Patent Description:
Image quality produced by an image forming apparatus such as an electrophotographic copying machine depends on uniformity of a charged state of the photoconductor element, which is affected by the surface roughness of a charging roll. Patent Documents <NUM> to <NUM> are known as conventional techniques that relate to a surface roughness of charging rolls.

Patent Document <NUM> describes a technique related to a charging member (charging roll) consisting of an electroconductive support, an electroconductive elastic layer laminated on the electroconductive support, and an electroconductive resin layer laminated as the outermost layer on the electroconductive elastic layer. The electroconductive resin layer contains a matrix material and at least one kind of particles selected from a group consisting of resin particles and inorganic particles, the particles containing first particles, in which A is <NUM> micrometers to <NUM> micrometers, B1/A is <NUM> to <NUM>, and Sm is <NUM> micrometers to <NUM> micrometers, where the thickness of a portion of the electroconductive resin layer formed by the matrix material alone is A [micrometers], the mean particle diameter of the particles is B1 [micrometers], and the inter-particle distance is Sm [micrometers].

Patent Document <NUM> discloses a technique that relates to an image forming apparatus including a positively-charged single-layer type electrophotographic photoconductor element; a charging device having a contact-type charging member for charging the surface of the photoconductor element; an exposure device for exposing the surface of the charged image carrier to form an electrostatic latent image on the surface of the image carrier; a developing device for developing the electrostatic latent image as a toner image; and a transfer device for transferring the toner image from the image carrier to a transfer object. The contact-type charging member is a charging roller made of electroconductive rubber and has an Asker-C rubber hardness of <NUM> degrees to <NUM> degrees, and a surface roughness of the charging roller of the contact-type charging member has a mean interval Sm of <NUM> micrometers to <NUM> micrometers between surface irregularities, and a ten-point height irregularity RZ of <NUM> micrometers to <NUM> micrometers.

Patent Document <NUM> discloses a technique that relates to a charging roller including an electroconductive support, a roll-shaped semi-electroconductive elastic layer formed on the electroconductive support, and a protective layer formed on the surface of the semi-electroconductive elastic layer. The protective layer is formed by application of a coating liquid for forming the protective layer containing fine particles that prevent adhesion of an external substance to the protective layer, with the volume average particle diameter of the fine particles being refined such that the surface roughness of the protective layer is equal to or less than <NUM> micrometer.

An object of Patent Documents <NUM> to <NUM> is to control a discharge between the charging roll and the photoconductor element to make the discharge as uniform as possible, which is achieved by adjusting a surface roughness of the outermost surface of the charging roll by use of fine particles in the surface layer, to thereby improve an image quality.

Patent Document <NUM> discloses an electroconductive roll, in particular a charging roll, as specified in the preamble of claim <NUM>.

Demand exists for image forming apparatuses that provide a high image quality.

The present invention provides a charging roll that reduces image unevenness.

A charging roll according to the present invention includes the features specified in claim <NUM>.

Preferably, the charging roll according to the present invention further includes the features specified in claim <NUM>.

An embodiment for carrying out the present invention will now be described in detail. In the drawings, the scale is not necessarily to scale, and some dimensions may be exaggerated for illustrative products or samples.

As shown in <FIG>, an image forming apparatus according to an embodiment of the present invention includes a photoconductor element <NUM>. Around the photoconductor element <NUM>, a developing section <NUM>, an exposure section <NUM>, a charging section <NUM>, a transfer section <NUM>, and a cleaning section <NUM> are arranged. In the developing section <NUM>, a developing roll <NUM>, a regulating blade <NUM>, and a supply roll <NUM> are disposed, and the toner <NUM> is stored. The charging section <NUM> is provided with a charging roll <NUM>. The transfer section <NUM> transfers the toner image onto a sheet <NUM> of paper, which is a recording medium. The toner image transferred by the transfer section <NUM> is fixed by a fusing section (not shown).

The cylindrical and rotating photoconductor element <NUM> and the cylindrical and rotating charging roll <NUM> are in contact with each other at the nip <NUM>. Discharge between the photoconductor element <NUM> and the charging roll <NUM> occurs in the region <NUM> in front of the nip <NUM> in the rotational direction of the photoconductor element <NUM> and the charging roll <NUM> (in some cases, in addition to discharge in the region <NUM> in front of the nip <NUM> discharge occurs in the region <NUM> behind the nip <NUM>), whereby the surface of the photoconductor element <NUM> is charged. Preferably, the charged state of the surface of the photoconductor element <NUM> is uniform in both the circumferential direction and the axial direction of the photoconductor element <NUM>.

<FIG> is a cross-sectional view showing an example of a charging roll according to an embodiment of the present invention. As shown in <FIG>, the charging roll <NUM> includes a core member <NUM>, a rubber base material <NUM> formed on the outer peripheral surface of the core member <NUM>, and a surface layer <NUM> coated on the outer peripheral surface of the rubber base material <NUM>. By coating the outer peripheral surface of the rubber base material <NUM> with the surface layer <NUM> having a coating composition that is formed to have a suitable surface state, uneven discharge between the photoconductor element <NUM> and the charging roller <NUM> can be prevented and uniform discharge can be provided to the photoconductor element <NUM> such that the developing section <NUM> adheres to the surface of the photoconductor element <NUM> an amount of toner that accurately corresponds to the latent image formed by the exposing section <NUM>.

The core member <NUM> can be formed of a material, including, but not limited to, a metal or resin material having excellent thermal conductivity and mechanical strength, for example, a metal material such as stainless steel, nickel (Ni), nickel alloy, iron (Fe), magnetic stainless steel, and cobalt-nickel (Co-Ni) alloy, or a resin material such as PI (polyimide resin). The structure of the core member <NUM> is not particularly limited, and it may be hollow or not hollow.

The rubber base material <NUM> is disposed on the outer peripheral surface of the core member <NUM>, and is formed of electroconductive rubber having conductivity. The rubber base material <NUM> may be composed of a single layer or two or more layers. In addition, an adhesion layer, an adjustment layer, etc. may be interposed between the core member <NUM> and the rubber base material <NUM>, as appropriate.

The rubber base material <NUM> can be formed by molding a rubber composition, which is obtained by adding a conductivity imparting material, a crosslinking agent, etc. to an electroconductive rubber, around the core member <NUM>. Examples of the electroconductive rubber include polyurethane rubber (PUR), epichlorohydrin rubber (ECO), nitrile rubber (NBR), styrene rubber (SBR), and chloroprene rubber (CR).

As the conductivity imparting material, an electronic conductivity imparting material such as carbon black or metal powder, an ionic conductivity imparting material, or a mixture thereof can be used.

Examples of the ionic conductivity imparting material include organic salts, inorganic salts, metal complexes, and ionic liquids. An example of an organic salt is sodium trifluoride acetate, and examples of the inorganic salt includes lithium perchlorate and quaternary ammonium salt. An example of a metal complex is ferric halide-ethylene glycol, and specific examples thereof include those described in <CIT>. The ionic liquid is a molten salt that is liquid at room temperature, and is referred to as a room temperature molten salt. The salt has a melting point of <NUM> degrees Celsius or less, preferably <NUM> degrees Celsius or less. Specific examples thereof include those described in <CIT>.

The crosslinking agent is not particularly limited, and examples thereof include sulfur and a peroxide vulcanizing agent.

Furthermore, a crosslinking aid, etc. that promotes action of the crosslinking agent may be added to the rubber composition, as appropriate. Examples of the crosslinking aid include inorganic materials, such as zinc oxide and magnesium oxide, and organic materials, such as stearic acid and amines. In addition, to shorten a time taken to achieve crosslinking, a thiazole-based or other crosslinking accelerator may be used. Other additives may be added to the rubber composition, as appropriate.

In this embodiment, the surface of the rubber base material <NUM> formed on the outer peripheral surface of the core member <NUM> is first ground to a predetermined thickness with a grinding machine, after which the surface of the rubber base material <NUM> is subjected to dry grinding with a grinding wheel. The surface layer <NUM> is then formed on the outer peripheral surface of the rubber base material <NUM>. Grinding is performed to adjust the surface roughness of the rubber base material <NUM> as appropriate, and to thereby adjust the surface state of the surface layer <NUM> formed on the outer peripheral surface of the rubber base material <NUM>.

In a case in which the surface roughness of the rubber base material <NUM> is to be minimized, the surface roughness (ten-point height irregularities) RZ according to JIS B <NUM> (<NUM>) of the rubber base material <NUM> is preferably equal to or less than <NUM> micrometers. The surface roughness RZ is measured by a contact-type surface roughness meter.

Dry grinding is performed, for example, in a state in which the rubber base material <NUM> is rotated, by moving the rotary grinding wheel along the axial direction of the core member <NUM> while the wheel is in contact with the rubber base material <NUM> (traverse grinding). In a case in which the surface roughness of the rubber base material <NUM> is to be minimized, the number of revolutions of the grinding wheel of the grinding machine may be gradually increased, for example, from <NUM> rpm, to <NUM> rpm, to <NUM> rpm. Alternatively, the coarseness of a grinding wheel may be progressively changed. For example, a GC (green carborundum) grinding wheel may be changed, for example, from a GC <NUM> wheel, to a GC <NUM> wheel, to a GC <NUM> wheel.

In addition, after the surface of the rubber base material <NUM> is dry-ground, the surface may be wet ground with a wet grinding machine in which a waterproof grinding paper such as waterproof sandpaper is employed, with the rubber base material <NUM> being brought into contact with the sandpaper under supply of a grinding liquid.

The rubber hardness of the base material <NUM> is measured by use of a durometer "Type A" according to JIS K <NUM> and ISO <NUM>, and the hardness is preferably within a range from <NUM> degrees to <NUM> degrees.

The surface layer <NUM> formed on the rubber base material <NUM> is thin, and thus a hardness of the surface of the charging roll <NUM> is affected by a hardness of the rubber base material <NUM>. In a case in which the hardness of the rubber base material <NUM> is less than <NUM> degrees, convex portions on the surface of the charging roll <NUM> are likely to be crushed and contaminate the photoconductor element <NUM>, and cause image defects. On the other hand, if the hardness of the rubber base material <NUM> is greater than <NUM> degrees, convex portions on the surface of the charging roll <NUM> may affect the image.

In this embodiment, a coating liquid is applied to the outer peripheral surface of the rubber base material <NUM> and dried and cured, thereby forming the surface layer <NUM>. Application of the coating liquid may be carried out by dip coating, roll coating, spray coating, or the like.

As shown in <FIG>, the cured surface layer <NUM> includes an electroconductive matrix <NUM> and particles <NUM> of a surface roughness enhancing material (also referred to as a roughness enhancing material), which may be, e.g., an electric insulator, dispersed in the electroconductive matrix <NUM>. The particles <NUM> of the roughness enhancing material provide the surface layer <NUM> with an appropriate surface roughness. The electroconductive matrix <NUM> serves to hold the particles <NUM> of the roughness enhancing material in position and serves to effect discharge to the photoconductor element <NUM>. The electroconductive matrix <NUM> contains a base material and an electroconductive material dispersed in the base material. As described above, discharge occurs between the charging roller <NUM> and the photoconductor element <NUM> in the region <NUM> (and in some cases in the region <NUM>, also).

In the example shown in <FIG>, the particles <NUM> of the roughness enhancing material are not completely embedded in the electroconductive matrix <NUM>. If the thickness of the electroconductive matrix <NUM> is small, the ability of the matrix to hold the particles <NUM> of the roughness enhancing material will also be low. Accordingly, it is preferable for the electroconductive matrix <NUM> to have a thickness that is appropriate relative to the diameter of the particles <NUM> of the roughness enhancing material. When the particles <NUM> of the roughness enhancing material are made of an electric insulator, when the thickness of the electroconductive matrix <NUM> is large, and when the electrical resistance of the electroconductive matrix <NUM> is large, discharge is less likely to occur. However, by increasing the proportion of the electroconductive material contained in the electroconductive matrix <NUM>, the electrical resistance of the electroconductive matrix <NUM> can be reduced to facilitate occurrence of discharge.

In the present embodiment, the surface state of the surface layer <NUM> is adjusted by dispersing the particles <NUM> of the roughness enhancing material in the surface layer <NUM> formed on the rubber base material <NUM>, of which the surface roughness is adjusted.

In the present embodiment, it would be preferable for the thickness of the electroconductive matrix <NUM> of the surface layer <NUM> to be within an appropriate numerical range. It is contemplated that if the thickness is too large, the surface roughness of the surface layer <NUM> will be too small resulting in image unevenness.

Furthermore, in the present embodiment, it would be preferable for the amount of the particles <NUM> of the roughness enhancing material in the surface layer <NUM> to be within an appropriate numerical range. It is contemplated that if the amount of the particles is large, the particles may overlap, causing the surface of the surface layer <NUM> to be rough, and resulting image unevenness.

In this embodiment, the composition of the coating liquid that is the material of the surface layer <NUM> contains at least the base material, the electroconductive material, and the particles <NUM> of the surface roughness enhancing material. After curing of the coating liquid, the base material and the electroconductive material become components of the electroconductive matrix <NUM>.

The coating liquid is obtained, for example, by dissolving in a diluent solvent the following components.

Base material, <NUM> to <NUM> parts by weight;.

It is contemplated that when the surface state of the surface layer <NUM> is appropriate, discharge between the charging roll <NUM> and the photoconductor element <NUM> will be substantially uniform in the gap before the nip, at which the charging roll <NUM> and the photoconductor element <NUM> are in contact with each other, so that uneven discharge will not occur upon image formation, whereby an image of a desired density will be formed, with an end result of provision of high image quality.

It is considered that the surface state of the surface layer <NUM> can be adjusted as appropriate by adjusting the particle diameter and the amount of the particles <NUM> of the surface roughness enhancing material.

The base material contained in the coating liquid is an electric insulator. Examples of the base material include urethane resin, acrylic resin, acrylic urethane resin, amino resin, silicone resin, fluorine resin, polyamide resin, epoxy resin, polyester resin, polyether resin, phenol resin, urea resin, polyvinylbutyral resin, melamine resin, nylon resin, etc. The base materials may be used alone or in combination.

Examples of the electroconductive material contained in the coating liquid include a carbon black such as acetylene black, Ketjen black, and Tokablack, a carbon nanotube, an ion such as lithium perchloride, an ionic liquid such as <NUM>-butyl-<NUM>-methylimidazolium hexafluorophosphate, and a metal oxide such as tin oxide, and an electroconductive polymer. These electroconductive materials may be used alone or in combination.

Examples of the particles <NUM> of the surface roughness enhancing material contained in the coating liquid include acrylic particles, urethane particles, polyamide resin particles, silicone resin particles, fluororesin particles, styrene resin particles, phenol resin particles, polyester resin particles, olefin resin particles, epoxy resin particles, nylon resin particles, carbon, graphite, carbide balloon, silica, alumina, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide, calcium sulfate, calcium carbonate, magnesium carbonate, calcium silicate, aluminum nitride, boron nitride, talc, kaolin clay, diatomaceous earth, glass beads, hollow glass spheres, etc. These particles may be used alone or in combination.

It is considered that there is a preferable range with respect to the relationship between the particle diameter and the amount of the particles <NUM> of the surface roughness enhancing material in the coating liquid in order to improve the image quality.

The diluent solvent contained in the coating liquid is not particularly limited, and examples thereof include an aqueous-based solvent or other solvents such as methyl acetate, ethyl acetate, butyl acetate, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), methanol, ethanol, butanol, <NUM>-propanol (IPA), acetone, toluene, xylene, hexane, heptane, and chloroform.

Hereinafter, working examples of the present embodiment will be described in greater detail.

A rubber composition obtained by adding <NUM> parts by weight of sodium trifluoroacetate (as a conductivity imparting material), <NUM> parts by weight of zinc oxide, <NUM> parts by weight of stearic acid, and <NUM> parts by weight of a crosslinking agent to <NUM> parts by weight of epichlorohydrin rubber ("Epichlomer CG-<NUM>" manufactured by Osaka Soda Co. , Osaka, Japan) was kneaded with a roll mixer.

The kneaded rubber composition was formed into a sheet material and wound around the surface of a core member <NUM> having a diameter of <NUM>. The sheet material was press-molded to form a rubber base material <NUM> made of crosslinked epichlorohydrin rubber.

The hardness of the resulting rubber base material <NUM> was measured using a durometer "Type A" according to JIS K <NUM> and ISO <NUM>. The measured hardness was within a range from <NUM> degrees to <NUM> degrees.

The surface of the rubber base material <NUM> was ground with a grinding machine. More specifically, the surface of the obtained rubber base material <NUM> was ground with a grinding machine to provide the rubber base material <NUM> with a predetermined thickness (<NUM>), followed by dry grinding in which the rotation speed of the grinding wheel of the grinding machine was gradually increased from <NUM> rpm, to <NUM> rpm, to <NUM> rpm. That is, in Experiment <NUM>, the surface roughness of the rubber base material <NUM> was minimized.

A coating liquid for forming the surface layer <NUM> on the outer peripheral surface of the rubber base material <NUM> described above was prepared.

The composition of the coating liquid is as shown in Table <NUM>.

Urethane beads manufactured by Negami Chemical Industrial Co. (Tokyo, Japan) were used as the urethane particles.

The relationship between the average particle diameter of the urethane beads and the product name is as follows. It is of note that in practice, one product contains particles having diameters that differ from the average particle diameter.

In Experiment <NUM>, samples having different surface conditions of the surface layer <NUM> were produced by applying coating liquids containing particles <NUM> of the surface roughness enhancing material having different particle diameters and in different amounts. The particle diameters and amounts of particles <NUM> in the samples are as shown in Table <NUM>. In Table <NUM>, samples <NUM> to <NUM> are samples of Experiment <NUM>. However, in sample <NUM>, the particles <NUM> of the roughness enhancing material are not included in the surface layer <NUM>.

The coating liquid having the above composition was stirred with a ball mill for <NUM> hours.

The surface layer <NUM> was formed by applying the coating liquid to the outer peripheral surface of the ground rubber base material <NUM>, to manufacture a charging roll <NUM>. Specifically, the coating liquid was stirred, and the liquid was spray-coated on the surface of the rubber base material <NUM>, and dried in an electric furnace at <NUM> degrees Celsius for <NUM> minutes to form the surface layer <NUM> on the outer peripheral surface of the rubber base material <NUM>, to produce a charged roll.

Measurement of Surface Area of Surface Layer per Unit Projected Area The surface area of the surface layer <NUM> per unit projected area of each sample of the charging roll <NUM> was measured. <FIG> is a diagram illustrating the surface area of the surface layer <NUM> per unit projected area of each sample of the charging roll <NUM>.

First, the surface of the central portion in the axial direction of the charging roll <NUM> was photographed with a non-contact type laser microscope. The laser microscope used was a "VK-X200" manufactured by Keyence Corporation (Osaka, Japan). Magnification was <NUM> times, and the photographic field of view was <NUM> micrometers along the circumferential direction of the charging roll <NUM> and was <NUM> micrometers along the axial direction of the charging roll <NUM>. The unit projected area of the samples is the area A of the photographed field of view, that is, X · Y = <NUM> × <NUM> (square micrometers).

Next, using Version <NUM><NUM>. <NUM> of the multi-file analysis application "VK-H1XM" produced by Keyence Corporation, the second-order curved surface correction was performed for the geometric data obtained by photographing. Second-order curved surface correction is a process of removing data components corresponding to the cylindrical surface of the charging roll <NUM> from the geometrical data obtained by photographing. In other words, it is a process of converting the geometric data on the cylindrical surface obtained by photographing into geometric data on a plane.

Thereafter, by means of the above application, the surface area S of the surface layer <NUM> in the photographed field of view was calculated. The surface area S is the surface area of the surface layer <NUM> including irregularities. Furthermore, the surface area of the surface layer <NUM> per unit projected area was calculated by dividing the surface area S by the area A of the photographed field of view. The surface area of the surface layer <NUM> per unit projected area thus obtained is shown in Table <NUM>.

An image evaluation test of the samples of the charging roll was conducted using a copying machine. The copying machine was a color multifunction peripheral (MFP) "bizhub C3850" (DC-voltage supply type) manufactured by Konica Minolta Inc. (Tokyo, Japan).

The applied charging voltage was measured with a tester. In Experiment <NUM>, a voltage (REF - <NUM> V), which was <NUM> V lower than the normal voltage (REF), was applied by way of an external power supply.

The charging roll was applied to the copying machine, and image unevenness was evaluated for images (halftone images and white solid images) printed under the conditions described below. The results are shown in Table <NUM>.

For the image unevenness evaluation, occurrence of local discharge was judged on the basis of the halftone images, and lightness was judged on the basis of the white solid images. Occurrence of local discharge was confirmed by visual detection of white spots, black spots, white streaks, or black streaks in the halftone images.

For the halftone images, occurrence of image unevenness caused by local discharge was evaluated by visual observation using the following criteria.

The L* value (lightness) was measured at seven points in each image by a chroma meter, "CR-<NUM>" manufactured by Konica Minolta Inc. The lightness was evaluated with the following evaluation criteria. The reason why the lightness was measured was to determine whether scumming, i.e., fogging (printing on a non-print area) occurred.

Samples in which image unevenness occurred due to local discharge or scumming were judged to be bad in image comprehensive judgment, and these were described in Table <NUM>.

A rubber composition obtained by adding <NUM> parts by weight of sodium trifluoroacetate (as a conductivity imparting material), <NUM> parts by weight of zinc oxide, <NUM> parts by weight of stearic acid, and <NUM> parts by weight of a crosslinking agent to <NUM> parts by weight of epichlorohydrin rubber ("Epichlomer CG-<NUM>" manufactured by Osaka Soda Co. was kneaded with a roll mixer.

The hardness of the resulting rubber base material <NUM> was measured using a durometer "Type A" according to JIS K <NUM> and ISO <NUM>. The measured hardness fell within a range from <NUM> degrees to <NUM> degrees.

The surface of the rubber base material <NUM> was ground with a grinding machine. More specifically, the surface of the obtained rubber base material <NUM> was ground with a grinding machine to provide the rubber base material <NUM> with a predetermined thickness (<NUM>), after which dry grinding was applied. In Experiment <NUM>, the rotation speed of the grinding wheel was not changed.

The composition of the coating liquid is shown in Table <NUM>.

In Experiment <NUM>, samples having different surface conditions on the surface layer <NUM> were produced by applying coating liquids containing particles <NUM> of the surface roughness enhancing material having different particle diameters and in different amounts. The particle diameters and amounts of particles <NUM> in the samples are shown in Table <NUM>. In Table <NUM>, samples <NUM> to <NUM> are samples of Experiment <NUM>.

Measurement of Surface Area of Surface Layer Per Unit Projected Area Using the same procedure as in Experiment <NUM>, the surface area of the surface layer <NUM> per unit projected area of each sample of the charged roll <NUM> was measured. The surface area of the surface layer <NUM> per unit projected area is shown in Table <NUM>.

An image evaluation test of the samples of the charging roll was conducted using a copying machine. The copying machine was a color multifunction peripheral (MFP) "MP C5503" (AC/DC voltage-superimposed supply type) manufactured by Ricoh Company, Ltd. (Tokyo, Japan).

The DC voltage was the normal voltage (REF), and the AC voltage Vpp was controlled by the copying machine.

In Experiment <NUM>, the alternating current was set at <NUM> mA, which is lower than the normal alternating current (REF) of the copying machine.

The charging roll was applied to the copying machine, and the image unevenness was evaluated for images (halftone images and white solid images) printed under the following printing conditions. The results are shown in Table <NUM>. For the image unevenness evaluation, occurrence of local discharge was judged on the basis of the halftone images. Occurrence of local discharge was confirmed by visual detection of white spots, black spots, white streaks, or black streaks in the halftone images. Occurrence of scumming, i.e., fogging was judged by visual detection in the white solid images.

Printing environment: The temperature was <NUM> degrees Celsius and the humidity was <NUM>%.

For the halftone images, occurrence of image unevenness caused by local discharge was judged by visual observation using the following criteria.

For the white solid images, occurrence of scumming, i.e., fogging (printing on a no-print area) was judged by visual observation.

Samples in which image unevenness caused by local discharge or scumming occurred were judged to be bad using comprehensive image judgment, as described in Table <NUM>.

As will be apparent from Table <NUM>, whereas image unevenness occurred in Samples <NUM> and <NUM>, good images were generated in the other samples.

Accordingly, it is preferable for the surface area of the surface layer <NUM> per unit projected area to be equal to or greater than <NUM> and to be equal to or less than <NUM>.

Claim 1:
A charging roll (<NUM>) comprising:
a core member (<NUM>), a rubber base material (<NUM>) disposed around the core member (<NUM>), and a surface layer (<NUM>) disposed around the rubber base material (<NUM>), wherein
a surface area of the surface layer (<NUM>) per unit projected area is equal to or greater than <NUM> and being equal to or less than <NUM>,
the surface layer (<NUM>) comprises an electroconductive matrix (<NUM>) comprising a base material formed of an electric insulator and an electroconductive material being dispersed in the base material, and particles (<NUM>) of a surface roughness enhancing material being dispersed in the electroconductive matrix (<NUM>),
the particles (<NUM>) of the surface roughness enhancing material are formed of an electric insulator,
characterized in that
the particles (<NUM>) of the surface roughness enhancing material are not completely embedded in the electroconductive matrix (<NUM>).