US3789223A - Charging method for relatively movable electrophotographic means and corona means - Google Patents

Abstract

An electrophotographic coating provided on a highly insulating backing having a conductive interlayer can be uniformly charged in one polarity by forming a conductive path in a certain region of said coating by means of light exposure and on said conductive path applying a corona discharge of the opposite polarity whereby another corona electrode imparts corona ions of the first polarity, in which the coating is to be charged, on an area of the coating which is sufficiently close to the exposed area.

Description

ljnited States Patent [191 Sato [ Jan. 29, 1974 CHARGING METHOD FOR RELATIVELY MOVABLE ELECTROPHOTOGRAPHIC MEANS AND CORONA MEANS [75] Inventor: Masamichi Sato, Asaka, Japan [73] Assignee: Fuji Photo Film Co., Ltd.,

Kanagawa, Japan 22 Filed: Aug.1l, 1971 21 App1.No.: 170,921

[30] Foreign Application Priority Data Aug. 11, 1970 Japan 45-70205 [52] US. Cl. 250/325, 317/262 A [51] Int. Cl G03g 13/02 [58] Field of Search 250/49.5 GC, 49.5 ZC;

317/262 A; 96/1 C, 1 PC [56] References Cited UNITED STATES PATENTS 10/1972 Ohta et a1 250/495 ZC X 11/1970 Tellin et a1 317/262 A 10/1960 Steinhilper 317/262 A X 2,833,930 5/1958 Walkup 317/262 A X 3,611,074 10/1971 Weichardt 317/262 A OTHER PUBLICATIONS Kuniki et 211.; West Germany, Offenlegungsschift (Printed Application) 1963, 615; July 9, 1970; 5 Sht. Dwg., 25 spec.

Primary Examiner-Walter Stolwein Attorney, Agent, or FirmGerald J. Ferguson, Jr.; Joseph J. Baker; J. T. Martin [5 7] ABSTRACT 1 Claim, 6 Drawing Figures CHARGING METHOD FOR RELATIVELY MOVABLE ELECTROPI-IOTOGRAPHIC MEANS AND CORONA MEANS DETAILED DESCRIPTION OF THE INVENTION This invention relates to a new method of corona charging for use in electrophotography.

An electrophotographic recording material generally comprises a photoconductive insulating coating provided on a conductive support. Typical examples of this are a metal plate coated with photoconductive selenium layer by vacuum evaporation, or a paper substrate coated or impregnated with a conductive polymer material overcoated with a homogeneous mixture of photoconductive ZnO and an insulating resinous binder.

Such photoconductive materials can be easily charged by a corona discharge. FIG. 1 illustrates the conventional corona charging method; in FIG. 1, an electrophotographic material comprising a photoconductive coating 12 and a conductive support 11 such as metal plate. A corona wire 13 is extended over the entire width of the photoconductive coating several centimeters thereabove. A shield case 14 provided close to the wire 13 surrounds the wire on three sides. The wire 13 is applied a high, for example, negative potential, while the shield case and the conductive support 11 are kept at ground potential.

With a wire potential of about 6,000 to 7,000 volts and a gap between the wire and the photoconductive surface of several centimeters, the photoconductive coating is charged negative by virtue of negative corona ions impinging onto the coating. In order to charge uniformly the entire surface of the coating, the corona charging device (comprising the wire and the shield case) may be moved at a constant speed in the direction shown by an arrow. Alternatively, the material to be charged may be moved under the stationary charging device.

In the case where an electrically conductive support is used for the electrophotographic material, which has a simple structure as shown in FIG. 1, it is quite easy to charge such material with a sufficient degree of uniformity by the use of the arrangement shown in FIG. 1.

But when an electrophotographic material having a cross-sectional structure such as is shown in FIG. 2, comprising a highly insulating support 21, a conductive intermediate layer 22 and a photoconductive insulating coating 12 is to be charged, the arrangement shown in FIG. 1 becomes inappropriate. The reason is the difficulty with which the intermediate layer 22 is earthed. It is especially the case where the insulating support comprises a plastic film such as polyester, polyethylene, polyvinyl chloride, cellulose triacetate. When the support 21 is ordinary paper, it can absorb moisture from the atmospheric air so as to lower the resistivity of the support to enable a substantially uniform charging by the conventional charging method. Because the conductive support is earthed through the conductive plate on which the electrophotographic material is put. With a highly insulating support such as polyester film, the electric resistance along its thickness cannot be neglected and the conventional charging method as shown in FIG. 1 is not applicable because the support is no longer earthed through the conductive plate.

If the insulating support 21 is rather thin, and the conductive layer 22 has a high degree of conductance as is the case with a vacuum deposited metal layer, sparking takes place during corona charging between the very edge of the layer 22 and a conductive backing of ground potential on which the material is placed, thus permitting a uniform charging of the photoconductive coating. However, such earthing by sparks causes the resulting potential value of the coating to fluctuate considerably, and at the same time, the sparks, in addition to danger, attend on undesirable light to which the photoconductive coating is sensitive. And the earthing by sparks is dangerous.

When the conductive layer is made of such materials as cuprous iodide, conductive carbon, or conductive polymeric material, which are far less conductive than metal, sparking hardly occurs, and thus the photoconductive coating cannot accept an ample amount of electrostatic charge.

Conventionally, in order to avoid this difficulty, one has resorted to use an electrophotographic material which has the intermediate layer exposed at edge portions thereof to facilitate earthing of the layer. However, it required rather complicated manufacturing procedures to produce materials of such structure, and, moreover, the earthing became incomplete for an intermediate layer having an insufficient level of lateral conductivity.

The present invention can provide a new method of charging which is suited even for an electrophotographic material having a structure as shown in FIG. 2. The method of the present invention is characterized by disposing two corona charging electrodes above a photoconductive insulating layer formed on a conductive layer, causing a relative movement between these electrodes and the material to be charged in such a manner that one electrode precedes the other, subjecting the photoconductive coating to a corona discharge of one polarity from the preceding electrode whereby the photoconductive coating is made electrically conductive by photoconduction caused by light irradiation concurrently with or immediately after or before said corona discharging, and causing a corona discharge of the opposite polarity from the succeeding electrode.

FIG. 3 illustrates a cross-sectional view of a device to carry out the invention, wherein 30 designates a succeeding charging unit and 31 a preceding one disposed close to the former. These two units are made to move at a constant speed in the direction shown by an arrow. An electrophotographic material 20 stands still. The succeeding unit 31 comprises a corona wire 32 and a shield case 33 which is earthed. The preceding unit 31 similarly comprises a corona wire 34, and a shield 35 of ground potential but is further provided with a cylindrical light source 36 inside the shield and above the wire 34.

As illustrated in FIG. 4, the device works in the following manner. The light emitted from the light tube 3 6 converts the photoconductive coating 12 electrically conductive at the irradiated area. The charges on the corona ions which have reached on the surface of the coating 12 can therefore penetrate through the coating to the conductive layer 22. The charge migration through the coating takes place quite easily either by holes or electrons. Now presume a positive high potential is applied on the wire 34, positive corona ions will impinge on the photoconductive insulating coating 12, and the holes are thought to migrate through 12 to the conductive layer 22. As the conductive layer 22 becomes excessive with positive charges, negative ions from the succeeding electrode 30 applied negative high voltage are easily attracted on the coating. If the light fatigue of the photoconductive coating 12 is not heavy, negative charges will remain on the surface of the coating so as to charge up it negatively. In the case where the coating is heavily light-fatigued by the light exposure at the previous treatment, the negative corona ions will be neutralized instantaneously, and the coating will fail to accept a substantial amount of charge. Accordingly, the charging method of the present invention cannot be utilized for those photoconductive insulating coatings which are heavily fatigued by light. Photoconductive compositions which are suited for the present method include those comprising amorphous selenium, homogeneous mixtures of photoconductive powder (ZnO, CdS) and resinous binder, and many organic photoconductors.

Apart from the embodiment shown in FIG. 3, wherein the light source 36 is disposed in the cylindrical shield case, the light source may be provided above the shield case which has an opening on its top. Light irradiation may be carried out immediately before or after the corona discharge from the preceding unit, by the use of a light source disposed on the front or backwall of the shield through the irradiation and the corona discharge are simultaneously carried out. Since the two discharge units are located close to each other, and functioning simultaneously, the discharge of the succeeding unit 32 is enhanced by the aid of the preceding one 31, thus the efficiency of charging becoming quite high.

FIG. 5 illustrates the cross-sectional view of another device for practicing the invention. In this embodiment, a preceding unit 50 which corresponds to 31 in FIG. 3 comprises a shield case 52 and a light source 36. A succeeding unit is indicated at 30. An electrophotographic material is first exposed uniformly to light from the source 36, followed by corona discharge with the unit 50. Then, with a brief interval, again it is charged by the second unit 30. Since the first discharge is carried out on the photoconductive coating still in the state of light-adaptation, the corona ions cannot be stored thereon. The coating restores its dark-adapted state until the second unit comes across. Thus, the corona ions from the second unit may be effectively stored on the coating.

FIG. 6 illustrates still another embodiment of a charging device to carry out the invention. In contrast to the device shown in FIG. 5 in which light is flooded prior to the first charging, this device performs a first charging, blank exposure and a second charging, successively. Hence, photoconductive coatings need not exhibit any substantial photoconduction memory effect. Instead, those exhibiting no memory effect may advantageously be subjected to the subsequent charging immediately after the light exposure. In the device of FIG. 6, the corona ions which might have accumulated on the coating after the first charging can be dissipated by the succeeding exposure. Therefore ions generated at the second corona unit can accumulate on the photoconductive surface to charge the coating.

As will be clear from the above descriptions, the present invention is characterized by irradiating the photoconductive insulating coating during or immediately before or after a first corona discharge so as to temporarily convert the coating electrically conductive, preventing the first corona ions to accumulate on the coating, and applying the second corona on an area contiguous to the irradiated area whereby the second corona ions can easily accumulate on the coating by virtue of the first corona discharge. If the light exposure is not carried out prior to the second charging, the first corona ions will remain on the surface of the coating, which will require a prolonged or intense second charging operation to neutralize the accumulated charge. This will result in a marked elongation of time required for charging.

It follows from the working mechanism on which the present method is based that the first and the second charging unit should face to a single electrophotographic material, or, in other words, the conductive layer of the material to be charged must be electrically connected between the two areas which the two charging electrodes are facing to. From this regard, the present method is especially suited for an electrophotographic web material.

The present method has also an advantage in charging an electrophotographic material which has potential acceptances for positive and negative polarities.

The following examples will serve to illustrate the present method.

Example I An electrophotographic material was prepared in the following manner. A roll of polyethylene terephthalate film microns thick and 250 mm wide was surface activated by ultraviolet irradiation, and vacuum deposited with aluminum. The thickness of the aluminum layer was 0.2 micron.

On this support material was applied on the aluminized side a photoconductive coating mixture to give a dried thickness of 15 micron.

The coating mixture was prepared as follows. Purified CdS was fired at 200 C for 8 hours in the presence of 1.0 mol ZnI The original CdS was obtained by a wet process, and more than 40 percent of the powder proved to be cubic crystal. The heat-treated powder was blended with a binder comprising a silicon resin and a epoxyester resin so that the powder occupy 35 percent by volume of the dried coating. The first corona unit 31 had following specifications: corona wire-O.1 mm diameter stainless steel; wire 34 to shield case 35 distance-20 mm; wire to coating surface distancel5 mm; light source 36-an 8w florescent tube with about 250 mm effective length and tube diameter of 14.7 mm; light level at the coating surface-950 luxes. The second unit was provided adjacent to the first, having the following specifications: corona wire 32O.l mm diameter stainless steel; wire to shield distance-l5 mm; wire to coating surface-12mm. Both of the shield cases were earthed. With the application of 7KV to the first corona wire 34 and +7KV to the second 32, the electrophotographic web 20 was made to move in the direction shown by the arrow at Scm/sec to be charged positively.

Example II Another photoconductive web was prepared by providing 25 micron thick selenium by vacuum-deposition on the same supporting film used in the first example. The same operations were conducted as in the first example resulting in a similarly satisfactory charge acceptance.

Example III An electrically conductive polymer layer was formed on a 100 micron thick triacetyl cellulose film by applying a cationic polymer Dow Chemical ECR-34 (trade name) of Dow Chemical Co., U.S., to give a dried coating weight of 1.5 g/m On this layer was then formed a photoconductive coating comprising poly-N- venylcarbazol having a thickness of 8 microns.

The photoconductive film thus prepared was subjected to charging by the use of the device shown in F IG. 6. The specifications were as follows: corona wire 340.l mm diameter stainless steel; wire to shield distamemm; and corona wire to coating surface distancel 5 mm. The light source and the second charging unit were common to those employed in Example I. The coating was charged negatively by applying +8KV to the first and 7KV to the second corona wire.

Example IV A thin polyethylene film 15 microns thick was laminated on 100 micron thick art paper. The surface of said film was activated by subjecting it to corona discharge. On the activated surface was coated a conductive layer in the same manner as described in Example III.

A coating mixture was coated thereon comprising 100 parts by weight of ZnO, 14 parts by weight of styrenated alkyd resin (Styresol 4,400 (trade name) from Japan Reichhold Chemical Co.) 6 parts by weight of polyiocyanate compound (Colonate L (trade name) from Nippon Polyurethane Industries) and 0.02 part by weight of cupric stearate to give a dried thickness of about 7 microns.

The ZnO paper was charged by the device shown in FIG. 5. The two corona units and the light source were all common to those used in Example Ill. The paper was driven at 3 cm/sec. With the other conditions equal to those in Example III, the paper was successfully charged negatively.

What is claimed is:

l. A method of charging by means of at least two corona discharge electrodes an electrophotographic lightsensitive material comprising a photoconductive insulating coating, an insulating substrate, and a conductive layer interposed between said photoconductive insulating coating and said insulating substrate, said conductive layer being non-grounded and not connected to any source of electric potential, which method comprises, prior to subjecting the electrophotographic material to corona charges of a polarity to which said material is to be ultimately charged, subjecting said material to corona charges of the opposite polarity and to light irradiation simultaneously with subjecting said material to the corona charges of said opposite polarity, the light used being able to make conductive the photoconductive coating of said material.

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