Xerographic charging

2,987,660

June 6, 1961

L. E. WALKUP

XEROGRAPHIC CHARGING

Filed June 6, 1955

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United States Patent Office _p ., ^2,9f ;⁶⁶⁰

Patented June 6, 1961

2,987,660 XEROGRAPHIC CHARGING

Lewis E. Walkup, Columbus, Ohio, assignor, by mesne assignments, to Haloid Xerox Inc., a corporation of New York

Filed June 6,1955, Ser. No. 513,247 5 Claims. (Cl. 317—262)

This invention relates in general to xerography and in particular to the sensitization or application of electric charge to the surface of an insulating or photoconductive insulating layer.

In the art of xerography, according to Carlson U.S. 2,297,691, it is usual to employ the simultaneous application of electric field and a pattern of activating radiation on a photoconductive insulating layer to form an electrostatic charge pattern, otherwise known as an electrostatic latent image. This electrostatic image then is capable of being utilized, such as for example, by the deposition of electroscopic material thereon to form a visible image.

Usually the order of procedure is to sensitize the xerographic plate by applying a uniform charge to the surface of the photoconductive insulator, after which exposure is made. The art of sensitizing the photoconductive insulating layer, as employed in that Carlson patent, has heretofore been a difficult and complex one, and normally sensitization may be accomplished by any of various means, such as for example, frictional means as disclosed in that Carlson patent, or ion charging means as shown in Carlson U.S. 2,588,699'. Frictional charging, however, is found to be difficult of operation and generally results in uneven or irregular charge across the surface as well as non-reproducibility of charge. Ion charging, on the other hand, while generally uniform and reproducible, requires expensive and relatively clumsy equipment and, conventionally, the use of exceptionally high voltages. Thus, ion charging generally comprises the application of charge to the photoconductive insulating surface by mechanically passing across the photosensitive surface a corona generating electrode maintained at a potential of several thousand volts, generally in the order of about 7,000 volts with respect to ground potential. This obviously necessitates high voltage equipment, safety and protective devices and, frequently, additional moving parts which are held at high electric potentials. Furthermore, ion charging is subject to considerable variation with changing ambient atmospheric conditions.

Methods of direct charging by means of contact between the surface to be charged and a charging body or electrode have from time to time been proposed and it has been found generally that these methods are incapable of charging the surface to a potential greater than about 500 volts or more below the potential of the charging electrode. Thus, in order to impose a charge of about 500 volts on the insulating surface, it is necessary to maintain the charging electrode at a potential of about 1,000 volts or perhaps even higher, even under optimum conditions and according to best techniques. Furthermore, the results of such charging have been highly irregular, showing a streaked or mottled pattern, depending on the mechanical arrangements employed. It has become apparent, therefore, that on a gross basis it is possible to apply a charge to an insulating layer such as a xerographic plate by means of a contacting charging electrode, but that the charge thus applied to the surface is not a xerographic sensitizing charge because it is either too weak or is so irregular as to be unserviceable for xerographic reproductions, or in most instances suffers both defects. For example, in xerography it is usual to develop, in line copy, a potential difference of 100 to 200 volts between image and background areas of photosensitive members originally charged to about 500 volts.

Similarly, in continuous tone reproductions, it is quite usual to develop well over ten distinguishable shades of gray on a photoconductor originally charged to perhaps 150 volts. In either case, there necessarily is a very 5 close tolerance of uniformity of charging since charge defects of only a few volts can destroy the balance between image areas of different but closely adjacent potentials.

In more mature research relating to the application of 10 electric charge to an insulating surface by means of a contacting electrode, it has been found that these defects are inherent in the systems of contact between members. A very substantial potential difference generally in the order of about 500 volts under best conditions and rang15 ing drastically upwards under less suitable conditions remains between the charging electrode and the charge voltage applied to the insulating surface, and this has been found to be an inherent result of a gap substantially in excess of molecular dimensions between the two surfaces.

Thus, when two surfaces are pressed into firm contact under pressures as high as many hundreds of pounds per square inch, there still remains between the two surfaces a minute air gap having a high threshold of electric breakdown such that a potential difference of at least 25 about 400 or 500 volts must exist between these two sursurfaces before charge migration between the surfaces occurs, and such a potential difference must be maintained in order to maintain continuing charge migration. Furthermore, it has been shown in the prior art that these 30 voltage losses become higher rather than lower as spaces and distances decrease in the range of a few microns. On the other hand, when two surfaces are thus closely pressed together, and particularly when they are moved with respect to one another, certain points or areas come into literal or molecular contact so that localized or spotty areas become highly charged. Alternatively, perhaps, localized or minor electrical effects locally trigger a charge migration with the same result that certain localized areas have charge migration between the surfaces in 40 excess of the overall or general charge migration to form a spotty or mottled transfer of electric charge between the surfaces. Since the charging of such surfaces is utilized to apply charge to an insulating surface having substantially no lateral electrical conductivity, the result is a 45 pattern of charge on the surface, which pattern cannot be smoothed out by presently known methods and which is overriding in its effect as against a useful pattern of charge later imposed by xerographic methods such as exposure.

5q Now, in accordance with the present invention, it has been found that a substantially uniform electric charge can be placed on an insulating surface by electrification with a conductive or electrolytic liquid and that the charge so applied is of substantially the same potential as 55 the potential on the contacting liquid and is substantially uniform across the entire area.

One of the principal purposes now in existence for the application of uniform charge to an insulating or photoconductive insulating surface is for the purpose of sensi60 tizing such a surface for use in the art of xerography, and accordingly, it is an object of the present invention to provide new methods, means and apparatus for the charging of insulating and photoconductive surfaces in general and for the charging and sensitization of insulating and 65 photoconductive insulating xerographic layers.

It is another object of the invention to provide new methods, means and apparatus for xerography, including a new xerographic machine.

According to Carlson Patent 2,297,691, the art of xerog70 raphy includes the repetitive use of a reusable photosensitive xerographic member for the formation and development of xerographic images. This means that it is

2,987,660 conventional in xerography to employ a reusable member and to form a visible image on such a reusable member, which visible image is then transferred to a second surface. The first insulating or photoconductive insulating surface is then run through the xerographic steps a further time to form a new image, which again is transferred. In the present state of the art, it is necessary to employ a cleaning operation between cycles, whereby the reusable member is freed of residual material and returned to its original clean condition before a subsequent operation. One method of accomplishing such cleaning within the scope of the present invention is to flush or wash the surface with a suitable liquid material, which preferably is inert with respect to both the xerographic plate surface and the developed image material. Such cleaning may optionally be coupled with suitable agitation to assure complete removal of foreign material from the plate surface. Such a liquid, of course, may be of any nature which accomplishes the job, but it has generally been found desirable to employ a material which is non-solvent for the developed image material as well as being inert toward the plate, and accordingly, an aqueous type electrolytic liquid is well adapted to the purpose.

It is, therefore, an additional object of the present invention to provide means, methods and apparatus for the simultaneous charging and cleaning of a xerographic member wherein said member is contacted with an electrolytic liquid matintained at a charging potential and wherein the electrolytic liquid is designed and adapted to clean the surface for subsequent reuse.

Additional objects of the invention will in part be obvious and will in part become apparent from the following specification and drawings, in which:

FIG. 1 is a diagrammatic illustration of a simplified charging system according to one embodiment of the 35 present invention.

FIG. 2 is a chart correlating applied voltage and charge potential.

FIG. 3 is a diagrammatic illustration of a portion of mechanism according to one embodiment of the invention.

FIG. 4 is an enlarged fragmentary view of liquid seal mechanism according to FIG. 3.

FIG. 5 is a diagrammatic view of a charging and cleaning system according to another embodiment of the invention.

FIG. 6 is a diagrammatic view of a xerographic machine according to one embodiment of the present invention.

FIG. 7 is a diagrammatic view of a xerographic machine according to another embodiment of the invention.

In FIG. 1 is illustrated diagrammatically one embodiment of the invention in which an insulating layer 11 supported on a conductive backing 12 is uniformly charged by immersion in and removal from an electrolytic liquid 14 or other conductive liquid which is maintained at a desired charging potential by means of an electrode IS immersed therein. According to this figure, a suitable container 16 is at least partially filled with a conductive liquid such as, for example, an electrolytic liquid, such as water, an aqueous ionic solution, or the like, or with other liquid conductive material, including molten metal such as mercury as well as aqueous or other conductive solutions. An electrode or metallic conductor or the like is also immersed in the liquid or, if desired, at least one position on the walls of the container may be caused to be an electrode by an electrical connection thereto. This electrode is electrically connected to one pole of a voltage source 17 and the conductive backing member 12 of the surface being charged is electrically connected to the opposite pole.

Care is taken to insulate the conductive backing surface or member 12 from the charged electrolytic liquid 14 and this may be done by any suitable means such as, for example, masking or the like. Thus, the insulating layer 11 may be positioned in contact with the electrolytic liquid and supported in such contact by a supporting conductive backing member, which is masked by the same or other insulating material against actual contact with the electrolytic liquid. A simple method of obtaining such 5 contact is by floating the insulating surface face down on the surface of the electrolytic liquid while maintaining the electrical contact with the back surface. If desired, the insulating layer 11 may be placed against an open face of the container or may be otherwise immersed and sup10 ported therein while avoiding electrical contact between the electrolytic liquid 14 and the conductive backing member 12.

After the insulating layer is contacted with the conductive liquid, and while the potential source is still con15 nected across the backing member 12 and electrode 15, the layer 11 is removed from contact with the liquid.

In FIG. 2 is illustrated a chart of experimental results showing the relationship between the potential on electrode 15 and the potential attained on the insulating layer 20 11. Illustrated in the figure is the substantially straight line charging relationship according to which the potential applied to the insulating layer 11 within the range of substantially zero to about 300 volts is about 20 volts less than the potential imposed on the electrode 15. It is ap25 parent, therefore, that the insulator 11 becomes charged to substantially the potential of electrode 15 so that the charge differential between the insulating layer and the liquid is in the order of 20 volts. In contrast to this, the comparable differential between the charge on solid bodies 30 separated by the distances of ordinary pressure contact is generally in the order of 400 or 500 volts. It is apparent, therefore, and has been determined at higher potentials, as well as those included in FIG. 2, that it is possible to charge an insulating surface to a desired potential through the use of working voltages only a few volts higher than the potential which is to be applied by the methods and apparatus of this invention, whereas charging between solid surfaces in pressure contact is variable and uneven over ranges in the order of 400 to 500 volts between adjacent areas on the same insulator.

It is further to be observed that the insulating layer 11 may be any suitable insulator or photoconductive insulator. Thus, for example, in the art of xerography it is usual to employ a photoconductive insulator, and the in45 vention is particularly adapted to the appropriate charging of such a photoconductive insulator. This may include, for example, photoconductive insulators such as vitreous or amorphous selenium as well as photoconductive insulators such as anthracene, sulfur, and the like, 50 and film-forming binder materials containing photoconductive crystalline or solid materials such as, for example, the sulfides, oxides, and selenides of zinc, cadmium, and the like supported in organic film-forming binders. In addition, the invention is useful for the charging of insu55 lating surfaces such as, for example, non-self supporting or self supporting films or organic resins, plastics, binders, and the like, including cellulose and cellulosic materials and insulating resins such as lacquer coatings and the like and resin films and layers including urea- and melamine60 type resins, vinyl resins, acrylic resins, and the like. Favorable results have been achieved with certain resins such as polystyrene resins, polyester resins, and polyvinyl resins, all of which may have certain applications and uses in methods and procedures similar to or a part of 65 those methods and procedures conventionally used in xerography. Illustratively, as a specific example, the data in FIG. 2 were recorded with an insulating layer of Mylar, which is understood to be a polyethylene terephthalate. film supplied under this name by E. I. du Pont 70 de Nemours, Wilmington, Del.

In FIG. 3 is shown diagrammatically a portion of apparatus which may be employed for xerographic charging. Illustrated in this figure is a portion of a cylindrical xerographic machine including a xerographic cylinder 75 21 having a photoconductive insulating or other insu2,987,660 age source whereby the brush is maintained at a charging potential. According to the operation of this embodiment of the invention, the xerographic cylinder is moved past a charging station comprising the brush, container and like assembly, while the brush is rotated into contact with the cylinder. In this manner, the entire surface of the layer 22 to be charged is brushed by the liquid bearing brush 27 with the liquid being maintained at the desired charging potential.

An additional advantage forming a specific feature of the present invention is illustrated in each of FIGS. 3 and 5. In the normal applications of the art of xerography, it is conventional to form a developed or powder image on the surface of the xerographic photosensitive member, such as for example, xerographic cylinder 21. This surface is, accordingly, reusable and it is necessary to remove residual image from this surface prior to recycling through the xerographic process. It has been found in prior art and operation of xerography that removal of such residual image can be achieved by any of various methods, all of which heretofore have had specific disadvantages. One of the particular problems is that with repetitive use there tends to form on the xerographic surface a thin film of the image material, which in the course of hundreds or thousands of repetitive cycles tends to interfere with xerographic operations. As illustrated in FIGS. 3 and 5, the surface of the xerographic cylinder is actually brushed by the mechanically operating fibers of the brush 27, which fibers are wet with water or an aqueous liquid generally a non-solvent for and immiscible with the image material, and the resulting mild scrubbing with this liquid, such as for example, clean water, is one of the most effective methods of removal of residual image material between xero10 lating surface 22 coated on its surface. A charging liquid container 24 is positioned at the lower portion of the cylinder and is provided with liquid seal gaskets 25 or the like to maintain a liquid 26 within the container. Positioned within the container 24 is an agitator member 27 such as for example a rotating or rotatable brush which desirably may be maintained in contact with the surface of the xerographic cylinder. One member forming a part of the container 24 or its contents, which part may for example be brush 27, is electrically connected to an electric potential such as a battery or D.C. potential source adapted to supply a potential in the range of xerographic operating potentials, such as for example, about 100 to 1,000 volts. The xerographic cylinder is grounded, optionally by a ground connection at its axis of rotation, and the container is filled at least to the point of contact with the lower surface of the xerographic cylinder with a conductive or electrolytic liquid, preferably of relatively low viscosity. This liquid may be any conductive or electrolytic liquid as desired, but favorable results have been achieved with water, such as tap water, which contains sufficient impurities to cause it to be adequately conductive. In general it has been found that for the application of a charge, even of several hundred volts, to an insulating layer disposed on a conductive backing, there is an extremely small flow of electric current in the order of a fraction of a micro coulomb per square inch. Thus, to apply a charge to an area as large as a square foot on an insulating layer whose thickness is in the order of %ooo inch, the flow of electricity in normal electrolytic terms is negligible and for this reason it is not necessary that the electrolytic liquid have added ionic materials or impurities in order to make it additionally conductive and ionic materials are not detectably plated out on the insulating surface. It has been found, 35 graphic^cycles. therefore, that sufficient conductivity is present even in ^τ~ ordinary distilled water. Furthermore, when present commercial xerographic plates are employed, these plates having selenium photoconductor layers, water is particularly

In FIG. 5 is also illustrated an optional mechanism according to a further embodiment of the invention whereby water or other liquid particles which may be carried along with the xerographic cylinder can, if desired, be desirable* as* not wetting the selenium surface so that it 40 removed from the. cylinder surface before the cylinder is readily removable therefrom, as not entering into or promoting detectable chemical reaction with said surface, and as being easily and readily completely removed by evaporation. A further advantage in an electrolytic type is recycled into the xerographic operations. According to this embodiment, a cleaning material 32 optionally in belt form may be passed around a roller 33 into con______________tact with the surface of the xerographic cylinder. Suitaof liquid, and particularly in one of low conductivity 45 bly, a second roller 34 operated through a belt 35 under such as water, is that it has high resistance to surges of charge as might be occasional where there are pinhole defects in the insulator.

In FIG. 4 is shown additional detail on the liquid seal from the cylinder surface, and is either maintained at the same potential as the electrolytic liquid or is electrically at a floating potential, not grounded or otherwise caused to dissipate or remove electric charge.

FIG. 6 illustrates a xerographic machine according to one embodiment of the present invention. According to this figure, a xerographic cylinder 21 of the type disclosed in FIGS. 3 and 5 is rotated past a series of xerographic processing stations, including an exposure station 39, a developing station 40, a transfer station 41 and a charging station 42. The exposure station 39 may consist of suitable exposure means, such as for example, a slit projection system including a lens to project onto the cylinder surface, a slit of optical image, or the like. This may operate suitably from any image source, such as for example, a cathode ray tube, advancing microfilm, or other advancing documentary or three-dimensional image to be reproduced. Illustrated in the figure, the development station is a developer elevator 44 including drive motor 36 moves the belt material slowly so as to present always a clean surface to the xerographic cylinder. This belt material 32 desirably is of a water-ab______ ~________________________________ ________ sorbent or liquid-absorbent material, such as for example, mechanism according to FIG. 3. According to'this fig- 50 ^{a felted cl}°* °^T thefike in order to dry residual liquid ure, a portion of the xerographic cylinder 21 having at least on its outer surface a conductive surface or layer 21α has an outer layer or coating 22 of a photoconductive insulating or insulating material. Mounted on the nearest adjacent portion of the container 24 is a liquid seal 55 gasket 29 secured to the container wall by means of a plurality of rivets 30, screws or the like. The liquid seal gasket may be of any desired material, either insulating or conductive, and desirably it will be of the rubber-like material whereby durability and water sealing qualities θθ are achieved. Since the sealing gasket 29 is mounted on the container 24 which itself may be at the desired charging potential, it is immaterial whether the sealing mechanism is electrically conductive or electrically insulating.

In FIG. 5 is illustrated a modified embodiment of the invention wherein xerographic cylinder 21, according to FIG. 3, is positioned above and not immersed in the charging liquid 26. In this embodiment, the agitating member 27, such as for example a brush, is partially im- 70 buckets 45 mounted on a belt 46 adapted to carry demersed in the liquid 26 contained in container 24 and emerges from the liquid to brush into contact with the insulating or photoconductive insulating layer 22 on the surface of the cylinder. According to this figure, as in FIG.

3, the xerographic cylinder is connected to a suitable volt- 75 caded across the cylinder surface. Desirably the de65 veloper material from a catching hopper 47 to a developer dispensing hopper 48. Optionally, the elevator is driven by suitable means, such as for example, a drive motor 49, whereby a constant supply of developer is cas3,987,660 veloper in such a system may be a developer of the type disclosed in Walkup Patent 2,638,416.

The xerographic cylinder is then moved to the transfer station 41 where the developed image may be transferred to a transfer member, such as, for example, a roll of 5 paper 50 or similar sheet material on which the image is to be placed to form a xerographic print. Desirably, the roll of paper may be passed by suitable mechanism from a feed roll 51 to a take-up roll 52 passing between the cylinder and a transfer inducing member 53, such 10 as, for example, a conductive surfaced roller appropriately connected to an electric potential in order to transfer the image material to the print receiving material. Desirably the paper 50 may pass through a fixing mechanism, such as for example, an oven 54 or the like, which 15 may, if desired, include heating coils 55 adapted to heat the paper and image material or other solvent or heat fusing means to cause the image to melt or fuse to the paper surface.

For recycling through the xerographic operation, the 20 cylinder is then passed to a charging station 42 wherein it is charged and optionally cleaned of residual developer material. This charging station 42 may be the apparatus illustrated diagrammatically either in FIG. 3 or FIG. 5, or a suitable modification thereof. 25

The xerographic cylinder is suitably moved through these stations by a drive means such as a motor 57 operating on a belt 58 to drive the cylinder. In addition, it is to be understood that appropriate light shielding or other mechanisms generally employed in the art of xerog- 30 raphy may suitably be employed in the apparatus in FIG. 6.

In FIG. 7 is illustrated a xerographic machine according to another embodiment of the present invention. According to this figure, a xerographic cylinder 21 is 35 passed through similar exposure stations 39 and charging stations 42. In this case, however, there is a combined developing and transfer station 60 through which a print receiving material, such as a strip of paper 50, is passed. At this station 60 the electrostatic image on ⁴θ the xerographic cylinder is optionally directly developed on the obverse side of the paper 50 or optionally the electric image itself may by suitable means be transferred to the paper for subsequent development. In this manner, a xerographic print is formed on the print ma- 45 terial which passes from feed roll 51 to take-up roll 52, optionally again passing through an oven or like fusing means 54. According to this figure, as in the previous one, the charging station 42 may be the charging apparatus illustrated in either FIG. 3 or FIG. 5 or a suit- 50 able modification thereof.

It is to be understood that the present invention may be modified as is desired according to those skilled in the art. Specifically it is to be understood that suitable modifications and variations of this apparatus may be 55 employed according to conventional xerographic machine construction without departing from the scope of the present invention. Thus, in the art of xerography and related arts where it is desired to employ an appropriate system for charging a photoconductive insulating layer 60 or an insulating layer, the charging method, means and apparatus of the invention may be employed with suitable modification as will be apparent.

The invention has been described in connection with certain apparatus and equipment, but it is to be under- 65 stood that it is presently believed to be equally applicable to other form of equipment for xerography and related arts wherein charging of an insulating surface is desired. It is specifically to be understood that the invention is applicable to applying a charge to an insulating or photo- 70 conductive insulating surface for the formation of either line copy reproductions or continuous tone reproductions, and it is furthermore understood that for the more critical reproduction requirements as are generally encountered in continuous tone operation, the increased 75 uniformity of charging according to the present invention is believed to be particularly advantageous. In addition, the charging mechanism according to the present invention is more compact than those of the prior art and is adapted to operate with the minimum necessary potential which is substantially equal to the charging potential desired. Therefore, the present invention is particularly adapted to small-size machines and hand operated cameras. These and other modifications and variations in the invention may be employed without departing from the scope thereof, and, accordingly, the invention is to be limited only by the appended claims.