DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The present invention is described below in detail. It is described chiefly taking the case of a charging member (one having the shape of a roller is herein often particularly called “charging roller”). The conductive member of the present invention is applicable not only to charging members, but also to various conductive members used in electrophotographic apparatus, such as developing members and transfer members.

[0048] The conductive member of the present invention comprises a support and provided thereon at least one cover layer, and a surface layer which is one of the cover layers of the conductive member contains fine particles.

[0049] The fine particles the surface layer of the conductive member contains may be of one kind or of two or more kinds. At least one kind thereof may be conductive fine particles and, in the case when two or more kinds of fine particles are used, insulating fine particles may be used. In the present invention, it is preferable to use the conductive fine particles and the insulating fine particles in combination; the latter being particles for controlling electrical resistance of the conductive member.

[0050] In the present invention, the conductive fine particles are meant to be fine particles having a volume resistivity of less than 1×10 ¹⁰ Ω·cm, and the insulating fine particles are meant to be fine particles having a volume resistivity of 1×10 ¹⁰ Ω·cm or more.

[0051] In the surface layer of the conductive member of the present invention, fine particles present at the lower part of the surface layer (hereinafter “surface layer lower part”) have an average particle diameter which is larger than the average particle diameter of fine particles present at the upper part of the surface layer (hereinafter “surface layer upper part”).

[0052] In the present invention, the surface layer lower part is the part corresponding to a range within 30% of the total layer thickness from the lowermost plane of the surface layer. The surface layer upper part is the part corresponding to a range within 30% of the total layer thickness from the uppermost plane of the surface layer.

[0053] The fine particles the surface layer lower part contains may preferably have an average particle diameter in the range of from 0.02 μm to 2.0 μm, and particularly preferably in the range of from 0.051 μm to 0.4 μm, and the fine particles the surface layer upper part contains may preferably have an average particle diameter in the range of from 0.001 μm to 1.0 μm, and particularly preferably in the range of from 0.001 μm to 0.05 μm.

[0054] If the average particle diameter of the fine particles the surface layer lower part contains and the average particle diameter of the fine particles the surface layer upper part contains deviate from the above ranges, the effect of the present invention can not be obtained in some cases even if the average particle diameter of the fine particles in the surface layer lower part is made larger than the average particle diameter of the fine particles in the surface layer upper part.

[0055] The fine particles in the surface layer lower part may also preferably be in a content larger than the content of the fine particles in the surface layer upper part. This is because a more remarkable effect can be obtained in regard to charging uniformity and improvement in pinhole leak-proofness.

[0056] Controlling the average particle diameter (preferably the content of fine particles also) of the fine particles can make the upper part of the surface layer of the conductive member have a higher electrical resistance than the lower part thereof, as so considered. In virtue of this difference in electrical resistance, electric charges can be retained in the vicinity of the surface of the conductive member to prevent any excess feed of electric charges and conversely supplement any insufficient feed of electric charges, so that proper feed of electric charges can be ensured.

[0057] Moreover, any pinhole leak levels at a low process speed can be kept from becoming poor. This is because the ability to retain electric charges in the vicinity of the surface of the conductive member acts effectively also on the prevention of pinhole leak.

[0058] Furthermore, the conductive member can also be improved in its durability. Since in the surface layer upper part of the conductive member the fine particles having a smaller average particle diameter than the surface layer lower part are present, the surface layer has higher reinforcing properties than that in a case in which any fine particles are not present at all or almost not present, bringing a dramatic improvement in durability, as so considered. Also, since the fine particles present in the vicinity of the surface of the conductive member have a smaller average particle diameter, this is very effective also for preventing the fine particles from coming off.

[0059] The fine particles the whole surface layer of the conductive member contains may preferably have particle diameters in the range of from 0.001 μm to 2 μm. If the fine particles have a particle diameter smaller than 0.001 μm, they may come not to contribute to the providing of conductivity (conductive fine particles) or the controlling of conductivity (insulating fine particles). If on the other hand the fine particles have a particle diameter larger than 2 μm, in the case of the conductive fine particles, they may provide so excessively low electrical resistance there that electric charges tend to flow there concentratedly to make pinhole leak levels poor. In the case of the insulating fine particles, they may come not to contribute to the controlling of conductivity.

[0060] How to form the surface layer of the conductive member of the present invention is described below.

[0061] As a method of forming the surface layer, it is preferable to use a method in which a binder material is dissolved and the fine particles are dispersed therein to prepare a coating fluid and this is coated by dipping or the like to form the surface layer.

[0062] The conductive member of the present invention is, as described above, the conductive member comprising a support and provided thereon at least one cover layer, and is characterized in that, of the cover layer(s), a layer corresponding to the surface layer of the conductive member contains the fine particles and that the fine particles present at the surface layer lower part have an average particle diameter which is larger than the average particle diameter of fine particles present at the surface layer upper part.

[0063] In order to control the average particle diameter of the fine particles in the surface layer of the conductive member in this way, it is preferable to use in combination at least two kinds of fine particles having different average particle diameter. Such at least two kinds of fine particles having different average particle diameter may be those comprised of the same material and having different average particle diameters, or may be those comprised of different materials and having different average particle diameters.

[0064] As a sure method by which the fine particles are made to differ in average particle diameter between the surface layer lower part and the surface layer upper part of the conductive member, the following method is available. When, e.g., the surface layer is formed by a coating process such as dipping, a plurality of (at least two) coating fluids in each of which the fine particles having different average particle diameters have been dispersed are prepared, and these coating fluids containing the fine particles having different average particle diameters are coated dividedly in several steps (at least two steps), followed by drying the resulting coatings (wet coatings) simultaneously to form the surface layer.

[0065] One and the same coating fluid may also be used, where a method is available in which coating is divided into several steps (at least two steps) and the coating fluid is allowed to stand in each step, controlling the time therefor. This method is a method in which the average particle diameter is controlled by utilizing the action that, when the coating fluids are allowed to stand for a long time, particles having large average particle diameter, particles having poor dispersibility or particles having large specific gravity settle down and the average particle diameter comes different for each portion of the coating fluid which forms the surface layer.

[0066] In the case of dipping, in order to make the layer thickness uniform in the lengthwise direction, it is preferable to change the rate or speed at the time of drawing-up appropriately (the rate or speed at the time of plunging has not especially anything to do with the control of layer thickness).

[0067] When the surface layer is formed by coating through several steps, the binder materials to be dissolved in the coating fluids may preferably be of the same type. As long as binder materials of the same type are used in the coating fluids, the surface layer thus formed can be formed in a single layer. In other words, if binder materials of different types are used in the coating fluids, an interface may be produced between coatings not to make the surface layer a single layer.

[0068] Also when the fine particles are made to differ in content between the surface layer lower part and the surface layer upper part of the conductive member, this can surely be achieved by a method similar to the above, namely, by a method in which coating fluids different in content of the fine particles are coated dividedly in several steps, followed by drying the resulting coatings (wet coatings) simultaneously to form the surface layer.

[0069] The content may also be controlled in the same way also when, as described above, one and the same coating fluid is used and the time for which it is allowed to stand is controlled.

[0070] To control the average particle diameter of the fine particles in the surface layer of the conductive member, in addition to the above methods, it is also effective to change dispersion conditions for coating fluids or dispersion power of dispersion machines to make the fine particles differ in average particle diameter.

[0071] In order to improve the dispersibility of the fine particles, it is preferable to subject the fine particles to surface treatment.

[0072] In order to control the average particle diameter, it is an effective method to properly separately coat a coating fluid in which fine particles subjected to surface treatment have been dispersed and a coating fluid in which fine particles not subjected to surface treatment have been dispersed.

[0073] As the surface treatment, coupling treatment and fatty-acid treatment are available. The coupling treatment may include treatment with a silane coupling agent and/or a titanate coupling agent. The fatty-acid treatment may include treatment with an acid such as stearic acid.

[0074] The fine particles are also classified into the conductive fine particles and the insulating fine particles as described previously.

[0075] The conductive fine particles may include metal oxide type conductive fine particles, metal type conductive fine particles, carbon black, and carbon type conductive fine particles, any of which may be used alone or in combination of two or more.

[0076] The metal oxide type conductive fine particles may include fine particles of zinc oxide, tin oxide, indium oxide, titanium oxide (such as titanium dioxide and titanium monoxide) and iron oxide. As the metal oxide type conductive fine particles, some exhibit sufficient conductivity by themselves, and some do not. In order to make the conductive fine particles have sufficient conductivity, i.e., in order to make the conductive fine particles have a volume resistivity of less than 1×10 ¹⁰ Ω·cm, a dopant may be added to these fine particles. In general, it is considered that fine metal oxide particles exhibit conductivity upon formation of excess electrons in virtue of the presence of lattice defects. Thus, the addition of a dopant accelerates the formation of the lattice defects, so that the sufficient conductivity can be attained. For example, as a dopant for zinc oxide, aluminum is used; as a dopant for tin oxide, antimony; and as a dopant for indium oxide, tin. Also, as titanium oxide provided with conductivity, it may include titanium oxide coated with conductive tin oxide.

[0077] The metal type conductive fine particles may include fine particles of silver, copper, nickel, zinc and so forth.

[0078] The carbon black may include acetylene black, furnace black and channel black.

[0079] The carbon type conductive fine particles may include fine particles of graphite, carbon fiber, activated carbon and charcoal.

[0080] As the conductive fine particles, among these, it is particularly preferable to use metal oxide type conductive fine particles or carbon black. This is because these fine particles have characteristic features that they have good dispersibility in the binder material such as resins and their average particle diameter can be controlled by dispersion with ease.

[0081] The insulating fine particles may include, e.g., metal oxide type insulating fine particles such as fine particles of silica, alumina, titanium oxide (such as titanium dioxide and titanium monoxide), zinc oxide, magnesium oxide, zirconium oxide and antimony trioxide; and barium sulfate, barium titanate, molybdenum disulfide, calcium carbonate, magnesium carbonate, dolomite, talc, caolin clay, mica, aluminum hydroxide, magnesium hydroxide, zeolite, wollastonite, diatomaceous earth, glass beads, bentonite, montmorillonite, asbestos, hollow glass balls, graphite, rice hulls, organometallic compounds and organometallic salts. Also usable are fine particles of known resins as exemplified by polyamide resins, silicone resins, fluorine resins, acrylic or methacrylic resins, styrene resins, phenolic resins, polyester resins, urethane resins, olefinic resins, epoxy resins, and copolymers, modified products and derivatives of any of these.

[0082] Of these, from the viewpoint of dispersibility in the binder material such as resins, it is particularly preferable to use metal oxide type insulating fine particles or fine resin particles.

[0083] When, for example, the conductive fine particles and the insulating fine particles are used in combination, those which are analogous in material may be used, e.g., the fine particles may be unified into the metal oxide type fine particles, or the insulating fine particles to be added may be made to be fine resin particles having chemically bonded moieties analogous to those of binder resins. This is preferable in order to control their dispersibility.

[0084] With regard to the control of conductivity, the charging uniformity and pinhole leak-proofness can further be improved when the binder material used in the surface layer of the conductive member has nitrogen atoms or carbon atoms in its structure. Nitrogen atoms and carbon atoms have unshared electron pairs in the atoms. It is considered that the presence of such electron pairs enhances the ability to retain electric charges. Also, among carbon atoms, it is further effective to use, in particular, a binder material having a polarized structure like carboxyl groups. From this viewpoint, a material having a urethane linkage or an amide linkage may preferably be used in the binder material used in the surface layer of the conductive member.

[0085] The durability of the conductive member can also be improved when the surface layer is made to have a higher hardness. The conductive member of the present invention contains the fine particles in the surface layer, and hence has a higher hardness than a case in which it does not contain the fine particles. However, it is preferable to further employ a high-hardness material also in the binder material.

[0086] The conductive member may also preferably have an appropriate conductivity and elasticity in order to ensure the charging ability (charging performance) to and uniform close contact with other members coming into contact with it, e.g., the electrophotographic photosensitive member. From such a viewpoint, the conductive member may preferably additionally have an elastic layer between the support and the surface layer. The elastic layer may preferably have a hardness lower than the hardness of the surface layer.

[0087] More specifically, the conductive member may preferably be so constructed as to be functionally separated into the elastic layer, which is to ensure the charging ability to and uniform close contact with the electrophotographic photosensitive member, and the surface layer, which is to ensure the durability of the conductive member.

[0088] The surface of the conductive member may also preferably have a high releasability. Stated specifically, the surface layer of the conductive member may preferably contain a releasing material and also the binder material of the surface layer of the conductive member may preferably be a resin.

[0089] The fact that the surface layer has a high releasability is exactly that the surface layer has a small coefficient of friction. Thus, any contaminants can be made to less adhere to the surface of the conductive member, and also its durability can be improved. At the same time, the relative movement between the conductive member and other members such as the electrophotographic photosensitive member can be made smooth, and hence any irregular state of movement, such as a stick slip, can be made to less come into being. As the result, various phenomena such as noise and irregular wear of the conductive member surface which are considered to be caused by non-uniform rotation can be prevented.

[0090] The fact that the surface layer has a high releasability is also that the conductive member may hardly contaminate other members coming into contact with it, e.g., the electrophotographic photosensitive member.

[0091] Where the releasing material is a liquid, it acts also as a smoothing agent (leveling agent) when the surface layer of the conductive member is formed, and hence the surface layer of the conductive member can be formed in smooth finish.

[0092] The releasing material is various in type and also classified in different ways. Considering it in the aspect of function, many materials are those which utilize low surface energy and those which utilize slidability. As their states also, they are available as liquids or as solids.

[0093] Those which are solids and have slidability are commonly known as solid lubricants. For example, those listed in KOTAI JUNKATSU HANDOBUKKU (Solid-Lubricant Handbook) (published by K.K. Yuki Shoboh; Second Edition, published on Mar. 15, 1982) may be used.

[0094] Compounds containing silicon atoms or fluorine atoms in the molecules may also be used in the form of oils or solids (releasing resins or powders, or polymers into part of which moieties having releasability have been introduced). The releasing materials may also include waxes and higher fatty acids (inclusive of salts or esters thereof and besides derivatives thereof).

[0095] Examples of layer construction of the conductive member are shown in FIGS. 1 to 8 .

[0096] FIG. 1 shows a conductive member having the shape of a roller. It is constituted of a support 2 a having conductivity (i.e., a conductive support), another cover layer (elastic layer) 2 b formed on the periphery of the support, and a cover layer (surface layer) 2 d further formed on the periphery of the elastic layer.

[0097] Other examples of construction are shown in FIGS. 2 to 4 .

[0098] As shown in FIG. 2 , the conductive member may have a triple-layer structure provided with another cover layer (resistance layer) 2 c between the elastic layer 2 b and the surface layer 2 d . It may also have, as shown in FIG. 3, a four-layer structure provided with another cover layer (second resistance layer) 2 e between the resistance layer 2 c and the surface layer 2 d , or may be provided with still another cover layer (resistance layer) to have a structure in which four or more cover layers are formed on the support 2 a . It may still also have, as shown in FIG. 4, a single-layer structure in which only one cover layer corresponding to the surface layer is formed on the support 2 a.

[0099] Without limitation to the roller shapes shown in FIGS. 1 to 4 , the conductive member of the present invention may further be of various shapes such as the shape of a sheet, the shape of a belt, the shape of a film and the shape of a plate, as shown in FIGS. 5 to 8 . In regard to those having the respective shapes, the layer construction described above may be employed.

[0100] The binder material used to form the surface layer of the conductive member of the present invention may preferably be a resin or an elastomer, and may more preferably be a resin as mentioned above.

[0101] The resin may include fluorine resins, polyamide resins, acrylic resins, polyurethane resins, silicone resins, butyral resins, styrene-ethylene/butylene-olefin copolymers (SEBC) and olefin-ethylene/butylene-olefin copolymers (CEBC).

[0102] The elastomer may include natural rubbers (which may be vulcanized), synthetic rubbers and thermoplastic elastomers.

[0103] The synthetic rubbers may include EPDM (ethylene-propylene-diene-methylene rubber), SBR (styrene-butadiene rubber), silicone rubber, urethane rubber, IR (isoprene rubber), BR (butadiene rubber), NBR (nitrile-butadiene rubber) and CR (chloroprene rubber).

[0104] The thermoplastic elastomers may include polyolefin type thermoplastic elastomers, urethane type thermoplastic elastomers, polystyrene type thermoplastic elastomers, fluorine rubber type thermoplastic elastomers, polyester type thermoplastic elastomers, polyamide type thermoplastic elastomers, polybutadiene type thermoplastic elastomers, ethylene-vinyl acetate type thermoplastic elastomers, polyvinyl chloride type thermoplastic elastomers and chlorinated polyethylene type thermoplastic elastomers.

[0105] Any of these binder materials may be used alone, may be a mixture of two or more types, or may form a copolymer.

[0106] The surface layer 2 d is endowed with conductivity by adding conductive fine particles. For the purposes of controlling conductivity, controlling surface properties and improving reinforcing properties, it may further be incorporated with insulating fine particles and different type of conductive fine particles. As these conductive fine particles and insulating fine particles, the fine particles described previously may be used.

[0107] These fine particles may also be those having been subjected to surface treatment, to modification, to introduction of functional groups or molecular chains and to coating, which may be of various types.

[0108] The elastic layer 2 b has an appropriate conductivity and elasticity in order to ensure the charging ability to the electrophotographic photosensitive member and the uniform close contact with other members coming into contact with it, such as the electrophotographic photosensitive member.

[0109] In the case when the conductive member has the shape of a roller, in order to ensure the good uniform close contact of the conductive member with other members coming into contact with it, such as the electrophotographic photosensitive member, the roller may preferably be formed into what is called a crown, which is a shape having the largest diameter at the middle and diameters made smaller toward the both ends. It may be formed into the crown by, e.g., sanding the elastic layer 2 b.

[0110] Since commonly the conductive member having the shape of a roller, such as the charging roller, is brought into contact with other members coming into contact with it, such as the electrophotographic photosensitive member, under application of a stated pressure on both ends of the support 2 a , the pressure is low at the middle and is larger toward the both ends. Hence, there is no problem as long as the conductive member having the shape of a roller has a sufficient straightness. If, however, it has an insufficient straightness, it may cause charge non-uniformity between the middle and the both ends and, corresponding to this non-uniformity, may cause density non-uniformity in images. It is formed into the crown in order to prevent this.

[0111] As materials (elastic materials) for the elastic layer 2 b , any materials may be used as long as they are elastomers such as synthetic rubbers and thermoplastic elastomers. As to the elastomers, the same elastomers as those described above may be used. A foam obtained by foam molding may also be used as the elastic material. Where it is necessary to ensure a nip between the conductive member and other members coming into contact with it, such as the electrophotographic photosensitive member (e.g., between the charging roller and the electrophotographic photosensitive member), a synthetic rubber material may preferably be used as the elastic material.

[0112] The elastic layer 2 b may preferably be endowed with conductivity to have an electrical resistance adjusted to less than 10 ⁸ Ω·cm, by adding to the above elastic material the above conductive fine particles or insulating fine particles, or by adding thereto a conducting compound such as an alkali metal salt or an ammonium salt, or by using these in combination. If the elastic layer 2 b has an electrical resistance of 10 ⁸ Ω·cm or more, the conductive member may have a lower charging ability to make it unable to satisfy the charging uniformity to the electrophotographic photosensitive member.

[0113] The elasticity and hardness of the elastic layer 2 b may be controlled by adding a softening oil, a plasticizer or the like or by foaming the elastic material.

[0114] The support 2 a may at least have conductivity, and a metallic material such as iron, copper, stainless steel, aluminum or nickel may be used. For the purpose of providing resistance to scratching, the metal surface thereof may further be subjected to plating to such an extent that its conductivity is not damaged.

[0115] The surface layer 2 d may preferably have an electrical resistance controlled to be higher than the electrical resistance of the elastic layer 2 b and to be not higher than 10 ¹⁶ Ω·cm. If the surface layer 2 d has an electrical resistance lower than that of the elastic layer 2 b , it may be unable to prevent leak due to pinholes and scratches of the electrophotographic photosensitive member surface. If it has an electrical resistance higher than 10 ¹⁶ Ω·cm, the conductive member (charging member) may have a lower charging ability to make it unable to satisfy charging uniformity.

[0116] The conductive member may be provided with the resistance layer 2 c at the position contiguous to the elastic layer 2 b , in order that the softening oil or plasticizer contained in the elastic layer can be prevented from bleeding out to the conductive member surface.

[0117] As materials constituting the resistance layer 2 c , the same materials as those used in the elastic layer 2 b may be used. The resistance layer 2 c may also preferably have conductivity or semiconductivity. As a material which provides conductivity, the above conductive fine particles of various types may be used. In this case, in order to achieve the desired electrical resistance, the above conductive fine particles of various types may be used in combination of two or more.

[0118] The resistance layer 2 c may preferably have an electrical resistance controlled to be not higher than the electrical resistance of the surface layer 2 d and not lower than the electrical resistance of the elastic layer 2 b . If its electrical resistance deviates from this range, it may be unable to satisfy charging uniformity.

[0119] Besides the foregoing various materials, a material having different function may appropriately be used in the elastic layer 2 b , the surface layer 2 d and the resistance layer 2 c . Such a different material may include, in the case of, e.g., the elastic layer 2 b , antiaging agents (antioxidants) such as 2-mercaptobenzimidazole, and lubricants such as stearic acid and zinc stearate.

[0120] The elastic layer 2 b , the surface layer 2 d and the resistance layer 2 c may also be subjected to surface treatment. The surface treatment may include surface processing treatment making use of ultraviolet rays or electron rays and surface-modifying treatment in which a compound is made to adhere to the surfaces of the layers or the latter is impregnated with the former.

[0121] The electrical resistance (volume resistivity; unit: Ω·cm) of the elastic layer 2 b , surface layer 2 d and resistance layer 2 c is measured with, e.g., a resistance measuring instrument, an insulation resistance meter HIRESTA-UP, manufactured by Mitsubishi Chemical Corporation.

[0122] With regard to the elastic layer 2 b , the elastic layer material itself is molded into a sheet with a thickness of 2 mm, 250 V of voltage is applied for 30 seconds in an environment of 23° C. and 55% RH to measure the volume resistivity.

[0123] With regard to the surface layer 2 d and the resistance layer 2 c , the same binder material as that used to form each layer is made into a coating fluid, and its clear coating fluid is coated on an aluminum sheet, where the volume resistivity of each layer is measured under the same conditions as those for the elastic layer 2 b.

[0124] The elastic layer 2 b , the surface layer 2 d and the resistance layer 2 c may be formed by any method without any particular limitations as long as it is suited for forming each layer in the desired thickness (with regard to the surface layer, a preferable method of forming it has been described above). Known methods concerning layer formation making use of polymeric materials such as resins may be employed.

[0125] These layers may each be formed by bonding a sheetlike or tubelike layer formed previously in a stated thickness, or by covering with the same, or may be formed by, or according to, a conventionally known method such as electrostatic spraying or dipping.

[0126] A method may also be used in which the layers are roughly formed by extrusion and thereafter their shapes are adjusted, or a method in which materials are cured into a stated shape in a mold, followed by forming.

[0127] The elastic layer 2 b may preferably have a layer thickness of 0.5 mm or more. If the elastic layer has a layer thickness of less than 0.5 mm, the elastic layer can not have appropriate elasticity, so that its contact with the electrophotographic photosensitive member may come improper to make the conductive member (charging member) not satisfy charging uniformity.

[0128] The surface layer 2 d may preferably have a layer thickness of from 1 μm to 1,000 μm. If the surface layer has a layer thickness of less than 1 μm, it tends to have non-uniform layer thickness, and any unevenness of the elastic layer may appear as it is to the surface of the conductive member to make the conductive member (charging member) not satisfy charging uniformity. At the same time, since the surface of the conductive member stands rough (greatly uneven), toner particles and external additives may come to tend to adhere to the conductive member surface. If on the other hand the surface layer is thicker than 1,000 μm, the appropriate elasticity given to the elastic layer may be lost, so that its contact with the electrophotographic photosensitive member may come improper to make the conductive member (charging member) not satisfy charging uniformity.

[0129] The resistance layer 2 c may also preferably have a layer thickness of from 1 μm to 1,000 μm.

[0130] To measure the layer thickness of the elastic layer 2 b , surface layer 2 d and resistance layer 2 c , layer sections are observed on an optical microscope and their thickness is actually measured. Stated specifically, the conductive member is cut with a cutting knife, and its cut section is observed on an optical or electron microscope and the thickness of each layer is measured.

[0131] In the present invention, as to the particle diameter and average particle diameter of the fine particles, 100 particles are picked up at random under observation on a TEM (transmission electron microscope), and the space between two horizontal lines which hold fine particles between them is regarded as the particle diameter of the particles, and its number-based average is regarded as the average particle diameter.

[0132] In the present invention, as to also the content of the fine particles contained in the surface layer (surface layer lower part and surface layer upper part), the area where the fine particles are present is calculated under observation on the transmission electron microscope, and the proportion of the area where the fine particles are present that is held in the whole area is regarded as their content.

[0133] In the present invention, as to still also the volume resistivity of the fine particles, the value measured by connecting MCP-PD41 to LORESTA-GP or HIRESTA-UP (all manufactured by Mitsuishi Chemical Corporation) is regarded as the volume resistivity of the fine particles. The quantity of a sample therefor may preferably appropriately be adjusted according to the density or the like of the fine particles. In the present invention, 1.5 g of the sample is weighed in regard to tin oxide, and 0.5 g in regard to carbon black, where applied pressure is set constant at 10.1 MPa (102 kgf/cm ² ). Applied voltage is fixed at 10 V when measured with LORESTA-GP. When measured with HIRESTA-UP, since the regions of resistance to be measured differ depending on applied voltage, the applied voltage is appropriately changed in accordance with the resistance value to be measured.

[0134] As to further the hardness of the elastic layer and surface layer, the value of microhardness measured with a microhardness meter MD-1 (manufactured by Kohbunshi Keiki K.K.) is regarded as the hardness. The microhardness is what is found when an indenter point (reverse-conical) of 0.16 mm in diameter at the root and 0.5 mm in length is pressed against a sample and the amount of indentation (displacement) of the indenter point at the time of pressing is indicated as hardness value. This enables measurement of the hardness of the surface and its vicinity of the conductive member. Hence, the hardness of materials used in the respective layers can be measured more faithfully. The measurement is also made in a peak hold mode in an environment of 23° C./55% RH. Stated in greater detail, in the case of the elastic layer, a sample is molded in the same manner as the sheet sample used to measure the electrical resistance and a measuring terminal is precisely pressed against it, where the value after 5 seconds is read. This is repeated several times, and its average value is regarded as elastic-layer hardness in the present invention. In the case of the surface layer, it is difficult to mold the material into a sheet of 2 mm thick. Accordingly, four sheets of 0.5 mm thick are prepared, and these are superposed together to make a sheet sample of 2 mm thick. The value measured in the same manner as the elastic layer is regarded as surface layer hardness in the present invention.

[0135] The construction of the process cartridge and electrophotographic apparatus of the present invention is described below.

[0136] FIG. 12 is a schematic illustration of the construction of the electrophotographic apparatus of the present invention.

[0137] The electrophotographic apparatus shown in FIG. 12 is an apparatus of a reverse development system utilizing transfer type electrophotography, and is an apparatus having employed the conductive member of the present invention as a charging member.

[0138] Reference numeral 1 denotes a rotating-drum type electrophotographic photosensitive member. This electrophotographic photosensitive member 1 is rotatingly driven at a stated peripheral speed (process speed) in the clockwise direction as shown by an arrow in the drawing. The process speed is set variable. As the electrophotographic photosensitive member 1 , a known electrophotographic photosensitive member may be employed which has a cylindrical support having conductivity and provided on this support a photosensitive layer containing an inorganic photosensitive material or an organic photosensitive material.

[0139] The electrophotographic photosensitive member 1 may further have a charge injection layer for charging the electrophotographic photosensitive member surface to stated polarity and potential.

[0140] Reference numeral 2 denotes a charging roller serving as the charging member (the conducting member of the present invention). The charging roller 2 and a charging-bias-applying power source S 1 which applies a charging bias to the charging roller 2 constitute a charging means. The charging roller 2 is kept in contact with the electrophotographic photosensitive member 1 under a stated pressure. In this apparatus, it is rotatingly driven in the direction following the rotation of the electrophotographic photosensitive member 1 . Only a stated DC voltage (in this example, set at −1,200 V) is applied to this charging roller 2 from the charging-bias-applying power source S 1 , thus the surface of the electrophotographic photosensitive member is electrostatically uniformly charged to stated polarity and potential (in this example, set at a dark-area potential of −600 V).

[0141] Reference numeral 3 denotes an exposure means. A known means may be used as the exposure means 3 . For example, a laser beam scanner is available. Of the electrophotographic photosensitive member 1 , the surface to be charged is exposed to laser light L corresponding to the intended image information, which is exposed through the exposure means 3 , so that the surface potential (set at a light-area potential of −350 V) of the electrophotographic photosensitive member at exposed light areas of the charged surface lowers (attenuates) selectively and an electrostatic latent image is formed on the electrophotographic photosensitive member 1 .

[0142] Reference numeral 4 denotes a developing means. A known means may be used as the developing means. For example, the developing means 4 in this example is so constructed as to have i) a toner-carrying member 4 a which is provided at an opening of a developing container holding a toner, and carries and transports the toner, ii) an agitation member 4 b which agitates the toner held in the developing container and iii) a toner control member 4 c which controls (regulates) the quantity of the toner held on the toner-carrying member 4 a (i.e., toner layer thickness). In the developing means 4 , a toner (a negative toner) standing charged electrostatically (in this example, at a development bias of −350 V) to the same polarity as the charge polarity of the electrophotographic photosensitive member 1 is made to adhere selectively to the exposed light areas of the electrostatic latent image on the electrophotographic photosensitive member surface to render the electrostatic latent image visible as a toner image. As its developing system, there are no particular limitations, and an existent system may be used. The existent system may include, e.g., a jumping developing system, a contact developing system and a magnetic-brush developing system. Especially in a full-color electrophotographic apparatus which reproduces full-color images, the contact developing system is preferred in order to, e.g., prevent the toner from scattering. As the toner-carrying member 4 a , used in the contact developing system, it may preferably contain a compound having an elasticity such as rubber, from the viewpoint of ensuring contact stability. For example, a developing roller having a support made of a metal or the like and provided thereon an elastic layer endowed with conductivity may be used. This elastic layer may be formed using as an elastic material a foam obtained by foam molding. An additional layer may also be provided thereon, or the layer may be subjected to surface treatment. The surface treatment may include surface processing treatment making use of ultraviolet rays or electron rays and surface-modifying treatment in which a compound is made to adhere to the layers or the latter is impregnated with the former.

[0143] Reference numeral 5 denotes a transfer roller as a transfer means. A known means may be used as the transfer roller 5 . For example, a transfer roller having a support made of a metal or the like and covered thereon with an elastic resin layer controlled to medium resistance may be used. The transfer roller 5 is kept in contact with the electrophotographic photosensitive member 1 under a stated pressure to form a transfer nip, and is rotated in the direction following the rotation of the electrophotographic photosensitive member 1 at a peripheral speed substantially equal to the peripheral speed of the rotation of the electrophotographic photosensitive member 1 . Also, a transfer voltage having the polarity opposite to the charge polarity of the toner is applied from a transfer bias-applying power source S 2 . A transfer material P is fed at a stated timing from a paper feed mechanism section (not shown) to the transfer nip, and is charged on its back, to the polarity opposite to the charge polarity of the toner by means of a transfer roller 5 to which a transfer voltage is kept applied, whereby the toner image on the side of the electrophotographic photosensitive member 1 surface is electrostatically transferred to the surface side of the transfer material P at the transfer nip.

[0144] The transfer material P to which the toner image has been transferred at the transfer nip is separated from the surface of the electrophotographic photosensitive member 1 , and is guided into a toner image fixing means (not shown), where the toner image is subjected to fixing. Then the image-fixed transfer material is put out as an image-formed matter. In the case of a double-side image-forming mode or a multiple-image-forming mode, this image-formed matter is guided into a recirculation delivery mechanism (not shown) and is again guided to the transfer nip.

[0145] Residues on the electrophotographic photosensitive member 1 , such as transfer residual toner, are collected from the surface of the electrophotographic photosensitive member 1 by a cleaning means (not shown) which is of, e.g., a blade type. Thereafter, the surface of the electrophotographic photosensitive member 1 is again electrostatically charged by the charging roller 2 , and images are repeatedly formed.

[0146] The electrophotographic apparatus in this example may be an apparatus having a process cartridge (not shown) which is so constructed that the electrophotographic photosensitive member 1 and the charging roller 2 are integrally supported by a supporting member such as a resin molded member, and is, in the state of this integral construction, set detachably mountable to the body of the electrophotographic apparatus. It may also be a process cartridge in which not only the electrophotographic photosensitive member 1 and the charging roller 2 but also the developing means 4 , the transfer means transfer roller 5 and so forth are integrally supported together.

[0147] The present invention is described below in greater detail by giving Examples.

EXAMPLE 1

[0148] A charging roller was produced in the following way. 1

	(by weight)
Epichlorohydrin rubber terpolymer	100	parts
(epichlorohydrin:ethylene oxide:allyl glycidyl ether =
40 mol %:56 mol %:4 mol %)
Light-duty calcium carbonate	30	parts
Aliphatic polyester type plasticizer	10	parts
Stearic acid	1	part
Antioxidant MB (2-mercaptobenzimidazole)	0.5	part
Zinc oxide	5	parts
Quaternary ammonium salt (having structure	2	parts.
represented by the following formula)
1

[0149] The above materials were kneaded for 10 minutes by means of an enclosed mixer adjusted to 50° C., to prepare a raw-material compound. To this compound, based on 100 parts by weight of the raw-material epichlorohydrin rubber, 1 part by weight of sulfur as a vulcanizing agent, and as vulcanization accelerators 1 part by weight of DM (dibenzothiazyl sulfide) and 0.5 part by weight of TS (tetramethylthiuram monosulfide) were added, and these were kneaded for 10 minutes by means of a twin-roll mill kept cooled to 20° C. The compound thus obtained was extruded by means of an extruder onto a stainless-steel mandrel of 6 mm in diameter so as to be made into a roller of 15 mm in outer diameter, which was then vulcanized by heating with steam, followed by sanding so as to come to 12 mm in outer diameter to form the elastic layer 2 b . The roller was in a length of 232 mm.

[0150] The surface layer 2 d was formed on the elastic layer to cover it. The surface layer 2 d was formed by coating the following surface layer coating fluids by dipping. The dipping was carried out twice.

[0151] First, as a coating fluid for first-time dipping, a liquid mixture was prepared in a container glass bottle, using the following materials. 2

[0152] Into this container, as dispersion media, glass beads (average particle diameter: 0.8 mm) were so packed as to be in a packing of 80%, followed by dispersion for 8 hours using a paint shaker dispersion machine. To the resulting liquid dispersion, a 1:1 mixture of hexamethylene diisocyanate (HDI) butanone oxime block product and isophorone diisocyanate butanone oxime block product (IPDI) was so added as to be NCO/OH=1.0 to prepare the coating fluid for first-time dipping. Thus, the coating fluid for first-time dipping was prepared.

[0153] Subsequently, as a coating fluid for second-time dipping, a coating fluid was prepared in the same manner as the coating fluid for first-time dipping but using as the fine particles the following particles instead and changing the paint shaker dispersion time to 16 hours. 3

[0154] Hydrophobic fine silica particles (product treated with hexamethyldisilazane; average particle diameter: 0.012 μm; volume resistivity: ₁₀ ¹⁶ Ω·cm) 10 parts.

[0155] On the surface of the above elastic layer, the above surface layer coating solutions were coated by dipping carried out twice. As to draw-up speed, the initial speed was set at 16 mm/s, and thereafter the speed was linearly reduced at a rate 1.125 mm/s per second. First, the coating fluid for first-time dipping was coated, followed by air drying at normal temperature for 10 to 30 minutes. Then, the roller was reversed, and the coating fluid for second-time dipping was coated in the same manner as the coating fluid for first-time dipping, followed by air drying at normal temperature for 30 minutes or more, and subsequently drying in a circulating hot air dryer at 160° C. for 1 hour. The surface layer having been dried was in a layer thickness of 15 μm.

[0156] On the charging roller thus produced, measurement was made on the following items.

[0157] Average particle diameter and content of fine particles in surface layer:

[0158] A section (inclusive of the surface layer) of the charging roller was cured with an acrylic resin, and this was cut with a microtome to prepare slices for transmission electron microscope photography. A transmission electron microscope photograph of this sample was taken and observed to determine average particle diameter by the method described previously.

[0159] The average particle diameter and content of the fine particles in the surface layer lower part and surface layer upper part of the charging roller of this Example are shown in Table 1.

[0160] Part of the transmission electron microscope photograph used to determine the average particle diameter and content is shown in FIGS. 9 to 11 . FIG. 9 shows how the surface layer stands in its total layer thickness; FIG. 10 , the surface layer lower part; and FIG. 11 , the surface layer upper part.

[0161] Measurement of hardness of elastic layer and surface layer:

[0162] The hardness of the elastic layer and surface layer was measured by the method described previously.

[0163] The hardness of the elastic layer was found to be 50°.

[0164] With regard to the surface layer, the hardness of the sheet sample prepared using the coating fluid for first-time dipping was 90° and the hardness of the sheet sample prepared using the coating fluid for second-time dipping was 95°; the both being higher than the hardness 50° of the elastic layer. Thus, the hardness of the whole surface layer can be deemed to be higher than the hardness of the elastic layer.

[0165] Evaluation of charging uniformity in applying only DC voltage to charging roller:

[0166] The above charging roller was set in the electrophotographic apparatus constructed as shown in FIG. 12 , and halftone images were reproduced in each environment of Environment 1 (temperature: 23° C.; relative humidity: 55%), Environment 2 (temperature: 32.5° C.; relative humidity: 80%) and Environment 3 (temperature: 15° C.; relative humidity: 10%). The electrophotographic apparatus used in this Example was drivable at process speeds of 94 mm/s and 30 mm/s. Here, the images were reproduced also controlling the applied voltage in each environment in such a way that the surface potential V _D of the electrophotographic photosensitive member 1 came to −600 V.

[0167] The results are shown in Table 1.

[0168] In Table 1, image levels are ranked as follows: Rank 1: very good; Rank 2: good; Rank 3: line-like and dot-like image defects are slightly seen on halftone images; and Rank 4: line-like and dot-like image defects are conspicuous.

[0169] Evaluation of pinhole leak-proofness of charging roller:

[0170] Pinholes of 0.1 mm in diameter and 0.2 mm in diameter were made at the surface of the electrophotographic photosensitive member, and this electrophotographic photosensitive member and the above charging roller were set in the electrophotographic apparatus constructed as shown in FIG. 12 , and halftone images were reproduced in each environment in the same manner as in the evaluation of charging uniformity. To the charging roller, a voltage formed by superimposing AC voltage on DC voltage was applied (DC: −600 V; AC: frequency of 1,000 Hz and VPP (peak-to-peak voltage) of 1,800 V).

[0171] The results are shown in Table 1.

[0172] In Table 1, image levels are ranked as follows: Rank 1: no leak is seen on halftone images; Rank 2: leak images of 3 mm or less in diameter are seen on both sides of the pinhole of 0.1 mm in diameter; Rank 3: leak images are seen at the pinhole of 0.1 mm in diameter; and Rank 4: leak images are seen at the pinhole of 0.2 mm in diameter.

[0173] Evaluation of running performance (durability) in applying only DC voltage to charging roller:

[0174] After the above charging uniformity and pinhole leak-proofness were evaluated, a continuous 10,000-sheet image reproduction running test was conducted in each environment. The images formed were visually observed to evaluate the running performance of the charging roller. In this evaluation, the wearing characteristics and initial-function maintenance ability of the charging roller can be evaluated by examining the images.

[0175] The results are shown in Table 2.

[0176] In Table 2, image levels are ranked as follows: Rank 1: no changes from initial-stage images; Rank 2: coarse images (due to slight wear) are slightly seen in halftone images; Rank 3: coarse images and dots (due to slight coming-off of fine particles which is caused by wear) appear slightly in halftone images; and Rank 4: coarse images and dots appear in halftone images.

EXAMPLE 2

[0177] As to the charging roller in this Example, the elastic layer was formed in the same manner as in Example 1.

[0178] The surface layer 2 d was formed on the elastic layer to cover it. The surface layer 2 d was formed by coating the following surface layer coating fluid by dipping. The dipping was carried out three times.

[0179] First, as a coating fluid for first-time dipping and second-time dipping, a liquid mixture was prepared in a container glass bottle, using the following materials as materials for the surface layer 2 d . 4

[0180] Into this container, as dispersion media, glass beads (average particle diameter: 1.0 mm) were so packed as to be in a packing of 70%, followed by dispersion for 7 hours using a paint shaker dispersion machine. To the resulting liquid dispersion, a 3:1 mixture of hexamethylene diisocyanate (HDI) butanone oxime block product and isophorone diisocyanate (IPDI) butanone oxime block product was so added as to be NCO/OH=1.1 to prepare the coating fluid for first-time dipping and second-time dipping.

[0181] Subsequently, as a coating fluid for third-time dipping, a coating fluid was prepared in the same manner as the coating fluid for first-time dipping and second-time dipping but using as the fine particles the following particles instead, changing the dispersion media glass beads for those having an average particle diameter of 0.8 μm and changing the paint shaker dispersion time to 25 hours. 5

[0182] On the surface of the above elastic layer, the above surface layer coating solutions were coated by dipping carried out three times. In the first-time dipping and second-time dipping, the draw-up speed was fixed to 7 mm/s. First, the coating fluid for first-time dipping was coated, followed by air drying at normal temperature for 10 to 30 minutes. Then, the roller was reversed, and the coating fluid for second-time dipping, the same coating fluid as the coating fluid for first-time dipping, was coated in the same manner. Thereafter, this was air-dryed at normal temperature for 10 to 30 minutes, and then the coating fluid for third-time dipping was coated. In the third-time dipping, the coating fluid was coated changing the draw-up speed in the same manner as in Example 1. The coating thus carried out was followed by air drying at normal temperature for 30 minutes or more, and subsequently drying in a circulating hot air dryer at 160° C. for 1 hour. The surface layer having been dried was in a layer thickness of 25 μm.

[0183] On the charging roller thus produced, the average particle diameter and content of the fine particles in the surface layer were measured in the same manner as in Example 1. The results are shown in Table 1.

[0184] The hardness of the sheet sample prepared using the coating fluid for first-time dipping and second-time dipping was 89° and the hardness of the sheet sample prepared using the coating fluid for third-time dipping was 86°; the both being higher than the hardness 50° of the elastic layer. Thus, the hardness of the whole surface layer can be deemed to be higher than the hardness of the elastic layer.

[0185] On the charging roller of this Example, the same evaluation as that in Example 1 was also made. The results are shown in Tables 1 and 2.

EXAMPLE 3

[0186] As to the charging roller in this Example, the elastic layer was formed in the same manner as in Example 1.

[0187] The surface layer 2 d was formed on the elastic layer by carrying out dipping twice. The surface layer 2 d was formed using twice the same one as the coating fluid for first-time dipping in Example 1. The draw-up speed was fixed to 7 mm/s.

[0188] First, the coating fluid for first-time dipping was coated, followed by air drying at normal temperature for 10 to 30 minutes. Here, the coating fluid was allowed to stand also for the same time. Thereafter, the roller was reversed, and the same coating fluid as the coating fluid for first-time dipping was coated. The coating thus carried out was followed by air drying at normal temperature for 30 minutes or more, and subsequently drying in a circulating hot air dryer at 160° C. for 1 hour. The surface layer having been dried was in a layer thickness of 20 μm.

[0189] On the charging roller thus produced, the average particle diameter and content of the fine particles in the surface layer were measured in the same manner as in Example 1. The results are shown in Table 1.

[0190] The hardness of the surface layer was measured in the same manner as in Example 1. The hardness of the sheet sample prepared using the coating fluid for dipping was 89°, which was higher than the hardness 50° of the elastic layer. Thus, the hardness of the whole surface layer can be deemed to be higher than the hardness of the elastic layer.

[0191] On the charging roller of this Example, the same evaluation as that in Example 1 was also made. The results are shown in Tables 1 and 2.

EXAMPLE 4

[0192] As to the charging roller in this Example, the elastic layer was formed in the same manner as in Example 1.

[0193] The surface layer 2 d was formed on the elastic layer by carrying out dipping once. The surface layer 2 d was formed using the same one as the coating fluid for first-time dipping in Example 1. The draw-up speed was the same as that in Example 1 except that the initial-stage speed was set at 25 mm/s.

[0194] The coating thus carried out was followed by air drying at normal temperature for 30 minutes or more, and subsequently drying in a circulating hot air dryer at 160° C. for 1 hour. The surface layer having been dried was in a layer thickness of 18 μm.

[0195] On the charging roller thus produced, the average particle diameter and content of the fine particles in the surface layer were measured in the same manner as in Example 1. The results are shown in Table 1.

[0196] The hardness of the sheet sample prepared using the coating fluid for dipping (equal to the hardness of the surface layer) was 88°.

[0197] On the charging roller of this Example, the same evaluation as that in Example 1 was also made. The results are shown in Tables 1 and 2.

EXAMPLE 5

[0198] As to the charging roller in this Example, the charging roller was produced in the same manner as in Example 2 except that, in the coating fluid for first-time dipping and second-time dipping, the conductive fine tin oxide particles were changed for surface-untreated ones (average particle diameter: 0.10 μm; volume resistivity: 10 Ω·cm). The surface layer having been dried was in a layer thickness of 40 μm.

[0199] On the charging roller thus produced, the average particle diameter and content of the fine particles in the surface layer were measured in the same manner as in Example 1. The results are shown in Table 1.

[0200] The hardness of the sheet sample prepared using the coating fluid for first-time dipping and second-time dipping was 90° and the hardness of the sheet sample prepared using the coating fluid for third-time dipping was 86°; the both being higher than the hardness 50° of the elastic layer. Thus, the hardness of the whole surface layer can be deemed to be higher than the hardness of the elastic layer.

[0201] On the charging roller of this Example, the same evaluation as that in Example 1 was also made. The results are shown in Tables 1 and 2.

EXAMPLE 6

[0202] A charging roller was produced in the following way. 6

[0203] The above materials were kneaded for 10 minutes by means of an enclosed mixer adjusted to 50° C., and then further kneaded for 20 minutes by means of an enclosed mixer kept cooled to 20° C. to prepare a raw-material compound. To this compound, based on 100 parts by weight of the raw-material NBR, 1 part by weight of sulfur as a vulcanizing agent and 3 part by weight of NOCCELER TS (as a vulcanization accelerator were added, and these were kneaded for 10 minutes by means of a twin-roll mill kept cooled to 20° C. The compound thus obtained was extruded by means of an extruder around the periphery of a stainless-steel mandrel of 6 mm in diameter so as to be made into a roller, which was then vulcanized by heating and shaped by forming, followed by sanding so as to come to 12 mm in outer diameter to form the elastic layer 2 b . The roller was in a length of 232 mm.

[0204] The surface layer 2 d was formed on the elastic layer to cover it. The surface layer 2 d was formed by coating the following surface layer coating fluids by dipping. The dipping was carried out twice.

[0205] First, as a coating fluid for first-time dipping, a liquid mixture was prepared by mixing the following materials. 7

[0206] Using glass beads (average particle diameter: 0.8 mm) as dispersion media and using a bead mill dispersion machine packed with this dispersion media in a packing of 80%, the above liquid mixture was circulated five times in this dispersion machine to effect dispersion. To the resulting liquid dispersion, a hexamethylene diisocyanate butanone oxime block product was so added as to be NCO/OH=1.0 to prepare a surface layer coating fluid. Thus, the coating fluid for first-time dipping was prepared.

[0207] Subsequently, as a coating fluid for second-time dipping, a coating fluid was prepared in the same manner as the coating fluid for first-time dipping but changing the carbon black for the following one and changing the bead mill dispersion for that of 100-time circulation. 8

[0208] Subsequently, the surface layer was formed by coating in the same manner as in Example 1. The surface layer was in a layer thickness of 21 μm.

[0209] On the charging roller thus produced, the average particle diameter and content of the fine particles in the surface layer were measured in the same manner as in Example 1. The results are shown in Table 1.

[0210] The hardness of the elastic layer and surface layer was measured in the same manner as in Example 1.

[0211] The elastic layer was found to have a hardness of 45°. With regard to the surface layer, the hardness of the sheet sample prepared using the coating fluid for first-time dipping was 80° and the hardness of the sheet sample prepared using the coating fluid for second-time dipping was 76°; the both being higher than the hardness 45° of the elastic layer. Thus, the hardness of the whole surface layer can be deemed to be higher than the hardness of the elastic layer.

[0212] On the charging roller of this Example, the same evaluation as that in Example 1 was also made. The results are shown in Tables 1 and 2.

EXAMPLE 7

[0213] As to the charging roller in this Example, the elastic layer was formed in the same manner as in Example 4.

[0214] The surface layer 2 d was formed on the elastic layer to cover it. The surface layer 2 d was formed by coating the following surface layer coating fluids by dipping. The dipping was carried out twice.

[0215] First, as a coating fluid for first-time dipping, a liquid mixture was prepared by mixing the following materials. 9

[0216] Using glass beads (average particle diameter: 0.8 mm) as dispersion media and using a bead mill dispersion machine packed with this dispersion media in a packing of 80%, the above liquid mixture was circulated ten times in this dispersion machine to effect dispersion. Thus, the surface layer coating fluid for first-time dipping was prepared.

[0217] Subsequently, as a coating fluid for second-time dipping, a liquid mixture was prepared in a container glass bottle, using the following materials. 10

[0218] Into this container, as dispersion media, glass beads (average particle diameter: 0.8 mm) were so packed as to be in a packing of 80%, followed by dispersion for 6 hours using a paint shaker dispersion machine.

[0219] Subsequently, the surface layer was formed by coating in the same manner as in Example 1. The surface layer was in a layer thickness of 25 μm.

[0220] On the charging roller thus produced, the average particle diameter and content of the fine particles in the surface layer were measured in the same manner as in Example 1. The results are shown in Table 1.

[0221] The hardness of the elastic layer and surface layer was measured in the same manner as in Example 1.

[0222] The hardness of the sheet sample prepared using the coating fluid for first-time dipping was 58° and the hardness of the sheet sample prepared using the coating fluid for second-time dipping was 65°; the both being higher than the hardness 50° of the elastic layer. Thus, the hardness of the whole surface layer can be deemed to be higher than the hardness of the elastic layer.

[0223] On the charging roller of this Example, the same evaluation as that in Example 1 was also made. The results are shown in Tables 1 and 2.

EXAMPLE 8

[0224] As to the charging roller in this Example, the elastic layer was formed in the same manner as in Example 4.

[0225] The surface layer 2 d was formed on the elastic layer to cover it. The surface layer 2 d was formed by coating the following surface layer coating fluids by dipping. The dipping was carried out twice.

[0226] First, as a coating fluid for first-time dipping, a liquid mixture was prepared in a container glass bottle, using the following materials. 11

[0227] Into this container, as dispersion media, glass beads (average particle diameter: 0.8 mm) were so packed as to be in a packing of 50%, followed by dispersion for 0.5 hour using a paint shaker dispersion machine to prepare the coating fluid for first-time dipping.

[0228] Subsequently, as a coating fluid for second-time dipping, a liquid mixture was prepared in a container glass bottle, using the following materials. 12