (B-7), no peak derived from conductive particles was confirmed in the particle size distribution curve measured by the above-mentioned laser diffraction scattering method in the particle size range of 0.01 μm to 1.00 μm.

[0138] [evaluation] The voltage at which discharge unevenness occurs and the rotation resistance when using the charging rollers of Examples 1 to 17 and Comparative Examples 1 to 8 were evaluated by the following method. The evaluation results are also shown in Tables 1 to 7 below.

[0139] [Uneven discharge] One of the charging rollers of the examples or comparative examples to be evaluated was attached to a color multifunction printer (TASKalfa® 308i manufactured by Kyocera Document Solutions Inc.). In this way, an evaluation machine was prepared. Using the evaluation machine, halftone images (image density 25%) were formed on a recording medium while changing the photoreceptor surface potential in an environment of 32.5°C temperature and 80% RH. The image was visually confirmed to have discharge unevenness (uneven image density) and the charging voltage applied to the charging roller when discharge unevenness occurred was confirmed. The developer and toner used were those that are standardly used in the above-mentioned color multifunction printer. Printing paper (Askul copy paper "Multi Paper Super White+") was used as the recording medium. Discharge unevenness was evaluated according to the following criteria. Since the surface potential commonly used with the TASKalfa® 308i is 450V or higher, the reference voltage for discharge unevenness was set to 450V.

[0140] (Evaluation criteria for uneven discharge) A (good): The charging voltage applied to the charging roller when uneven discharge occurred was 450 V or more. B (poor): The charging voltage applied to the charging roller when uneven discharge occurred was less than 450V.

[0141] [Rotational resistance] The charging rollers of the examples and comparative examples to be evaluated were left in an LL environment (temperature 10.0°C, humidity 10% RH) for 24 hours. One of the charging rollers left in this LL environment was then mounted in a color multifunction printer (TASKalfa® 308i manufactured by Kyocera Document Solutions Inc.), and a charging voltage (DC voltage) of 500 V was applied under the LL environment, and the rollers were rotated at a rotation speed of 100 mm / sec to measure the rotational resistance. An Advantest R8340 was used to measure the rotational resistance. The charging voltage used to evaluate the rotational resistance was set to 500 V because this is an appropriate voltage at which stable resistance measurements can be made. If the voltage is too low, repetitive stability will be low, and if it is too high, a breakdown mode will occur.

[0142] (Rolling resistance evaluation criteria) A (Good): Rotational resistance is less than 6.5 log Ω B (bad): Rotation resistance is 6.5 log Ω or more

[0143] [Table 1]

[0144] [Table 2]

[0145] [Table 3]

[0146] [Table 4]

[0147] [Table 5]

[0148] [Table 6]

[0149] [Table 7]

[0150] FIG. 4 also shows the relationship between the ratio of the surface area of ​​the first conductive particles to the total surface area of ​​the conductive particles in the surface layer of each charging roller manufactured in the examples and some comparative examples (specifically, Examples 1 to 17 and Comparative Examples 1, 2, and 4 to 8 related to FIG. 4 ) (referred to as the "CB surface area ratio" in FIG. 4 ) and the water contact angle of the thermoplastic resin that is the binder resin in the surface layer of these charging rollers.

[0151] The charging rollers of Examples 1 to 17 included a conductive shaft, an elastic layer formed on the outer periphery of the conductive shaft, and a surface layer formed on the outer periphery of the elastic layer. The surface layer contained a binder resin and conductive particles. The binder resin contained only a thermoplastic resin. The conductive particles included first conductive particles and second conductive particles. The first conductive particles were carbon black particles. The particle size distribution of the conductive particles had a peak in the range of 0.01 μm or more and 1 μm or less.

[0152] Furthermore, if the water contact angle of the thermoplastic resin is y and the ratio of the surface area of ​​the first conductive particles to the surface area of ​​the entire conductive particles (CB surface area ratio) is x, as shown in Figure 4, the charging rollers of Examples 1 to 17 satisfied the above-mentioned formulas (1) to (3).

[0153] In other words, the charging rollers of Examples 1 to 17 were located within the region surrounded by the straight lines where x and y are represented by the following formulas (I) to (IV). y=40 (I) y=(800 / 13)x+(408 / 13) ···(II) y=-400x+3440 (III) y≒180 (however, y<180) (IV)

[0154] The charging rollers of Examples 1 to 17 did not exhibit uneven discharge when the charging voltage was less than 450 V, and the rotation resistance was less than 6.5 log Ω when a charging voltage (DC voltage) of 500 V was applied in an LL environment. Therefore, the effect of suppressing uneven discharge in the formed image was high, and uneven discharge could be sufficiently suppressed.

[0155] On the other hand, the charging rollers of Comparative Examples 1, 2, and 4 to 8 did not satisfy the formulas (1) to (3), as shown in Figure 4. For this reason, the charging rollers of Comparative Examples 1, 2, and 4 to 8 either had high rotation resistance or caused discharge unevenness at low charging voltages, and were unable to sufficiently suppress discharge unevenness.

[0156] Furthermore, the particle size distribution of the conductive particles in the charging rollers of Comparative Examples 3 and 7 did not have a peak in the range of 0.01 μm or more and 1.00 μm or less. Therefore, in the surface layer forming solutions of Comparative Examples 3 and 7, the conductive particles quickly aggregated, and the dispersed state of the conductive particles could only be maintained for a short time, or the dispersion was difficult to progress, and in neither case was it possible to obtain a surface layer in which the conductive particles were sufficiently dispersed. Therefore, the charging rollers of Comparative Examples 3 and 7 experienced discharge unevenness at charging voltages of less than 450 V. Therefore, Comparative Examples 3 and 7 demonstrate that when the particle size distribution of the conductive particles does not have a peak in the range of 0.01 μm or more and 1.00 μm or less, discharge unevenness occurs at low charging voltages, and discharge unevenness cannot be sufficiently suppressed.

[0157] Furthermore, the charging roller of Comparative Example 4 used a thermosetting resin as the binder resin, and the water contact angle of the binder resin was less than 40°. Furthermore, the charging rollers of Comparative Examples 5 and 6 used a thermoplastic resin as the binder resin, but the water contact angle of the binder resin was less than 40°. For this reason, the charging rollers of Comparative Examples 4, 5, and 6 experienced discharge unevenness at a charging voltage of less than 450 V. Therefore, it can be seen from Comparative Examples 4, 5, and 6 that when the water contact angle of the binder resin is less than 40°, discharge unevenness occurs at a low charging voltage, and discharge unevenness cannot be sufficiently suppressed.

[0158] Furthermore, the charging rollers of Comparative Examples 2 and 8, in which the conductive particles contained only the second conductive particles (metal oxide particles) and did not contain the first conductive particles (carbon black), had high rotational resistance. Comparative Examples 2 and 8 show that when the conductive particles do not contain the first conductive particles, the rotational resistance becomes high. [Industrial Applicability]

[0159] The charging roller and process cartridge according to the present invention can be used as components of an image forming apparatus, and the image forming apparatus and image forming method according to the present invention can be used to form an image on a recording medium. [Explanation of symbols]

[0160] 1: Image forming device 31: Exposure equipment 33: Transfer belt (transfer receiving body) 50: Photosensitive member (image carrier) 51: Charging roller (charging device) 51a: Conductive shaft 51b: Elastic layer 51c: Surface layer 52: Developing roller (developing device) 53: Primary transfer roller (transfer device) 101: Binder resin 102: Conductive particles 102a: First conductive particle 102b: Second conductive particle 103: Resin particles 111: First process cartridge (process cartridge) 112: Second process cartridge (process cartridge) 113: Third process cartridge (process cartridge) 114: 4th process cartridge (process cartridge) T: Toner

Claims

1. a conductive shaft; an elastic layer formed on the outer periphery of the conductive shaft; a surface layer formed on the outer periphery of the elastic layer; Equipped with the surface layer contains a binder resin and conductive particles, the binder resin contains only a thermoplastic resin, the conductive particles include first conductive particles and second conductive particles, the first conductive particles are carbon black particles; the particle size distribution of the conductive particles has at least one peak in the range of 0.01 μm or more and 1 μm or less; a water contact angle of the thermoplastic resin and a ratio of the surface area of ​​the first conductive particles to the surface area of ​​the entire conductive particles satisfy the following formulas (1) to (3). 40≦y<180 (1) y≦(800 / 13)x+(408 / 13)...(2) y≦-400x+3440...(3) (In the formulas (1) to (3), y represents the water contact angle of the thermoplastic resin; x represents the ratio of the surface area of ​​the first conductive particle to the surface area of ​​the entire conductive particle.

2. The charge roller of claim 1 , wherein the second conductive particles are metal oxide particles.

3. 3. The charging roller according to claim 1, wherein x is 0.14% or more and 8.50% or less.

4. The BET specific surface area of ​​the first conductive particles is 20 m 2 / g or more 150m 2 3. The charging roller according to claim 1, wherein the surface roughness is 0.1 / g or less.

5. The charging roller according to claim 1 , wherein the content of the first conductive particles in the conductive particles is 0.3% by mass or more and 30.0% by mass or less.

6. 3. The charging roller according to claim 1, wherein the thermoplastic resin is a thermoplastic polyamide resin.

7. 3. The charging roller according to claim 1, wherein the content of the conductive particles in the surface layer is 15.0 parts by mass or more and 230.0 parts by mass or less with respect to 100.0 parts by mass of the binder resin.

8. 3. The charging roller according to claim 1, wherein the particle size distribution of the carbon black particles has a peak in the range of 0.01 μm or more and 1 μm or less.

9. The charging roller according to claim 1 or 2, wherein the surface layer further contains resin particles.

10. The charging roller according to claim 9 , wherein the resin particles are acrylic resin particles.

11. 10. The charging roller according to claim 9, wherein the content of the resin particles in the surface layer is 20.0 parts by mass or more and 80.0 parts by mass or less with respect to 100.0 parts by mass of the binder resin.

12. A process cartridge that is detachably mounted in an image forming apparatus, A process cartridge comprising the charging roller according to claim 1 or 2.

13. an image carrier; a charging device that charges the surface of the image carrier; an exposure device that exposes the charged surface of the image carrier to light to form an electrostatic latent image on the surface of the image carrier; a developing device that develops the electrostatic latent image into a toner image; a transfer device that transfers the toner image from the image carrier to a transfer target; Equipped with The image forming apparatus, wherein the charging device is the charging roller according to claim 1 .

14. 14. The image forming apparatus according to claim 13, wherein the image carrier is a positively charged single-layer photoreceptor.

15. An image forming method including a charging step of charging a surface of an image carrier by a charging device, The image forming method, wherein the charging device is the charging roller according to claim 1 or 2.

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