Method For Charging Electrophotographic Material

Abstract

An electrophotographic material consisting of a high-insulating base and a photoconductive insulating layer coated directly thereon, is exposed uniformly on the side of the surface of the photoconductive insulating layer, to lights which are absorbable by the photoconductive insulating layer. Then, corona ions with a desired polarity produced by a corona discharge unit are sprayed onto the surface of the photoconductive insulating layer while the surface of the photoconductive insulating layer is simultaneously exposed to corona ions with the opposite polarity produced by another corona discharge unit. Thus, a conductive layer is temporarily produced on the surface of the photoconductive insulating layer so that the photoconductive insulating layer is charged or sensitized.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus for charging electrophotographic material consisting of a high-insulating base and a photoconductive insulating layer coated thereon.

2. Description of the Prior Art

Electrophotographic material, which consists of a base having relatively high resistance, e.g., a paper base, and a photoconductive insulating layer coated thereon, is normally charged by the double-corona charging method, whereby electrical charges of one polarity are applied to the surface of the electrophotographic material while electric charges of the opposite polarity are applied to the backface of the paper base.

However, this method is not able to provide the charging for high-insulating electrophotographic material, such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyamide, polyimide, polyvinyl chloride, diacetyl cellulose and polyacetyl cellulose. The charging of these types of electrophotographic material which have a high-insulating base, has been accomplished by providing a conductive layer between the base and the photoconductive insulating layer.

A method for charging an electrophotographic material having a high-insulating base and a photoconductive insulating layer coated directly thereon, without a conductive layer therebetween, is disclosed in British Pat. No. 971,281 Specification. In the method, light having a wavelength which penetrates the base and is absorbed into the photoconductive layer, is irradiated onto the material from the base-side thereof so as to make the contacting portion, of the photoconductive insulating layer with the base, temporarily conductive. Then the charging is provided by corona discharge during the period when the temporary conductive layer exists. For applying charges by this method, it is necessary to ground the conductive layer and therefore a connector is provided in contact with the surface of the photoconductive insulating layer.

However, the connector which contacts to the surface of photoconductive insulating layer, tends to produce line-cuts thereon which produce line-scars in the developed image and accordingly a significant deterioration of image quality. Furthermore, since the connector is merely contacted onto the surface of photoconductive insulating layer, there is a non-uniformity of charging due to imperfections in the conJact. It is therefore necessary to more strongly press the connector onto the surface of photoconductive insulating layer in order to provide a better contact, so that sharper line-cuts are produced thereon. In addition, since it is necessary that lights pentrate the base, it is impossible to use a non-transparent material such as veneer, plastics, concrete or the like.

SUMMARY OF THE INVENTION

This invention has been made with the intention of overcoming the above-described disadvantages in the prior electrophotographic material, and accordingly an object of this invention is to provide a novel and effective charging method and apparatus for electrophotographic material consisting of a high-insulating base and a photoconductive insulating layer coated thereon with no conductive layer.

Another object of this invention is to provide a method and apparatus for charging electrophotographic material having a base of opaque or non-transparent material.

According to this invention, electrophotographic material, which consists of a high-insulating base and a photoconductive insulating layer coated directly thereon, is uniformly exposed to light having a spectrum absorbable in the photoconductive insulating layer on the side of the surface of the photoconductive insulating layer. Next, corona ions, with a first polarity are emitted from a corona discharge unit and are sprayed onto the surface of the photoconductive insulating layer, while at least a portion of the photoconductive insulating layer is exposed to corona discharge ions with the opposite polarity. Thus, a conductive layer is formed temporarily on the surface of the photoconductive insulating layer, so that it is possible to charge, or sensitize, the electrophotographic material.

As described above, this invention provides a novel method and apparatus to enable the charging of an electrophotographic material by uniformly exposing the surface of a photoconductive insulating layer to form a conductive layer on the surface. It is, therefore, possible that an electrophotographic material consisting of a high-insulating base and a photoconductive insulating layer coated thereon is charged without providing any special conductive layer. Accordingly, it is possible to obtain a good electrophotographic image with no line-scar because it is not necessary to ground the material. Further, it is possible to use, as an insulating base, opaque or non-transparent insulating material such as veneer, plastics, concrete, wall, floor or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the electrophotographic material used for this invention.

FIG. 2 is a schematic sectional view for the illustration of the principle of this invention.

FIG. 3 shows an example of the electrophotographic apparatus according to this invention, wherein (a) is a schematic sectional view thereof and (b) is a schematic sectional side view thereof.

FIG. 4 is a schematic sectional view of another example of the electrophotographic apparatus according to this invention.

FIG. 5 is a graph of the results of tests carried out in accordance with Example V set forth below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, there is shown electrophotographic material 10 used for this invention. Photoconductive insulating layer 11 is made of vacuumdeposited non-crystalline selenium, the mixture of photoconductive powder (e.g., zinc oxide, cadmium sulfide) and insulating resin and a, and a photoconductive organic material or the like. It is desirable that it exhibits a slow decay after the exposure light is turned off. When a photoconductive insulating material is subject to repeat cycles of charging and exposure, an effect known as fatigue sometimes occurs. The effect is manifested as an increase in the dark decay rate of the potential, i.e., decreased retentivity. High-insulating base 12 is made of well-drive paper, non-electrosensitive paper, polyethylene terephthalate, polypropylene, polycarbonate, polyamide, polyimide, polyvinyl chloride, diacetyl cellulose or polyacetyl cellulose, or it may be made of other non-transparent insulating material.

The principle of this invention will be made clear in relation to the case wherein the photoconductive insulating layer is made of N-type semiconductor, e.g., by kneading together photoconductive zinc oxide powder and insulating resin, with reference to FIG. 2.

The electrophotographic material 10 is exposed, on the side of the surface of photoconductive insulating layer 11, to the lights of a light source (not shown). The light is absorbable into the photoconductive insulating layer 11 to make it conductive. Main corona discharge unit 20 and sub-corona discharge unit 21 are arranged above the electrophotographic material 10 and having corona wires 22 and 23, respectively. Negative high voltage (e.g., - 6 kV) is applied to the corona wire 22 of the main corona discharge unit 20 to emit negative corona ions onto the photoconductive insulating layer 11, and positive high voltage (e.g., + 6 kV) is applied to the corona wire 23 of the sub-corona unit 21 to emit positive corona ions onto the photoconductive insulating layer 11.

Fatigue is caused within the photoconductive insulating layer 11 by uniform exposure, so that there are relatively freely movable trapped electrons therein. These electrons are repulsed and driven away by the negative corona ions so that the trap centers are neutralized. Thus, the area R exposed to the negative corona discharge recovers from fatigue and is capable of being charged. On the other hand, the electrons in the area R, driven away by the negative corona ions, pass through the area outside the area R which has not recovered from fatigue and permits relatively free movement of electrons therein. These electrons are attracted by the positive corona ions emitted from the corona wire 23 for neutralization. When the trap centers in the area R are neutralized, the negative corona ions are stored in the surface of the photoconductive insulating layer within the area R and therefore positive charges equal to them are produced outside the area R. The area R is then expanded outwardly as a function of time so that the negative charging is provided over the large area.

Thus, a conductive layer is formed on the surface of the photoconductive insulating layer in accordance with the uniform exposure of the surface to lights and thus the electrophotographic material is charged. It is, therefore, possible to use electrophotographic material consisting of an opaque insulating base, such as veneer, plastics, concrete, wall or floor, and a photoconductive insulating layer directly coated thereon without the necessity of any special conductive layer.

FIG. 3 shows an example of electrophotographic apparatus for practicing the method of this invention, wherein (a) is the schematic front sectional view thereof and (b) is the schematic side sectional view thereof. Main corona discharge unit 30 is composed of corona wire 31, shield case 32 and insulating supporter 33 for corona wire 31. Sub-corona discharge unit 34 is composed of corona wire 35, shield case 36 and insulating supporter 37 for corona wire 35. The corona wires 31 and 35 are positioned at right angles with respect to each other, the corona wire 31 being at a right angle to the forward direction of the electrophotographic material 10 and the corona wire 35 is parallel to the forward direction.

Before the electrophotographic material 10 is exposed to corona discharge, it is uniformly exposed, on the side of the surface of the photoconductive insulating layer, to the lights from a light source 38. The light is absorbed by the photoconductive insulating layer to make the layer conductive. In order to provide negative charging, a negative high voltage may be applied to the corona wire 31 of the main corona discharge unit 30 to produce corona discharge, while a positive high voltage may be applied to the corona wire 35 of the sub-corona discharge unit 34. To provide positive charging, the polarity of high voltages applied to the corona wires may be opposite to the above case.

Though two sub-corona units 34 are used in the above example, it is possible to use one unit. In addition, if the corona ions emitted from the corona wire 31 of the main corona discharge unit 30 and the corona ions emitted from the corona wire 35 of the sub-corona discharge unit 34 overlap each other, the quantity of charge is decreased. It is, therefore, desired to provide a partition, and the shield plate of the one corona discharge unit may also serve as the partition.

FIG. 4 is a schematic sectional view of another example of the apparatus according to this invention. Electrophotographic material 10 is uniformly exposed to light from source 42 on the side of the surface of photoconductive insulating layer. The layer is then exposed to positive corona ions produced by the sub-corona discharge unit 41 followed by exposure to negative corona ions produced by the main discharge unit 40 so as to become negatively charged. In this apparatus, the corona wire is made longer than the width of the material to uniformly charge the whole surface.

It may be possible to use, as the corona discharge electrode, a needle electrode or strip electrode instead of wire electrodes. It is necessary that the light source emit the lights in a spectrum which is absorbable by the photoconductive insulating layer. For example, if the photoconductive insulating layer is made of zinc oxide, a tungsten light source may be used for pre-exposure. This photoconductive insulating layer is photosensitive only to ultraviolet and near ultraviolet lights and extremely absorbative to the spectral lights, so that only ultraviolet or near ultraviolet lights are available for exposure. When the photoconductive insulating layer contains dye-sensitized zinc oxide, however, its sensitive spectrum is extended up to visible spectrum from the near ultraviolet spectrum, and the photoconductive insulating layer exhibits a low absorption coefficient and high photosensitivity to visible lights. Accordingly, irradiating visible lights onto the surface thereof, some of the lights may reach the backface thereof so that the electrification coefficient of the photoconductive insulating layer is increased.

Though ultraviolet and near ultraviolet lights are absorbed in the surface of the photoconductive insulating layer, experiments show that the after effect on photoconductivity caused by the ultraviolet lights tends to continue for a relatively long period. The practice of this invention is achieved with preferred results with any one of the following combinations: (1) dye-sensitized layer and ultraviolet lights; (2) dye-sensitized layer and visible lights; (3) dye-sensitized layer and ultraviolet and visible lights; and (4) non-sensitized layer and ultraviolet lights.

In addition, if pre-exposure is provided a relatively long time before corona discharge, it will be necessary to determine the factor relating to exposure quantity, light quality and time in accordance with the characteristics of the photoconductive insulating layer.

In the following, examples of this invention will be described for the best understanding of this invention.

EXAMPLE I

The electrophotographic material is made by kneading 100 parts photoconductive zinc oxide power (SaZeX No. 2000, Sakai Chemical Industry Co. Ltd.), 14 parts insulating resin (Styresol No. 4400, Japan Reichhold Chemical, Inc.) and 7 parts polyisocyanate compound (Desmodul L, Bayer A.G.), then painting the mixture onto a film of polyethylene terephthalate, of 150.mu. thickness, to a thickness of about 7.mu. and then hardening the material in a constant temperature bath at 40.degree.C for 17 hours. After putting the electrophotographic material on a high-insulating plate (polymethylmethacrylate resin plate of 10 mm thickness) in a dark place and exposing it uniformly on the side of the surface of the photoconductive insulating layer, to a tungsten lamp with an exposure quantity of 200,000 Lux.sup.. sec (10,000 Lux .times. 20 sec), corona discharge units are arranged above the electrophotographic material as shown in FIG. 3.

In the main corona discharge unit 30, the distance between the corona wire and the surface to be charged is 15 mm, the space between the corona wire and the shield case is 15 mm, the diameter of the wire is 0.05 mm, the length of the wire is 30 mm, the distance from the lower edge of the shield case to the surface to be charged is 8 mm, and the material of the wire is tungsten.

In the sub-corona discharge unit 34, the distance between the corona wire and the surface to be charged is 10 mm, the space between the corona wire and the shield case is 15 mm, the wire diameter is 0.05 mm, the wire length is 50 mm, the distance between the lower edge of the shield case and the surface to be charged is 2 mm, and the wire material is tungsten. Applying -8 kV to the corona wire of the main corona discharge unit and +9 kV to the corona wire of the sub-corona discharge unit, the electrophotographic material is moved in a direction at a right angle to the corona wire of the main corona discharge unit at the speed of 50 mm/sec and the discharging is finished within 30 sec after the above exposure. The electrophotographic material is thus charged uniformly at the surface to a potential of -140 V except for both edge portions thereof. Further, the area under the sub-corona discharge unit is not charged.

EXAMPLE II

The same electrophotographic material as in Example I is put on the similar insulating plate as in Example I and pre-exposed uniformly to the lights of 200,000 Lux.sup.. sec (10,000 Lux.sup.. 20 sec). Then, two needle corona discharge electrodes are arranged above the material at right angles to the surface to be charged. The distance between the corona discharge electrodes is 150 mm, the main corona discharge unit 30 being positioned above the center portion of the charged surface and the sub-corona discharge unit 34 being positioned above the edge portion of the charged surface. The distance between the top of the main discharge unit electrode and the charged surface is 50 mm, and the distance between the top of the sub-corona discharge unit electrode and the charged surface is 20 mm. Applying -9 kV to the main corona discharge unit and +5 kV to the sub-corona discharge unit, the electrophotographic material is moved below both electrodes at the speed of 30 mm/sec and the discharging is finished within 30 sec after the previous exposure, with the result that the center portion of the surface to be charged is charged to a -130 V surface potential.

EXAMPLE III

The same material as in Example I is made zonal into a roll and transported at a speed of 30 mm/sec. The light source and corona discharge units are arranged as shown in FIG. 4. The light source consists of six tungsten lamps of 200 W arranged in double lines and provides an exposure quantity of 8000 Lux.sup.. sec during the material passage. In addition, a shielding plate is provided to prevent lights of the light source from leaking onto the charged area. The electrophotographic material passes below the light source, then below the sub-corona discharge unit 41 and then below the main corona discharge unit 40. In each corona discharge unit the corona wire is made of stainless steel with a 5 mm diameter, the distance between the shield case and the wire is 15 mm, and the distance between the corona wire and the charged surface is 20 mm. The distance between the first and the second corona discharge units is 100 mm. The length of each corona wire is 50 mm longer than the width of the electrophotographic material. Applying +9.5 kV to the corona wire of the first corona discharge unit 41 and -8.5 kV to the corona wire of the second corona discharge unit 40, the electrophotographic material is charged to a surface potential of -140 V.

EXAMPLE IV

The electrophotographic material is made by painting the same mixture as in Example I onto a veneer plate of 8 mm thickness and hardening as in Example I. This material is charged, according to the process as in Example I, to a surface potential of -120 V. Next, after contacting a positive transparency onto the material and then exposing it, image development is made by a cascade developer containing toners with positive charges to obtain a positive image.

EXAMPLE V

Using the same electrophotographic material as in Example I and charging it under the following conditions, measurements of the relation between exposure quantity and surface potential were made: The material was placed on the same insulating plate as in Example I, and two needle electrodes were arranged thereabove with a distance of 80 mm between electrodes. The distance between the top of each needle electrode and the photoconductive insulating layer was 30 mm. The one needle electrode was applied with -5 kV and the other with a +5 kV, for a duration of 5 sec and 30 sec.

The measured results are shown in FIG. 5.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

We claim:

1. A method for charging to a first polarity electrophotographic material consisting of an insulating base and a photoconductive insulating layer directly coated thereon and including a semiconductor having majority carriers of said first polarity, said method comprising:

a. uniformly exposing the surface of said photoconductive layer to light of a wavelength absorbable by said photoconductive insulating layer such that said surface becomes conductive; and, simultaneously while said surface is still conductive:

b. exposing a first area of said surface of said photoconductive insulating layer to first corona discharge ions of a first polarity; and

c. exposing a second area of said surface of said photoconductive insulating layer to second corona discharge ions of the opposite polarity to said first corona discharge ions, whereby said surface of said photoconductive insulating layer becomes charged to said first polarity.

2. The method as defined in claim 1 wherein said photoconductive layer is made of N-type semiconductor material, and said first polarity is negative.