JP2014224897A - Transfer belt and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent growth of a crack formed in a surface layer of a transfer belt, to prevent image noise.SOLUTION: An intermediate transfer belt 40 is formed by laminating at least an elastic layer 42, a stress relaxation layer 44 softer than the elastic layer, and a surface layer 43 harder than the elastic layer, in this order from the inside to the outside where a surface faces the last medium. In the intermediate transfer belt, the stress relaxation layer is more elastic than the elastic layer, thereby relaxing stress concentration at a tip 55a in a depth direction of a crack 55 formed in the surface layer. Enlargement of the crack can be prevented, accordingly.

Description

  The present invention relates to a transfer belt used for transferring a toner image formed on a photoreceptor to a final medium such as paper in an electrophotographic image forming apparatus, and an image forming apparatus including the transfer belt.

  In an image forming apparatus using an electrophotographic system such as a printer or a copying machine, a toner image formed on a photoreceptor is transferred to a final medium such as paper. As this transfer process, a process in which a toner image on a photosensitive member is temporarily copied onto an intermediate transfer belt (primary transfer) and then transferred from the intermediate transfer belt to a final medium (secondary transfer) is often employed. Yes. The intermediate transfer belt is wound around a plurality of rollers arranged on the inner side thereof, and rotates according to the rotational drive of the rollers.

As an intermediate transfer belt, a base layer made of a synthetic resin such as polyimide, an elastic layer made of a rubber material such as nitrile butadiene rubber (NBR), and a surface layer made of ceramics such as silicon dioxide (SiO ₂ ) are conventionally used from the inside. The thing of the 3 layer structure laminated | stacked in this order toward the outer side is known (refer the following patent document 1).

  The elastic layer is the most elastic layer among the three layers. The elastic layer secures a sufficient nip width and enhances the followability with respect to the final medium, and enables good transfer even to a final medium with unevenness such as embossed paper. The surface layer is a harder layer than the elastic layer. The surface layer is for improving the secondary transfer efficiency by enhancing the releasability of the toner on the intermediate transfer belt. That is, the intermediate transfer belt described in Patent Document 1 has both high followability with respect to the final medium and high toner releasability.

JP 2011-197230 A

  However, in the multi-layer type transfer belt configured as described above, the surface layer is harder than the elastic layer. Therefore, the surface layer sometimes cannot follow the elastic layer that expands and contracts by being driven by a roller. As a result, fine cracks may occur in the surface layer.

Incidentally, an electrophotographic image forming apparatus is provided with a charging device for charging a photosensitive member and a transfer roller facing the photosensitive member with an intermediate transfer belt interposed therebetween. The transfer roller is applied with a bias voltage for transferring the toner image formed on the photosensitive member to the intermediate transfer belt. Ozone (O ₃ ) is generated by discharge due to the use of this charging device or discharge accompanying voltage application to the transfer roller. This ozone exists in the vicinity of the intermediate transfer belt. Therefore, when a fine crack occurs in the surface layer as described above, this ozone enters the inside of the crack and hardens the inner surface of the crack due to ozone deterioration. When the crack portion whose inner surface is cured in this way is bent and extended along the outer peripheral surface of the roller as the intermediate transfer belt is driven to rotate, stress concentrates on the tip in the depth direction of the crack. Therefore, the crack expands in the depth direction. When the crack becomes deep, stress concentration occurs at both ends along the width direction orthogonal to the traveling direction of the intermediate transfer belt. For this reason, the crack also expands in the width direction of the intermediate transfer belt. Such expansion of cracks is repeated as long as image formation is performed.

  Therefore, fine cracks formed in the surface layer gradually expand as image formation is repeated. Eventually, the crack grows deeper as it penetrates the elastic layer, which is the lower layer of the surface layer, and grows larger as the width dimension on the surface of the surface layer exceeds 15 mm. If a minute crack grows into such a large crack, image noise is generated on the final medium after printing. Note that the fine cracks just formed are not enough to generate image noise. Further, the transfer belt of Patent Document 1 has a groove formed along the width direction of the belt, but the crack extends along the width direction of the transfer belt. It was difficult to stop the progress of crack growth.

  The present invention has been made in view of the above circumstances. That is, an object of the present invention is to provide a transfer belt and an image forming apparatus capable of preventing the occurrence of image noise by suppressing the growth of cracks formed in the surface layer of the transfer belt.

  In order to solve this problem, the transfer belt of the present invention is an endless transfer belt that is used in an electrophotographic image forming apparatus and is rotationally driven when a toner image is transferred, and includes at least an elastic layer and an elastic layer. A stress relaxation layer that is softer and a surface layer that is harder than the elastic layer are laminated in this order from the inside toward the outside where the final medium faces.

  According to the transfer belt of the present invention, the stress relaxation layer that is softer than the elastic layer is provided between the elastic layer and the surface layer. Therefore, the expansion (growth) of cracks formed along with the rotation driving of the transfer belt can be stopped by the stress relaxation layer. That is, when a crack is formed in the surface layer as the transfer belt rotates, and the crack expands, the depth of the crack reaches the stress relaxation layer. Here, since the stress relaxation layer is a softer layer than the elastic layer, when the transfer belt receives a tensile force along the belt traveling direction, it extends better than the elastic layer along the plane direction perpendicular to the thickness direction. Therefore, the stress concentration at the tip in the depth direction of the crack reaching the stress relaxation layer is relaxed. Therefore, it is possible to prevent the crack from growing deep enough to penetrate the stress relaxation layer and reach the elastic layer.

  And since the growth to the depth direction of a crack can be prevented in this way, the stress concentration to the both ends along the width direction of the transfer belt in a crack can also be prevented. Therefore, it is possible to prevent cracks from growing along the width direction of the transfer belt. Therefore, according to the present invention, it is possible to prevent image noise from occurring in the printed image on the final medium due to the enlarged crack.

  Here, in the transfer belt of the present invention, the JIS K 6253 durometer type A hardness (hereinafter referred to as “hardness”) in the elastic layer is 40 degrees or more and less than 80 degrees, and the stress relaxation layer has a hardness of less than 40 degrees. The hardness of the surface layer is desirably 80 degrees or more. This is because the growth can be suitably suppressed even if cracks are formed in the surface layer. Therefore, according to the present invention, it is possible to perform printing with no secondary image noise and high secondary transfer efficiency.

  In the transfer belt of the present invention, the tensile breaking elongation of the surface layer may be 200% or less, and the tensile breaking elongation of the stress relaxation layer may be 600% or more. This is because even with this configuration, the growth of cracks formed in the surface layer can be suitably suppressed. That is, according to this configuration, printing with high secondary transfer efficiency can be performed without causing any image noise.

  In the transfer belt of the present invention, it is preferable that a base layer harder than the surface layer is further provided inside the elastic layer. If comprised in this way, it can prevent that each other layer extends indefinitely by having a base layer harder than any other layer. Therefore, good printing can be performed.

  In the transfer belt of the present invention, the elastic layer is preferably made of nitrile butadiene rubber. This is because nitrile butadiene rubber is particularly suitable as an elastic layer of a transfer belt and can particularly improve secondary transfer efficiency.

  The present invention also extends to an image forming apparatus including the above-described transfer belt and an image forming unit that forms a toner image transferred onto a final medium using the transfer belt on a photosensitive member.

  In this image forming apparatus, the transfer belt described above may be an intermediate transfer belt that receives the primary transfer of the toner image formed on the photoconductor by the image forming unit and performs the secondary transfer to the final medium. The secondary transfer belt may be a secondary transfer belt that faces the intermediate transfer belt with the final medium sandwiched during secondary transfer in which the toner image formed by the image forming unit and transferred from the photosensitive member to the intermediate transfer belt is transferred to the final medium. . When cracks formed on the surface layer expand (grow) not only in the intermediate transfer belt but also in the secondary transfer belt, image noise occurs in the final medium. The transfer belt having the above configuration is used as the secondary transfer belt. This is because such image noise can be prevented from occurring.

  According to the present invention, there is provided a transfer belt and an image forming apparatus capable of preventing the occurrence of image noise by suppressing the growth of cracks formed in the surface layer of the transfer belt.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. FIG. 2 is a schematic cross-sectional view of an intermediate transfer belt provided in the image forming apparatus. It is a figure for demonstrating the suppression principle of the crack expansion by a stress relaxation layer. FIG. 6 is a diagram illustrating a modification example of an intermediate transfer belt. It is a figure which shows the example of a change of an image forming apparatus, and is a figure which shows a part of image forming apparatus using a secondary transfer belt. It is a figure for demonstrating the expansion principle of the crack in the conventional intermediate transfer belt which does not have a stress relaxation layer.

(First embodiment)
DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the best mode for embodying the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram illustrating a configuration example of an image forming apparatus according to the present invention. An image forming apparatus 1 shown in FIG. 1 is an electrophotographic tandem digital color printer (hereinafter simply referred to as “printer”). Of course, in addition to the printer, the present invention can also be applied to a copier having a scanner or a multi-function machine having these functions combined.

  The image forming apparatus 1 includes an intermediate transfer belt (intermediate transfer member) 40 at a substantially central portion inside thereof. The intermediate transfer belt 40 is an endless (endless) belt, and is looped around the outer periphery of the drive roller 12, the tension roller 13, and the driven rollers 14 and 15. The intermediate transfer belt 40 rotates counterclockwise as the driving roller 12 rotates. The tension roller 13 is urged by the spring 10 from the inside to the outside of the intermediate transfer belt 40. Therefore, tension is always applied to the intermediate transfer belt 40.

  Below the intermediate transfer belt 40, there are four imaging units (corresponding to image forming units) 2Y, 2M, corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (K). 2C and 2K are arranged along the intermediate transfer belt 40 in this order. Each of the imaging units 2Y, 2M, 2C, 2K has a photoreceptor 21Y, 21M, 21C, 21K, respectively. Around each of the photoconductors 21Y, 21M, 21C, and 21K, charging devices 22Y, 22M, 22C, and 22K are sequentially arranged along a rotation direction at the time of image formation (also referred to as a positive rotation direction, clockwise in the figure), Exposure devices 23Y, 23M, 23C, and 23K, developing devices 24Y, 24M, 24C, and 24K, cleaning devices 25Y, 25M, 25C, and 25K, and erasers (static discharge devices) 26Y, 26M, 26C, and 26K are arranged. Yes. In the following description, subscripts representing the respective colors are omitted except when it is necessary to distinguish yellow (Y), magenta (M), cyan (C), or black (K).

  The charging device 22 is, for example, a scorotron, and uniformly charges the surface of the photoreceptor 21. The exposure device 23 exposes the charged photoreceptor 21 based on image data to be imaged. As a result, an electrostatic latent image based on the image data is formed on the photoreceptor 21. The developing device 24 converts the electrostatic latent image formed on the photoreceptor 21 into a visible image with toner. That is, a toner image is formed on the photoreceptor 21.

  The cleaning device 25 includes a strip-shaped cleaning blade 38. The cleaning blade 38 scrapes off residual toner remaining on the photosensitive member 21 after primary transfer described later. The eraser 26 includes an eraser lamp, and removes an afterimage of the electrostatic latent image on the surface of the photoconductor 21 after the primary transfer by irradiation of the eraser lamp.

  Further, as shown in FIG. 1, primary transfer rollers 30Y, 30M, and 30C for primary transfer, which will be described later, are located at positions facing the photoreceptors 21Y, 21M, 21C, and 21K with the intermediate transfer belt 40 interposed therebetween. 30K. A secondary transfer roller 16 for secondary transfer described later is pressed against the portion of the intermediate transfer belt 40 supported by the driving roller 12. A nip portion between the secondary transfer roller 16 and the intermediate transfer belt 40 becomes a secondary transfer region 17. In the secondary transfer region 17, the toner image transferred onto the intermediate transfer belt 40 is transferred onto the paper (final medium) P.

  A paper feed cassette 91 is detachably disposed below the image forming apparatus 1. The paper P stored in a state of being stacked in the paper feed cassette 91 is pulled out one by one from the uppermost one by the rotation of the paper feed roller 92 and sent out to the transport path 93. The conveyance path 93 continues from the paper feed cassette 91 to the paper discharge tray 98 through the nip portion of the timing roller pair 94, the secondary transfer region 17, and the fixing unit 95. The paper P sent out from the paper feed cassette 91 is conveyed to the timing roller pair 94 and is sent out to the secondary transfer area 17 at a predetermined timing.

  The fixing unit 95 has a hollow cylindrical shape, and includes a heating roller 96 having a heater 99 therein, and a pressure roller 97 that is in pressure contact with the heating roller 96 and rotates. The toner image is fixed to the paper P by passing the paper P on which the toner image is transferred by the secondary transfer through the nip formed by the heating roller 96 and the pressure roller 97.

  An image forming operation of the image forming apparatus 1 having such a configuration will be briefly described. In the full color mode for outputting a color image, for example, when an image signal is input to the control unit 70 of the image forming apparatus 1 from an external device such as a personal computer, the control unit 70 outputs the image signal to yellow, cyan, magenta, and black. A digital image signal that has undergone color conversion is created, and based on the digital image signal, the exposure device 23 of each imaging unit 2 emits light to expose the photoconductor 21. Thereby, electrostatic latent images for the respective colors are formed on the surfaces of the photoreceptors 21Y, 21M, 21C, and 21K, respectively. Note that the surface of each photoconductor 21 is uniformly charged by the charging device 22 before exposure.

  The electrostatic latent images formed on the photoreceptors 21Y, 21M, 21C, and 21K are developed by the developing devices 24Y, 24M, 24C, and 24K, respectively, and become toner images of the respective colors. Each color toner image is rotated counterclockwise in FIG. 1 by applying a bias potential to the primary transfer rollers 30Y, 30M, 30C, and 30K (applying a bias potential opposite to the toner charging polarity). The images are sequentially transferred onto the intermediate transfer belt 40 and superimposed. As a result, a toner image is formed on the intermediate transfer belt 40 as a color image in which the toner images of the respective colors are superimposed. This is called primary transfer.

  The toner image transferred onto the intermediate transfer belt 40 reaches the secondary transfer area 17 as the intermediate transfer belt 40 moves. On the other hand, the paper P sent out from the paper feed cassette 91 to the transport path 93 is transported to the secondary transfer region 17 by the timing roller pair 94 in accordance with the timing when the toner image reaches the secondary transfer region 17. A large bias potential is applied to the secondary transfer roller 16 on the side opposite to the charged polarity of the toner as compared to the potential applied to the primary transfer roller 30. As a result, the toner image is transferred from the intermediate transfer belt 40 to the paper P in the secondary transfer region 17. This is called secondary transfer.

  The sheet P on which the toner image is transferred is sent to the fixing unit 95 through the conveyance path 93. Therefore, the toner image is fixed on the paper P by passing through the nip formed by the heating roller 96 and the pressure roller 97. The paper P on which the toner image is fixed is discharged to a paper discharge tray 98.

  The residual toner remaining on the intermediate transfer belt 40 without being transferred onto the paper P is scraped off by the belt cleaning device 9 and removed from the outer peripheral surface of the intermediate transfer belt 40. Thereafter, the rotational drive of the photoreceptor 21 and the intermediate transfer belt 40 is stopped.

  Next, the intermediate transfer belt 40 provided in the image forming apparatus 1 will be described in detail. As shown in FIG. 2, the intermediate transfer belt 40 has a four-layer structure having a base layer 41, an elastic layer 42, a stress relaxation layer 44, and a surface layer 43 in this order from the inside to the outside. The surface layer 43 is the outermost layer in the intermediate transfer belt 40. The intermediate transfer belt 40 may include layers other than these four layers.

  The base layer 41 is a core material for ensuring the rigidity of the intermediate transfer belt 40, and is the hardest of the four layers. The base layer 41 is made of a synthetic resin. Specifically, polyimide, polyamide, PC (polycarbonate), PVDF (polyvinylidene fluoride), PAT (polyalkylene terephthalate), blend material of PC and PAT, blend of ETFE (ethylene-tetrafluoroethylene copolymer) and PC It is formed from a material and a thermoplastic resin such as a blend material of ETFE and PAT. Since the rigidity of the intermediate transfer belt 40 cannot be ensured if the base layer 41 is too thin, the thickness of the base layer 41 is preferably about 50 to 150 μm. In this embodiment, the thickness of the base layer 41 is about 80 μm.

  The elastic layer 42 is for ensuring a sufficient nip width of the intermediate transfer belt 40 with respect to the secondary transfer roller 16. The hardness of the elastic layer 42 (JIS K6253 durometer type A hardness, hereinafter referred to as “hardness” means this hardness) is preferably 40 degrees or more and less than 80 degrees. If the hardness is too high, a sufficient nip width cannot be ensured and transfer efficiency may be reduced, an image may be lost, or the intermediate transfer belt 40 may be difficult to rotate. Because. Further, if the hardness is too low, the surface of the intermediate transfer belt 40 may be wrinkled in the secondary transfer region 17 and image noise may be caused.

  The elastic layer 42 is formed from a rubber material such as nitrile butadiene rubber (NBR), chloroprene rubber, polybutadiene rubber, isoprene rubber, urethane rubber, EPDM (ethylene propylene diene rubber), acrylic rubber, and silicon rubber. The thickness of the elastic layer 42 is preferably about 100 to 300 μm. This is because if it is less than 100 μm, it is difficult to ensure a sufficient nip width during transfer. Further, if it exceeds 300 μm, the surface of the intermediate transfer belt 40 is likely to be wrinkled during transfer. In this embodiment, the elastic layer 42 has a thickness of about 200 μm.

  The stress relaxation layer 44 is the softest layer among the four layers. Although described in detail later, the stress relaxation layer 44 is a layer for suppressing the expansion of the crack 55 (see FIG. 3) formed in the surface layer 43. The hardness of the stress relaxation layer 44 is preferably less than 40 degrees, and particularly preferably about 20 degrees or more and less than 40 degrees. This is because if the hardness is 40 degrees or more, the effect of stopping the expansion of the crack 55 is weak. Moreover, if the hardness is too low, it takes time to restore the stress relaxation layer 44 after deformation, and there is a concern that image noise may occur. This is because there is no problem if the hardness of the stress relaxation layer 44 is 20 degrees or more.

  A material suitable for forming the stress relaxation layer 44 is the same as the material suitable for forming the elastic layer 42 described above. However, the material constituting the stress relaxation layer 44 and the material constituting the elastic layer 42 do not need to match. The thickness of the stress relaxation layer 44 is preferably about 10 to 30 μm. This is because if the thickness is less than 10 μm, the effect of suppressing the expansion of the crack 55 (see FIG. 3) is weak. On the other hand, if it exceeds 30 μm, it takes too much time to restore after elastic deformation, causing image noise. That is, since the low hardness rubber material constituting the stress relaxation layer 44 has low resilience, it takes time to restore after deformation. For this reason, the deformation does not return until it is transferred, causing image noise. In this embodiment, the thickness of the stress relaxation layer 44 is about 15 μm.

  The surface layer 43 is a harder layer than the elastic layer 42. The surface layer 43 is for improving the releasability of the toner in the intermediate transfer belt 40. The hardness of the surface layer 43 is desirably 80 degrees or more, and particularly desirably about 85 to 100 degrees. This is because the harder the surface layer 43, the better the toner releasability. However, if the surface layer 43 is too hard, the deformation of the elastic layer 42 is suppressed and it becomes difficult to ensure a sufficient nip width at the time of transfer.

  The surface layer 43 may be provided separately from the stress relaxation layer 44 or may be formed by subjecting the stress relaxation layer 44 to a surface treatment. When the stress relaxation layer 44 is formed by subjecting it to a surface treatment, it is not necessary to separately prepare a material constituting the surface layer 43, and the advantage that the surface layer 43 can be prevented from peeling off from the stress relaxation layer 44. There is.

When the surface layer 43 is provided separately from the stress relaxation layer 44, the surface layer 43 can be formed of, for example, a fluororesin or ceramics. The fluororesin that forms the surface layer 43 includes polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polychloro Examples include trifluoroethylene (PCTFE), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF). it can. Examples of the ceramic forming the surface layer 43 include silicon oxide such as silicon dioxide (SiO ₂ ), aluminum oxide, titanium oxide, and zinc oxide. When the surface layer 43 is formed of ceramics, for example, a plasma CVD method under atmospheric pressure can be suitably used.

  The surface layer 43 may be formed of the above-described fluororesin alone, or may be formed by dispersing the fluororesin in a binder exemplified below. This is because the fluororesin alone is inferior in film formability, but if a fluororesin dispersed in a binder is used, the desired surface layer 43 can be easily formed. As the binder, thermoplastic resins such as polyurethane, polyolefin, polyester, polyamide, polystyrene, acrylic resin, styrene-acrylic copolymer, polycarbonate, vinyl chloride, or thermosetting resin can be used. The content of the fluororesin is not particularly limited, but is preferably 10 wt% or more in order to reduce the friction coefficient of the surface layer 43 to some extent or to ensure toner releasability. . In order to form the surface layer 43 using a fluororesin dispersed in a binder, for example, a binder and fluororesin dissolved in an appropriate solvent are applied onto the elastic layer 42 and then dried. You can do it.

  On the other hand, when the surface layer 43 is formed by applying a surface treatment to the stress relaxation layer 44, a known surface hardening modification such as an oxidation treatment or an isocyanate treatment is applied to the stress relaxation layer 44 made of the rubber material. Apply quality treatment. In the isocyanate treatment, an isocyanate compound is impregnated on the surface of the stress relaxation layer 44 and then heated to react. In the isocyanate treatment, the surface of the elastic layer 42 can be cured to a desired hardness by adjusting the amount of the isocyanate compound impregnated, the heating temperature, and the heating time. Examples of the isocyanate compound used for the isocyanate treatment include aromatic isocyanate and aliphatic isocyanate. Specific examples include toluene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), and crude MDI. When the stress relaxation layer 44 is NBR, hypochlorite can also be used. A rubber material for constituting the surface layer 43 is provided separately from the stress relaxation layer 44 so as to overlap the stress relaxation layer 44, and a surface treatment is applied to the rubber material provided separately from the stress relaxation layer 44. You may form the surface layer 43 of the hardness of. Alternatively, the surface layer 43 having a desired hardness may be formed by filling an appropriate amount of a resin crosslinking agent (for example, a phenol resin) into a separately provided rubber material (NBR or the like).

  The thickness of the surface layer 43 is desirably about 10 to 30 μm. The thinner the surface layer 43 is, the better. However, if the surface layer 43 is too thin, stable manufacturing becomes difficult. In this embodiment, the thickness of the surface layer 43 is about 15 μm.

The volume resistivity of the base layer 41, the elastic layer 42, the stress relaxation layer 44, and the surface layer 43 is adjusted to the following values in order to perform good transfer. In adjusting the volume resistivity, a conductive agent (conductive value adjusting conductive agent) such as carbon black, graphite, and metal powder such as nickel is appropriately added.
Volume resistivity of base layer 41: 10 ⁷ to 10 ¹¹ Ω · cm
Volume resistivity of the elastic layer 42: 10 ^8.5 to 10 ^11.5 Ω · cm
Volume resistivity of the stress relaxation layer 44: 10 ¹⁰ to 10 ¹³ Ω · cm
Volume resistivity of surface layer 43: 10 ¹⁰ to 10 ¹³ Ω · cm

  By the way, the intermediate transfer belt 40 configured as described above is bent and stretched at a position wound around the rollers 12, 13, 14, and 15 (see FIG. 1) as it is rotationally driven during image formation. (Receives tensile force). Therefore, fine cracks are generated on the surface 43 a of the surface layer 43. The fine cracks expand in the depth direction and the belt width direction as the image forming apparatus 1 is used.

  FIG. 6 is a diagram showing a conventional intermediate transfer belt 200 that does not have the stress relaxation layer 44. A conventional intermediate transfer belt 200 shown in FIG. 6 has a three-layer structure of a base layer 201, an elastic layer 202, and a surface layer 203. In the intermediate transfer belt 200, when a crack 255 occurs in the surface layer 203, stress concentration occurs at the tip 255a in the depth direction of the crack 255 as the intermediate transfer belt 200 is driven to rotate, and the crack 255 extends in the depth direction. It expands (see the broken line in FIG. 6). If the crack 255 is deepened, both end portions of the crack 255 in the belt width direction (direction perpendicular to the paper surface in FIG. 6) when the intermediate transfer belt 200 receives a tensile force (see symbols e and f in FIG. 6). In addition, since stress concentration occurs, the crack 255 also expands in the belt width direction. Since the crack 255 is expanded as long as the intermediate transfer belt 200 is used, the crack 255 eventually expands to the base layer 201 through the elastic layer 202. If the crack 255 is greatly enlarged so far, image noise is generated in the printed image.

  Therefore, in this embodiment, in order to prevent the occurrence of such image noise, the stress relaxation layer 44 described above is provided between the surface layer 43 and the elastic layer 42 as shown in FIG. With such a stress relaxation layer 44, even if a crack is formed in the surface layer 43 as the intermediate transfer belt 40 is driven to rotate, the crack can be prevented from expanding (growing) indefinitely.

  That is, as shown in FIG. 3, even if a crack 55 is generated in the surface layer 43 and the crack 55 reaches the stress relaxation layer 44, the stress relaxation layer 44 extends better than the elastic layer 42. Therefore, the stress concentration at the tip 55a in the depth direction of the crack 55 is alleviated. Therefore, the crack 55 is caused by stress (when the intermediate transfer belt 40 wound around the rollers 12, 13, 14, and 15 is subjected to a tensile force (see arrows a and b in FIG. 3)). Although it expands somewhat along the surface direction of the relaxation layer 44 (see arrows c and d in FIG. 3), the expansion of the crack 55 in the depth direction is suppressed. Therefore, since the crack 55 does not become deep, the crack 55 is also prevented from expanding in the belt width direction (direction perpendicular to the paper surface in FIG. 3). Therefore, according to the intermediate transfer belt 40 of the embodiment, it is possible to prevent the crack 55 from growing to the elastic layer 42. Therefore, it is possible to prevent the occurrence of image noise due to the enlarged crack.

Next, an experiment conducted to confirm the effect of the present invention will be described based on Table 1 below. In Examples 1 to 8 and Comparative Examples 1 to 6 of this experiment, an intermediate transfer belt manufactured by a method in which each layer under the following conditions is applied in order from the inside is used as a sample.
Base layer: made of polyimide, thickness 80 μm, volume resistivity 10 ¹⁰ Ω · cm
Elastic layer: made of nitrile butadiene rubber, thickness 200μm, volume resistivity 10 ⁹ Ω · cm
Stress relaxation layer: made of nitrile butadiene rubber, thickness 15 μm, volume resistivity 10 ¹¹ Ω · cm
Surface layer: Made of nitrile butadiene rubber, thickness 15 μm, volume resistivity 10 ¹¹ Ω · cm

  In each sample (intermediate transfer belt) of Examples 1 to 8 and Comparative Examples 1 to 6, the hardnesses of the elastic layer, the stress relaxation layer, and the surface layer are adjusted as shown in Table 1 below. The hardness of the elastic layer, the stress relaxation layer, and the surface layer was adjusted by adjusting the filling amount of the resin crosslinking agent (specifically, phenol resin) into the rubber material (nitrile butadiene rubber). In addition, surface treatment is applied to the surface layer to align the surface friction coefficient of each sample. The surface treatment is a treatment in which the surface layer is brought into contact with the treatment liquid. The treatment liquid is a mixture of t-butyl hypochlorite, ethyl acetate, and tertiary butyl alcohol in a weight ratio of 2:10:88. After contact at room temperature for 30 seconds, it was washed with water. By performing such surface treatment, the friction coefficient of the surface layer is reduced without impairing the flexibility of the surface layer.

  The intermediate transfer belts of Examples 1 to 8 and Comparative Examples 1 to 6 manufactured in this way were mounted on a Konica Minolta bizhub C650, which is an image forming apparatus, and 10,000 images with a printing rate of 5% for each color were printed. Later, cyan solid images were printed on smooth paper and rough paper (embossed paper). Thereafter, the image density (secondary transfer efficiency) was measured for the solid image printed on the rough paper, and the image noise was visually confirmed for the solid image printed on the smooth paper. The image noise includes those caused by wrinkles formed on the surface of the intermediate transfer belt and those caused by cracks. A transmission density measuring machine used for measuring the image density is TD904 manufactured by Macbeth. The evaluation results are as shown in Table 1 above. In Table 1, the meanings of “◯”, “Δ”, and “×” in the “Evaluation result” column are as follows.

Evaluation result of image density (secondary transfer efficiency) ○: Transmission density is 0.9 or more. Good transfer.
Δ: Transmission density is 0.8 or more and less than 0.9. The transfer is slightly thin but has no practical problem.
X: Transmission density is less than 0.8. This transfer has a problem in practical use.
-Evaluation results of image noise (noise caused by wrinkles and noise caused by cracks) ○: No image noise occurred.
Δ: Image noise is slightly generated, but there is no practical problem.
X: Image noise which is a problem in practical use is generated.

From the experimental results shown in Table 1 above, in order to prevent the occurrence of image noise while ensuring good secondary transfer efficiency, the hardness H2 of the surface layer, the hardness H2 of the stress relaxation layer, and the hardness H3 of the elastic layer It was found that the relationship should be kept in the following range (Examples 1 to 8).
Surface layer hardness H1: 80 degrees or more (H1 ≧ 80)
Stress relaxation layer hardness H2: less than 40 degrees (40> H2)
Hardness H3 of elastic layer: 40 degrees or more and less than 80 degrees (80> H3 ≧ 40)

  Further, it was found that the problem of image noise does not occur when H2 is 20 degrees or more (Examples 1, 2, 5, and 6). Further, although H1> H3> H2 is satisfied, if H2 exceeds the above range and is large, crack expansion cannot be sufficiently suppressed, and there is no problem in practical use. It was found that noise was generated (Comparative Example 1). In addition, although H1> H3> H2, the surface of the intermediate transfer belt is wrinkled when H3 is outside the above range and is small, but there is no practical problem, and image noise is caused by the wrinkle. It was found that this occurred (Comparative Example 2). In addition, although H1> H3> H2, it is found that when H1 is out of the above range, there is no practical problem, but the secondary transfer efficiency and the belt cleaning performance are deteriorated ( Comparative Example 4).

  It was also found that when H3 is large enough to deviate from the relationship of H1> H3> H2, the secondary transfer efficiency with respect to rough paper decreases at a level that causes a practical problem (Comparative Example 3). Further, it was found that if H2 is large enough to deviate from the relationship of H1> H3> H2, the expansion of cracks cannot be suppressed, and image noise caused by cracks occurs at a level that causes a practical problem (Comparative Example). 5 and 6).

  As described above in detail, in the intermediate transfer belt 40 in the image forming apparatus 1 according to the first embodiment, the elastic layer 42, the stress relaxation layer 44 that is softer than the elastic layer 42, and the surface layer 43 that is harder than the elastic layer 42. Are stacked in this order from the inner side toward the outer side having the surface (front surface 43a) facing the final medium (paper). Therefore, the expansion (growth) of the crack 55 (see FIG. 3) formed with the rotation of the intermediate transfer belt 40 can be stopped by the stress relaxation layer 44. That is, when the crack 55 is formed in the surface layer 43 with the rotation of the intermediate transfer belt 40 and the crack 55 expands, the depth of the crack 55 reaches the stress relaxation layer 44. Here, since the stress relaxation layer 44 is a softer layer than the elastic layer 42, when the intermediate transfer belt 40 receives a tensile force along the belt traveling direction, the elastic layer 42 extends along the surface direction perpendicular to the thickness direction. Stretches better. Therefore, the stress concentration at the tip 55a in the depth direction of the crack 55 reaching the stress relaxation layer 44 is relaxed. Therefore, the crack 55 can be prevented from growing deep enough to penetrate the stress relaxation layer 44 and reach the elastic layer 42.

  Since the crack 55 can be prevented from growing in the depth direction in this way, stress concentration at both ends along the width direction of the intermediate transfer belt 40 in the crack 55 can also be prevented. Therefore, it is possible to prevent the crack 55 from growing along the width direction of the intermediate transfer belt 40. Therefore, according to this embodiment, it is possible to prevent image noise from being generated in the printed image on the final medium due to the enlarged crack 55.

  The tip 55a of the crack 55 formed in the stress relaxation layer 44 receives a tensile force along the surface direction of the stress relaxation layer 44 as the intermediate transfer belt 40 is driven to rotate, so that it slightly expands along the surface direction. . As a result, the crack 55 appearing on the surface 43a of the surface layer 43 also becomes somewhat larger along the belt traveling direction. However, the degree is about 1/10 compared to the case where the stress relaxation layer 44 is not provided and the lower layer of the surface layer 43 is the elastic layer 42. Therefore, when the crack 55 is enlarged to such an extent, no image noise occurs in the printed image.

  Here, in the intermediate transfer belt 40 of this embodiment, the hardness of the elastic layer 42 is 40 degrees or more and less than 80 degrees, the hardness of the stress relaxation layer 44 is less than 40 degrees, and the hardness of the surface layer 43 is 80 degrees or more. . Therefore, even if the crack 55 is formed in the surface layer 43, the growth can be suitably suppressed. Therefore, printing with no secondary noise and high secondary transfer efficiency can be performed.

  Further, the intermediate transfer belt 40 of this embodiment further includes a base layer 41 that is harder than the surface layer 43 inside the elastic layer 42. Thus, the presence of the base layer 41 that is harder than any of the other layers (the elastic layer 42, the stress relaxation layer 44, and the surface layer 43) prevents the other layers 42, 43, and 44 from extending indefinitely. it can. Therefore, good printing can be performed.

  In the intermediate transfer belt 40 of this embodiment, the elastic layer 42 is particularly preferably made of nitrile butadiene rubber. This is because nitrile butadiene rubber is particularly suitable as the elastic layer 42 of the intermediate transfer belt 40 and can particularly improve the secondary transfer efficiency. Nitrile butadiene rubber is more susceptible to ozone degradation than other rubber materials. However, according to this embodiment, even if the elastic layer 42 is provided by nitrile butadiene rubber, the expansion of cracks can be suppressed, so that image noise is reduced. There is no risk.

(Second Embodiment)
Next, a second embodiment will be described. In the second embodiment, the softness of each layer (the base layer 41, the elastic layer 42, the stress relaxation layer 44, and the surface layer 43) in the intermediate transfer belt 40 is defined by the tensile elongation at break rather than the hardness. The tensile elongation at break indicates how far the sample (each layer) can extend without breaking. The tensile elongation at break here is a value measured using a precision universal testing machine Autograph AG-10kN manufactured by Shimadzu Corporation. A specific formula for calculating the tensile elongation at break is as follows.
Tensile elongation at break (%) = (L−L ₀ ) / L ₀ × 100
L: Length of sample at break L ₀ : Length of sample before test

  The configuration of the intermediate transfer belt of this embodiment is the same as that of the intermediate transfer belt 40 of the first embodiment (see FIG. 2). That is, the base layer 41, the elastic layer 42, the stress relaxation layer 44, and the surface layer 43 are laminated in this order from the inside to the outside. In the description of the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals and the description thereof is omitted. The constituent material, layer thickness, and volume resistivity of each layer (base layer 41, elastic layer 42, stress relaxation layer 44, and surface layer 43) constituting the intermediate transfer belt 40 of this embodiment are the same as those in the first embodiment. .

  The base layer 41 of this embodiment has a tensile breaking elongation of 10% or less (for example, about 5%).

  The elastic layer 42 of this embodiment has a tensile elongation at break of greater than 200% and less than 600% (desirably greater than or equal to 250% and less than or equal to 500%). If the tensile elongation at break is too large, it is too soft and the surface of the intermediate transfer belt 40 can be wrinkled in the secondary transfer region 17, which may cause image noise. On the other hand, if the tensile elongation at break is too small, a sufficient nip width is not secured and transfer efficiency is lowered. Therefore, the tensile breaking elongation of the elastic layer 42 should be in the above range.

  The stress relaxation layer 44 of this embodiment has a tensile breaking elongation of 600% or more. The reason is as follows. That is, when the tensile elongation at break of the stress relaxation layer 44 is 600% or more, even if a tensile force is applied to the intermediate transfer belt 40 that is wound around the rollers 12, 13, 14, and 15 and is driven to rotate, The relaxation layer 44 can sufficiently extend. Therefore, the stress concentration generated at the tip 55a (see FIG. 3) in the depth direction of the crack 55 is alleviated, and the expansion of the crack 55 can be suppressed. Therefore, the tensile elongation at break of the stress relaxation layer 44 is preferably 600% or more. In order to increase the tensile breaking elongation of the rubber material (decrease the hardness), the crosslinking density should be lowered. Note that the expansion of the crack 55 (see FIG. 3) generated in the surface layer 43 cannot be suppressed only by the elastic layer 42 having no stress relaxation layer 44 and having a tensile breaking elongation of about 250% to 500%.

  The surface layer 43 of this embodiment has a tensile elongation at break of 200% or less. As the tensile elongation at break of the surface layer 43 increases, the crack 55 (see FIG. 3) is less likely to occur. However, if the tensile elongation at break of the surface layer 43 is large, the releasability of the toner is deteriorated. In consideration of the releasability of the toner, it is preferable that the tensile elongation at break is small. Specifically, it is preferably 50% or more and 200% or less. This is because if it is 200% or less, the releasability of the toner can be kept good. Further, if it is 50% or more, the surface layer 43 is too hard, so that deformation of the elastic layer 42 is suppressed, and a problem that a sufficient nip width cannot be secured can be prevented.

Next, experiments conducted for confirming the effects of the present invention will be described based on Table 2 below. In Examples 1 to 4 and Comparative Examples 1 to 6 of this experiment, an intermediate transfer belt manufactured by a method in which each layer under the following conditions is applied in order from the inside is used as a sample.
Base layer: made of polyimide, thickness 80 μm, volume resistivity 10 ¹⁰ Ω · cm
Elastic layer: made of nitrile butadiene rubber, thickness 200μm, volume resistivity 10 ⁹ Ω · cm
Stress relaxation layer: made of nitrile butadiene rubber, thickness 15 μm, volume resistivity 10 ¹¹ Ω · cm
Surface layer: Made of nitrile butadiene rubber, thickness 15 μm, volume resistivity 10 ¹¹ Ω · cm

  In each sample of Examples 1 to 4 and Comparative Examples 1 to 6 (intermediate transfer belt), the tensile rupture elongation of the stress relaxation layer and the surface layer is adjusted as shown in Table 2 below, and the tensile rupture elongation of the elastic layer is It is adjusted to 400%. The tensile breaking elongation of the elastic layer, the stress relaxation layer, and the surface layer was adjusted by adjusting the filling amount of the resin crosslinking agent (specifically, phenol resin) into the rubber material (nitrile butadiene rubber). In addition, surface treatment is applied to the surface layer to align the surface friction coefficient of each sample. The surface treatment is a treatment in which the surface layer is brought into contact with the treatment liquid. The treatment liquid is a mixture of t-butyl hypochlorite, ethyl acetate, and tertiary butyl alcohol in a weight ratio of 2:10:88. After contact at room temperature for 30 seconds, it was washed with water. By performing such surface treatment, the friction coefficient of the surface layer is reduced without impairing the flexibility of the surface layer.

  The intermediate transfer belts of Examples 1 to 4 and Comparative Examples 1 to 6 manufactured in this way are mounted on a bizhub C650 manufactured by Konica Minolta, which is an image forming apparatus, as in the experiment shown in Table 1 above. After printing 10,000 images with a printing rate of 5%, a cyan solid image was printed on smooth paper and rough paper (embossed paper). Thereafter, the image density (secondary transfer efficiency) was measured for the solid image printed on the rough paper, and the image noise was visually confirmed for the solid image printed on the smooth paper. The image noise includes those caused by wrinkles formed on the surface of the intermediate transfer belt and those caused by cracks. A transmission density measuring machine used for measuring the image density is TD904 manufactured by Macbeth. The evaluation results are as shown in Table 2 above. In Table 2, the meanings of “◯”, “Δ”, and “×” in the “Evaluation Result” column are the same as the meanings of “O”, “Δ”, and “×” shown in Table 1.

From the experimental results shown in Table 2 above, in order to prevent the occurrence of image noise while ensuring good secondary transfer efficiency, the relationship between the tensile rupture elongation S1 of the surface layer and the tensile rupture elongation S2 of the stress relaxation layer is It turned out that it should just keep to the following ranges (Examples 1-4).
Surface layer tensile elongation at break S1: 200% or less (200% ≧ S1)
Tensile elongation at break S2 of stress relaxation layer: 600% or more (S2 ≧ 600%)

  In particular, it was found that when S2 is out of the above range (Comparative Examples 1 to 4), the effect of suppressing the expansion of cracks cannot be obtained and image noise occurs. The degree of the generated image noise is larger as the value of S2 is smaller. This is because the smaller the tensile rupture elongation of the stress relaxation layer, the larger the crack.

  Further, it was found that when S1 is out of the above range (Comparative Examples 5 and 6), the toner releasability is not sufficiently secured and the secondary transfer efficiency is lowered. The secondary transfer efficiency decreases as the value of S1 increases. This is because the larger the tensile elongation at break of the surface layer, the softer the surface layer, and the worse the toner releasability. When the tensile elongation at break of the surface layer was 600% (Comparative Example 6), wrinkles were formed on the surface of the intermediate transfer belt, and image noise due to the wrinkles was also generated.

  As described above in detail, in the intermediate transfer belt 40 in the image forming apparatus according to the second embodiment, the elastic layer 42, the stress relaxation layer 44 that is softer than the elastic layer 42, and the surface layer 43 that is harder than the elastic layer 42. Are stacked in this order from the inner side toward the outer side with the surface (front surface 43a) facing the final medium (paper). The tensile elongation at break in the surface layer 43 is 200% or less. The tensile breaking elongation of the stress relaxation layer 44 is 600% or more. Therefore, the expansion (growth) of the crack 55 (see FIG. 3) formed with the rotational drive of the intermediate transfer belt 40 can be stopped by the stress relaxation layer 44 based on the same principle as in the first embodiment. Therefore, according to this embodiment, it is possible to prevent image noise from being generated in the printed image on the final medium due to the enlarged crack 55 (see Table 2). In the second embodiment, the degree of softness of each layer (the base layer 41, the elastic layer 42, the stress relaxation layer 44, and the surface layer 43) is defined by tensile elongation at break. The hardness of each layer is shown in the first embodiment. There is nothing to prevent it from becoming hard.

  In addition, embodiment mentioned above is only a mere illustration, and does not limit this invention at all. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, in the above embodiment, the intermediate transfer belt 40 having the base layer 41 is used, but a belt without the base layer 41 may be used. That is, for example, an intermediate transfer belt 140 shown in FIG. 4 may be used which includes only the elastic layer 142, the stress relaxation layer 144, and the surface layer 143. In the case of such an intermediate transfer belt 140, the elastic layer 142, the stress relaxation layer 144, and the surface layer 143 may have the same configuration as the elastic layer 42, the stress relaxation layer 44, and the surface layer 43 of the embodiment. .

  In the embodiment, the secondary transfer is performed by using the secondary transfer roller 16, but the secondary transfer may be performed by using the secondary transfer belt 80 as shown in FIG. The secondary transfer belt 80 is stretched over a plurality of rollers 81, 82, 83 and rotates as the rollers 81, 82, 83 are driven to rotate. The secondary transfer belt 80 forms a nip portion 85 serving as a secondary transfer region between the secondary transfer belt 80 and the intermediate transfer belt 40. In other words, in this configuration, the secondary transfer is performed with the paper (final medium) sandwiched between the secondary transfer belt 80 and the intermediate transfer belt 40. In such a configuration, a secondary transfer belt 80 having the same configuration as that of the intermediate transfer belt 40 of the embodiment is preferably used. That is, it is preferable to use a layer having an elastic layer 42, a stress relaxation layer 44, and a surface layer 43 as shown in FIG. Also in the secondary transfer belt 80, it is known that image noise occurs in the final medium when cracks formed in the surface layer expand (grow). This is because a sufficient bias necessary for transferring the toner image is not applied to the secondary transfer belt 80. However, if configured as described above, generation of image noise can be prevented. Various modifications regarding the intermediate transfer belt 40 described above can also be applied to the secondary transfer belt 80.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 2 ... Imaging unit (image forming unit)
DESCRIPTION OF SYMBOLS 21 ... Photoconductor 40 ... Intermediate transfer belt 41 ... Base layer 42 ... Elastic layer 43 ... Surface layer 43a ... Surface layer surface 44 ... Stress relaxation layer 80 ... Secondary transfer belt

Claims (7)

In an endless transfer belt which is used in an electrophotographic image forming apparatus and is driven to rotate when transferring a toner image,
At least an elastic layer, a stress relaxation layer that is softer than the elastic layer, and a surface layer that is harder than the elastic layer are laminated in this order from the inner side to the outer side that faces the final medium. And transfer belt. The transfer belt according to claim 1,
JISK6253 durometer type A hardness (hereinafter referred to as “hardness”) in the elastic layer is 40 degrees or more and less than 80 degrees,
The stress relaxation layer has a hardness of less than 40 degrees;
The transfer belt according to claim 1, wherein the surface layer has a hardness of 80 degrees or more. The transfer belt according to claim 1 or claim 2,
The tensile elongation at break of the surface layer is 200% or less,
The transfer belt, wherein the stress relaxation layer has a tensile elongation at break of 600% or more. A transfer belt according to any one of claims 1 to 3,
A transfer belt, further comprising a base layer harder than the surface layer inside the elastic layer. A transfer belt according to any one of claims 1 to 4,
The transfer belt, wherein the elastic layer is made of nitrile butadiene rubber. A transfer belt according to any one of claims 1 to 5,
An image forming apparatus comprising: an image forming unit that forms a toner image transferred onto a final medium using the transfer belt on a photoconductor. The image forming apparatus according to claim 6, wherein
The transfer belt is an intermediate transfer belt that receives a primary transfer of a toner image formed on a photoconductor by the image forming unit and performs secondary transfer to a final medium, or a photoconductive belt formed by the image forming unit. An image forming apparatus comprising: a secondary transfer belt facing the intermediate transfer belt with the final medium sandwiched during secondary transfer for transferring the toner image transferred from the body to the intermediate transfer belt to the final medium.

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