Electrophotographic Plate

Abstract

An electrophotographic plate having high resolving power and high sensitivity comprising a base, a composite photoconductive layer on the base, and an insulating layer overlying the composite photoconductive layer. The composite photoconductive layer comprises two adjacent layers, one layer contacting the insulating layer and comprising fine photoconductive particles and binder, and the other contacting the base and comprising relatively coarse photoconductive particles and binder.

Parent Case Text

This is a division of U.S. Pat. application Ser. No. 563,899, filed July 8, 1966.

Description

This invention relates to electrophotography, and more particularly to an electrophotographic plate for use in electrophotography.

U.S. Pat. No. 3,124,456 discloses the use of a photosensitive plate where a photoconductive layer, comprising CdS or CdSe and binder resin, is adhered to a photoconductive base and a translucent insulating layer is applied over the photoconductive layer. Original image irradiation and electrostatic charging are made simultaneously from the translucent insulating layer side to form an electrostatic image on the insulating layer by utilizing the difference of the charges built up in accordance with the difference of the time constants caused by the difference of the values of the resistances of the photoconductive layer at the light-and-dark portions of the original image. However, to obtain good electrostatic images with this method, the capacitance of the translucent insulating layer must be larger than that of the photoconductive layer, and as a practical matter, the thickness of the translucent insulating layer is limited to the range of from 2 - 6.mu.. With such a thin insulating layer, break-down of the surface occurs easily and long repetitive use can hardly be expected. Furthermore, in the method described above where the electrostatic image formed depends upon the change of the impedance of the photoconductive layer, when the thickness of the translucent insulating layer is increased, contrast decreases and the quality of the image deteriorates.

U.S. Pat. No. 3,041,167 teaches the use of a photosensitive plate in which the photoconductive layer is protected by an overcoating layer, which is thin in comparison to the photoconductive layer. When the electrostatic image is formed using said photosensitive plate according to the Carlson process, it is necessary to repeat the copying cycle, and accordingly, prior to applying the sensitizing charge, the plate is charged to a polarity opposite to that of the sensitizing charge; and after charging, light is irradiated on the whole surface, and thereafter, a sensitizing charge is made, followed by image exposure. According to this process, the photosensitive plate is revived; however, the obtainable electrostatic contrast is only 300 - 500 volts, which is almost the same as that obtainable with a photosensitive plate which has no overcoating layer. According to the process of U.S. Pat. No. 3,041,167, after the sensitizing charge, the light image is irradiated so that in the light exposed area, a carrier is injected from the conductive base side to weaken the external field, and contrast is obtained due to the difference of the capacitances between the exposed area and the unexposed area.

According to the invention set forth in copending and commonly assigned United States patent application Ser. No. 563,899, filed July 8, 1966, an electrostatic image is formed on a photosensitive plate having a conductive base, a photoconductive layer overlying the base and exhibiting p-type or n-type semiconductivity and an insulative layer overlying the photoconductive layer. The photosensitive plate is characterized in having carrier charge of a polarity corresponding to the conductivity type of the photoconductive layer injectable from the conductive base into the photoconductive layer and bound in the region of the interface between the insulative and photoconductive layers. The electrostatic image is formed on the photosensitive plate by applying a first charge of a polarity opposite to the conductivity type of the photoconductive layer substantially uniformly onto the insulative layer to inject and bind carrier charge in the region of the interface between the insulative and photoconductive layers. Then the photoconductive layer is exposed to a pattern of image light while a corona discharge of a polarity opposite to that of said first charge is applied onto the insulative layer, and thereafter the photoconductive layer is exposed to activating light to discharge bound carrier charge remaining in the region of the interface and form a high contrast electrostatic image.

An object of the present invention is to provide an improved photosensitive plate having high sensitivity and high contrast, which is usable over a long term and which can be used with any of the above described processes as well as with other electrophotographic processes.

A still further object of the present invention is to provide an inexpensive, highly sensitive photosensitive plate, which comprises two photoconductive layers, one layer in contact with the translucent insulating layer and composed of a mixture of fine particles of a photoconductor and a binder, and the other layer interposed between the base and the first photoconductive layer and composed of large particles of a photoconductor and a binder.

The above objects and various other objects and the advantages of the present invention will become clear from the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a four layer photosensitive plate constructed in accordance with the present invention, and having a base of conductive material; and

FIG. 2 shows a four layer photosensitive plate constructed in accordance with the present invention, and having a base of insulative material.

The fundamental structure of an electrophotographic plate of the present invention includes a base, a composite photoconductive layer (described more fully hereinafter) coated or sprayed on the base, and a layer of translucent insulating material overlying the photoconductive layer. Depending on the electrophotographic process used, the base may be of conductive or insulative material. The photoconductive layer comprises two layers, one layer adjoining the translucent insulating layer and containing fine particles of photoconductor, and the other layer adjoining the base and containing highly photosensitive photoconductor, for example, a photoconductor of larger particles.

Thus, the photoconductive layer on the side where light is radiated comprises a photoconductor of fine particles, and therefore, it is possible to obtain an image of high resolving power. At the back of the photoconductor of fine particles, there is the layer containing the photoconductor of larger particles, and since the photoconductor of larger particles is highly sensitive, the sensitivity of photosensitive plate is high, and therefore it is possible to obtain an electrophotographic plate of high sensitivity and high resolving power.

In FIG. 1, 1g is a translucent insulating layer, 2g and 3g are photoconductive layers, layer 2g comprising a photoconductor of finer particles than those of 3g, and 4g is a conductive base.

In FIG. 2, 1h is a translucent insulating layer, 2h and 3h are photoconductive layers, and 2h comprises finer photoconductor particles than 3h, and 4h is an insulating base.

As to fine particle photoconductors, zinc oxide or zinc sulfide, and cadmium oxide-selenium compounds, whose average granular diameter is less than several .mu. can be used. Excellent results can be expected where zinc oxide is used. Zinc oxide with granular diameter less than 1.mu., is readily available and inexpensive, and it is possible to increase sensitivity very simply by means of dyes. Excellent results are obtained where particles of cadmium sulfide, cadmium selenide activated by copper or the like are used as high sensitivity large particle photoconductors. These compounds are readily available, and they have the advantage of having high photoconductivity. Similar results are obtained where the base is a metal, and insulating layers are provided.

Especially excellent results can be obtained where cadmium sulfide is used in layer 3, and zinc oxide is used in layer 2 in combination with any of the above-mentioned kinds of photoconductors, in which case the thickness of photoconductive layer 2 containing zinc oxide is from 5 to 20.mu., and the thickness of photoconductive layer 3 is from 10 to 100.mu..

EXAMPLE

50g of varnish (containing 20 percent of a vinyl chloride vinyl acetate copolymerized resin), 20g of CdS, and 56g of thinner were mixed, and the resulting mixture was coated on an aluminum plate to a thickness of between 50 and 100.mu. when dried; and 50g of ZnO and 50g of silicon resin varnish (containing 50 percent of resin component), 15cc of 0.1 percent Rose Bengale ethanol solution and 100cc of thinner were mixed in a porcelain ball mill for 2 hours, and a liquid of pink paint form was obtained; and the resulting solution was spray coated on the above prepared CdS film so as to have the thickness of about several .mu. when dry, and the film was dried. Finally, 12.mu. of polyethylene terephthalate resin film (sold by the DuPont Company under the trademark Mylar) was adhered on ZnO layer using an adhesive.

A +6KV charge was applied to the side of the movable electrode in a light area, and a corona discharge was made between the electrode and the aluminum plate to charge it positively; and then in a dark area, microfilm was enlarged about eight times by means of an enlarger which had 150 W tungsten lamp, and the image was projected on the photosensitive plate at an exposure of F 5.6; and while exposure was being made through the movable electrode having the protecting wall of the structure which allows light to pass, the protecting wall touching the photosensitive film, a corona discharge of 6KV was made, and the exposure and charge was made for about 2 seconds average on the respective portions of the photosensitive plate. After applying an overall exposure for about 2 seconds with a tungsten lamp of 100 W at the distance of 50 cm from the plate, a toner having the negative electrostatic charge was used, and cascade development was made by using glass balls as carriers, and a clear positive image enlarged from the original copy of the microfilm was obtained.

It is apparent that the present invention can be modified by the expert in the art within the scope of the disclosure herein and the following claims.

Claims

What is claimed is:

1. An electrophotograpic plate comprising:

a. a base,

b. a composite photoconductive layer on said base, and

c. an insulating layer on said photoconductive layer;

d. said composite photoconductive layer comprising at least two adjacent layers, one layer contacting said insulating layer and comprising binder and fine photoconductive particles characterized in having high resolving power, and the other layer comprising binder and photoconductive particles characterized in having high sensitivity, wherein the photoconductive particles of said other layer are relatively coarse as compared to those of said one layer.

2. An electrophotographic plate according to claim 1, wherein said fine and coarse photoconductive particles are of different material, and said coarse photoconductive particles have substantially higher sensitivity than said fine photoconductive particles.

3. An electrophotographic plate according to claim 1, wherein said fine photoconductive particles comprise ZnO and said one layer of same has a thickness of from 5 to 20.mu., and said coarse photoconductive particles comprise CdS and said other layer of same has a thickness of from 10 to 100.mu. .