Title:
ELECTRODE CONFIGURATION FOR ELECTROPHOTOGRAPHY
United States Patent 3597072
Abstract:
A persistent internal polarization (PIP) electrophotography printing or copying system wherein one electrode is nonremovable and has a discontinuous configuration, such as a wire screen mesh, which is embedded in or attached to the surface of a PIP layer and the other electrode is transparent so that each electrode is capable of simultaneously applying an electric field to the PIP layer, while permitting radiation to reach the PIP layer.


Inventors:
Brown, Felix H. (East Lansing, MI)
Clark, Robert N. (Mason, MI)
Application Number:
04/764683
Publication Date:
08/03/1971
Filing Date:
10/03/1968
Assignee:
OWENS-ILLINOIS INC.
Primary Class:
Other Classes:
365/147, 399/266, 430/51, 430/66
International Classes:
G03G5/024; G03G5/14; G03G15/056; (IPC1-7): G03G15/00
Field of Search:
355/3,17,16,12 96
View Patent Images:
US Patent References:
3457070ELECTROPHOTOGRAPHY1969-07-22Watanabe et al.
3288602Xerographic plate and method1966-11-29Snelling et al.
3268331Persistent internal polarization systems1966-08-23Harper
3199086Devices exhibiting internal polarization and apparatus for and methods of utilizing the same1965-08-03Kallmann et al.
3146688Xerographic machine1964-09-01Clark et al.
3005707Devices exhibiting persistent internal polarization and methods of utilizing the same1961-10-24Kallmann et al.
2912592Memory device1959-11-10Mayer
Primary Examiner:
Matthews, Samuel S.
Assistant Examiner:
Greiner, Robert P.
Claims:
We claim

1. A PIP electrophotographic printing or copying machine wherein a persistent electrostatic latent image is formed in a photoconductive body exhibiting persistent internal polarization when sandwiched between a pair of conductive electrodes wherein an electric field exists between the electrodes and through the photoconductive body when flooding radiation is impressed on said photoconductive body through one of said pair of electrodes and then an image is impressed on said photoconductive body through the other of said pair of electrodes while reversing the polarity of said electric field, said one of said electrodes comprising a completely transparent plate of glass with a conductive coating thereon, said other of said electrodes comprising a substantially transparent foraminous wire screen mesh, and each of said electrodes remaining adjacent to and in contact with said photoconductive body during toning and transfer of said electrostatic latent image from said photoconductive body.

2. A PIP electrophotographic printing or copying machine wherein a persistent electrostatic latent image is formed in a photoconductive body exhibiting persistent internal polarization when sandwiched between a pair of conductive electrodes wherein an electric field exists between the electrodes and through the photoconductive body when flooding radiation is impressed on said photoconductive body through one of said pair of electrodes and then an image is impressed on said photoconductive body through the other of said pair of electrodes while reversing the polarity of said electric field, said one of said electrodes comprising a completely transparent plate of glass with a conductive coating of tin oxide thereon, said other of said electrodes comprising s substantially transparent foraminous wire screen mesh which is completely embedded in the surface of said photoconductive body, and each of said electrodes remaining adjacent to and in contact with said photoconductive body during toning and transfer of said electrostatic latent image from said photoconductive body.

Description:
BACKGROUND OF THE INVENTION

This invention relates to novel apparatus and process for practicing electrophotographic printing or copying. More particularly, this invention relates to printing apparatus and process utilizing photoconductive insulating materials and the principles of persistent internal polarization.

Persistent internal polarization (abbreviated herein as PIP) involves the separation of positive and negative charges in a photoconductive insulating material by subjecting it to irradiation and an electric field. The charges are subsequently trapped and remain fixed or frozen so as to form an internal polarization field for a period of time sufficient to permit toning. PIP and the theory thereof are well known in the electrophotography art. See, for example, Electrophotography, by R. M. Schaffert, The Focal Press, London and New York (1965), pages 59 through 77, and Persistent Internal Polarization, by Kallmann and Rosenberg, The Physical Review, Volume 97, No. 5 Mar. 15, 1955), pages 1596 through 1610, both of which are incorporated herein by reference.

In general, a PIP electrophotography system includes a layer of photoconductive insulating material sandwiched between a pair of field producing electrodes. The phenomenon of PIP can be achieved in any material which exhibits the following characteristics:

1. The material must have a high resistivity in the dark (a low density of free charge carriers), whereby it is a good insulator in the absence of irradiation.

2. The material must be photoconductive. In other words, it must have decreased resistivity when excited with appropriate irradiation.

Thus, a PIP material is one which will become persistently internally polarized due to the separation of positive and negative charges when it is subjected to irradiation and the action of an electric field.

Typical PIP materials contemplated herein comprise binder dispersions of photoconductors and binder free thin films of photoconductors.

Examples of inorganic photoconductors contemplated in the process of this invention include, not by way of limitation, appropriately activated zinc sulfide, cadmium sulfide, zinc selenide, cadmium selenide, cadmium oxide, zinc-cadmium selenides, and zinc-cadmium sulfides. Examples of organic photoconductors include anthracene, chrysene, and poly (vinylcarbazole).

Examples of resin binders contemplated herein include, not by way of limitation, cellulose acetate, cellulose ether, cellulose ester, silicones, vinyl resins, alkyds, and/or epoxy resins.

When it is desired to form a latent electrostatic image in the PIP material, it is flooded with radiation and an electric field is applied so as to polarize the PIP layer. After termination of the flooding radiation, the polarity of the electric field across the PIP material is reversed and the PIP materials exposed to an image or other pattern of activating radiation. The reversal of the electric field will cause rapid depolarization of that portion of the PIP material rendered photoconductive under the influence of the imagewise radiation.

If the exposure to the image is continued for a sufficient time period, the irradiated area of the PIP layer will repolarize and assume a polarization opposite to that of the nonirradiated or dark portion of the PIP layer. Thus, the image is simulated by an internal latent electrostatic image or pattern detectable at the surface of the PIP material.

This latent electrostatic image is subsequently developed with charged or dipolar toner particles so as to produce a visible reproduction of the image which is capable of being viewed, photographed, or transferred, utilizing known methods in the electrophotography printing or copying art.

It should be noted that, due to the characteristics of the PIP material, the latent electrostatic image produced in the PIP material will typically remain fixed such that a finite number of reproductions can be made. The image can be erased by overall irradiation with or without an electric field, thereby returning the PIP material to a prepolarized or neutral condition capable of being used for the formation of a new electrostatic image.

The irradiation of the PIP material (for polarization and/or imaging) can be accomplished by means of any form of electromagnetic or particulate radiation or energy, visible or invisible, which will excite the PIP material so as to permit charge separation in an electric field. Such radiation includes, not by way of limitation, visible light, infrared, ultraviolet, X-rays, gamma rays, and beta rays. For printing or copying purposes, the typical radiation is light in the visible range.

In the prior electrophotographic printing and copying art, simultaneous application of the electric field and the light from an image to a PIP material has been obtained by means of at least one continuous electrode which is substantially transparent. The subsequent development and transfer of the electrostatic image using electroscopic powders or liquids has to date required that the continuous electrode be removable.

A nonremovable, discontinuous wire mesh electrode has been introduced which does not need to be removed for toning and transfer purposes. However, since such a discontinuous electrode is not completely transparent, it impedes the impingement of light on the PIP material in the regions of the strongest field, thereby slowing the polarization and image formation in the PIP material. This speed of polarization and image formation becomes particularly important when it is desired to use a PIP system in a printing or copying machine.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided a novel PIP system which includes both a nonremovable electrode configuration, which does not have to be removed for image development and transfer purposes, and a completely transparent electrode which permits total impingement of light radiation on the PIP layer to facilitate rapid polarization and image formation within the PIP layer.

In a specific practice hereof, this invention features the combination of a thin mesh grid electrode placed on or embedded in the top surface of a PIP layer and a transparent electrode, such as a tin oxide-coated glass. Thus, a sandwich configuration is achieved whereby the base electrode is a conductive glass, the PIP layer in a suitable binder is applied on top of the base electrode to a suitable thickness, and the mesh electrode is placed on top of the PIP layer and embedded in or attached to the PIP layer.

In operation, an electric field is applied between the conductive glass electrode and the embedded mesh electrode and the flooding radiation is impinged upon the PIP layer through the transparent conductive glass electrode. The imaging radiation may then be impinged upon the PIP layer through either the conductive glass electrode or the embedded screen mesh electrode. Formation of an image occurs more rapidly when the image radiation is impinged upon the PIP layer through the conductive glass electrode than it does when passed through the embedded screen mesh electrode. However, successful image formation can also be accomplished when the image radiation is passed through the embedded screen mesh electrode as it contains sufficient openings (60 to 90 percent) to be substantially transparent.

Other features and advantages of the subject invention will become obvious to those skilled in the art upon reference to the following detailed description and the drawings illustrating a preferred embodiment of the invention.

IN THE DRAWINGS

FIG. 1 is a schematic view of a PIP system having both a nonremovable, discontinuous electrode and a transparent conductive glass electrode in accordance with this invention.

FIG. 2 is a schematic view of the PIP system of FIG. 1 upon the formation of a latent electrostatic image.

FIG. 3 is a schematic view of a nonremovable, discontinuous electrode in the form of a flat wire mesh embedded in the surface of a PIP layer.

DESCRIPTION OF A PREFERRED EMBODIMENT

In the drawings, the numeral 10 refers to a body of PIP material as previously described. The PIP body 10 is sandwiched between a pair of electrodes 12 and 14 which are connected to a DC source E. For the purposes of explanation, the electrode 12 is connected to the positive terminal of the DC source E and, accordingly, the electrode 14 is connected to the negative terminal of the DC source E.

The electrode 12 is a transparent, conductive glass electrode and may, for example, be a tin oxide-coated glass.

The electrode 14 is discontinuous and may take the form of a mesh which is partly or entirely embedded in the top surface of the PIP body 10. The electrode 14 should be sufficiently embedded in the top surface so as to be flush with the outer face thereof. One possible such mesh material would be an electroformed nickel mesh which can be obtained commercially as fine as 2,000 lines per inch. The light transmission of such mesh may be varied by controlling the space and wire dimensions.

To initially prepolarize the PIP body 10, the system is flooded with light through the conductive glass electrode 12 as shown in FIG. 1. Under the combined action of the light and the DC source E, it is shown schematically that negative charges are effectively conducted to the edge of the PIP body 10 adjacent to the electrode 12 connected to the positive terminal of the DC source E, and, conversely, positive charges are effectively conducted to the edge of the PIP body 10 adjacent to the embedded mesh electrode 14 which is connected to the negative terminal of DC source E.

When the system is subjected only to imagewise radiation while the polarity of the DC source E is reversed, the PIP body 10 reacts as shown in FIG. 2. In FIG. 2 it is seen that only those areas of the PIP body subjected to the imagewise radiation undergo internal polarization under the force of the field produced by the reversed polarity of source E. The PIP system has thus produced a latent electrostatic image (as represented schematically by the middle four negative charges on the right side of PIP body 10 adjacent to embedded mesh electrode 14 in FIG. 2) which is capable of being toned and transferred through the use of charged electroscopic particles (not shown). As previously described, the image-wise radiation may be impinged upon the PIP layer 10 through either the conductive glass electrode 12 or the embedded mesh electrode 14.

A continuous electrode has an inherent disadvantage in that it must be removed in order to develop the latent electrostatic image as it acts as a shield between the latent electrostatic image and the electroscopic particles because of the overall presence of image charges. In contrast to this, the discontinuous electrode (embedded mesh electrode 14) of this invention with its high percentage of openings appreciably reduces the shielding of the electroscopic particles from the latent electrostatic image and, therefore, need not be removed during the transfer or printing stages. Thus, a discontinuous electrode has a distinct advantage in that its nonremovability saves considerable time and facilitates the transfer speed necessary to successfully use a PIP system in a printing or copying machine.

In commonly available copying machines, it has been found that the toning of a large solid area often results in decreased toner density; that is, deterioration of the image in areas furthest away from the edges. In other words, the middle portions of a large solid area which has been toned and transferred from such an image often appear less distinct than do the edge portions. The use of a discontinuous electrode (such as a mesh) electrically breaks up the large areas, thereby resulting in uniform development over the large solid areas of the latent image.

An advantage of a nonremovable electrode, such as embedded mesh electrode 14, is its ability to avoid dust collection between the electrode and the PIP layer. Removable electrodes frequently pick up dust particles and other foreign matter which, when positioned between the electrode and the PIP layer, distort the effect of the field lines of the PIP layer. A nonremovable electrode embedded in the top surface of the PIP layer such as the mesh of this invention completely eliminates the possibility of dust particles gathering between the electrode and the PIP layer, thereby insuring against distortion of the field lines.

Another advantage accrues in this PIP system. The possibility exists that in transferring toner from the PIP drum to the substrate to be printed, the application of the electric field which transfers the electrostatically charged powder may deteriorate the PIP image and thus minimize the possibility of repeatedly toning and transferring from a single imaging. With the embedded screen mesh of this invention, this potential difficulty is completely eliminated. For this purpose, the electric field used to transfer the toner particles to a substrate to be printed is applied to both the mesh electrode 14 and the transparent electrode 12 so that both electrodes are at the same potential. The other electrode is placed on the opposing side of the substrate to be printed. This produces an electrostatic field between the embedded screen mesh and the substrate, which moves the toner particles to the substrate surfaces. It also provides that there will be no applied force within the PIP layer and thus eliminates any degradation effect which transfer electric fields frequently cause.

Given the combination of the nonremovable, embedded mesh electrode and the transparent conductive glass electrode of this invention, there are a number of variations which can be utilized within the scope of this invention. All of the presently known methods of polarizing the PIP material can be used, including the image reversal method, as well as the direct polarization method. Either an electrostatic toner dipolar-type a dipolar type toner can be used. The embedded mesh electrode can be of a screen mesh either woven or electrically formed mesh with various thickness to aperture ratios; or it can be of metal evaporated onto the PIP layer surface by standard evaporization techniques. The transparent electrode 12 may be formed of any transparent glass or plastic with a conductive coating, such as tin oxide, deposited thereon.

Thus, the invention as described herein provides the combination of a nonremovable electrode, which can be used to simultaneously apply an electric field and permit the imaging radiation to reach the PIP layer and which does not have to be removed for image transfer and printing purposes, and a transparent conductive coated electrode, which is completely transparent to allow rapid transfer of light to the PIP layer to facilitate fast polarization of the PIP layer. It should be noted that although this invention has been described in connection with a planar system, it is well suited to be used in conjunction with a rotary drum system.

Although but one embodiment of the subject invention has been shown and described in detail, it should be clear to those skilled in the art that, in accordance with the preceding description, many changes and modifications may be made thereto without departing from the scope of this invention. Therefore, this invention is not intended to be limited except as defined in the claims hereinafter.