Intermediate transfer member and electrophotographic apparatus including same

Abstract: An intermediate transfer member is formed by at least a base layer (62) and a surface layer (63). The base layer (62) is formed by epichlorohydrin rubber and acrylonitrile-butadiene rubber mixed in a certain weight ratio (preferably 1:9 to 9:1). The surface layer (63) is formed by urethane resin or elastomer and a fluorine-containing compound powder. The fluorine-containing compound powder may preferably be contained in an amount of 20 - 80 wt. % based on the surface layer. The intermediate transfer member is suitable for full-color image formation by electrophotography and is effective in improving a transfer performance and providing clear images without causing transfer failure even in repetitive use for a long period.

Description: FIELD OF THE INVENTION AND RELATED ARTS The present invention relates to an intermediate transfer member for temporarily holding an image in an image forming process according to electrophotography, and an electrophotographic apparatus including the intermediate transfer member. An electrophotographic apparatus including an intermediate transfer member is very effective for forming a color image by sequentially superposing and transferring a plurality of component color images. For example, it is possible to decrease color deviation in superposing respective color toner images compared with a transfer process described in Japanese Laid-Open Patent Application JP-A 63-301960. Moreover, it is possible to transfer an image from the intermediate transfer member onto a recording medium or transfer-receiving material without necessitating holding means, such as gripper means, sucking means or curvature means (as disclosed in Figure 1 of JP-A 63-301960), so that the recording medium can be selected from a wide variety of materials, including thin paper (40 g/m 2 ) to thick paper (200 g/m 2 ), wide to narrow medium, and long to short medium. Accordingly, transfer can be performed onto an envelope, a post card and even label paper, etc. Because of such advantageous features, color copying machines and color printers using intermediate transfer members have already been available on the market. However, a conventional intermediate transfer member has caused the following difficulties when actually used repetitively in various environments. (1) When the intermediate transfer member is used for a long period of time, a transfer efficiency from a photosensitive drum to the intermediate transfer member and a transfer efficiency from the intermediate transfer member to, e.g., paper or an OHP sheet are lowered. For this reason, in order to effect cleaning of a large amount of a transfer residual toner, a load on the photosensitive drum, the intermediate transfer member or a cleaning device becomes large, thus shortening the life of these members. In addition, the cleaning device is considerably complicated in structure and also becomes expensive. (2) The intermediate transfer member is deteriorated by, e.g., ozone with repetitive use thereof to change its surface property and its resistance in some cases. If the intermediate transfer member is considerably deteriorated, cracks in an elastic layer and a coating layer of the intermediate transfer member and a peeling of a surface layer thereof are caused, thus failing to maintain a good transfer efficiency and a uniform image obtained at an initial stage. US 5,530,532 which corresponds to JP-A-7 72740 discloses an intermediate transfer member, having a defined surface roughness in relation to toner particles to be used, for an image forming apparatus that uses toner particles in a liquid developer. The intermediate transfer member has a cushion layer formed from rubber (e.g. acrylonitnile-butadiene-copolymer or epichlorohydrin) and a conductive polymer. The cushion layer has a surface layer which may include microparticles of a fluororesin. SUMMARY OF THE INVENTION An object of the present invention is to provide an intermediate transfer member excellent in transfer efficiency and durability. Another object of the present invention is to provide an electrophotographic apparatus including such an intermediate transfer member and capable of providing clear images without causing transfer failure of a toner even when used for a long period. The above objects are achieved by an intermediate transfer member according to claim 1 and a method for forming an image according to claim 12. According to a first aspect of the invention, there is provided an intermediate transfer member for temporarily holding a toner image, comprising a base layer and a surface layer wherein the surface layer comprises a powder of a fluorine-containing compound, characterised in that the base layer comprises epichlorohydrin rubber and acrylonitrile-butadiene rubber in a mixing ratio of from 1:9 to 9:1 and the surface layer comprises urethane resin or urethane elastomer. According to a second aspect the invention, there is further provided an image forming apparatus comprising an electrophotographic photosensitive member, charging means for charging the electrophotographic photosensitive member, image exposure means for exposing the charged electrophotographic photosensitive member to an image, to form an electrostatic latent image, developing means for developing the electrostatic latent image to form a toner image on the electrophotographic member, and an intermediate transfer member as described above for temporarily holding a toner image received from the electrophotographic photosensitive member. Other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: Figure 1 is a perspective view for illustrating an embodiment of the intermediate transfer member according to the invention. Figure 2 is a view for illustrating a method for measuring an electric resistance of an intermediate transfer member. Figures 3 and 4 are side views each illustrating an embodiment of an electrophotographic apparatus according to the invention. Figure 5 is a side view for illustrating an embodiment of an apparatus for performing a durability test of the intermediate transfer member according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinbelow, the intermediate transfer member according to the present invention will be described with respect to some embodiments in the form of a drum but need not be restricted to such a drum form. Figure 1 is a perspective illustration of an embodiment of the intermediate transfer member according to the present invention. Referring to Figure 1, the intermediate transfer member comprises an electroconductive support 61 in the form of a cylinder, a base layer 62 disposed thereon, and a surface layer 63 disposed on the base layer 62. In the case of an intermediate transfer member in the form of a belt, the support 61 is not used. The surface layer 63 comprises a fluorine-containing compound powder and a binder. By incorporating the fluorine-containing powder, the surface of the intermediate transfer member is provided with a sufficient lubricating property, thus improving a secondary transferability (a transferability of a toner image from the intermediate transfer member to a secondary image-carrying member, such as a recording paper) and a durability. Further, it is possible to prevent a filming phenomenon thereby to reduce an abrasion of a photosensitive member. The binder for the surface layer 63 may preferably have a property allowing a sufficient mixing and dispersion of the fluorine-containing compound powder. If the binder is not appropriately selected, the fluorine-containing compound powder is not dispersed or is dispersed but results in a brittle state, so that the resultant surface layer is liable to be cracked or damaged with repetitive use and becomes ununiform in the case of a remarkable dispersion failure. Further, the surface layer is accompanied with a problem that the fluorine-containing compound powder once incorporated into the surface layer is dropped out to provide a roughened surface thereto and a lowering in a transfer efficiency. In view of the above circumstances, the binder of the surface layer may preferably comprise a urethane resin or a urethane elastomer exhibiting a sufficient strength and dispersibility. Such a urethane resin or a urethane elastomer used as the binder of the surface layer in the present invention may preferably have an elongation of at least 150 %, a tensile strength of at least 300 kgf/cm 2 and a tensile stress of at most 250 kgf/cm 2 at an elongation of 100 %, as measured according to JIS K-6301, in order to further improve a durability of the surface layer. The urethane resin or the urethane elastomer may more preferably have an elongation of at least 250 %, a tensile strength of at least 400 kgf/cm 2 and a tensile stress of at most 200 kgf/cm 2 at an elongation of 100 %; further preferably an elongation of at least 350 %, a tensile strength of at least 450 kgf/cm 2 and a tensile stress of at most 150 kgf/cm 2 at an elongation of 100 %. The fluorine-containing compound powder described above may preferably be contained in the surface layer in an amount of 20 - 80 wt. %, more preferably 30 - 70 wt. %, based on the surface layer. If the content is below 20 wt. %, an impartment of a lubricity to the surface layer becomes insufficient, thus resulting in a lowering in a secondary transfer efficiency and an occurrence of a filming. If the content exceeds 80 wt. %, a resultant surface layer becomes brittle and lowers its adhesiveness to an underlying layer, thus causing deficiencies, such as peeling or crack with respect to the surface layer in repetitive use for a long period. Examples of the fluorine-containing compound powder may include powders of resins, such as tetrafluoroethylene resin, trifluorochloroethylene resin, tetrafluoroethylene-hexafluoropropylene resin, vinyl fluoride resin, vinylidene fluoride resin, difluorodichloro-ethylene resin, copolymers of the above resins, and fluorinated carbon. These may be used singly or in combination of two or more species. Among these, tetrafluoroethylene resin (PTFE) powder is particularly preferred. The fluorine-containing compound powder may appropriately selected from commercially available powders of the above resins so as to have a desired molecular weight and a desired particle size. The fluorine-containing compound powder may preferably have a relatively lower molecular weight (e.g., weight-average molecular weight (Mw) of 10 4 - 10 5 , particularly 10 4 - 5x10 4 ) in view of a lubricity of the surface layer and may preferably have an average particle size of 0.02 - 5 µm, more preferably 0.05 - 10 µm, further preferably 0.1 - 1.0 µm. The above-described fluorine-containing compound powder may be mixed and dispersed in the binder (e.g., urethane resin or elastomer) by appropriately using a known mixing device. More specifically, in the case of using an urethane elastomer as the binder, a mixing device, such as a roll mill, a kneader or a Banbury mixer, may preferably be used. If a liquid urethane binder is used, a mixing device, such as a ball mill, a bead mill, a homogenizer, a paint shaker, a nanomizer or the like may preferably be adopted. The base layer 62 of the intermediate transfer member of the present invention comprises epichlorohydrin rubber and acrylonitrile-butadiene rubber (NBR). These rubbers are selected from rubbers and elastomers showing a solubility parameter (SP) value closer to those of the urethane resin or the urethane elastomer used as the binder for the surface layer 63 in view of an adhesiveness between the base layer 62 and the surface layer 63. As a rubber or elastomer showing an SP value close to that of the urethane resin or elastomer, NBR or urethane rubber may generally be exemplified. However, a base layer consisting of NBR alone is inferior in ozone resistance and causes therein, e.g., a crack or a fissure when used for a long period. Accordingly, in the present invention, by mixing NBR with epichlorohydrin rubber having a good ozone resistance and a good mutual solubility with not only NBR but also the urethane resin or elastomer as the surface layer binder, it is possible to provide an intermediate transfer member having a good adhesiveness of the base layer to the surface layer and an excellent ozone resistance. In addition, both of NBR and epichlorohydrin rubber have a relatively low electrical resistance. Accordingly, by using these rubbers in combination to constitute the base layer, it is possible to control an electric resistance of a resultant intermediate transfer member to some extent without dispersing an electroconductive filler within the base layer, thus resulting in the intermediate transfer member with little irregularity in resistance. The above-mentioned epichlorohydrin rubber (hereinafter sometimes referred to as "CHR") and NBR are mixed in a mixing ratio by weight of (CHR:NBR) = 1:9 to 9:1, more preferably 6:4 to 8:2. In case where CHR is used in a small amount and NBR is used excessively, a resultant intermediate transfer member has an insufficient ozone resistance to case a deficiency, such as a crack in the base layer in some cases. In case where CHR is used excessively and NBR is used in a small amount, the base layer has a poor adhesiveness to the surface layer to cause a peeling of the surface layer in some cases when used for a long period. Examples of epichlorohydrin rubber (CHR) used in the present invention may include epichlorohydrin homopolymer, epichlorohydrin-ethylene oxide copolymer, epichlorohydrin-allyl glycidyl ether copolymer and epichlorohydrin-ethylene oxide-allyl glycidyl ether terpolymer. In the base layer, CHR and NBR may preferably be used in a total amount of 60 - 100 wt. %, more preferably 80 - 100 wt. %, based on a binder component of the base layer. The base layer used in the present invention may preferably a hardness of 10 - 70 degrees, more preferably 20 - 55 degrees, as measured according to JIS-A. The electroconductive support 61 may preferably comprise a metal or alloy, such as aluminum, iron, copper or stainless steel, or an electroconductive resin containing electroconductive carbon or metal particles dispersed therein. The support may have a shape of a drum or a belt as described above, inclusive of a drum equipped with a shaft piercing therethrough and a drum inside of which has been reinforced. In the present invention, in order to control an electrical resistance of the intermediate transfer member, powders of electroconductive materials, such as carbon black, graphite, carbon fiber, metal compounds, organic metal salts and electroconductive polymers, may be added in the base layer and/or the surface layer. The intermediate transfer member according to the present invention may preferably have an electrical resistance of 10 1 - 10 13 ohm, particularly 10 2 - 10 10 ohm. Incidentally, the life of the intermediate transfer member may be determined by several factors one of which is a lowering in transfer characteristics due to a change in resistance. The intermediate transfer member is generally prepared by controlling electrical properties represented by a resistance so as to provide optimum transfer characteristics but in many case, is gradually changed in its resistance by, e.g., voltage application in repetitive use for a long period. Such a change in resistance can be corrected to a certain degree by using, e.g., a transfer voltage-adjusting mechanism provided within an apparatus body of a printer or a copying machine. However, if the resistance change is not within an allowable range, appropriate transfer characteristics cannot be attained, thus requiring replacement of the intermediate transfer member. In view of the above factor, in the present invention, the intermediate transfer member may preferably have resistances R1 (ohm) and R2 (ohm) satisfying the following relationships: 0.1 ≦ R2/R1 ≦ 10 and 5.0x10 4 ohm ≦ R2 ≦ 5.0x10 9 ohm, wherein R1 denotes an initial resistance (ohm) of the intermediate transfer member and R2 denotes a resistance (ohm) after a direct current of 5 mA is continuously applied for 5 hours per a surface area of the intermediate transfer member of 1 m 2 . In a more preferred embodiment, the intermediate transfer member may preferably satisfy the following relationship: 0.5 ≦ R2/R1 ≦ 5. When a resistance (electrical resistance) of the intermediate transfer member is increased in a long term use, it is necessary to increase a transfer voltage in order to maintain a transfer efficiency, thus resulting in a large-sized apparatus. Further, if the resistance of the intermediate transfer member is considerably increased compared with an initial value thereof, a lowering in transfer efficiency cannot be suppressed only by control and adjustment on the apparatus body side. As a result, the transfer efficiency of a toner is lowered, thus causing a lowering in image density and an increase in transfer residual toner (developer) on the surface of the intermediate transfer member. On the other hand, the resistance of the intermediate transfer member is lowered in some cases in a long term use. For instance, the use of a large amount of a high electroconductive material causes the lowering of the resistance. In this case, a local electroconductive path is formed by repetitive voltage application for a long period, whereby a withstand voltage of the intermediate transfer member is lowered to cause a so-called leak. As a result, a lack of image due to a local transfer failure or a lowering in an overall transfer efficiency leading to a cleaning failure are caused. This phenomenon is found to be largely affected by an amount of a current rather than a magnitude of an applied voltage. Accordingly, it is possible to expect a possibility of a fluctuation in resistance in a long term use by applying an excessive current for several hours. In this respect, the resistance R2 of the intermediate transfer member after the current application may desirably be set in the above-described range (5.0x10 4 - 5.0x10 9 ohm), particularly in a range from 5.0x10 5 ohm to 1.0x10 9 ohm, in order to retain a good transfer efficiency even after the long term use and obviate a large-sized apparatus body to reduce the cost of an apparatus body. In order to control the change in resistance of the intermediate transfer member of the present invention, the above-mentioned electroconductive powder may appropriately be selected and mixed in the intermediate transfer member or amounts of electroconductive powders and binders for respective layers may appropriately be controlled by forming the base layer and/or the surface layer each in plural layers, thus suppressing the resistance change. It is also possible to minimize the resistance change by controlling, e.g., a dispersion state of respective constituents (e.g., electroconductive powder). The electrical resistances at an initial stage (R1) and after the current application (R2) of the intermediate transfer member referred to herein is based on values measured in the following manner. Further, the current application at that time is effected in the manner shown below successively. <Measurement of resistance> (1) Under an environment of 23 °C and 65 %RH, an intermediate transfer member 6 and a metal roller 200 (outer diameter = 40 mm) are pressed against each other at a linear pressure of 40 g/cm while keeping respective shafts (axes) in parallel and are connected to a DC power supply 202, a resistor 201 disposed on a downstream side of the intermediate transfer member 6, and a potential meter 203, as shown in Figure 2. In the case of the intermediate transfer member 6 in the form of a belt, an aluminum cylinder having an outer diameter corresponding to an inner diameter of the belt is used as a core member and subjected to the measurement. (2) The metal roller 200 is driven in rotation so that the mating intermediate transfer member 6 is rotated at a peripheral speed of 120 mm/sec. (3) A constant voltage of 1 kV is applied from the DC power supply 202 to read a potential difference Vr between both terminals of the resistor 201 having a known resistance value sufficiently lower than the measurement sample by the potential meter 203. (4) A current I is calculated from the measured potential difference Vr. The resistance of the intermediate transfer member 6 is calculated as applied voltage (1 kV)/current I. <Application of current> (1) Similarly as in the above manner, the intermediate transfer member 6 is disposed and driven in rotation. (2) An applied direct current is adjusted so as to be a constant current of 5 mA per a surface area of 1 m 2 with respect to the intermediate transfer member 6. The direct current application is continued for 5 hours. The base layer of the intermediate transfer member may preferably have a thickness of at least 0.5 mm, more preferably at least 1 mm, particularly 1 - 10 mm. The surface layer of the intermediate transfer member may preferably have a thickness sufficiently small so as not to impair the resilience of the base layer, more specifically at most 1 mm, further preferably at most 500 µm, and particularly 5 - 100 µm. The intermediate transfer member according to the present invention may be produced, e.g., in the following manner. First of all, a metal roller as a cylindrical electroconductive support (core metal) is provided. A rubber is molded or formed into a base layer to be disposed on the metal roller by melt molding, injection molding, dip coating, spray coating, etc. A material for a surface layer is molded or formed into a surface layer to be disposed on the base layer by melt molding, injection molding, dip coating, roller coating, spray coating, etc., to prepare an intermediate transfer member. An electrophotographic apparatus will now be described with reference to Figure 3. The apparatus includes a rotating drum-type electrophotographic photosensitive member (hereinafter called "photosensitive drum") 1 repetitively used as a first image-bearing member, which is driven in rotation in a clockwise direction indicated by an arrow at a prescribed peripheral speed (process speed). The photosensitive drum 1 may preferably be one having an outermost layer (protective layer) containing particles of polytetrafluoroethylene (PTFE) (tetrafluoroethylene resin), so as to improve a transfer characteristic from the photosensitive drum as the first image-bearing member (primary transferability), thus attaining a good image quality free from image defects, such as a hollow dropout and a high primary transfer efficiency. For instance, if a transfer characteristic from the intermediate transfer member to a secondary image-bearing member such as recording paper (secondary transferability) is insufficient, a transfer residual toner on the intermediate transfer member is increased. As a result, a substantial transfer sufficiency is not improved and image defects, such as secondary transfer are caused to occur. However, the intermediate transfer member according to the present invention is not accompanied with such a problem and can improve a substantial transfer efficiency and image quality in combination with the photosensitive drum using the protective layer. During the rotation, the photosensitive drum 1 is uniformly charged to a prescribed polarity and potential by a primary charger (corona discharger) 2 and then exposed to imagewise light 3 (indicated by an arrow) supplied from an imagewise exposure means (not shown, e.g., an optical system including means for color separation-focusing exposure of a color original image, a scanning exposure system including a laser scanner for emitting laser beam modulated corresponding to time-serial electrical digital pixel signals of image data) to form an electrostatic latent image corresponding to a first color component image (e.g., a magenta color component image) of an objective color image. Then, the electrostatic latent image is developed with a magenta toner M (first color toner) by a first developing device (magenta developing device 41). At this time, second to fourth developing devices (cyan developing device 42, yellow developing device 43 and black developing device 44) are placed in an operation-off state and do not act on the photosensitive drum 1, so that the magenta (first color) toner image thus formed on the photosensitive drum 1 is not affected by the second to fourth developing devices 42, 43 and 44 An intermediate transfer member 6 is rotated in a counterclockwise direction at a peripheral speed equal to that of the photosensitive drum 1. As the magenta toner image formed and carried on the photosensitive drum 1 passes through a nip position between the photosensitive drum 1 and the intermediate transfer member 6, the magenta toner image is transferred onto an outer surface of the intermediate transfer member 6 under the action of an electric field caused by a primary transfer bias voltage applied to the intermediate transfer member 6 (primary transfer). The surface of the photosensitive drum 1 after the transfer of the magenta (first color) toner image onto the intermediate transfer member 6 is cleaned by a cleaning device 5. Thereafter, a cyan (second color) toner image, a yellow (third color) toner image and a black (fourth color) toner image are similarly formed on the photosensitive drum 1 are successively transferred in superposition onto the intermediate transfer member 6 to form a synthetic color toner image corresponding to an objective color image. A transfer roller 91 is supported on a shaft in parallel to the intermediate transfer member 6 and so as to be in contact with a lower (but outer) surface of the intermediate transfer member 6. During the sequential transfer steps for transferring the first to fourth color images from the photosensitive drum 1 onto the intermediate transfer member 6, the transfer roller 91 can be separated from the intermediate transfer member 6. For the secondary transfer, the transfer roller 91 is abutted against the intermediate transfer member 6, a transfer-receiving material 10 as a second image-bearing member is supplied via paper supply cassette 11 to a nip position between the intermediate transfer member 6 and the secondary transfer roller 91 at a prescribed time and, in synchronism therewith, a secondary transfer bias voltage is applied to the transfer roller 91 from a power supply 12. Under the action of the secondary transfer bias voltage, the synthetic color toner image on the intermediate transfer member 6 is transferred onto the transfer-receiving material (second image-bearing member) 10 (secondary transfer). The transfer-receiving material 10 carrying the toner image is introduced into a fixing device 13 to effect heat fixing of the toner image. After completion of image transfer onto the transfer-receiving material 10, a transfer residual toner (a portion of toner remaining on the intermediate transfer member 6 without being transferred onto the transfer-receiving material 10) is cleaned by abutting a cleaner 7 against the intermediate transfer member 6. Figure 4 shows another embodiment of the electrophotographic apparatus of the present invention. The apparatus employs similar structural members and systems as in the apparatus shown in Figure 3 except that an intermediate transfer member 65 in the form of a belt and a transfer charger 92 are used instead of the intermediate transfer member 6 in the form of a drum and the transfer roller 91, respectively. Referring to Figure 4, the intermediate transfer member 65 is supported about four rollers. Hereinbelow, the present invention will be described more specifically with reference to Examples and Comparative Examples, wherein "part(s)" used for describing a composition means "part(s) by weight". Example 1 On an aluminum cylinder (outer diameter (OD) = 182 mm, length (L) = 320 mm, thickness (T) = 5 mm), a rubber compound of the following composition was transfer-molded to prepare a roller having a 5 mm-thick base layer. (Rubber compound) NBR 35 parts Epichlorohydrin rubber 65 parts Paraffin oil 2 parts Carbon black 1 part Calcium carbonate 10 parts Sulfur (vulcanizing agent) 1 part Zinc oxide (vulcanization aid) 2 parts Thiuram compound (vulcanization promoter 2 parts Separately, a surface layer paint of the following composition was prepared. (Surface layer paint) Polyester polyurethane prepolymer (containing dimethylformamide) 100 parts Hardener (isocyanate containing ethyl acetate) 4 parts Tetrafluoroethylene resin powder (average particle size (Dav = 0.3 µm) 100 parts Dispersion aid 5 parts DMF (dimethylformamide) 120 parts The paint was applied by spraying onto the outer surface of the roller and dried at 80 °C for 1 hour, followed by curing (hardening) at 120 °C for 2 hours to form first and second intermediate transfer members each having a ca. 60 µm-thick tough surface layer. The tetrafluoroethylene powder occupied 55 wt. % of the total solid components of the surface layer. The first intermediate transfer member was incorporated in a full-color electrophotographic apparatus as shown in Figure 3 including an OPC photosensitive member (as a first image-bearing member) having a photosensitive layer and a protective layer thereon, and subjected to measurement of transfer efficiencies in an environment of temperature of 23 °C and humidity of 65 %RH according to a mono-color mode using a cyan toner, thereby to obtain a primary transfer efficiency (from the photosensitive member to the intermediate transfer member) of 95 % and a secondary transfer efficiency (from the intermediate transfer member to plain paper of 80 g/m 2 (as a secondary image-bearing member)) of 94 %. Herein, the respective transfer efficiencies are calculated according to the following equations. Primary transfer efficiency (%) = [(Toner density on the intermediate transfer member)/(Residual toner density on the photosensitive member + Toner density on the intermediate transfer member)] x 100 Secondary transfer efficiency (%) = [(Toner density on the plain paper)/(Residual toner density on the intermediate transfer member + Toner density on the plain paper)] x 100 Then, by using the image forming apparatus, a continuous full-color image forming test in an environment of 23 °C/65 %RH was performed under the following conditions. Photosensitive member: OPC photosensitive member having a laminar structure of an electroconductive support, an undercoating layer, a charge generation layer, a charge transportation layer and a protective layer containing tetrafluoroethylene resin powder. Dark part potential: -750 volts Developer: non-magnetic mono-component toners of four colors (cyan, magenta, yellow and black) Primary transfer voltage: +700 volts Secondary transfer voltage: +2500 volts Process speed: 120 mm/sec Developing bias voltage: -500 volts As a result, a good image quality was confirmed. Then, the first intermediate transfer member 6 was incorporated in a durability testing apparatus 102 as shown in Figure 5 and was abutted on an aluminum cylinder 101 (outer diameter = 80 mm) at a total pressure of 5 kg, followed by a continuous load rotation test of 10 5 revolutions at a peripheral speed of 120 mm/sec. This test was performed at a constant ozone concentration of 10 ppm within the durability testing apparatus. After the durability test, a continuous full-color image forming test was performed on 10,000 sheets of plain paper of 80 g/m 2 , whereby images similar to those at the initial stage were obtained. The secondary transfer efficiency was 93 % and accordingly, substantially no lowering in (secondary) transfer efficiency was caused. Further, as a result of evaluation of a surface state of the intermediate transfer member by eye observation, no peeling and crack were observed at the surface layer, and no toner filming was observed either. Separately, the second intermediate transfer member prepared as described above was subjected to measurement of an initial resistance R1 and a resistance R2 after continuous direct current application (5 mA for 5 hours) in the above-described manner with reference to Figure 2. The results of this example are summarized in Table 1 appearing hereinafter. Example 2 Two (first and second) intermediate transfer members were prepared (and evaluated in the same manner as in Example 1 except that the composition of the rubber compound was changed as follows. (Ingredient) (wt.part(s)) NBR 15 Epichlorohydrin rubber 85 Paraffin oil 2 Carbon black 1 Calcium carbonate 10 Sulfur (vulcanizing agent) 1 Zinc oxide (vulcanization aid) 2 Thiuram compound (vulcanization promoter) 2 With respect to the first intermediate transfer member, sufficient transfer efficiencies and a good image quality were obtained. Further, as a result of the continuous image formation of 10,000 sheets, a very slight peeling of the surface layer was observed and the transfer efficiencies were somewhat lowered when compared with those at the initial stage. However, these phenomena did not affect resultant images and were evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 3 Two (first and second) intermediate transfer members were prepared (and evaluated in the same manner as in Example 1 except that the composition of the rubber compound was changed as follows. (Ingredient) (wt.part(s)) NBR 85 Epichlorohydrin rubber 15 Paraffin oil 2 Carbon black 1 Calcium carbonate 10 Sulfur (vulcanizing agent) 1 Zinc oxide (vulcanization aid) 2 Thiuram compound (vulcanization promoter) 2 With respect to the first intermediate transfer member, sufficient transfer efficiencies and a good image quality were obtained. Further, as a result of the continuous image formation of 10,000 sheets, a very slight crack in the intermediate transfer member presumably attributable to the influence of ozone was observed but did not affect resultant images thus being evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 4 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the content (55 wt. %) of the tetrafluoroethylene resin powder (Dav. = 0.3 µm) contained in the surface layer was changed to 23 wt. %. As a result, sufficient transfer efficiencies and a good image quality were obtained with respect to the first intermediate transfer member. Further, as a result of the continuous image formation of 10,000 sheets, a very slight toner filming was observed on the surface of the intermediate transfer member and the transfer efficiencies were somewhat lowered when compared with those at the initial stage. However, these phenomena did not affect resultant images and were evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 5 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the content (55 wt. %) of the tetrafluoroethylene resin powder (Dav. = 0.3 µm) contained in the surface layer was changed to 76 wt. %. As a result, sufficient transfer efficiencies and a good image quality were obtained with respect to the first intermediate transfer member. Further, as a result of the continuous image formation of 10,000 sheets, a very slight peeling of the surface layer was observed but did not affect resultant images, thus being evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 6 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the tetrafluoroethylene resin powder (Dav. = 0.3 µm) contained in the surface layer was changed to tetrafluoroethylene-hexafluoropropylene resin powder (Dav. = 1.0 µm). As a result, sufficient transfer efficiencies and a good image quality were obtained with respect to the first intermediate transfer member. Further, as a result of the continuous image formation of 10,000 sheets, a very slight toner filming was observed on the surface of the intermediate transfer member and the transfer efficiencies were somewhat lowered when compared with those at the initial stage. However, these phenomena did not affect resultant images and were evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 7 A rubber compound having a composition identical to that prepared in Example 1 was subjected to extrusion molding, vapor vulcanization and polishing to form a rubber belt (OD = 150 mm, width (W) = 320 mm, T = 0.8 mm). A surface layer paint having a composition identical to that prepared in Example 1 was applied onto the rubber belt in the same manner as in Example 1 to form two (first and second) intermediate transfer member. The first intermediate transfer member in the form of a belt was incorporated in an electrophotographic apparatus as shown in Figure 4 and subjected to measurement of transfer efficiencies and observation of image quality in the same manner as in Example 1, so that sufficient transfer efficiencies and a good image quality were obtained. Then, the first intermediate transfer member (belt form) was wound about an aluminum cylinder (OD = 148.4 mm, L = 320 mm, T = 2 mm) and subjected to the durability test (10 5 revolutions) and the continuous full-color image formation of 10,000 sheets in the same manner as in Example 1. As a result, at the surface of the first intermediate transfer member, a very slight crack was observed but was evaluated as a practically acceptable level since resultant images were not adversely affected by the crack. The second intermediate transfer member was also wound about an aluminum cylinder (OD = 148.4 mm, L = 320 mm, T = 2 mm) and subjected to measurement of resistances R1 and R2 in the same manner as in Example 1. The results are summarized as in Table 1. Example 8 On an aluminum cylinder (OD = 182 mm, L = 320 mm, T = 3 mm), a rubber compound of the following composition was transfer-molded to prepare a roller having a 5 mm-thick base layer. (Rubber compound) NBR 35 parts Epichlorohydrin rubber 65 parts Electroconductive carbon black 2 part Paraffin oil 3 parts Calcium carbonate 10 parts Sulfur (vulcanizing agent) 2 part Zinc oxide (vulcanization aid) 2 parts Thiuram compound (vulcanization promoter) 3 parts Separately, a surface layer paint of the following composition was prepared. (Surface layer paint) Polyurethane prepolymer (solid content = 35 %) 100 parts Hardener (solid content = 30 %) 50 parts Tetrafluoroethylene resin powder 100 parts Dispersion aid 5 parts DMF (dimethylformamide) 120 parts The paint was applied by spraying onto the outer surface of the roller and dried at 80 °C for 1 hour, followed by curing (hardening) at 120 °C for 2 hours, thus effecting removal of solvent and crosslinking to form first and second intermediate transfer members each having a ca. 40 µm-thick tough surface layer. The tetrafluoroethylene powder occupied 67 wt. % of the total solid components of the surface layer. With respect to the first intermediate transfer member, performed in the same manner as in Example 1, sufficient transfer efficiencies and a good image quality were obtained. Further, the continuous load rotation test of 10 5 revolutions and the continuous image formation of 10,000 sheets were performed in the same manner as in Example 1. As a result, a very slight peeling of the surface layer was observed but did not affect resultant images, thus being evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 9 Two (first and second) intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the composition of the rubber compound was changed as follows. (Ingredient) (wt.part(s)) NBR 5 Epichlorohydrin rubber 95 Paraffin oil 2 Carbon black 1 Calcium carbonate 10 Sulfur (vulcanizing agent) 1 Zinc oxide (vulcanization aid) 2 Thiuram compound (vulcanization promoter) 2 With respect to the first intermediate transfer member, sufficient transfer efficiencies and a good image quality were obtained. Further, as a result of the continuous image formation of 10,000 sheets, a peeling of the surface layer was observed and the transfer efficiencies were somewhat lowered when compared with those at the initial stage. However, these phenomena did not affect resultant images and were evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 10 Two (first and second) intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the composition of the rubber compound was changed as follows. (Ingredient) (wt.part(s)) NBR 95 Epichlorohydrin rubber 5 Paraffin oil 2 Carbon black 1 Calcium carbonate 10 Sulfur (vulcanizing agent) 1 Zinc oxide (vulcanization aid) 2 Thiuram compound (vulcanization promoter) 2 With respect to the first intermediate transfer member, sufficient transfer efficiencies and a good image quality were obtained. Further, as a result of the continuous image formation of 10,000 sheets, a crack in the intermediate transfer member presumably attributable to the influence of ozone and a slight hollow dropout by transfer were observed but did not affect resultant images thus being evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 11 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the content (55 wt. %) of the tetrafluoroethylene resin powder contained in the surface layer was changed to 16 wt. %. As a result, sufficient transfer efficiencies and an almost good image quality were obtained with respect to the first intermediate transfer member. Further, as a result of the continuous image formation of 10,000 sheets, a slight toner filming was observed on the surface of the intermediate transfer member and, a slight follow dropout by transfer was confirmed. However, these phenomena did not substantially affect resultant images and were evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Example 12 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the content (55 wt. %) of the tetrafluoroethylene resin powder (Dav. = 0.3 µm) contained in the surface layer was changed to 84 wt. %. As a result, sufficient transfer efficiencies and a good image quality were obtained with respect to the first intermediate transfer member. Further, as a result of the continuous image formation of 10,000 sheets, a slight peeling and crack were observed on the surface of the intermediate transfer member and the transfer efficiencies were somewhat lowered when compared with those at the initial stage. However, these phenomena did not affect resultant images and were evaluated as a practically acceptable level. The second intermediate transfer member was subjected to measurement of resistances R1 and R2. The results are also shown in Table 1. Comparative Example 1 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the surface layer was not formed. As a result, at the initial state, transfer efficiencies (primary transfer efficiency = 90 %, secondary transfer efficiency = 73 %) were inferior to those in Example 1 and low-density images presumably attributable to transfer failure were observed, thus being evaluated as at a practically unacceptable level. For this reason, the durability test was not performed. The results are summarized in Table 2. Comparative Example 2 Two intermediate transfer members were prepared and evaluated in the same manner as in Example 1 except that the tetrafluoroethylene resin powder was not used. As a result, at the initial state, transfer efficiencies (primary transfer efficiency = 91 %, secondary transfer efficiency = 76 %) were inferior to those in Example 1 and low-density images presumably attributable to transfer failure were observed, thus being evaluated as at a practically unacceptable level. For this reason, the durability test was not performed. The results are summarized in Table 2.

Claims: An intermediate transfer member (6;65) for temporarily holding a toner image, comprising a base layer (62) and a surface layer (63) wherein the surface layer (63) comprises a powder of a fluorine-containing compound, characterised in that : the base layer (62) comprises epichlorohydrin rubber and acrylonitrile-butadiene rubber in a mixing ratio epichlorohydrin rubber: acrylonitrile-butadiene rubber of 1:9 to 9:1 by weight; and the surface layer (63) comprises urethane resin or urethane elastomer. A member according to claim 1, wherein the surface layer (63) comprises 20-80 wt.% of said powder of a fluorine-containing compound. A member according to claim 2, wherein the surface layer (63) comprises 30-70 wt.% of said powder of a fluorine-containing compound. A member according to any of claims 1 to 3, wherein said mixing ratio is 6:4 to 8:2 by weight. A member according to any preceding claim, which exhibits a resistance R1 at an initial stage and a resistance R2 after a direct current of 5 mA is continuously applied for 5 hours per a surface area of the member of 1 m 2 , said resistances R1 and R2 satisfying the following relationship: 0.1 ≤ R2/R1 ≤ 10, and 5.0x10 4 ohm ≤ R2 ≤ 5.0x10 9 ohm. A member according to claim 5, wherein said resistances R1 and R2 satisfy the following relationship: 0.5 ≤ R2/R1 ≤ 5. A member according to claim 5, wherein said resistances R2 is in a range of 5.0x10 5 - 1.0x10 9 ohm. A member according to any preceding claim, further comprising a cylindrical electroconductive support (61). An electrophotographic apparatus, comprising: an electrophotographic photosensitive member (1); charging means (2) for charging the electrophotographic photosensitive member; image exposure means (3) for exposing the charged electrophotographic photosensitive member to an image, to form an electrostatic latent image; developing means (41,42,43,44) for developing the electrostatic latent image to form a toner image on the electrophotographic member; and an intermediate transfer member (6,65) according to any of claims 1 to 8 for temporarily holding a toner image received from the electrophotographic photosensitive member. An apparatus according to claim 9, wherein said toner image comprises plural toner images of different colours. An apparatus according to claim 9, wherein said electrophotographic photosensitive member comprises at least an outermost layer comprising tetrafluoroethylene resin. A method for forming an image which comprises supplying a transfer-receiving medium (10) to an apparatus as claimed in any of claims 9 to 11 and operating said apparatus to form a toner image and transfer it to said transfer-receiving medium (10).

Inventors: ASHIBE TSUNENORI, KOBAYASHI HIROYUKI, KUSABA TAKASHI, NAKAZAWA AKIHIKO, SHIMADA AKIRA, SHIMOJO MINORU, TANAKA ATSUSHI, ASHIBE, TSUNENORI, KOBAYASHI, HIROYUKI, KUSABA, TAKASHI, NAKAZAWA, AKIHIKO, SHIMADA, AKIRA, SHIMOJO, MINORU, TANAKA, ATSUSHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: G03G15
IPC codes: G03G15