Non Air-polluting Corona Discharge Devices

Abstract

A corona generating device, adapted to operate in air, to apply an electrostatic charge to a surface, includes an elongated, knife-edged, corona-generating electrode enclosed within a housing. The housing includes a foraminous screen that has an optical transmission of less than 50 percent to prevent ultraviolet light produced by the corona from reaching the surface to be charged. The screen and the interior of the housing are formed, or coated, with a catalytic material, such as silver, to convert any ozone formed by the corona discharge, to oxygen, thereby preventing pollution of the air with toxic ozone.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to apparatus, adapted to operate in air, for applying an electrostatic charge to a surface; and, more particularly, to non air-polluting corona discharge devices. The improved corona discharge devices of the present invention are particularly useful for electrostatically charging recording elements employed in the electrostatic printing art.

In electrophotographic printing processes in common use it is necessary to apply a uniform electrostatic charge, in darkness, to the surface of a photoconductive layer of a recording element. When the recording element is exposed to a light image, developed with a pigmented toner, and fixed, it provides a copy of the image to which it was exposed. It has been proposed to charge the surface of the recording element electrostatically by subjecting its photoconductive surface to a corona discharge, in air, from a corona discharge device disposed above the photoconductive surface. While such a corona discharge device is suitable for charging a photoconductive surface for many practical purposes, it produces an ultraviolet glow which adversely affects uniform charging. Also, the production of a corona discharge in air usually generates ozone, which also adversely affects uniform charging and which, even in very small quantities, is considered to be toxic to humans. A concentration of 1 part ozone in 20,000 parts of air irritates mucous membranes, when breathed, and is poisonous.

I have observed that ultraviolet light and ozone, both formed during a corona discharge, discharge an electrostatically charged electrophotographic recording element. In fact, I have observed that ozone alone, in the absence of light, discharges a charged photoconductive layer of zinc oxide in a resin binder very quickly, actually in a matter of about one second. It is believed that the spurious discharges of a charged photoconductive layer of a recording element, caused by both the ozone and the ultraviolet light during an electrophotographic process, produce unwanted, spurious toning defects in the finished electrophotographic print. This is especially observable in reversal prints wherein the spurious discharges cause unwanted toned spots in background (white) areas of the prints. The spurious toning defects degrade the quality of the finished electrophotographic print.

SUMMARY OF THE INVENTION

The novel apparatus for applying an electrostatic charge to a surface comprises a housing, having a foraminous screen as a portion thereof. The screen is adapted to be disposed adjacent the surface to be charged. A corona generating electrode is enclosed within the housing and insulated therefrom. The electrode is disposed adjacent to the screen and is adapted to generate, when energized, a corona discharge in the general direction of the screen. A catalyst is disposed within the housing to convert any ozone that may be formed during the generation of the corona discharge to oxygen, thereby preventing toxic ozone from polluting the atmosphere.

In a preferred embodiment of the novel corona discharge device, the screen has an optical transmission of less than 50 percent, and a baffle plate disposed between the electrode and the surface to be charged to shield the surface from both ultraviolet light and ozone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view, in side elevation, of one embodiment of a non air-polluting corona discharge device disposed to apply an electrostatic charge to an electrophotographic recording element;

FIG. 2 is a cross-sectional view taken along the line 2--2 in FIG. 1; and

FIG. 3 is a cross-sectional view, in side elevation, of another embodiment of a non air-polluting corona discharge device disposed to apply an electrostatic charge to a recording element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, there is shown, in cross-section, a non air-polluting corona discharge device 10 for applying an electrostatic charge to a surface 12 of a photoconductive layer 14 of a recording element 16. The recording element 16 is of the type commonly used in the electrostatic printing art. The recording element 16 comprises a relatively electrically conductive substrate 18, such as paper, and the photoconductive layer 14, such as of zinc oxide in a suitable resin binder thereon. The recording element 16 is on an electrically conductive support plate 20, disposed beneath the corona device 10, and adapted to be moved, by any suitable means, in the direction of the arrow 52.

The corona discharge device 10 comprises an enclosed, electrically conductive housing 22, formed by a downwardly extending shield 24, a horizontally extending foraminous screen 26, a horizontally extending baffle plate 28, and an upwardly extending foraminous screen 30. The positional orientation described herein is merely relative with respect to the horizontally disposed recording element 16. It is within the contemplation of the present invention, however, to orient the recording element 16 and the corona discharge device 10 in positions other than horizontal. The screens 26 and 30 are fixed to the shield 22 and to the baffle plate 28 by any suitable means, as, for example, by welding or soldering.

Means are provided to generate a corona discharge, that is, a source of ions, when energized by a suitable power supply. To this end, an elongated, corona generating electrode 32 is disposed within the interior 34 of the housing 22. The electrode 32 is an elongated band, or strip, formed with a knife edge 36 pointing in the general direction of the screen 26, for the purpose hereinafter appearing. The electrode 32 is insulated from the conductive baffle plate 28 by insulating threaded studs 38 that extend upwardly from the baffle plate 28. The studs 38 may be adhered to the baffle plate 28 by any suitable means, such as by epoxy glue, and the electrode 32 may be adjustably mounted on the threaded studs 38 with nuts.

The screens 26 and 30 should have an optical transmission of less than 50 percent. The term "optical transmission," as used herein, is defined as "the ratio of the area of the holes to the area of both the holes and the non-holes of the screen, considering the screen as a two dimensional planar object." Thus, the optical transmission of a foraminous screen is essentially the percentage of light it will transmit when exposed to light. I have found that the optical transmission of the screen 26 should be less than 50 percent to prevent both ultraviolet light and ozone that may be formed during the generation of a corona discharge from passing from the interior 34 of the corona discharge device 10 onto the surface 12 of the recording element 16. Also, the electrode 32 and the baffle plate 28 are disposed with respect to each other so that the baffle plate 28 prevents ultraviolet light, emanating from the knife edge 36 of the electrode 32, from reaching the surface 12 of the recording element 16. Thus, the baffle plate 28 is disposed between the surface 12 of the recording element 16 and the electrode 32. The junction 40, between the baffle plate 28 and the screen 26, is disposed so that any ultraviolet light emanating from the knife edge 36 of the electrode 32 does not reach the surface 12 to be charged.

Means are provided to energize the corona discharge device 10. To this end, the negative terminal of a unidirectional power supply 42, represented symbolically herein as a battery, is connected to the electrode 32 by a conductor 44 that passes through an insulator 46 positioned in the screen 30. The positive terminal of the power supply 42 is connected to a point of reference potential, such as ground. The shield 24 of the housing 22 is connected to ground through a variable resistor 48 so that the potential of the shield 24 may be biased with respect to the positive terminal of the power supply 42. The support plate 20 is connected to ground through a variable voltage source 50, the positive terminal of the voltage source 50 being connected to the plate 20 and the negative terminal being grounded. Thus, the substrate 18 of the recording element 16 can be biased positively with respect to the electrode 32 and the housing 22 to cause a negative corona discharge to be attracted to the positively biased surface 12 of the recording element 16.

A negative electrostatic charge is applied to the surface 12 of the recording element 16 by the corona discharge device 10 as follows: A negative voltage of at least 5,000 volts with respect to ground is applied to the electrode 32. The variable resistor 48 is adjusted so that the voltage of the housing 22 is between 0 and -500 volts with respect to ground. The variable voltage source 50 is adjusted so as to provide a voltage to the support plate 20 of between about 0 and +400 volts with respect to ground. The support plate 20 should be maintained at a positive potential with respect to the housing 22. The corona discharge device 10 is disposed so that the baffle plate 28 and screen 26 are parallel to, and about 5mm above, the surface 12 of the recording element 16. Under these conditions, a negative corona discharge enanates from the knife edge 36 of the electrode 32 in the direction of, and through, the screen 26. The electric wind produced by the corona discharge blows the negative ions on the corona through the screen 26 and onto the surface 12 of the recording element 16.

The recording element 16 is usually adapted to be moved in the direction of the arrow 52. It is also within the contemplation of the present invention to move the corona discharge device 10 with respect to either a fixed or a moving recording element 16. Any radiation in the form of light that may emanate from the knife edge 36 of the electrode 32 during the generation of the corona discharge is prevented from reaching the surface 12 by both the baffle plate 28 and by the screen 26 (because of its limited optical transmission of less than 50 percent).

I have observed that ozone is generated within the corona discharge device 10 during the generation of the corona discharge. I have also observed that ozone, alone and in darkness, discharges a charged recording element 16. It is also well known that ozone, even in the relatively small quantities of 50 parts of ozone to 1 million parts of air, is toxic to humans. An important feature of the novel corona discharge device 10 is its provision for preventing, or substantially reducing, the pollution of the atmosphere by the ozone formed during the generation of the corona discharge. To this end, the inner surface of the shield 24 and the inner surface of the baffle plate 28 are coated with a catalytic coating 54 of a catalyst for converting to oxygen any ozone that may be formed. I have found silver to be substantially the best coating 54 for catalyzing the chemical reaction:

Other suitable catalysts for the catalytic coating 54 are gold, nickel, platinum, iron, and their alloys. Certain oxides are also suitable catalysts. These are, for example, nickel sesquioxide (Ni.sub.2 O.sub.3), barium monoxide (BaO), cupric oxide (CuO), and silver peroxide (Ag.sub.2 O.sub.2). Some of these metal oxides may be formed by first plating the shield 22 and the baffle plate 28 with a layer of the metal of the oxide and then oxidizing the deposited metal layer to form the required metal oxide. For example, a layer of nickel when heated at a temperature of between 400.degree. and 480.degree. C. is converted partially to the nickel sesquioxide (Ni.sub.2 O.sub.3). Cupric oxide may be formed, for example, by heating a copper coating in air until the surface of the copper turns black.

The catalytic coating 54 of platinum can be formed by coating the interior of the corona discharge device 10 with platinum, treating the coating with aqua regia having an excess of nitric acid, evaporating the acid, and heating the treated surface until it turns black. The screens 26 and 30 are also either coated with, or comprise, at least one of the aforementioned catalytic materials. Hence, ozone formed within the interior 34 of the corona discharge device 10 is acted upon by the catalytic coating 54 and the catalytic material on the screens 26 and 30 to convert the ozone to oxygen.

Referring now to FIG. 3, there is shown an improved corona discharge device 60 which differs from the corona discharge device 10 in that the corona discharge device 60 does not have a baffle plate to function as a light shield. The corona discharge device 60, however, employs a screen 62, similar in structure and function to the screen 26 of the corona discharge device 10 with the exception that the screen 62 may have an optical transmission that is less than that of the screen 26. Thus, the optical transmission of the screen 62 is between about 20 percent and 50 percent. Parts of the corona discharge device 60 in FIG. 3 that are similar to those of the corona discharge device 10 in FIG. 1 in both structure and function are given the same reference numerals.

The corona discharge device 60 comprises an enclosed housing 64 of which the screen 62 is a part. The housing 64 comprises a rectangular box-like structure having side walls 66 and 68 and end walls (not shown). A top wall 70 of the housing 64 comprises a screen 70 similar in structure and function to the screen 30. An elongated electrode 72 is disposed within the interior 74 of the housing 64. The electrode 72 is supported within the interior 74 by electrically insulating rods 76 and is disposed so that a knife edge 78 of the electrode 72 causes a corona discharge therefrom to flow in the general direction of the screen 62 and onto the surface 12 of the recording element 16.

The corona discharge device 60 is energized from a source 42 of unidirectional voltage, the negative terminal of the voltage source 42 being connected to the electrode 72 through an insulator 80 in the wall 66, the positive terminal being connected to a point of reference potential, ground. The housing 64 is connected to ground through a variable resistor 48 to bias it negatively with respect to ground. The support plate 20 is connected to the positive terminal of a voltage source 50, the negative terminal being grounded.

In operation, the corona discharge device 60 is adapted to provide a negative electrostatic charge onto the surface 12 of the recording element 16 through the screen 62. The electrode 72 is maintained at a voltage of at least 5,000 volts with respect to ground; the resistor 48 is adjusted to provide a negative bias of between 0 and -500 volts to the housing 64, and the variable voltage source 50 is adjusted to provide voltage of between 0 and +400 volts to the support plate 20. When thus energized, a corona discharge is directed onto the surface 12 by the electric wind produced, providing the surface 12 with a uniform negative electrostatic charge. Because the optical transmission of the screen 62 is less than 50 percent, most of the ultraviolet light emanating from the knife edge 78 of the electrode 72 is substantially filtered (blocked) so that only a relatively small amount of the ultraviolet light reaches the surface 12. The values of voltage and dimensions given herein are not critical. They are merely illustrative and are not to be construed in a limiting sense.

The corona discharge device 60 is provided with means to convert ozone formed during the corona discharge to oxygen in a manner similar to that described for the corona discharge device 10. Thus, the inner surface of the enclosed housing 64 is coated with a catalytic coating 54 of one of the aforementioned catalytic materials that catalyzes the chemical reaction wherein oxone is converted to oxygen. Both of the screens 62 and 70 are either made of, or coated with, a catalytic material of the type described, such as silver, gold, nickel, platinum, iron, or their alloys, nickel sesquioxide, or copper oxide, for example.

Some catalytic materials 82, in the form of compounds, such as barium monoxide and silver peroxide, that do not lend themselves easily to being applied in the form of the catalytic coating 54 may be disposed within the interior 74 of the housing 64 in foraminous screen containers 84. The screen containers 84 may be secured, by any suitable means, to the insulating rods 76, for example. The screen containers 84 may be refilled if the catalytic materials 82 should become poisoned in time and with use.

A fan 86 is disposed by any suitable means adjacent to the screen 70 and adapted to rotate in a direction, when suitably energized, to cause air to flow gently through the housing 64 in a direction indicated by the arrows 88. Thus, the fan 86 comprises air moving means to cause ozone to flow through the catalytic materials 82, wherein the ozone is converted to oxygen, as well as means to prevent ozone from reaching the surface 12 to be charged. Although the fan 70 is illustrated in FIG. 3 as being outside of the housing 64, it may be included within the housing 64 and disposed to cause air to flow from the interior of the housing 62 out through the screen 70, in the direction of the arrows 88.

While the novel corona discharge device 10 and 60 have been described in circuits wherein negative corona discharges were produced, it is also within the contemplation of the present invention to reverse the polarities of the voltage sources so that the novel corona discharge devices 10 and 60 produce positive corona discharges.

Claims

I claim:

1. Apparatus, adapted to operate in air, for applying an electrostatic charge to a surface, said apparatus comprising:

an electrically conductive housing comprising a foraminous screen as a portion thereof, said screen being adapted to be disposed adjacent said surface to be charged,

a corona generating electrode enclosed within said housing and insulated therefrom, said electrode being disposed adjacent to said screen and adapted to generate, when energized, a corona discharge in the general direction of said screen, said corona discharge producing ozone in air, and

a catalyst within said housing and spaced from said corona generating electrode for converting said ozone to oxygen.

2. Apparatus, adapted to operate in air, for applying an electrostatic charge to a surface as described in claim 1, wherein

said screen has an optical transmission of less than 50 percent.

3. Apparatus, adapted to operate in air, for applying an electrostatic charge to a surface as described in claim 1, wherein

said catalyst comprises a metal selected from the group consisting of silver, gold, nickel, platinum, iron, and alloys thereof.

4. Apparatus, adapted to operate in air, for applying an electrostatic charge to a surface as described in claim 1, wherein

said screen comprises a catalyst for converting ozone to oxygen, said catalyst comprising a metal selected from the group consisting of silver, gold nickel, platinum, iron, and alloys thereof.

5. Apparatus, adapted to operate in air, for applying an electrostatic charge to a surface as described in claim 1, wherein

said catalyst comprises a metal oxide selected from the group consisting of nickel sesquioxide, barium monoxide, cupric oxide, and silver peroxide, and

means within said housing containing said metal oxide so that said ozone may come in contact with said metal oxide.

6. In apparatus, adapted to operate in air, for applying an electrostatic charge to a surface, wherein a corona generating electrode is enclosed within an electrically conductive housing, the improvement comprising:

a foraminous screen comprising a portion of said housing, said screen being adapted to be disposed adjacent said surface to be charged, said screen having an optical transmission of less than 50 percent;

said electrode comprising an elongated strip of metal elongated in a direction substantially parallel to said surface, formed with a knife edge pointing in the general direction of said screen, said electrode being adapted to generate, when energized, a corona discharge with an electric wind in the general direction of said screen, said corona discharge producing ozone in air, and

a catalyst spaced from said electrode and cooperatively associated with said housing for converting said ozone to oxygen.

7. In apparatus, adapted to operate in air for applying an electrostatic charge to a surface, as described in claim 6,

said catalyst comprising an oxide selected from the group consisting of nickel sesquioxide, barium monoxide, cupric oxide, and silver peroxide;

foraminous means disposed within said housing for containing said catalyst,

a second foraminous screen, spaced from said first-mentioned foraminous screen, and comprising another portion of said housing; and

air moving means cooperatively associated with said housing for moving ozone within said housing through said second screen.