US3748127A - Treatment of reusable photoconductive surfaces with lewis acids or bases - Google Patents

Abstract

AN ELECTROSTATOGRAPHIC IMAGING SYSTEM EMPLOYING A LIQUID DEVELOPER AND A CYCLING OR REUSABLE PHOTOCONDUCTOR IS PROVIDED WHEREIN A SMALL AMOUNT OF A LEWIS ACID OR LEWIS BASE IS CYCLICALLY APPLIED TO THE PHOTOCONDUCTOR SURFACE. THE LEWIS ACID OR BASE IS PREFERABLY A CONSTITUENT OF THE LIQUID DEVELOPER AND ACTS TO MAINTAIN OR REJUVENATE THE ELECTRICAL PROPERTIES OF THE PHOTOCONDUCTOR.

Description

United States Patent U.S. Cl. 96-1 LY 5 Claims ABSTRACT OF THE DISCLOSURE An electrostatographic imaging system employing a liquid developer and a cycling or reusable protoconductor is provided wherein a small amount of a Lewis acid or Lewis base is cyclically applied to the photoconductor surface. The Lewis acid or base is preferably a constituent of the liquid developer and acts to maintain or rejuvenate the electrical properties of the photoconductor.

This is a continuation of application Ser. No. 838,328, filed July 1, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to imaging systems, and more particularly, to improved developer systems and techniques.

The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic xerographic process, as taught by C. F. Carlson in U.S. Pat. 2,297,691 involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as toner. The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently afiixed to a support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image directly be charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.

Similar methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. Included within this group are the cascade development technique disclosed by E. N. Wise in US. Pat. 2,618,552; the powder cloud technique disclosed by C. F. Carlson in US. Pat. 2,221,776 and the magnetic brush process disclosed, for example, in US. Pat. 2,874,063.

Development of an electrostatic latent image may also be achieved with liquid rather than dry developer materials. In conventional liquid development, more commonly referred to as electrophoretic development, an insulating liquid vehicle having finely divided solid material dispersed therein contacts the imaging surface in both charged and uncharged areas. Under the influence of the electric field associated with the charged image pattern the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This "ice electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration.

A further technique for developing electrostatic latent images is the liquid development process disclosed by R. W. Gundlach in US. Pat. 3,084,043 hereinafter referred to as polar liquid development. In this method, an electrostatic latent image is developed or made visible by presenting to the imaging surface a liquid developer on the surface of a developer dispensing member having a plurality of raised portions or lands defining a substantially regular patterned surface and a plurality of portions depressed below the raised portions or valleys. The depressed portions of the developer dispensing member contain a layer of conductive liquid developer which is maintained out of contact with the electrostatographic imaging surface. Development is achieved by moving the developer dispensing member loaded with liquid developer in the depressed portions into developing configuration with the imaging surface. The liquid developer is believed to be attracted from the depressed portions of the applicator surface in the charged field or image areas only. The developer liquid may be pigmented or dyed. The development system disclosed in US. Pat. 3,084,043 differs from electrophoretic development systems where substantial contact between the liquid developer and both the charged and uncharged areas of an electrostatic latent imaging surface occurs. Unlike electrophoretic development systems, substantial contact between the polar liquid and the areas of the electrostatic latent image bearing surface not to be developed is prevented in the polar liquid development technique. Reduced contact between a liquid developer and the nonimage areas of the surface to be developed is desirable because the formation of background deposits is thereby inhibited. Another characteristic which distinguishes the polar liquid development technique from electrophoretic development is the fact that the liquid phase of a polar developer actually takes part in the development of a surface. The liquid phase in electrophoretic developers functions only as a carrier medium .for developer particles.

While capable of producing satisfactory images these liquid development systems in general sufler deficiencies in certain areas and are in need of further development and improvement. Particularly troublesome difiiculties are encountered in liquid development systems employing a reusable or cycling photoconductor surface. Eln these systems a photoconductor such as a selenium or selenium alloy drum as the photoconductor surface is charged, exposed to a light and shadow image and developed by bringing the image bearing surface into developing configuration with an applicator containing developing quantities of liquid developer thereon. The liquid developer is transferred according to the appropriate technique from the developer applicator onto the image bearing surface in image configuration. Thereafter, the developer pattern on the photoconductor surface is transferred to copy paper and the liquid developer may be absorbed by the paper to form a permanent print. During the transfer operation not all the liquid developer is transferred to the copy paper and a considerable quantity remains on the photoconductor surface. The photoconductor drum on further movement may come in contact with a cleaning device which may be in the form of an absorbent web material. This absorbent web absorbs most of the liquid residue and smoothes out the remaining residue on the photoconductor surface to the configuration of a very thin layer. The photoconductor is then in position to commence the charge, expose, develop, transfer and clean sequence once again. On repeated cycling there is a progressive accumulation of liquid developer on the photoconductor surface since in each cycle not all the developer is transferred to the copy paper. This progressive accumulation of developer residue results in an overall loss of density, deterioration of fine detail and contributes to increased background deposits on the final copy particularly since accurate imaging on the photoconductor may be inhibited.

Procedures to remove the developer liquid from the surface of the photoconductor have been employed. However, to provide the necessary removal of ink film the cleaning step must be so severe and complete that there is a progressive degradation of the photoconductor surface lessening its useful life span. The severity of the cleaning step is dictated by the fact that in cleaning a film from a surface the film is progressively split so that on each separate cleaning about one half the film remains on the photoconductor surface. The cleaning solvents generally necessary to provide adequate cleaning frequently are major contributors to the chemical attack of the photoconductor surface. In some instances and with complete removal of the ink film the electrical prop erties of the photoconductor are virtually destroyed by the cleaning operation after only a small number of cycles.

Particularly troublesome is the fact that with prolonged cycling and cleaning the electrical properties of the photoconductor are progressively destroyed. Particularly noticeable is the fact that the photoconductor will accept and retain less and less charge resulting in progressively deteriorating resolution of the developed image and transferred copy prints. These disadvantages are particularly noticeable when employing a selenium or selenium alloy photoconductor. With complete cleaning of the imaging surface after the first cycle, which produces a print of good quality, the print obtained in the second cycle is already of markedly poorer quality and by the time the print from the third or fourth cycle is obtained little image contrast may be observed to remain. In addition, the more severe the cleaning operation, the longer it may generally take to obtain adequate cleaning and the greater the possibility of damage to the photoconductor. It is, therefore, clear that there is a continuing need for a better liquid development system employing a cycling or reusable photoconductor.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a developing system which overcomes the above noted deficiencies.

It is another object of the invention to provide a continuous liquid development system having increased photoconductor life span.

It is another object of this invention to provide a liquid development system employing a photoconductor capable of repeated recycling.

It is another object of this invention to provide a liquid development system permitting relative ease in cleaning photoconductor surfaces.

It is another object of this invention to provide a liquid development system wherein the photoconductor may be repeatedly recycled without substantial loss of electrical properties.

It is another object of this invention to provide an additive which will permit increased photoconductor life.

It is another object of this invention to provide a liquid developer which will enable recycling of a photoconductor without substantial loss in print quality.

It is another object of this invention to provide a method for stabilizing the electrical properties of a photoconductor.

It is another object of this invention to provide a method for rejuvenating a recycling photoconductor.

It is another object of this invention to provide a liquid development system superior to known systems.

It is another object of this invention to provide a liquid developer superior to known liquid developers.

The above objects and others are accomplished, generally speaking, by providing an electrostatographic imaging system wherein an additive is added to the surface of a cycling or reusable photoconductor to improve the cycling ability of the photoconductor. More specifically, in a liquid development system employing a reusable photoconductor a small amount of a Lewis acid or Lewis base may be added directly to the liquid developer as a constituent thereof or it may be added directly to the surface of the photoconductor. In the latter instance the Lewis acid or base is generally added in the presence of excess developer remaining on the photoconductor surface after the transfer of developer to copy paper in any one cycle.

The additives of this invention may generally be described as Lewis acids and Lewis bases which are compatible with the remainder of the constituents of the developer and which have no otherwise deleterious effect on the photoconductor. Preferably, the additives do not render the developer too electrically conductive when employed as a constituent of the developer on the photoconductor or too electrically conductive when separately added to the photoconductor surface because then the recharging of the photoconductor becomes too diflicult and the lateral conductivity becomes excessive for good image resolution.

For any particular photoconductor the additive is spe cially selected between the two groups of materials, Lewis acids and bases. Generally for photoconductors which are positively charged, Lewis bases are employed and for phtotoconductors which are negatively charged Lewis acids are employed.

Typical examples of Lewis bases include among others the triarylmethane dyes such as crystal violet, malachite green, para-rosaniline, basic fuchsin, xanthene dyes such as rhodamine B, eosin, erythrosine and fluoroscein; aniline dyes such as nigrosine and aniline black, soluble porphyrin derivatives such as tetraphenyl porphine, and copper chlorophyllin; thionine dyes such as methylene blue and thionine; amines such as triphenylamine, polycyclic aromatic and heterocyclic compounds such as anthracene, pyrene, fluorene, acridene, carbazole, and their basic derivatives; aromatic hydroquinones, diamino phenyl oxazoles and triazines.

Typical Lewis acids include among others indanthrone dyes such as anthrazol blue IBC; azo dyes such as naphthol blue black B, be'nzoazurin G; aromatic compounds such as 2,4,7 trinitro fiuorenone, tetrachloro phthalic anhydride, chloranil, fluoranil, anthraquinone, and 2-dicyanomethylene-1,3-indanedione.

Many of the Lewis acids and bases listed above are colored dyes and it is generally preferred to employ such materials since they incidentally provide additional colorant to the developer.

The Lewis acids and bases of this invention may be employed in any suitable liquid developer which may comprise one or more vehicles, one or more pigments, dispersants and other materials.

While developers which are somewhat conductive may be employed, it is preferred to employ a relatively insulating developer in development systems which employ a reusable or cycling photoconductor in order to provide adequate charge retention on the photoconductor. If, for example, a highly conductive developer is employed it may tend to discharge a charged portion of the photoconductor when placed on the photoconductor. Further, since a thin film of developer residue may be present on the surface of a recycling photoconductor adequate recharging of the photoconductor for each cycle and adequate image definition by the charged portions after exposure are difficult to maintain. For these reasons it is preferred to employ a vehicle having a conductivity less than about 10 (ohm-cm).

Any suitable vehicle contributing to these properties may be used. Typical vehicles within this group include mineral oil, the vegetable oils including castor oil and its oxidized derivatives, peanut oil, coconut oil, sunflower seed oil, corn oil, rapeseed oil, and sesame oil. Also included are mineral spirits, fluorocarbon oils such as Du Ponts Freon solvents and Krytox oils; silicone oils, kerosene, carbon tertrachloride, toluene, oleic acid and fatty acid esters.

In addition to the above vehicles an auxiliary or secondary vehicle may be employed to impart or adjust any one or more of the properties of the liquid developer. Any suitable material may be employed as the secondary vehicle and may, for example, have dispersant properties, contribute to viscosity adjustment, or confer wetting properties to the pigment employed or act as a fixing agent. In addition, the secondary vehicles preferably exhibit properties in common with the principal vehicle of being nonodorous, non-hygroscopic and of low volatility to provide a stable developer with a non-offensive odor. An additional function of secondary vehicles may be to help the developer penetrate into the copy paper. Typical materials that may be employed as secondary or primary vehicles include dibutyl phthalate, butyl isodecyl phthalate, butyl octyl phthalate, diisooctyl phthalate, di(2-ethyl hexyl) phthalate, isooctyl isodecyl phthalate, normal octyl decyl phthalate, diisodecyl phthalate, ditridecyl phthalate, isodecyl tridecyl phthalate, diisooctyl adipate, di(2-ethyl hexyl) adipate, isooctyl isodecyl adipate, normal octyldecyl adipate, diisodecyl adipate, diisooctyl sebacate, di 2- ethyl hexyl sebacate, polyadipate ester, isooctyl palmitate, butyl stearate, butyl oleate, triethylene glycol dicaprylate, triethylene glycol caprylate-caprate, triethylene glycol dipelargonate, diethylene glycol dipelargonate, butanediol dicaprylate, tr iisooctyl trimellitate, tri Z-ethyl hexyl trimellitate, and mixed normal trialkyl trimellitates.

Any suitable colorant may be employed in the developer including both pigments and dyes. It is preferred that the colorant be fast to light in order to obtain image permanence. Typical pigments include carbon black, charcoal and other forms of finely-divided carbon, quinacridones, phthalocyanine blues, iron oxide, ultramarine blue, zinc oxide, titanium dioxide, and benzidine yellow. Typical dyes include Irgracet Black RI, oil red, oil blue, and oil yellow. Preferred pigments in obtaining maximum dispersion of the pigment include surface resinated carbon blacks such as Microlith CT manufactured by Ciba.

A dispersant is generally employed to aid in dispersing the pigment and other additives in the vehicles. Any suitable dispersant may be employed that is compatible with and soluble in the vehicle. Typical materials include alkylated polyvinyl pyrrolidone and wood rosin derivatives such as Staybelite ester, manufactured by Hercules Powder Co. and interpolymers of n-octadecyl vinyl ether and maleic anhydride, such as Gantrez AN 819 4 manufactured by GAF Corp.

Additional materials may be added to the developer for particular functions such as, for example, viscosity controlling additives or additives which contribute to fixing the pigment on the copy paper may be employed.

The proportions of the several constituents in the developer may be varied over a wide range depending on individual properties of the constituents and operational considerations of the specific development scheme. A significant factor in determining proportions is the speed of development since with higher speeds lower viscosity developers must be used than at lower speed. One skilled in the art may readily determine the appropriate viscosity for any given development speed.

In liquid development systems having development speeds of from about 5 to about 20 inches per second, for example, developer viscosities of from about 300 to about 1800 centipoises measured at 25 C. are preferred to provide ease of operation and desired print quality.

The viscosity is in part dependent on the pigment loading of the vehicle. As more pigment is added the viscosity of the developer increases and operable development speed is lowered. The balance between pigment loading to obtain maximum image density and development speed to maintain maximum development speed may be readily determined by one skilled in the art.

The several constituents may generally be present in a developer in amounts according to the following weight percentages:

Vehicle (including principal and secondary vehicle):

from about 40 to about 90 Wt. percent Colorant pigment or dye: up to about 60 wt. percent Dispersants: up to about 20 wt. percent Lewis acid or base: from about .05 to about 1.0 wt.

percent Typical developers have the following composition by weight:

Total vehicle: from about 40 to about wt. percent Colorant: from about 15 to about 60 wt. percent Dispersants: from about 1 to about 20 wt. percent Lewis acid or base: from about .05 to about 1.0 wt.

percent The Lewis acid or base should be present at least in amounts necessary to yield the superior and unexpected recycling ability herein disclosed. They may be soluble or insoluble in the developer. For better cycling characteristics it is prefered that they at least be molecularly dispersed in the developer in order to insure complete availability to the entire photoconductor surface and to insure maximum rejuvenation and stabilization of the photoconductor. To provide maximum stabilization of the photoconductor the Lewis acid or base preferably is at least partially soluble in the developer vehicles. Optimum restoration of the electrical properties of the photoconductor is obtained when the additive is present in amounts to form saturated solutions with the developer vehicle.

Within the broad range of proportions set forth above a prefererd range of proportions for the constituents of the developer providing good print quality and ease of operation at development speed of from about 2 to about 20 inches per second are the following:

Total vehicle: from about 65 to about 85 wt. percent Primary vehicle: from about 20 to about 85 wt. percent Secondary vehicle: from about 0 to about 45 wt. percent Colorant: from about 15 to about 40 wt. percent Dispersant: from about 5 to about 15 wt. percent Lewis acid or base: from about 0.1 to about 0.50 wt.

percent Particularly preferred recycling ability with a selenium surface photoconductor is obtained with a developer composition containing about 0.3% by Weight of the entire developer of nigrosine in a developer having a conductivity from about 10- to 10* (ohm-cm.)-

The developers of this invention may be prepared by simply mixing the several constituents. However, to provide homogeneity it is generally prefered to combine the constituents of the vehicle first while heating them and then adding dispersant, pigment or dye and additive. The pigment may be comminuted separately or together with the vehicle. In addition, suitable control, suspending and fixing agents as are well known in the art may be added in conventional manner.

In addition to providing improved cycling ability to recycling photoconductors the addition of the Lewis bases and acids of this invention also provide greater ease in the periodic cleaning of the photoconductor surface.

Development of an electrostatic latent image may be achieved with any suitable development technique. A preferred technique producing better quality images employs the use of a patterned applicator surface having a substantially uniform distributor of raised portions or lands and depressed portions or valleys as described in U.S. Pat. 3,084,043 and discussed above. In this development technique the liquid developer is present in the depressed portions of the applicator surface while the raised portions are substantially free of developer. During development the liquid developer is pulled from the depressed portions to the photoconductor in image configuration. A gravure roll applicator having a patterned surface of from about 100 to about 300 lines per inch is a typical applicator for this technique.

The mechanism behind which the Lewis acid and bases function to yield the surprisingly superior recycling ability is not fully understood. It is observed, however, that with the addition of the additives of this invention that the photoconductors maintain their ability to accept and hold charge for every cycle. It is further surprising that such a small quantity of the additives of this invention provides such strikingly improved recycling ability. In some cases, as may be observed from the comparative examples herein set forth, the addition of a Lewis acid or base to the developer actually provides the essential means by which the photoconductor can be continuously recycled yielding prints of acceptable quality on each cycle. In some cases the Lewis acid or base may be adsorbed onto the photoconductor surface and due to an afiim'ty between the additive and the photoconductor will remain there despite a simple cleaning by wiping.

As previously stated the Lewis bases are effective agents generally for photoconductors that are charged positively while the Lewis acids are effective generally for photoconductors that are charged negatively. All those photoconductors that may be employed in a development system which uses a reusable or cycling photoconductor possess the problems discussed above to varying degrees and the novel developers of this invention are effective in minimizing these problems.

Typical photoconductors that may be positively charged include selenium and selenium alloys, polyvinyl carbazole sensitized with 2,4,7 trinitrofluorenone according to the technique of US. Pat. 3,307,861 and phthalocyanine binder coatings as described by J. F. Byrne in US. application Ser. No. 375,191, filed June 15, 1964, now abandoned. Typical photoconductors that may be negatively charged include cadmium sulfo selenide and cadmium sulfide binder coatings. The photoconductor may be employed in any suitable structure including plates, endless belts and drums and may be employed in the form of a binder layer.

In the recycling photoconductor systems of the present invention any suitable cleaning system may be employed. A typical cleaning system scrubs the ink film on the photoconductor surface obliterating the image pattern by smearing the developer over the surface. The residual developer is subsequently picked up by an absorbent web which may absorb the developer. For example, a squeegee roller may be used as the scrubbing or obliterating device and an absorbent web wrapped around a portion of the photoconductor drum and moving slowly counter to the direction of rotation of the drum may be used. It is preferred that the residual developer left on the photoconductor surface after transfer of developer to copy paper be smeared or spread across substantially the entire photoconductor surface to thereby insure contact of substantially the entire surface with the developers of this invention. Without this preferred smearing, rejuvenation of the photoconductor substantially only in the charged areas from some preceding cycle will be attained. The cleaning operation is a particularly advantageous operation to pro- 'vide this uniform distribution of developer on the photoconductor surface since both this function and cleaning may be accomplished with the same material in the same operation. While acceptable rejuvenation of the photoconductor may be obtained with periodic cleaning on every other cycle for example, best results are observed when the photoconductor is cleaned on every cycle.

Alternative techniques for supplying the additives of this invention include providing a separate applicating step after transfer of developer to copy paper, for the additives which may be applied directly to the photoconductor in liquid form. This may take the form of a roller or web type applicator. In addition the additives may be supplied directly to the photoconductor during the cleaning operation. They may, for example, be supplied from a cleaning web.

DESCRIPTION OF PREFERRED EMBODIMENTS The following preferred examples further define, describe and compare preferred materials, methods and techniques of the present invention: Examples II, III, VII are included for comparative purposes to show the surprisingly superior and unexpected results obtained in the practice of this invention. In the examples all parts and percentages are by weight unless otherwise specified.

Example I A commercial Xerox Type E vitreous selenium xerographic plate (Type E available from Xerox Corporation, Rochester, N.Y.) comprising a surface layer of selenium about 50 microns thick on a conductive aluminum plate is positively charged with a potential of 500 volts and exposed to a light and shadow image in conventional manner. The electrostatic latent image thus formed is developed by moving a patterned surface applicator roll having developing quantities of developer in the depressed portions thereof past the image bearing surface so that liquid developer is pulled out of the depressed portions to the image bearing surface in image configuration. The speed of development is about 10 inches per second. The developer employed is of the following composition:

Parts by weight Drakeol 9 is a mineral oil manufactured by Pennsylvania Refining Co. having a kinematic viscosity of about 45.7-18.1 centistoke at 25 Co. and a specific gravity of about 0.85. Microlith CT is a resinated predispersed carbon black pigment composed of about 40% carbon black pigment and 60% ester gum resin, manufactured by Ciba. Paraflint RG is a hard synthetic wax available in flake form from Moore & Munger. Ganex V21 6 is an alkylated polyvinyl pyrrolidone compound manufactured by GAF Corp. which serves as additional pigment dispersant and may also be regarded as a secondary vehicle. Nigrosine S81] is a spirit soluble nigrosine dye manufactured by American Cyanamid.

The developer is prepared by combining the mineral oil and Ganex V216 in a suitable vessel while stirring, heating to about C. and then adding the pigment and other ingredients while continuing the stirring.

The developer on the photoconductor is transferred to bond paper in image configuration. The selenium plate is then wiped with absorbent cotton to spread the residual developer over the entire surface and is then manually cleaned with absorbent cotton substantially to a bare selenium surface removing substantially all residual developing liquid from the selenium surface. The resolution of the first print obtained is about 10 line pairs per millimeter. The clean photoconductor plate is again subjected to the same cycle of charging, exposing, developing, transferring and cleaning. The resolution of the second print is also about 10 line pairs per millimeter. After 250 prints no significant change in print quality is observed.

9 Example II the prints obtained is observed to gradually decrease from.

about lines pairs per millimeter on the first print to 8 line pairs per millimeter for the second, 4 line pairs per millimeter for the fourth, 3 line pairs per millimeter for the tenth and 1 line pair per millimeter for the fifteenth.

Example III The procedure employed in Example II is repeated except that after each cycle the selenium plate is not cleaned down to the selenium surface. Results similar to those obtained in Example II are obtained on repeated cycling.

Example IV The procedure of Example II is repeated except that after each transfer of developer from the selenium plate to the copy paper a small quantity of nigrosine in an ethanol solution is applied to the selenium plate and uniformly distributed over the plate by wiping with a cloth. The plate is then cleaned down to the selenium surface by removing substantially all the residual liquid on the plate. Both the first and the fiftieth print obtained in this manner have a resolution of about 10 line pairs per millimeter.

Example V The procedure of Example I is repeated except that about 0.3 part by weight of triphenyl amine is substituted for nigrosine. Results after repeated recycling similar to those obtained in Example I are observed.

Example VI A xerographic plate comprising a surface layer of cadmium sulfo selenide glass binder layer 150 microns in thickness coated on a stainless steel foil sheet and prepared in the manner disclosed in US. Pat. 3,151,982 is charged negatively to a potential of 600 volts and thereafter exposed to a light and shadow image in conventional manner. The resultant electrostatic latent image is developed in the manner described in Example I except that the nigrosine is replaced by an equal quantity of 2- dicyanomethylene 1,3 indanedione. The developer is transferred and the xerographic plate cleaned in the manner described in Example I. The resolution of the first print obtained is about 10 line pairs per millimeter. The clean cadmium sulfo selenide plate is again subjected to the same cycle of charging, exposing, developing, transferring and cleaning. The resolution of the second print is also about 10 line pairs per millimeter. After 250 prints no significant change in print quality is observed.

Example VII The procedure of Example VI is repeated with a fresh cadmium sulfo selenide glass plate of the same type except that the Z-dicyanomethylene 1,3 indanedione is omitted from the liquid developer. The first print is observed to have good contrast and a resolution of about 10 line pairs per millimeter. Upon repeated cycling the resolution of the prints is observed to drop to 6, 4, 3 and 2 line pairs per millimeter respectively in the third, tenth, thirtieth and one hundredth print.

Example VIII The procedure of Example I is repeated except that q 10 15, 1964, now abandoned. The plate is charged positively with a potential of 400 volts and exposed to a light and shadow image in conventional manner. The resultant electrostatic latent image is developed in the manner described in Example I with the following composition:

Parts by weight Light mineral oil 45 Microlith CT 27 Ganex V216 23 Methyl violet tannate 5 Parafiint RGW 1 Triphenyl amine 0.4

Example IX The procedure of Example I is repeated except that the selenium plate is replaced by a xerographic plate comprising a photocoductive layer of polyvinyl carbazole which is negatively charged to 500 volts and exposed to a projected light and shadow image in conventional manner. The electrostatic latent image is developed in the manner described in Example I 'with the developing composition described in Example VIII except that the triphenyl amine is replaced by an equal amount of tetrachloro phthalic anhydride. The developer on the photoconductor is transferred to Xerox 4024 copy paper in image configuration. Thereafter the photoconductor is cleaned and subjected to the cycling procedures as described in Example I. The resolution of the first print obtained is about 10 line pairs per millimeter. The resolution of the second and one hundredth print is also about 10 line pairs per millimeter.

Example X The procedure of Example I is repeated with the following composition:

Pale is castor oil supplied by Baker Castor Oil Co. Rucoflex TG-8 is triethylene dicaprylate supplied by Hooker Chemical Co. With repeated cycling according to the technique of Example I prints of similar quality are obtained.

The above comparative examples show the surprisingly superior and unexpected results in terms of electrical properties and cycling ability of photoconductor. Comparison of Examples I and II clearly show the distinct advantages of employing the additives of this invention as constituents of the developer. Example III demonstrates that a simple cleaning of photoconductor surface does not permit acceptable cycling ability. Example IV demonstrates that the additives of this invention may be added directly to a photoconductor surface. Examples V to IX demonstrate the effectiveness of several additives of the invention when employed with different photoconductors.

Additional advantages of the materials and techniques of this invention include the relatively simple operation of a system for restoring and/or stabilizing the electrical properties of a photoconductor with simple control over the quantity of additive needed. Also, the necessary restoration may be accomplished in every cycle without disruption of the continuous operation and without the use of additional procedures, techniques or equipment.

In addition, the rejuvenation is accomplished at the same speed as the development is obtained and the system may, therefore, be capable of high speed development.

Although specific materials and operational techniques are set forth in the above exemplary embodiments using the developer composition and development techniques of this invention, these are merely intended as illustrations of the present invention. There are other developer materials and techniques such as those listed above which may be substituted for those in the examples with similar results.

Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure which modifications are intended to be included within the scope of this invention.

What is claimed is:

1. A method of cyclically developing electrostatic latent images on a reusable photoconductor comprising the steps of forming either an electrostatic negatively or positively charged pattern on a reusable photoconductor by placing a uniform electrostatic charge on the photoconductive insulating layer and exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light, developing the electrostatic charge pattern on the photoconductor with a composition comprising a liquid developer having an electrical conductivity of less than about (ohmcm.)- said developer containing a Lewis acid when said photoconductor is charged negatively and containing a Lewis base when said photoconductor is charged positively, said Lewis acid and Lewis base being present in a proportion effective to stabilize the electrical properties of said reusable photoconductor, transferring the developer from the photoconductor to a receiving surface in image configuration, substantially uniformly distributing the residual developer over the photoconductor surface, cleaning the surface to prepare it for the next imaging cycle and repeating the method sequence at least one additional time.

2. The method of claim 1 wherein said composition comprises by weight from about 40-80% of a total liquid vehicle, from about 15-60% of a colorant and from about 0.05-1% of the Lewis compound.

3. The method of claim 1 wherein said Lewis base is nigrosine.

4. The method of claim 3 wherein said photoconductor is selected from the group consisting of selenium and selenium alloys.

5. In an imaging system comprising placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light-and-shadow image to dissipate the charge on the areas of the layer exposed to the light, developing the resulting electrostatic latent image, transferring the image, and cleaning the photoconductive surface, the method of improving the electrical properties of the reusable photoconductor surface comprising substantially uniformly distributing before cleaning over the surface of the photoconductor a Lewis acid when said photoconductor accepts a negative charge and a Lewis base when said photoconductor accepts a positive charge.

References Cited UNITED STATES PATENTS 3,084,043 2/1963 Gundlach 961 LY 3,362,907 1/1968 Matkan et al 252-62.1 3,383,316 5/1968 Matkan 25262.1 3,427,247 2/1969 Peck 25262.1 X 3,081,263 3/1963 Metcalfe et al 96-1 R X 3,150,976 9/ 1964 Johnson 252-62.1 X 3,296,140 1/1967 Zabiak 252-62.1 3,399,140 8/1968 Fischer et al. 25262.1

FOREIGN PATENTS 1,057,432 2/1967 Great Britain 25262.1

ROLAND E. MARTIN, JR., Primary Examiner US. Cl. X.R.

961.4; 1l737 LE; 256--62.l