Abstract:
A charging device for electrically charging an image bearing member, includes a contact charging member contactable to the image bearing member to charge the image bearing member, the contact charging member being supplied only with a DC voltage; and a contact member contacted to the charging member, wherein the image bearing member is adapted to carry a toner image of toner which contains an externally added material comprising silica, wherein work functions of the charging member, the contact member and the silica satisfy,
 
(the work function of the contact member)&gt;(the work function of the silica)&gt;(the work function of the charging member).

Description:
FIELD OF THE INVENTION AND RELATED ART 
   The present invention relates to a charging apparatus which charges an image bearing member such as an electrophotographic photosensitive member, an electrostatically recordable dielectric member, etc., by placing a charging member in contact with the image bearing member. It also relates to a process cartridge having such a charging apparatus. Further, it relates to an image forming apparatus which employs such a charging apparatus and an electrophotographic image forming method. 
   Here, “image forming apparatus employing an electrophotographic image forming method” means an apparatus which forms an image on recording medium (for example, recording paper, OHP sheet, etc.) with the use of an electrophotographic image forming method. As examples of an image forming apparatus, an electrophotographic copying machine, an electrophotographic printer (for example, laser printer, LED printer, etc.), a facsimileing machine, a wordprocessor, and a multi-functional machine (multifunctional printer, etc.) made up of the combination of the preceding apparatuses, can be included. 
   “Process cartridge” means a cartridge in which at least one among a charging apparatus, a developing apparatus, and a cleaning apparatus, and a photosensitive member as an image bearing member, are integrally disposed, and which is removably mountable in the main assembly of an image forming apparatus. Thus, “process cartridge” includes a cartridge in which at least a charging apparatus as a processing means, and an electrophotographic photosensitive drum are integrally disposed, and which is removably mountable in the main assembly of an image forming apparatus. 
   In an electrophotographic image forming apparatus, generally, the following processes are carried out: a charging process for charging a photosensitive drum as an image bearing member; an exposing process for forming an electrostatic latent image on a photosensitive drum with the use of an exposing means such as scanning a photosensitive drum with a beam of laser light; a developing process for developing an electrostatic latent image into a visible image, with the use of toner; a transferring process for transferring a toner image on a photosensitive drum, onto recording medium; a fixing process for fixing a toner image on recording medium; and a cleaning process for cleaning a photosensitive drum after a transferring process. An electrophotographic image is formed by sequentially carrying out these processes. 
   For the purpose of giving desired properties to toner, various external particulate additives are added to toner, causing external additive particles to adhere to the surface of a toner particle. The inherent polarity to which external additives become charged is the same as the inherent polarity to which toner becomes charged. 
   Charging apparatuses for charging a photosensitive drum can be roughly classified into two types: noncontact type and contact type. 
   A charging apparatus of the noncontact type generally employs a corona discharger, which is disposed next to a photosensitive drum, with no contact between the corona discharger and photosensitive drum. To the metallic wire (discharge wire) of the corona discharger, high voltage (5-8 kV of DC voltage) is applied to cause the wire to discharge corona, by which the peripheral surface of a photosensitive drum is charged. 
   A charging apparatus of the contact type is such a charging apparatus that charges the peripheral surface of the photosensitive drum by placing a charging member (which hereinafter may be be referred to as charge roller), for example, a charge roller, in contact with the photosensitive drum, and applying voltage to the charge roller. It is superior to a charging apparatus of the noncontact type in that it is lower in ozone emission, low in power consumption, etc. 
   Charging apparatuses of the contact type may be roughly classified into two groups, that is, a group of the so-called DC type, and a group of the so-called AC type. In the case of the DC type group, only DC voltage is applied to a charge roller to charge a photosensitive drum, whereas in the case of the AC type, the combination of DC voltage and AC voltage is applied to a charge roller to charge a photosensitive drum. 
   A charging method of the contact/DC type is advantageous over a charging method of the contact/AC type in that it does not produce charging noises and electrostatic noises, is lower in power source cost, is low in ozone emission, low in the overall cost, etc. Further, it is smaller in the amount of electric current involved with the electric discharge for charging the peripheral surface of a photosensitive drum to a preset potential level. In other words, its merits are substantial. 
   Generally, in the case of an image forming apparatus of the reversal development type, negative voltage is applied as bias to a charge roller, and a photosensitive drum is charged to negative polarity. 
   After the end of a transfer process, unwanted substances such as transfer residual toner, paper dust, etc., are present on a photosensitive drum. Thus, an image forming apparatus is designed so that these unwanted substances are removed through a cleaning process, in which they are scraped away with the use of a cleaning blade or the like, and stored in a waste toner bin. However, it is difficult to scrape away all the unwanted substances on a photosensitive drum through a cleaning process; a very small amount of the unwanted substances, for example, a very small amount of toner, etc., slips by a cleaning blade. 
   The small amount of residual toner, which has slipped by the cleaning blade, contains such toner particles that had been positively charged through a transfer process, and such toner particles that had been given a small amount of negative charge through a transfer process. These residual toner particles are given electric charge by the electrical discharge caused through the following charging process, acquiring therefore a certain amount of negative electric charge. Negatively charged residual toner particles are not likely to adhere to a charge roller. However, there are residual toner particles that were not given a sufficient amount of electrical charge, and these residual toner particles adhere to a charge roller. 
   These residual toner particles are likely to nonuniformly adhere to the peripheral surface of a charge roller, and the nonuniform adhesion of these residual toner particles to the peripheral surface of a charge roller is likely to make the charge roller nonuniform in surface properties. A charge roller which is nonuniform in surface properties is likely to nonuniformly charge a photosensitive drum in terms of potential level, creating the problem that an image suffering from defects attributable to the nonuniform charging of a photosensitive drum is yielded. This problem is more likely to occur when the charging method of the DC/contact type, which applies only DC voltage to a charge roller, is employed than when the charging method of the AC/contact type is employed. 
   Thus, in order to prevent the above described toner particle adhesion to a charging roller as a charging member of the contact type, from causing the charge roller to nonuniformly charge a photosensitive drum, such a technology has been proposed that a rubbing sheet is placed in contact with a charge roller to disperse the toner particles having adhered to the charge roller. 
   However, it has been discovered that even when the above described technology in accordance with the prior art is employed, a photosensitive drum is unsatisfactorily charged because the external additives of toner, which will be described later, adhere to the abovementioned member rubbing sheet and accumulate thereon. Thus, it has been desired to improve this technology. 
   To describe in more detail the abovementioned problem, toner contains various external additives which are in the form of a microscopic particle. These microscopic particles of the external additives are generally negatively charged particles as are the toner particles themselves. Further, they sometimes become separated from toner particles as they are put through a development process, a transfer process, and cleaning process. The occurrence of this phenomenon is sometimes rendered more conspicuous by such factors as the cumulative usage of an image forming apparatus which is required to be durable, type of external additive, condition under which toner is manufactured, etc. In addition, compared to the particle diameter of toner (roughly 6 μm in volume average diameter), the particle diameter of external additive is extremely small (roughly 0.01 μm in volume average diameter). Therefore, external additive is difficult to scrape away from the peripheral surface of a photosensitive drum with the use of a cleaning apparatus, sometimes slipping by the cleaning apparatus by a greater amount than the amount by which the residual toner slipped by the cleaning apparatus, and reaching a charge roller. As the external additive slips by a cleaning apparatus and reaches a charge roller, it is given electrical charge, which is the same in polarity as that given to toner, through a charging process. However, the substantial amount of the external additive having slipped by the cleaning apparatus receives an unsatisfactory amount of electric charge, being thereby rendered likely to adhere to a charge roller. 
   There has been observed the following phenomenon: As a charge roller is rotated, the external additive having adhered to a charge roller adheres to the abovementioned rubbing sheet as the external additive moves through the area in which the charge roller is in contact with the sheet. The severity of the additive adhesion to the sheet was affected by the additive type, and the material for the sheet. Besides, external additives are more likely to agglomerate than toner. Therefore, external additives continue to accumulate onto the body of external additives having adhered to the sheet, forming eventually microscopic projections on the sheet. 
   Once the microscopic projections are formed of external additives, on the sheet, they leave, on the peripheral surface of a charge roller, such scratches that extend in the circumferential direction of the charge roller, and/or prevent the rubbing sheet from evenly contacting the charge roller, preventing thereby the rubbing sheet from evenly dispersing the external additives having adhered to the charge roller. As a result, a photosensitive drum is unsatisfactorily charged across the areas which correspond to the abovementioned streaky scratches of the charge roller, causing thereby an image forming apparatus to yield an image suffering from image defects which extend in the vertical direction. The possibility of the formation of an image suffering from this image defect is much higher when the external additives include silica than otherwise. 
   The present invention is a further development of the above described technology in accordance with the prior art, and its primary object is to prevent the phenomenon that the additives of toner adhere to a charging member, transfer from the charging member to a particle dispersing member placed in contact with the charge roller, accumulating thereon, and cause the charging member to unsatisfactorily charge an object to be charged. 
   SUMMARY OF THE INVENTION 
   Accordingly, the object of the present invention is to provide a charging device, a process cartridge and an image forming apparatus, wherein occurrence of improper charging attributable accumulation of the extenally added material. 
   According to an embodiment of the present invention, there is provided a charging device for electrically charging an image bearing member, comprising a contact charging member contactable to the image bearing member to charge the image bearing member, said contact charging member being supplied only with a DC voltage; and a contact member contacted to said charging member, wherein said image bearing member is adapted to carry a toner image of toner which contains an externally added material comprising silica, wherein work functions of said charging member, said contact member and the silica satisfy (the work function of said contact member)&gt;(the work function of the silica)&gt;(the work function of said charging member). 
   These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of the image forming apparatus in the first embodiment of the present invention. 
       FIG. 2  is a perspective view of the charge roller, and its adjacencies, in the first embodiment of the present invention. 
       FIG. 3  illustrates a cell dedicated to measuring the work function of external additive, wherein (a) is a schematic plan view of the cell dedicated to measuring the work function of external additive; (b) is a schematic side view of the cell dedicated to measuring the work function of the external additive; and (c) is a perspective view of the cell dedicated to measuring the work function of the external additive. 
       FIG. 4   a  is a schematic view of a test sample cut from the cylindrical image forming apparatus member to measure the work function of the member. 
       FIG. 4   b  is a schematic drawing of a surface analyzing apparatus. 
       FIG. 5  is a graph of the work function of the sheet A, which was measured using the surface analyzing apparatus. 
       FIG. 6  is a diagram showing the order in terms of work function among the rubbing sheets, external additive, and charge roller. 
       FIG. 7  is a schematic drawing of the rubbing sheet, in the second embodiment, which is equipped with a driving mechanism. 
       FIG. 8  is a schematic drawing of an image forming apparatus in which a process cartridge is removably mountable. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, the preferred embodiments of the present invention will be described with reference to the appended drawings. Incidentally, the measurements, materials, and shapes of the structural components in the following embodiments of the present invention, and the positional relationships among them, are to be changed as necessary according to the structure of an apparatus to which the present invention is applied, and various conditions under which the present invention is applied. In other words, they are not intended to limit the scope of the present invention. 
   Embodiment 1 
   (Image Forming Apparatus) 
     FIG. 1  is a schematic drawing of an image forming apparatus having a charging apparatus  200 , which is in accordance with the present invention. This image forming apparatus is provided with a photosensitive drum  100  (electrophotographic photosensitive member in the form of drum) as an image bearing member, on which an electrostatic latent image is formed. The photosensitive drum  100  is disposed roughly in the center of the image forming apparatus. In the adjacencies of the peripheral surface of this photosensitive drum  100 , a charging apparatus, an exposing apparatus, a developing apparatus, a transferring apparatus, a cleaning apparatus, etc., which are processing means, are disposed in a manner of surrounding the peripheral surface of the photosensitive drum. 
   The charging apparatus  200  is a charging means, which uniformly charge the photosensitive drum  100  with the use the charging method of the DC/contact type. The exposing apparatus  300  is a latent image forming means, which forms an electrostatic latent image, which reflects printing data and image formation data, on the charged photosensitive drum  100 . The developing apparatus  400  is a developing means, which develops the formed electrostatic latent image into a visible image, with the use of developer. The transferring apparatus  500  is a transferring means which transfers the toner image on the photosensitive drum  100 , onto a recording medium  900 . The cleaning apparatus  600  is a cleaning means which removes the toner, etc., which remain on the photosensitive drum  100  after the toner image transfer. Designated by a referential symbol  700  is a cassette as a paper feeding apparatus which supplies the main assembly of the image forming apparatus with the recording medium  900 . Designated by a referential symbol  800  is a fixing apparatus which permanently fixes the transferred image on the recording medium  900 . 
   The charging apparatus  200  has a charge roller  201  as a charging member of the contact type. The charging apparatus  200  also has an electric power source for charging charge bias to the charge roller  201 . Further, it has a sheet  203 , which is kept in contact with the charge roller  201  to disperse the unwanted substances having adhered to the charge roller  201 . 
   The charge roller  201  is made up of: a metallic core, which is 6 mm in diameter; a roughly 3 mm thick electrically conductive elastic layer formed of urethane rubber, around the metallic core; a several micrometer thick high resistance layer formed of urethane rubber in which carbon black has been dispersed, around the conductive elastic layer. As the material for the high resistance layer, acrylic resin, Nylon resin, fluorinated resin, or the like, may be used instead of urethane rubber. The charge roller  201  is disposed roughly in parallel to the photosensitive drum  100 . It is rotatably supported at the lengthwise ends of its metallic core by electrically conductive supporting members (unshown), which are kept pressed toward the photosensitive drum  100  so that the charge roller  201  is allowed to move toward, or away from, the photosensitive drum  100 . With the employment of this structural arrangement, the charge roller  201  remains under a preset amount of pressure, being thereby kept pressed on the photosensitive drum  100 , against the elasticity of the conductive elastic layer of the photosensitive drum  100 , forming a charging nip. 
   The charge roller  201  is placed in contact with the photosensitive drum  100 , and is rotated by the rotation of the photosensitive drum  100 . To the charge roller  201 , charge bias is applied from the electric power source through the abovementioned electrically conductive supporting members and metallic core. In this embodiment, a DC voltage of roughly −1,000 V is applied between the photosensitive drum  100  and charge roller  201 , whereby the peripheral surface of the photosensitive drum  100  is charged to −500 V (dark point voltage: VD). 
     FIG. 2  is a top perspective view of the charge roller  201  and its adjacencies. A sheet  203  as a member to be kept in contact with the charge roller  201  is 25-100 μm in thickness. It is roughly the same in length as the elastic layer of the charge roller  201 . It is supported by a sheet supporting member  204 ; it is pasted to the sheet supporting member  204  by one of its long edges, with the use of a piece of two-sided adhesive tape, so that it does not become wavy. The sheet supporting member  204  is rigidly fixed to a stationary cleaning blade supporting member  603  of the cleaning apparatus  600 . The cleaning apparatus is designed so that the sheet  203  is kept pressed upon the peripheral surface of the charge roller  201  by a preset amount of pressure, so that the free longer edge of the sheet  203  is on the downstream side of the longer edge of the sheet  203 , by which the sheet  203  is attached to the sheet supporting member  204 , in terms of the rotational direction of the charge roller  201 , and also, so that the contact area between the sheet  203  and charge roller  201  is substantial in width in terms of the circumferential direction of the charge roller  201 . Further, the sheet  203  is placed in contact with the charge roller  201  so that there is no gap at all in the contact area between the sheet  203  and charge roller  201 . With the employment of the above described structural arrangement, as the charge roller  201  is rotated, the sheet  203  rubs the charge roller  201 , uniformly dispersing the unwanted substances having adhered to the charge roller  201 . 
   In this embodiment, the following three sheets A, B, and C were prepared as the sheet  203 . 
   Sheet A: polyimide sheet (commercial name: Upilex; product of Ube Industries Ltd.). 
   Sheet B: polyimide sheet (commercial name: Kapton; product of Toray •Dupon Co. Ltd.). 
   Sheet C: PPS (polyphenylenesulfide) sheet (commercial name: Torelina; product of Toray Industries Ltd.). 
   At this time, polyimide sheet will be described. Generally, polyimide is manufactured by polycondensation of aromatic tetracarboxyl acid dianhydride and aromatic diamine. Various monomers are industrially manufactured as the material for diamine, offering a large number of choices. However, the number of the choices of dianhydride of aromatic tetracarboxyl acid which are industrially manufactured is small. Thus, it is said that the properties of polyimide are dependent upon the choice of dianhydride of aromatic tetracarboxyl acid. As the material for the sheet A, that is, a polyimide sheet, biphenyl tetracarboxyl dianyhydride (BPDA) is used as the choice of the aromatic tetracarboxyl acid dianhydride. As the material for the sheet B which also is a polyimide sheet, piromerit acid dianhydride is used. 
   The photosensitive drum  100  is charged across its peripheral surface to −500 V (VD: dark point voltage level) by the charging apparatus  200 . Thereafter, it is exposed by the exposing apparatus  300 ; it is exposed to a beam of laser light projected by the an exposing apparatus  300  while being modulated with printing data and image formation data. As a result, an electrostatic latent image is formed on the photosensitive drum  100 . The potential level of an exposed point of the photosensitive drum  100  was −100 V (VL: light point voltage level). Designated by a referential symbol  300   a  is a deflection mirror for deflecting the beam of laser light outputted from the exposing apparatus  300 , toward the points of the photosensitive drum  100 , which are to be exposed. 
   The developing apparatus  400  is a developing apparatus of the reversal development type, which adheres negatively charged toner to the exposed points (light points) of the electrostatic latent image on the photosensitive drum  100 . This developing apparatus has a hopper portion  406  and a development chamber  407 , which are separated by a partitioning wall  405 . In the hopper portion  406 , a stirring apparatus  404  is disposed, which sends toner into the development chamber  407 . 
   In the development chamber  407 , a development roller  401  is disposed, which develops the electrostatic latent image on the photosensitive drum  100  into a toner image (image formed of toner). This development roller  401  is 16 mm in diameter. It has a substrate layer, and a surface layer coated on the substrate layer. The substrate layer is formed of silicon rubber, and the surface layer is formed of acrylic, urethane, or the like rubber. Also in the development chamber  407 , a supply roller  402  is disposed, which is for supplying the development roller  401  with toner. The supply roller  402  is 16 mm in diameter, and is formed of urethane sponge. Further, a metallic development blade  403  for regulating the thickness in which toner is borne in a layer on the development roller  401  is disposed in the development chamber  407 . 
   The developing apparatus  400  is structured so that the abovementioned stirring apparatus  404 , development roller  401 , and supply roller  402  can be externally driven. During a development process, the stirring apparatus  404 , development roller  401 , and supply roller  402  are continuously rotated to supply the photosensitive drum  100  with toner. 
   The development roller  401  is disposed in contact with the photosensitive drum  100  to develop the electrostatic latent image. To the development roller  401 , development bias is applied from an electric power source  408 . In this embodiment, the electrostatic latent image formed on the photosensitive drum  100  is developed in reverse by applying a DC voltage of roughly −300 V between the photosensitive drum  100  and development roller  401 . 
   The transferring apparatus  500  has a transfer roller  501 , which is 12 mm in diameter and is formed of EPDM sponge. This transfer roller  501  is placed in contact with the photosensitive drum  100  with the application of a preset amount of pressure, forming a transfer nip. To the transfer roller  501 , a transfer bias power source  502  is connected, which is controlled so that a preset amount of voltage is applied to the transfer roller  501  during image formation. 
   A recording medium  900  stored in a cassette  700  as a paper feeding apparatus is delivered by a paper feeding roller  701  to a registration roller  702 , in synchronism with the progression of the formation of a toner image on the photosensitive drum  100 . Then, the recording medium  900  is delivered by the registration roller  702  to the transfer nip, that is, the contact area between the transfer roller  501  and photosensitive drum  100 , in synchronism with the arrival of the leading edge of the toner image being formed on the photosensitive drum  100 , at the transfer nip. After arriving at the transfer nip, the recording medium  900  is conveyed through the transfer nip while remaining pinched by the transfer nip. While the recording medium  900  is conveyed through the transfer nip, a DC voltage of roughly −1,000 V, as a transfer bias, is applied to the transfer roller  501  from the power source  502 . As a result, the toner image on the photosensitive drum  100  is electrostatically transferred onto the recording medium  900  as if it were rolled out of the photosensitive drum  100 . 
   After coming out of the transfer nip, the recording medium  900  is separated from the peripheral surface of the photosensitive drum  100 , and then, is conveyed to the fixing apparatus  800 , by which the unfixed toner image on the recording medium  900  is fixed, that is, turned into a permanent image, by heat and pressure. After coming out of the fixing apparatus  800 , the recording medium  900  is discharged as a copy from the image forming apparatus. 
   Meanwhile, the transfer residual toner, that is, the toner remaining on the photosensitive drum  100  after the recording medium  900  having come out of the transfer nip is separated from the photosensitive drum  100 , is removed from the peripheral surface of the photosensitive drum  100 ; it is wiped away by the cleaning blade  601  of the cleaning apparatus  600 , which is formed of urethane rubber. The removed transfer residual toner is stored in a waste toner container  602 . Thereafter, the cleaned portion of the peripheral surface of the photosensitive drum is charged again by the charging apparatus  200 , for the following image formation cycle. 
   The toner used in this embodiment is a nonmagnetic single-component developer. As the bonding resin for the toner, one of such resins as copolymer of styrene and acrylic, copolymer of styrene and methacrylic, polyester, and the like is used. As the controlling agent for charging toner to the negative polarity, organic metallic complexes, and chelate compounds are effective. More specifically, mono-azoic metallic compounds, acetyl-acetonic metallic compounds, aromatic hydroxy carboxyl acids, aromatic dicarboxylic metallic compounds, are usable to ensure that toner is charged to the negative polarity. 
   The amount (ratio) of a coloring agent for toner is 0.1-60, preferably, 0.5-50, units of mass per 100 units of mass of bonding resin. The amount (ratio) of polarity controlling agent is 0.1-15, preferably, 0.5-10, units of mass per 100 units of mass of the bonding resin. 
   Further, for the purpose of giving toner properties that are desirable for powder, microscopic powder of an inorganic substance, such as silica, alumna, titanium oxide, hydrotalcite compound, di-tertiary butyl salicylic aluminum complex, or the like are added, as external additives, to toner, alone or in combination. 
   These inorganic powders are desired to be 20 m 2 /g-400 m 2  in specific surface. Further, these inorganic powders may be surface-treated. As the surface treatment agent for these inorganic powders, silane coupling agent, titanium coupling agent, silicone oil, and the like can be listed. Preferably, these inorganic powders are treated with silane coupling agent or silicone oil coupling agent; they may be surface-treated with the combination of silane coupling agent and silicone oil coupling agent. The amount (ratio) by which the abovementioned microscopic powders are to be added to toner is 0.03-5, preferably, 0.03-3, units of mass per 100 units of mass of toner. 
   In this embodiment, a product yielded by adding 1.5 parts of silica (external additive α), 0.1 part of hydrotalcite compound, 0.1 part of di-tertiary butyl salicylic acid aluminum complex, to 100 parts of toner, in terms of mass, was used as the developer. 
   When these additive were used as the external additives, defective images were formed, the defects of which were attributable to the adhesion of silica to the abovementioned sheet of the charging apparatus  200 . Therefore, silica was evaluated as the external additive α. 
   (Measurement of Work Function) 
   The nonuniform charging of the photosensitive drum  100 , which is traceable to the nonuniform toner adhesion to the charge roller  201 , can be prevented by the dispersion of the toner having adhered to the charge roller  201 , by the sheet  203  placed in contact with the charge roller  201 . 
   However, as described above, toner contains the external additives, which are extremely small in particle diameter (roughly 0.01 μm in volume average diameter) compared to the particle diameter (roughly 6 μm in volume average particle diameter) of toner. These external additives adhere to the abovementioned sheet  203  of the charging apparatus  200 , and accumulate thereon. Thus, even if the transfer residual toner is uniformly dispersed by the sheet  203 , the unsatisfactory charging of the photosensitive drum  100 , which is traceable to the adhesion of the external additives to the sheet  203  and the resultant accumulation of the external additives on the sheet  203 , still occurs. 
   In this embodiment, therefore, the charging apparatus  200  was designed so that the relationship in terms of work function among the charge roller  201  as the charging member, the sheet  203  as the residual toner dispersing member, and silica satisfies an inequality: sheet  203 &gt;silica&gt;charge roller  201 . With the employment of this design, the unsatisfactory charging of the photosensitive drum  100 , which is traceable to the adhesion of the external additive (silica) of the toner to the sheet  203 , and the resultant accumulation of the external additive (silica) on the sheet  203 , is prevented. 
   Next, this embodiment will be evaluated regarding the relationships among the charge roller  201 , sheet  203  placed in contact with the charge roller  201 , and external additive α, in terms of the work function which is measured using the following measuring method. 
   Work function (φ) is known as the energy necessary to free an electron from a substance. The smaller the work function of a substance, the easier it is to free an electron from the substance; the greater the work function of a substance, the more difficult to free an electron from the substance. Therefore, a substance which is smaller in work function is characterized in that it is likely to be positively charged, whereas a substance which is larger in work function is characterized in that it is likely to be negatively charged. 
   The value of the work function of a given substance is measured using the following measuring method, and is expressed, in numerical value, as the amount of energy (eV) necessary to free an electron from the substance. It can be used for evaluating various substances as the materials for the components of the image forming apparatus, the external additives, etc., in terms of the inherent polarity to which they become charged. 
   The work function a substance is measured using a surface analyzing apparatus AC-2 (product of Riken Instrument Co., Ltd.) which employs the low energy electronic measurement method). In this embodiment of the present invention, a heavy hydrogen lamp is used with the surface analyzing apparatus, and its luminous energy is set to 500 nW. A light separating apparatus is used to choose one of the monochromatic lights, and a beam of the selected monochromatic light is projected on a text sample so that the spot size becomes 4 mm square, and an energy scan range falls in the range of 3.4-6.2 eV. The length of measurement is 10 seconds per spot. While the test sample is illuminated, the photoelectrons freed from the surface of the test sample are detected, and the result of the detection was process with the use of a work function computation software. The work function was measured at a repeatability level (standard deviation) of 0.02 eV. Incidentally, as for the measurement conditions for ensuring the data reproducibility, test samples were left unattended for 24 hours at 25° C. and 55% RH. 
   In order to measure the work functions of test samples of external additives, a cell such as a cell  10 , shown in  FIG. 3 , which is dedicated to work function measurement, is used.  FIG. 3(   a ) is a plan view of the cell  10 , and  FIG. 3(   b ) is a partially cutaway side view of the cell  10 .  FIG. 3(   c ) is a perspective view of the cell  10 . This cell  10  is 30 mm in diameter and 5 mm in height. It is formed of stainless steel, and has a recess  10   a  for holding an external additive. The recess  10   a  is 15 mm in diameter and 3 mm in depth. Silica is placed in the recess  10   a , using a measurement spoon, without packing the silica. Then, the body of the silica in the recess  10   a  is flattened using a knife edge so that the top surface of the body of the silica become level with the top edge of the recess  10   a . Then, the cell  10  is unmovably attached to a preset position of the sample table, to measure the test sample. 
   In order to measure the work function of a cylindrical member of an image forming apparatus, for example, a charge roller, the cylindrical member of an image forming apparatus is to be cut crosswise to obtain a 1-1.5 cm wide piece of the cylindrical member. Then, the piece is to be cut in parallel to the axial line of the cylindrical member to obtain a test piece a having the shape shown in  FIG. 4(   a ). Then, the test piece a is to be immovably attached to the preset location of the sample table  11  so that its surface to be irradiated with the beam of measurement light L, that is, the surface facing the direction from which the beam of measurement light L is projected, is flat and smooth as shown in  FIG. 4(   b ). With the employment of this arrangement, it is ensured that the freed photoelectron  12  is detected by a detecting device  13  (photomultiplier tube). 
   When a flat member of an image forming apparatus, such as the sheet  203  placed in contact with the charge roller  201 , is sampled, a test piece which is no smaller than 1 cm square is to be cut from the flat member, and this piece is immovably attached to the sample table  11 , for measurement. This is because the measurement light is controlled so that the spot it forms on the test piece is 4 mm square, as described above. 
   In this surface analysis test, the beam of monochromatic light is gradually increased in excitation energy from the preset value. As it is increased, a photoelectron begins to be freed when the excitation energy reaches a certain value [eV] . This value, that is, the threshold value, is the work function [eV] of the test sample. 
     FIG. 5  is a graph showing the results of the analysis of the abovementioned sheet A formed of polyimide sheet (commercial name: Upilex; product of Ube Industries Ltd.), which were obtained with the use of a surface analyzing apparatus. In  FIG. 5 , the excitation energy [eV] is represented by the horizontal axis, and the normalized yield of photon (n-th power of photon yield per unit amount of photon) is represented by the vertical axis. Where the excitation energy is greater than the value at which a photoelectron begins to be freed, the relationship between the excitation energy and the amount of the freed photoelectrons is linear; the value of Y/eV is constant. In  FIG. 5 , the work function of the sheet A is represented by the value of excitation energy [eV] at the point (A) at which the graph bends. In other words, in  FIG. 5 , the work function of the sheet A is 5.31 [eV]. 
     FIG. 6  is a diagram showing the relationships among the work function values of the charge roller  201 , three sheets A, B, and C, placed in contact with the charge roller  201 , and the external additive α (silica), which were measured with the use of the above described method. 
   Among those components and the external additive, the charge roller  201  is most likely to become positively charged, and the sheet C is most likely to become negatively charged. 
   In this embodiment 1, the combination of the charge roller  201 , sheet B, and external additive α, the work functions of which were measured with the use of the above described method, and the combination of the charge roller  201 , sheet C, and external additive α, the work functions of which were also measured with the use of the above described method, were employed. For comparison, the combination of the charge roller  201 , sheet B, and external additive α was employed. Each of these combinations was used to assembly a charging apparatus. Then, each of the assembled charging apparatuses was employed by the above described image forming apparatus to actually form images. The evaluations of the yielded images are given in Table 1. 
   
     
       
             
             
           
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Occurrences of stripes 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               sheet A (Emb.) 
               Non 
             
             
                 
               sheet B (Comp. Ex.) 
               Yes 
             
             
                 
               sheet C (Emb.) 
               Non 
             
             
                 
                 
             
           
        
       
     
   
   The reason why the tests yielded the above results can be thought to be as follows. In the case of the charging apparatus  200  in this embodiment, the relationships among the work functions of the charge roller  201 , sheet  203 , and silica satisfied the inequality: sheet&gt;silica&gt;charge roller. It can be said that when this inequality was satisfied, the sheet  203  was more likely to become negatively charged than silica, and the charge roller  201  was more likely to become positively charged than silica. Thus, as silica was given negative electrical charge through the charging process, it was repelled by the sheet  203 . As a result, silica did not adhere to the sheet  203 , being instead adhered to the charge roller  201  which had become positively charged. As silica was adhered to the charge roller  201 , it moved with the peripheral surface of the charge roller  201 , being thereby uniformly dispersed by the sheet  203  instead of accumulating on the sheet  203 . Therefore, the charge roller  201  is prevented from unsatisfactorily charging the photosensitive drum  100  in a pattern of circumferential stripes. As the charge roller  201  is further rotated, the silica on the charge roller  201  is again given negative electric charge through the charging process. Then, it is electrostatically moved onto the photosensitive drum  100  by the electric field formed between the charge roller  201  and photosensitive drum  100 . Incidentally, during this process, some of the silica particles are given an unsatisfactory amount of electric charge. However, as they are repeatedly subjected to the charging process, they are eventually given a satisfactory amount of negative electric charge, being thereby moved onto the photosensitive drum  100 . This is thought to be why the formation of an image suffering from the image defects attributable to silica was prevented in this embodiment. 
   Next, referring to  FIG. 6 , the test in which the sheet A was used, and the test in which the sheet B was used, will be described. 
   (Test in which Sheet A was Used) 
   In the test in which sheet A was used, the work function of the sheet A was 5.31 [eV], and the work function of the charge roller  201  was 4.7 [eV], as shown in  FIG. 6 . Thus, as the sheet A and charge roller  201  rubbed against each other in the contact area between the sheet A and charge roller  201 , the sheet A became negatively charged, whereas the charge roller  201  became positively charged. Since the silica having become negatively charged is 5.01 [eV] in work function, it was repelled by the sheet A having become negatively charged to a potential level, the absolute value of which was greater than that of the potential level to which the silica had become negatively charged. As a result, the silica did not adhere to the sheet A, being instead attracted by the charge roller  201  having become positively charged. Thus, the silica adheres to the charge roller  201 . Then, it moved with the peripheral surface of the charge roller  201  as the charge roller  201  was rotated. 
   Regarding the electrostatic force generated in the contact area between the sheet A and charge roller  201 , generally, the magnitude of electrostatic force generated between two objects is determined by the difference in work function between the two objects. The greater the difference, the greater the electrostatic force, and the smaller the difference, the smaller the electrostatic force. When the sheet A is employed as the sheet  203 , the electrostatic force generated in the contact area between the charge roller  201  and sheet  203  is greater than when the sheet B, which will be described later, is used as the sheet  203 , because of the relationship between the sheets A and B in terms of which is greater in work function. In addition, the absolute value of the potential level to which the sheet A is negatively charged is greater than that of the potential level to which silica is negatively charge. Therefore, such force that causes the silica and sheet A to repel each other is generated. Thus, it does not occur that the silica adheres to the sheet  203  (A), in the contact area between the sheet  203  (A) and charge roller  201 . Consequently, the silica is evenly distributed on the charge roller  201 , being thereby prevented from causing the photosensitive drum  100  to be unsatisfactorily charged in the pattern of circumferential stripes. After being evenly distributed on the charge roller  201 , the silica is again given negative electrical charge, moving therefore onto the photosensitive drum  100 , because such an electric field that causes the negatively charged particles to move onto the photosensitive drum  100  is present between the photosensitive drum  100  and charge roller  201 . After moving onto the photosensitive drum  100 , the silica is eventually recovered by the developing apparatus  400 , transferring apparatus  500 , and cleaning apparatus  600 . Incidentally, in the above, the electrostatic force between the sheet A and charge roller  201  was described. However, the formation of a defective image, the flaws of which are attributable to the silica adhesion, can be prevented, for the same reason as given above, by using sheet C which is far greater in work function than the sheet A. In other words, all that is necessary to prevent the formation of the above described defective image is to select, as the material for the sheet  203 , which is placed in contact with the charge roller  201 , such a substance that satisfies the following relationships in terms of the work function value: particle dispersing member&gt;silica&gt;charging member. Incidentally, the usage of the sheet C as the sheet  203  renders greater the difference in work function between the sheet  203  and charge roller  201  than the usage of the sheet A as the sheet  203 , generating thereby a greater amount of electrostatic force between the sheet  203  and charge roller  201 . In other words, the usage of the sheet C increases the amount of force which causes the sheet  203  and silica to repel each other. This is thought to be why the force which works to prevent silica from adhering to the sheet  203  is greater when the sheet C is used than when the sheet B is used. 
   (Test in which Sheet B was Used) 
   In comparison, in the test in which sheet B was used as the sheet  203 , the work function of the sheet  203  (B) was 4.79 [eV], and the work function of the charge roller  201  was 4.7 [eV], as shown in  FIG. 6 . Thus, as the sheet B and charge roller  201  rubbed against each other in the contact area between the sheet B and charge roller  201 , the sheet B became negatively charged relative to the charge roller  201 , whereas the charge roller  201  became positively charged relative to the sheet B. However, the external additive α (silica) having become negatively charged was 5.01 [eV] in work function. Further, relative to the electrical charge of silica, the electrical charge of the sheet B and the electrical charge of the charge roller  201  are both on the positive side. Therefore, it is possible that there was a case in which the silica moved with the peripheral surface of the charge roller  201  as the charge roller  201  was rotated, and a case in which the silica adhered to the sheet B. In addition, the electrostatic force in the contact area between the sheet B and charge roller  201  was smaller than that generated when the sheet A was used as the sheet  203 . Thus, when the sheet B was used as the sheet  203 , it was difficult to evenly disperse the silica having adhered to the charge roller  201 . Therefore, the silica adhered to the sheet B, and accumulated thereon, forming minuscule projections, which scratched the charge roller  201 . This seems to be why the photosensitive drum  100  was unsatisfactorily charged in the pattern of circumferential streaks. 
   Therefore, it is desired that the sheet A or sheet C, which are greater in work function than the external additive α (silica) is used as the sheet  203 , as is in this embodiment (Embodiment 1). 
   The charging apparatus  200 , in which the sheet A was used as the sheet  203 , was fitted in an image forming apparatus. Then, 10,000 copies were actually formed using the image forming apparatus. The images were satisfactory, proving that silica was dispersed on the charge roller  201 , instead of adhering to the sheet A, being prevented from causing the photosensitive drum  100  to be unsatisfactorily charged in the pattern of circumferential streaks. 
   Embodiment 2 
   In the second embodiment, a structural arrangement is made so that the sheet  203  can be reciprocally moved, while being kept in contact with the charge roller  201 , roughly in parallel to the lengthwise direction (axial line) of the photosensitive drum  100 . More specifically, referring to  FIG. 7 , designated by a referential symbol  206  is a reciprocally movable member for supporting the sheet  203 . The reciprocally movable sheet supporting member  206  is supported roughly in parallel to the charge roller  201  by a guiding member (unshown) so that it can be slid in the lengthwise direction of the charge roller  201 . The sheet  203 , which is placed in contact with the charge roller  201 , is supported by the reciprocally movable sheet supporting member  206 ; one of the long edges of the sheet  203  is pasted to the abovementioned reciprocally movably sheet supporting  206 , with the use of a piece of two-sided adhesive tape, so that the sheet  203  does not become wavy. The sheet  203  is tilted so that its long edge by which the sheet  203  is not pasted to the supporting member  206  is on the downstream side of its long edge by which the sheet  203  is pasted to the supporting member  206 , in terms of the rotational direction of the charge roller  201 . Further, the sheet  203  is kept in contact with the charge roller  201  with the application of a proper amount of pressure for causing the sheet  203  to contact the charge roller  201  by the middle portion instead of the edge, and also, so that no gap is created in the contact area between the charge roller  201  and sheet  203 . The reciprocally movable supporting member  206  is reciprocally moved by a reciprocal driving mechanism  205  such as a cranking mechanism. 
   With the provision of this structural arrangement, not only is it possible to obtain the effect of preventing the adhesion of the external additive to the sheet  203 , by utilizing the relationships in terms of work function among the charge roller  201 , sheet  203 , and external additive, but also, it is possible to obtain the effect of more efficiently dispersing the external additive by the sheet  203 . Therefore, it possible to prevent the photosensitive drum  100  from being unsatisfactorily charged in the pattern of circumferential streaks; it is possible to uniformly charge the photosensitive drum  100 . 
   Embodiment 3 
   The third embodiment is one of the cases in which the charging apparatus  200  is incorporated into a process cartridge removably mountable in the main assembly of an image forming apparatus. 
     FIG. 8  is a schematic drawing of the image forming apparatus structured so that a process cartridge P in accordance with the present invention is removably mountable. 
   The process cartridge P in this embodiment is made up of a development process unit D and an image formation process unit C, which are integrally joined. 
   The development process unit D is equipped with a hopper  406  in which toner is held, a development roller  401  as a developer bearing member, a supply roller  402  for supplying the development roller  401  with toner, and a development blade  403  as a developer regulating means. 
   The image formation process unit C is equipped with a photosensitive drum  100  as a latent image bearing member, and a charge roller  201  for uniformly charging the peripheral surface of the photosensitive drum  100 . The image formation process unit C is also provided with a sheet  203  placed in contact with the charge roller  201 , and a cleaning blade  601  for removing the unwanted substances, such as the transfer residual toner, on the photosensitive drum  100 . 
   The process cartridge P is provided with a mounting means (unshown) for removably mounting the process cartridge P into the image forming apparatus main assembly, and is removably mountable in the image forming apparatus main assembly. 
   The process cartridge P in this embodiment employs the sheet A and charge roller  201 , which are used in the first embodiment, and which satisfy the following inequity regarding the relationship among the work functions of sheet, silica, and charging member: sheet&gt;silica&gt;charging member. 
   As a print start signal is inputted into an image forming apparatus main assembly which contains the above described process cartridge P, driving force is transmitted from the image forming apparatus main assembly to the photosensitive drum  100 , development roller  401 , and supply roller  402 . Further, biases are applied from the main assembly to the charge roller  201  and development roller  401 . The photosensitive drum  100  is exposed for image formation, by the exposing apparatus  300 . As a result, an electrostatic latent image is formed on the photosensitive drum  100 . The latent image is developed by the development roller  401  into a toner image, and the toner image is transferred by a transfer roller  501  onto a recording medium  900 . The recording medium  900  is conveyed to a fixing apparatus  800 , in which the toner image is fixed by heat and pressure. Meanwhile, the unwanted substances, such as the toner, paper dust, external additive, etc., remaining on the photosensitive drum  100  after the transfer are removed by the cleaning blade  601 , and are accumulated in a waste toner container  602 . Thereafter, the peripheral surface of the photosensitive drum  100  is charged again by the charge roller  201  to be put through the above described steps. 
   Because the relationships in terms of work function among the sheet A, silica, and charge roller  201  are: sheet A&gt;silica&gt;charge roller  201 , the silica particles having slipped by the cleaning blade  601  adhere to the charge roller  201 , but, they do not adhere to the sheet A. Thus, they are evenly dispersed on the charge roller  201 . 
   The process cartridge P in this embodiment was attached to the image forming apparatus main assembly, and this image forming apparatus was used to form 10,000 images. As a result, excellent images were obtained, proving that the unsatisfactory charging of the photosensitive drum  100 , which is in the form of circumferential streaks and attributable to silica, did not occur. 
   The materials for the sheet  203  do not need to be limited to the substances listed in the first embodiment. For example, in a case in which silica is used as the external additive, a sheet formed of a substance which is far greater in work function than the materials for the sheets A and C may be employed as the sheet  203 . In other words, all that is necessary is that the materials for the charge roller  201 , and the sheet  203  which is placed in contact with the charge roller  201 , are correctly selected so that the particle dispersing sheet (sheet  203 ), silica, and charge roller satisfy the following inequality in terms of work function: particle dispersing member&gt;silica&gt;charge roller. With this assurance, it is possible to prevent the external additive from adhering to the particle dispersing member, and therefore, it is possible to prevent the photosensitive drum from being nonuniformly charged in the pattern which results in the formation of an image suffering from unwanted vertical streaks; it is possible to uniformly charge the photosensitive drum. 
   While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims. 
   This application claims Priority of Japanse Patent Applications Nos. 045457/2005 and 039570/2006 filed Feb. 22, 2005 and Feb. 16, 2006, which are hereby incorporated by reference.