Electrophotographic Copying Process for Forming Positive or Negative Images

The invention relates to an electrophotographic copying process, and more particularly, to 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.

A variety of electrophotographic copying processes have been proposed in the prior art 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 applied 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, 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 ray 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, 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 a 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 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 of a high image quality to be obtained selectively and easily.

The copying procedure to produce a positive copy image differs from the copying procedure to produce a negative copy image only in the charged quantity in a charging step and in respect of the regions of wavelengths which are used in the immediately following uniform exposure. Accordingly, in accordance with the invention, a change between a positive and a negative latent image can be easily achieved. For either a positive or a negative latent image, the toner is deposited on those regions where the charge of the latent image is present, thus resulting in an increased magnitude of adherence of the toner and hence an improved image quality of the copy image.

The process of the invention is also adapted to be used in a multi-cycle process in which an electrostatic latent image which is once formed on a photosensitive member is repeatedly subjected to a developing and a transfer step to produce a plurality of positive or negative copies. In this instance, the second or outer photoconductive material is only required to have a sensitivity to the ultraviolet and does not require any sensitizing treatment to impart a sensitivity to the visible region of the spectrum. Accordingly, it is a simple matter to provide a photoconductive layer which maintains an excellent charge retention over time, and this means that the electrostatic latent image can be maintained stabilized over a number of copies. In particular, when a negative copy image is to be produced, the electrostatic latent image is formed by the charges which are trapped on the both surfaces of the first photoconductive layer, and thus is not in direct contact with a developer, preventing any leakage of the charge defining the latent image through the developer as a number of copies are being produced.

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.

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), (III) and (IV) 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 forme, 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, as indicated in FIG. 8(II), the photosensitive member is subjected to an inverse charging in darkness so that the surface potential reaches approximately minus 1000 volts of the opposite polarity by utilizing a corona charger 28. As a result, in the bright area of the image, the surface potential on the second photoconductive layer 24 is eliminated and the charges of opposite polarities from each other are trapped in the interface between the first and the second photoconductive layers 23, 24 and in the interface between the conductive layer 22 and the first photoconductive layer 23, respectively, with result that the surface potential assumes minus 1000 volts. In addition, in the dark area of the image, the negative charge is trapped on the second photoconductive layer 24 and the positive charge is trapped in the interface between the conductive layer 22 and the first photoconductive layer 23, with result that the surface potential assumes minus 1000 volts.

After charged with the reverse polarity, the photosensitive member 25 for electrophotography is subjected to a uniform exposure by visible light 31 of the first wavelength region, as shown in FIG. 8(III). This causes the charges in the bright area to be completely eliminated and in the dark area the positive charge which is trapped in the interface between the conductive layer 22 and the first photoconductive layer 23 migrates to the interface between the first and the second photoconductive layers 23, 24, whereby these charges are trapped on the opposite surfaces of the second photoconductive layer 24, thus forming the positive electrostatic latent image the surface potential of which assumes minus 500 volts.

With the positive electrostatic latent image thus formed, as indicated in FIG. 8(III), a positive copy image can be produced by performing a developing and a transfer step with a positively charged toner 29.

After the positive copy image has been obtained, the photosensitive member 25 is subjected to a uniform exposure 32 by ultraviolet ray of the second wavelength region, as indicated in FIG. 8(IV). This causes the charges which are trapped on the opposite surfaces of the second photoconductive layer 24 in the dark area of the image to be neutralized, whereby the surface potential reduces to zero to remove the latent image, allowing the photosensitive member to be prepared for the next formation of an electrostatic latent image.

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), the surface potential is reduced to substantially zero volts in darkness. This neutralization step can be achieved in this instance by using a corona charger 28 which is to be used for the process of FIG. 8(II) with its charged quantity changing, and can also be achieved by using an a.c. corona charger, or by using 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, respectively, 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 32 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 thereof. 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, forming the negative electrostatic latent image the surface potential of which assumes approximately minus 500 volts.

As indicated in FIG. 10(III), a toner 29 which is positively charged is deposited on the latent image to convert it into a visible image, which is then transferred onto a record sheet to produce a negative copy image. After the copy image has been obtained, as shown in FIG. 10(IV), the photosensitive member is subjected to a uniform exposure 31 by visible light in the first wavelength region, in contrast to the procedure of FIG. 8(IV), whereby the latent image is removed, thus preparing the photosensitive member for the next formation of a latent image.