Electrophotographic Copying Process for Producing a Plurality of Copies

The invention relates to an electrophotographic copying process for producing a plurality of copies, and more particularly, to such an electrophotographic copying process which permits either a positive or a negative copy image, as considered with respect to the image of an original, to be selectively obtained and which is adapted to obtain a plurality of copy images by repeatedly utilizing an electrostatic latent image once it is formed on a photosensitive member for electrophotography.

A variety of electrophotographic copying processes for producing a plurality of copies has been proposed in the prior art. For example, there is a toner image transfer system which obtains the same copy image on a plurality of record sheets by repeatedly subjecting an electrostatic latent image which is once formed on a photosensitive drum to only a developing and a transfer step, and also a latent image transfer system which obtains the same copy image on a plurality of image receiving sheets by repeating a step for forming an electrostatic latent image on an image receiving sheet by modulating a corona ion current in accordance with the electrostatic latent image which is once formed on a photosensitive member in the form of a screen and subsequently a step for developing the electrostatic latent image on the image receiving sheet and the like. In these conventional electrophotographic copying processes for producing a plurality of copies, it is necessary that an electrostatic latent image is maintained stabilized over all copying steps for producing a plurality of copies in order to obtain all the copies of high image quality. However, with these conventional processes, since a charge defining an electrostatic latent image is generally on the surface of the uppermost layer of a photosensitive member, the charge leaks through a developer during the developing step in the toner image transfer system, while a latent image is disturbed in the latent image transfer system in which a photosensitive member in the form of a screen is employed. As the result, it is difficult to maintain a latent image stabilizer over all copying steps for producing a plurality of copies, which prevents from obtaining a plurality of copies of high image quality.

On the other hand, a variety of conventional electrophotographic copying processes has also been proposed which permit a positive or a negative copy image, as considered with respect to the image of an original, to be selectively and arbitrarily obtained. One technique utilizes a photosensitive member 1 for electrophotography as shown in FIG. 1 which includes a conductive layer 2 on which a photoconductive layer 3 is laminated. FIG. 2(I) illustrates a procedure followed when a positive copy image of an original is desired. As shown, the photosensitive member is initially charged to the negative polarity in a uniform manner and is then subjected to an imagewise irradiation to form an electrostatic latent image. A toner 4 which is charged to the positive polarity is deposited on the latent image for developing purpose, and the toner image is transferred onto a record sheet to provide a positive copy image On the other hand, FIG. 2(II) illustrates a procedure followed when a negative copy image is desired. Initially the photosensitive member is uniformly charged to the positive polarity, in contradistinction to the initial charging to the negative polarity as illustrated in FIG. 2(I), and is then subjected to an imagewise exposure to form an electrostatic latent image. Again a toner 4 which is charged to the positive polarity is deposited on the latent image to produce a toner image, which is then transferred onto a record sheet to produce a negative copy image. To enable these procedures, it is essential that the photoconductive layer 3 exhibits substantially uniform charge retention and light sensitivity when it is charged to either the positive or the negative polarity. Zinc oxide is known as a suitable material to form the photoconductive layer 3 which satisfies such requirement.

However, it is to be noted that the above requirement is not satisfied by a number of photosensitive materials including Se, Se alloys, PVK (polyvinyl carbazole) containing sensitizer or the like which are frequently used in an electrophotographic system of toner image transfer type. Accordingly, the choice of material which forms the photoconductive layer 3 is greatly limited when the above procedures are to be adopted. In addition, as will be apparent from the above description, such an electrophotographic copying process utilizing an electrostatic latent image, the charge of which is on the surface of a photoconductive layer 3, is not necessarily adapted to obtain a plurality of copy images by repeatedly subjecting the electrostatic latent image which is once formed on the photosensitive member 1 to only a developing and a transfer step. When the above procedure illustrated in FIG. 2(II) is employed to produce a negative copy image, the toner 4 which is charged to the positive polarity is deposited on a region corresponding to the bright areas of a light image where no charge of the latent image is present, and hence it exhibits a reduced adherence, thus causing an inconvenience that a negative image of a satisfactory optical density cannot be obtained.

Another electrophotographic copying process which selectively produces a positive and a negative copy image is illustrated in FIG. 3 where a photosensitive material 5 for electrophotography is employed which comprises a conductive layer 6 carrying a successive lamination of a photoconductive layer 7 and another photoconductive layer 8 which is sensitive to the ultraviolet region of the spectrum. When it is desired to obtain a positive copy image of an original, a source of radiation 9 which supplies a radiation including ultraviolet rays is utilized to illuminate an original 10, as shown in FIG. 4(I), and the light image of the original is projected through a projection lens 11 onto the photosensitive member 5 while simultaneously utilizing a corona charger 12 to charge the photosensitive member 5 to the negative polarity, for example, thus forming an electrostatic latent image. A toner which is charged to the positive polarity is deposited principally on the dark areas of the light image to form a toner image, which is then transferred onto a record sheet to produce a positive copy image. When it is desired to produce a negative copy image, an ultraviolet cut-off filter 13 is interposed between the source 9 and the original 10, as shown in FIG. 4(II), thus allowing the original 10 to be illuminated by visible light. The resulting light image is projected onto the photosensitive member 5 through the projection lens 11 while simultaneously charging the photosensitive member to the same polarity as used during the formation of the positive image by means of the corona charger 12, thus forming an electrostatic latent image. A toner which is charged to the positive polarity is deposited principally on the bright areas of the light image to form a toner image, which is then transferred onto a record sheet to produce a negative copy image.

In the process described immediately above, the toner is deposited on those areas of the light image where the charge of the latent image is present when either a positive or a negative image is to be produced, and hence the resulting toner image has an increased magnitude of adherence, permitting a copy image of a relatively high optical density to be obtained. However, as the latent image charge is on the surface of photoconductive layer 8 which is the uppermost layer sensitive to ultraviolet rays, the process is not necessarily adapted to produce a plurality of copies. With this process, the positive or the negative copy image is selectively produced by the use of either radiation containing ultraviolet ray or visible light, and this causes inconveniences as mentioned below.

Specifically, the distribution of radiation from a usual light source contains little or no emission of ultraviolet ray. In addition, a projection lens generally exhibits a reduced transmissivity to the ultraviolet ray. The combination of these facts makes it difficult to achieve a selective projection of radiation including ultraviolet ray in one instance and visible light in another by utilizing the same light source and the same projection lens. Furthermore, with this process, there is a high residual potential in the non-image region, namely, in the bright areas of the light image where the positive image is to be obtained as illustrated in FIG. 4(I), or in the dark areas of the light image where the negative image is to be obtained as illustrated in FIG. 4(II), resulting in an image which is highly influenced by fogging. This is because an electrostatic latent image having a high contrast cannot be formed in either instance because of the incapability of providing a photoconductive layer 7 which satisfies the both requirements for producing the positive and the negative image. More specifically, when a positive image is to be produced by a procedure as illustrated in FIG. 4(I), a sufficiently high dark resistance and a high sensitivity is required for the both photoconductive layers 7 and 8. By contrast, when a negative image is to be produced by a procedure illustrated in FIG. 4(II), a high sensitivity and a reduced dark resistance is required of the photoconductive layer 7, and this requirement is opposite from the requirement imposed upon the photoconductive layer 7 when producing a positive image.

It is an object of the invention to eliminate the above disadvantages of the conventional processes by providing an electrophotographic copying process which permits both a positive and a negative copy image, as considered with respect to the image of an original, to be obtained selectively and arbitrarily and which is adapted to obtain a plurality of either positive or negative copy images by repeatedly utilizing an electrostatic latent image once it is formed on the photosensitive member for electrophotography.

The copying procedure to produce a positive copy image differs from the copying procedure to produce a negative copy image only in the presence or absence of an inverse charging step and in respect of the polarities of toner charge in accordance with the regions of wavelengths which used in the immediately following step of uniform exposure. Therefore, in accordance with the invention, a change between a positive and a negative latent image can be easily achieved. The formation of the positive image, which generally represents a preponderance of proportion of the need to produce copies, can be completed with a reduced number of steps. In particular, since either a positive or a negative electrostatic latent image is formed by a charge which is trapped on the opposite surfaces of the first photoconductive layer 23 (as will be explained later in FIG. 7), both of the latent images are not in direct contact with a developer, thereby preventing any leakage of the charge defining the latent image through the developer. Further, toner is deposited on those regions where the charge of the electrostatic latent image is present, thus resulting in an increased magnitude of adherence of the toner. It is to be understood therefore that the process of the invention can effectively achieve its object because the latent image potential can be maintained stabilized over a number of copies and an improved image quality of the copy image can be obtained over a number of copies.

It is also to be understood that the process of the invention is effectively applicable to an electrophotographic system of the type utilizing a photosensitive member in the form of a screen on which an electrostatic latent image is formed to modulate a current of corona ions to cause a transfer of the latent image onto a record sheet, whereupon it is developed with toner.

In addition, polarities of charging simultaneously with an imagewise exposure for forming a positive and a negative electrostatic latent image can be made opposite from each other and as a result of this, the resulting positive and negative electrostatic latent image have a potential of the same polarity, thus enabling to then develop with the toner which is charged with the same polarity. Specifically, the process begins with a step for imagewise exposure simultaneously with charging, as will be explained later. At this time, the charging for a negative latent image is of the polarity opposite to that for a positive latent image. As the present instance employs a charge of positive polarity for a positive latent image, a charge of the negative polarity is to be employed for forming a negative latent image. Subsequently, an inverse charging with the positive polarity is performed in darkness. Thereupon a uniform exposure with ultraviolet rays is applied. As such, it is possible to form an electrostatic latent image with the charge of positive polarity which is trapped only at a bright area of the image. This means that the resulting positive and the negative electrostatic latent image have a potential with the same polarity (as will be understood by referring to FIG. 8).

FIG. 1 is a schematic cross section of one form of a photosensitive member for electrophotography which is used in the conventional electrophotographic copying process;

FIGS. 2(I) and (II) schematically illustrate copying steps utilized with the photosensitive member shown in FIG. 1;

FIG. 3 is a schematic cross section of another form of photosensitive member for electrophotography used in the prior art;

FIGS. 4(I) and (II) schematically illustrate copying steps used with the photosensitive member shown in FIG. 3;

FIG. 5 is a schematic cross section of one form of photosensitive member for electrophotography which is used in the present invention;

FIG. 6 graphically shows the spectral photocurrent response and absorption spectral response of PVK (polyvinyl carbazole);

FIG. 7 is a schematic cross section of another form of photosensitive member used in the present invention;

FIGS. 8(I), (II) and (III) schematically illustrate a sequence of copying steps when a positive copy image is to be produced in accordance with the process of the invention;

FIG. 9 graphically illustrates a change with time of the surface potential of the photosensitive member during the copying steps illustrated in FIG. 8;

FIGS. 10(I) to (IV) schematically illustrate a sequence of copying steps when a negative copy image is to be produced in accordance with the process of the invention; and

FIG. 11 graphically shows a change with time of the surface potential of the photosensitive member during the copying steps illustrated in FIG. 10.

Referring to FIG. 5, there is shown one form of photosensitive member for electrophotography which may be used in the electrophotographic copying process of the invention, in schematic cross section. A photosensitive member 21 shown comprises a conductive layer 22 carrying a successive lamination of a first photoconductive layer 23 which is sensitive to visible light (radiation in a first wavelength region), and a second photoconductive layer 24 which transmits the visible light and which is sensitive to radiation different from the radiation in the first wavelength region or to ultraviolet ray or light in the visible region which is close to the ultraviolet region of the spectrum (radiation in the second wavelength region). The conductive layer 22 is also effective as a support for the entire photosensitive member 21, and may be formed by a metal such as aluminium or a polyester film having a metallized surface. The first photoconductive layer 23 which is sensitive to radiation in the first wavelength region may comprise Se, Se alloys, amorphous silicon, CdS, ZnO and PVK containing a sensitizer such as TNF (2,4,7-trinitro-9-fluorenone) or the like, which are in themselves known in the art. The second photoconductive layer 24 which is sensitive to radiation in the second wavelength region may comprise PVK, amylhydrazones, oxazoles, pyrazolidones, 4-5 diphenyl imidazoles, 1,3,4 triazoles, oxydiazoles, perillenes, for example. However, since the material which forms the first photoconductive layer 23 is generally sensitive to ultraviolet ray in addition to visible light, it is desirable that the material to form the second photoconductive layer 24 be selected in connection with the material to form the first photoconductive layer 23 so that radiation in the second wavelength region cannot reach the first photoconductive layer 23, or stated differently, so that the second photoconductive layer has a good absorption of radiation in the second wavelength region.

FIG. 6 graphically shows the spectral photocurrent response and the absorption spectrum of PVK which may be used as a material to form the second photoconductive layer 24. The sample comprises 15 .mu.m thick PVK layer which is sandwiched between Au and Nesa glass and the measurement is made in a high vacuum. Curves 1 and 3 indicate the spectral photocurrent response when the Nesa glass is connected to the ground and a voltage of plus 2 and 50 volts, respectively, is applied to the Au electrode. Curves 2 and 4 indicate the spectral photocurrent response when the Au electrode is connected to the ground while a voltage of minus 2 and 50 volts, respectively, is applied to the Nesa glass. Curve 5 represents the absorption spectrum. As will be apparent from FIG. 6, PVK exhibits a reduced absorption in a region of wavelengths from 310 to 350 nm and exhibits little absorption of light having a wavelength greater than 350 nm. When such PVK is used to form the second photoconductive layer 24, a material may be chosen to form the first conductive layer 23 which is sensitive to radiation of wavelengths greater than 350 nm and is not sensitive to radiation of wavelengths below such value.

FIG. 7 shows, in schematic cross section, another form of photosensitive member for electrophotography which may be used in carrying out the electrophotographic process of the invention. Photosensitive member 25 shown comprises a ultraviolet absorbing filter layer 26 which is interposed between a first photoconductive layer 23 and a second photoconductive layer 24. In all other respects, the photosensitive member is similar to that shown in FIG. 5. The ultraviolet absorbing filter layer 26 comprises a light transmitting resin such as polyvinyl chloride, polymethylmethacrylate, polyethylene or the like in which a ultraviolet absorber is blended. A ultraviolet absorber comprises benzophenones or triazoles including

2,2'-dihydroxy-4,4'-dimethoxybenzophenone

2,2'-dihydroxy-4-methoxybenzophenone

2-(2'-hydroxy-5'-methylphenyl)benzotriazole

2-(2'-hydroxy-5'-methylphenyl)-5,6-dichlorobenzotriazole A preferred thickness of the ultraviolet absorbing filter layer 26 is less than several microns. An increased thickness results in the presence of residual charge, which in turn causes a fogging in the background. A preferred proportion of the ultraviolet absorber is from 5 to 100 parts by weight with respect to 100 parts by weight of the resin when the film thickness is 1 micron.

When the ultraviolet absorbing filter layer 26 is thus formed between the first and the second photoconductive layers 23, 24, ultraviolet ray is absorbed by the filter layer 26 and cannot reach the first conductive layer 23 if the latter is sensitive to ultraviolet radiation, thus effectively preventing the first conductive layer 23 from responding to radiation in the second wavelength region. Consequently, the choice of materials to form the first and the second photoconductive layers 23, 24 is greatly facilitated.

It should be understood that instead of forming a ultraviolet absorbing filter layer 26 independently, a ultraviolet absorber of the kind described above may be dispersed into the materials which formed the first and the second photoconductive layers 23, 24 in the vicinity of a boundary therebetween to provide an effective filtering action. Alternatively, a ultraviolet absorber of a relatively low concentration may be uniformly dispersed throughout the material which forms the second photoconductive layer 24.

The electrophotographic copying process of the invention will now be described assuming that the photosensitive member 25 shown in FIG. 7 is used in which both the first and the second photoconductive layers 23, 24 have an equal capacitance and in which the first photoconductive layer 23 is capable of withstanding a voltage in excess of 500 volts while the second photoconductive layer 24 is capable of withstanding a voltage in excess of 1000 volts. In the present invention, both a positive and a negative electrostatic latent image can be selectively formed on the photosensitive member 25, but the formation of the positive latent image will be described first, followed by a description of subsequent steps to produce a positive copy image.

When a positive electrostatic latent image is to be formed, an image 30 of an original formed by visible light in the first wavelength region is projected onto the photosensitive member 25 while simultaneously projecting a corona ion current of a positive polarity from a corona charger 27 so that the surface potential reaches a value of 1000 volts, for example, as illustrated in FIG. 8(I). As a result of such imagewise irradiation which occurs simultaneously with the charging, charges of opposite polarities are trapped on the opposite surfaces of the second photoconductive layer 24 in a bright area of the image, with the surface potential reaching a level of plus 1000 volts, as indicated by broken lines in FIG. 9. In a dark area of the image, charges of opposite polarities are trapped on the surface of the second photoconductive layer 24 and on the boundary surface between the conductive layer 22 and the first conductive layer 23, and the surface potential reaches a level of plus 1000 volts as before, as indicated by a solid line in FIG. 9. However, the amount of the latter charge trapped is approximately one-half the amount of the charge trapped in the former, so that the rising rate of the latter charging is more rapid than that of the former. Substantially the same surface potential can be reached in both the bright and the dark area of the image by continuing the charging step over a sufficient length of time until a saturation is reached or by utilizing a Scorotron charger for the charger 27 to apply a bias voltage to the Scorotron grid which is substantially equal to the desired charging potential.

Subsequent to the imagewise irradiation simultaneous with the charging, the photosensitive member is subjected to a uniform exposure 31 by ultraviolet ray of the second wavelength region, as indicated in FIG. 8(II). Upon irradiation of the ultraviolet ray, the latter is absorbed by the second photoconductive layer 24 and the ultraviolet absorbing filter layer 26, and thus cannot reach the first conductive layer 23. Thus, during this step, the first photoconductive layer 23 serves as an insulating layer. By contrast, the second photoconductive layer 24 produces carrier pairs in response to the irradiation of the ultraviolet ray, whereby the charges trapped on the surface of the second photoconductive layer 24 and the boundary between the second photoconductive layer and the ultraviolet absorbing filter layer 26 are neutralized in the bright area of the image, whereby the surface potential is reduced to substantially zero volts as indicated in FIG. 9. In the dark area of the image, the charge present on the surface of the second photoconductive layer 24 migrates to the interface between the first and the second photoconductive layers 23, 24 where it is trapped, and consequently the surface potential is slightly attenuated from the initial potential of plus 1000 volts. In this manner, a positive electrostatic latent image is formed by the charge which is trapped in the dark area of the image.

After the positive latent image is formed, a developing step in which a toner 28 which is negatively charged is deposited thereon to convert it into a visible image, as shown in FIG. 8(II), and a subsequent transfer step in which the positive toner image is transferred onto a record sheet are repeated, thus producing a plurality of positive copy images.

After a plurality of the positive copy images has been obtained, the photosensitive member 25 is subJect to a uniform exposure by visible light 32 of the first wavelength region, as indicated in FIG. 8(III). This causes the charges which are trapped on the first and the second photoconductive layers 23, 24 as well as in the interface between the first photoconductive layer 23 and the conductive layer 22 to be neutralized, whereby the surface potential reduces to zero to remove the latent image, allowing the photosensitive member for be prepared to the next formation of an electrostatic latent image in response to a next imagewise irradiation.

FIGS. 10 and 11 illustrate the successive steps which are followed when a negative electrostatic latent image is formed to produce a negative copy image. In this instance, as illustrated in FIG. 10(I), the photosensitive member is subjected to an imagewise irradiation 30 simultaneously with a charging thereof as described above in connection with FIG. 8(I), producing a surface potential in the bright and the dark area of the image on the order of plus 1000 volts, for example, as indicated in FIG. 11.

Subsequently, as indicated in FIG. 10(II), a corona charger 29 is moved over the photosensitive member in darkness so that the surface potential is reduced to substantially zero volts. This neutralization step can be achieved by utilizing a d.c. corona chrager of the opposite polarity from that used during the step of FIG. 10(I), by using an a.c. corona charger or by utilizing a Scorotron charger with its grid connected to the ground. As a result of such neutralization, the charge is completely eliminated from the dark area of the image, which therefore assumes a surface potential of zero volts. In the bright area of the image, the charge which has been trapped between the first and the second photoconductive layers 23, 24 during the previous step remains unchanged, but a charge of the opposite polarity from the trapped charge and which is one-half the amount of such charge is trapped on the surface of the second photoconductive layer 24 and in the interface between the first photoconductive layer 23 and the conductive layer 22, so that the surface potential is apparently zero volts, with result that the entire surface potential assumes zero volts.

After the neutralization step, the photosensitive member 25 is subjected to a uniform exposure 31 by ultraviolet rays in the second wavelength region, as indicated in FIG. 10(III). Upon irradiation of the ultraviolet rays, the second photoconductive layer 24 produces carrier pairs, with the holes acting to neutralize approximately one-half the negative charge which has been trapped in the interface between the first and the second photoconductive layers 23, 24 while the electrons neutralize the positive charge which is on the surface of the second photoconductive layer. As a consequence, in the bright area of the image, the negative charge is trapped in the interface between the first and the second photoconductive layers 23, 24 and the positive charge is trapped in the interface between the conductive layer 22 and the first photoconductive layer 23. Thus the negative electrostatic latent image is formed and its surface potential will fall to approximately minus 500 volts.

As indicated in FIG. 10(III), the negative latent image thus formed can be repeatedly subjected to a developing and a transfer step with a toner 33 which is positively charged to produce a plurality of negative images. After a given number of copies have been obtained, the photosensitive member, as shown in FIG. 10(IV), is subJected to a uniform exposure 32 by visible light in the first wavelength region, in the same manner as the procedure of FIG. 8(III), whereby the latent image is removed, thus preparing the photosensitive member for the next formation of a latent image.