US 3808026 A
Electrophotographically produced electrostatic latent images are developed with minimal toner coloration of background or highlights. Residual electrostatic charge or potential is neutralized before development by a uniform application to the latent image surface of particles carrying a charge opposite in polarity from that of the electrostatic image. The quantity and population density of these charged particles are selected to just neutralize said residual potential. These particles are either colorless or have a coloration that is non-contrasting with the image background. After neutralizing the residual potential, development of the latent image is effected by colored toner particles, which will deposit in the image charge areas only.
Description (OCR text may contain errors)
United States Patent [191 Sato et a1. 1
[451 Apr. 30, 1974 LIQUID DEVELOPMENT OF ELECTROSTATIC LATENT IMAGE  Assignees Xerox Corporation, Stamford, Conn.
 Filed: Mar. 13, 1972  Appl. No.: 234,192
 US. Cl. 117/37 LE, 96/1 LY, 355/10 51 Int. Cl C03g 13/10  Field of Search 117/37 LE; 96/1 LY; 355/10; ll8/D1G. 23, 637
 References Cited UNITED STATES PATENTS 3,527,684 9/1970 York et a1 117/37 LE 3,081,263 3/1963 Metcalfe et al. 96/1 LY 3,212,888 10/1965 Weugebauer 117/37 LE 3,010,842 11/1961 Richer 117/37 LE 3,300,410 H1967 Oliphant 1 17/37 LE 2,991,754 7/1961 Johnson 355/10 Metcalfe et a1 96/ 1.5
3,212,887 lO/1965 Miller et a1. 96/1.2 3,236,776 2/1966 Tomanek 96/1 LY 3,262,806 Gource l17/l7.5
Primary ExaminerWilliam D. Martin Assistant Examiner-M. Sofocleous  ABSTRACT Electrophotographically produced electrostatic latent images are developed with minimal toner coloration of background or highlights. Residual electrostatic charge or potential is neutralized before development by a uniform application to the latent image surface of particles carrying a charge opposite in polarity from that of the electrostatic image. The quantity and population density of these charged particles are selected to just neutralize said residual potential. These particles are either colorless or have a coloration that is non-contrasting with the image background. After neutralizing the residual potential, development of the latent image is effected by colored toner particles, which will deposit in the image charge areas only.
7 Claims, 4 Drawing Figures LIQUID DEVELOPMENT OF' ELECTROSTATIC LATENT IMAGE INTRODUCTION AND BRIEF SUMMARY OF INVENTION The present invention relates to the development of electrostatic latent images, and more particularly it is concerned with the elimination or minimizing of toner deposition onimage background or highlight areas.
In electrophotography it would be desirable for the charge on electrophotographic surfaces to be discharged completely in the image highlight or background areas. In practice however, a residual charge or potential is usually present in these areas, and on development of the electrostatic latent image, this residual charge causes undesirable toner deposition, resulting in background coloration and dull highlights. In one .known method of preventing toner deposition due to residual potential, a bias potential is applied from an external source to the conductive backing of the electrophotographic coating during development, tocompensate or neutralize the residual potential. The bias potential approach complicates the developing apparatus, and it hasa further disadvantage in that toner deposits on the counter electrode, and this tends to smudge the image surface of the electrophotographic material being developed.
In accordance with the present invention the foregoing disadvantages are largely overcome, and developed prints are produced with bright highlights and minimal background toner coloration. These results are attained by applying uniformly over the electrostatic latent image bearing surface of the electrophotographic material, a dispersion of colorless charged particles and causing the particles to uniformly adhere to the image surface area. These charged particles have a charge of opposite polarity from that of the latent image, and are deposited in a quantity and population density selected substantially to just neutralize the residual potential. This first deposition is followed by a .usual development process employing conventional colored toner particles. Since the colorless particle deposit substantially neutralizes the residualpote'ntial over the image surface, deposition of colored toner on the highlight and background areas during the developmentprocess is essentially eliminated. It is stated that the charged particles are colorless. This condition is preferred, but obviously the particles could have a coloration that is not contrasting with the background coloration. For example, if the photoelectric image surface is white, these particles could be white, if desired.
It is apparent that deposition of the colorless particles must be uniform over the entire image area, and it must also be controlled so that it effects only aneutralization of the residual potential. Care must be taken to avoid preferential deposition of this material in development quantities over theimage charge areas, for such deposition would diminish the density of the image attainable in the development step. Accordingly, pursuant to the present invention a suspension of the neutralizing particles in a liquid vehicle is applied as a very thin uniform film over the entire latent image bearing surface, and
at the same time, a development electrode is placedas close as possible and in parallel relation to said surface so as not to allow the particles to deposit preferentially onto the areas of high charge density. The presence of maximum density over background obtained by subsequent development with colored toner.
It is accordingly one object of thepresent invention to provide for reduced'background deposition of toner and brighter highlights'in the development of electrostatic'latentimages on electrophotographic surfaces.
Another object of thepresent invention is to provide such results by neutralizing the residual, potential on the electrophotographic surfaces.
And still another object of the present invention is to effect such neutralization by the uniform deposition overjthe latent image surface of charge neutralizing particles that do not affect'the color rendition of the developertoner particles and do nothave a coloration that'contrasts with that of the background.
Other objects and advantages of the present invention will become apparent to those skilled in the art.
from a consideration of the following detailed description of exemplary embodiments of the invention, had
in conjunction with the accompanying drawings in which like numerals refer to like or corresponding parts, and wherein:
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a three part drawing depicting an image charge pattern on an electrophotographic surface in part A, the application thereto of residual potential neutralizing particles in part B, and the application of developer toner thereto in part C and FIG. 2 is a schematic representation of apparatus for practicing the present invention.
DETAILED DESCRIPTION Referring to FIG. 1, numeral 10 designates a conventional sheet of electrophotographic paper having a photoconductive surface on one side. As is well understood in the art, an electrostatic latent image is formed thereon by first applying a uniform electrostatic charge to the photoconductive surface, and then exposing the surface to an optical image while the back of the sheet 10 is grounded..Asa result, the electrostatic charge is discharged in the illuminated areas and retained in the non-illuminated areas, providing an electrostatic latent image.
A negative charge electrostatic latent image is depicted in FIG. 1A. Portion ll depicts an area that was illuminated during the exposure step, and the charge is removed therefrom except for a small residual charge or potential. During development, a small amount of toner would adhere to portion 12, thereby toning the background, if area 12 is background, or dulling the highlight, if area 12 is a highlight.
In accordance with the present invention, this residual potential is neutralized by applying a small quantity of positively charged particles uniformly over the photocond'uctive surface of sheet 10. This result is depicted in FIG. 18, where particles 13 have been uniformly applied over the entire latent image. The concentration of particles 13 is selected-so as just to neutralize the residual charge in area 12 of sheet 10, as shown in FIG. 1B. A similar concentration ofcharged particles 13 is also present in area 11, but since the electrostatic charge in area 11 is so much higher than in area 12, area-11 remains unneutralized. The unneutralized charge in area 11 corresponds to the full image charge, since only the non-image residual charge portion has been neutralized.
In order not to interfere with the subsequently developed image, and indeed to improve it as is the purpose of this invention, the charged particles 13 are selected to be either colorless or of the same color as the surface of paper 10. r
After the residual charge is neutralized by application of particles 13, the sheet is developed in a conventional way, and colored toner particles 14 are caused to adhere to the surface of sheet 10 in the charged area 11, as depicted in FIG. 1C. No toner particles adhere in the area 12, because all residual charge has been fully neutralized by particles 13. Thus, the background or highlight area 12 remains clear.
FIG. 2 schematically shows an apparatus for practicing the process described above and illustrated in FIG. 1. Tank contains a dispersion or suspension 21 of positively charged colorless particles 13 in an electrically insulating liquid vehicle. A series of rolls 22 are partially immersed in the dispersion 21 and are paired with companion pressure rolls 23. Photoconductive sheet 10 is caused to enter the nips of the paired rollers 22 and 23 and to pass therethrough with its electrostatic latent image surface faced downward toward rolls 22, rolls 22 and 23 rotating in the directions indicated by the associated arrows. As rolls 22 rotate, each carries a film of dispension 21 on its surface to the image surface of sheet 10, causing a controlled and uniform deposition of particles 13 on the surface of sheet 10. Rolls 22 are preferably polished stainless steel and function as development electrodes. By this operation, and by control of the concentration of particles 13 in the dispersion 21 and control of the thickness of the dispersion film carried on the rolls 22, the residual charge or potential on the image surface of electrophotographic sheet' l0 is substantially exactly neutralized by the particles 13, deposited on sheet 10, as depicted in FIG. 1B.' 7
After the sheet 10 passes the rolls Hand 23, it follows a feed path indicated by the dashed line 24 and the arrow 28. Any excess dispersion 21 carried by sheet 10 v is removed by squeeze rolls 25. The sheet 10 is then directed and fed by rolls 26 into developer tank 29 containing a conventional developer suspension 30 of colored toner particles 14 in an electrically insulting liquid In the foregoing operations, in order to obtain a completely uniform application of charged particles 13 to the surface of sheet 10, it would be necessary to have an infinitely small gap between rolls 22 and the surface of sheet 10. However, as a practical matter a finite gap is always necessary in order to accomodate the presence of a film of dispersion 21. Therefore, it is impossible to eliminate completely the tendency of the toner particles 13 to adhere somewhat preferentially to the areas of higher or image charge density. From a practical point of view, however, such preferential deposition is negligible, since the amount of toner deposition in the actual development step remains quitedominant. In order to realize a more perfectly uniform deposition of charged particles 13, one may resort to a method in which a bias potential far larger than the maximum potential in the latent image is applied to the development electrode during the charged particle deposition. It will of course be appreciated by those skilled' in the art that before sheet 10 is treated by despersion-21, it may be prewetted with a suitable electrically non-conducting liquid free of any particles or toner, to prevent mechanical absorption of the dispersion 21 into the porous paper sheet 10.
To insure proper operation of the present method, it is of course necessary that the charge forming the latent image should decay at only a very slow rate during the processing steps. Since the first coating step does not contribute to the optical density of the finally developed image,'but rather to the reduction of highlight or background coloration, dissipation of charge in this phase of the process must be minimized. The rate of charge decay on the surface of sheet 10 depends largely on the composition of the photoconductive insulating coating and on the treating liquid with which the surface is treated. The decay rate is smaller in the more insulating treating liquids, and in those which have a lesser tendency to dissolve or swell the resinous ingredients included in the photoconductive coating. A variety of resinous materials are suitable for the photoconductive coating, when during this development process the coating is wetted with insulating liquids having low dissolving power, such as halogenated and isoparaffinic hydrocarbon solvents. When using insulating liquids such as Decalin, Tetralin, kerosene, or'cyclohexane, which have a higher dissolving power, resinous photoconductor compositions having polar structures are desired. Suitable examples include alkyd, epoxyester, or linear polyester resins cross-linked by suitable reagents, such as polyisocyanate compounds. With photoconductive insulating coatings having these binder compositions, surface charges decay very slowlyin the insulating processing liquids.
The dispersion 21 used to effect the uniform coating of charged particles can be prepared according to the methods known for the preparation of conventional liquid electrophoretic developers used in electrophotography. Suitable white pigments that may be used for the particles 13 in this dispersion include titanium oxide,
zinc oxide and, as the most desirable, various body pigments such as calcium carbonate, alumium hydroxide, barium sulfate, alumium'oxide', talc, silica, calcium silicate, magnesium carbonate, magnesium oxide, etc. While no elaborate charge control techniques are necessary so long as the pigment particles have a distinct tendency to be charged in a definite polarity in insulating liquids, any of the known liquid developer charge control techniques may be employed, such as the incorporation of resinous control agent in cases where the electrophore'tic property of the pigment particles are unsuitable. Colorless particles 13 may be formed from clear resin particles or low-molecular weight resin powders. The desirable range of particle size is less than several microns in diameter.
' To further illustrate the practice of the present invention the following specific examples are presented:
. EXAMPLE I coating comprising photoconductive zinc oxide and alkyd resin binder cured by a polyisocyanate compound, was charged to -l00 volts, and exposed .to a continuous tone image to form an electrostatic latent image. Both of the above-described suspensions included positively charged particles or toner. The latent image bearing sheet was then processed in the manner.
EXAMPLE II Calcium carbonate was used in place of the aluminum hydroxide for the coating dispersion 21 described in Example I. .A commercially available black devel- 'oper for Electrofax copier was used as the liquid developer. The electrostatic latent image was formed on a zinc oxide/resin binder type photoconductive sheet, using an alkyd resin binder cured with cobalt naphthenate. When the electrostatic latent image was treated and developed in accordance with the procedures described above, an excellent image was obtained, free of background coloration.
EXAMPLE lll A methylethyl ketone solution of vinyl chloride/vinyl acetate copolymer (copolymer composition VCliVAc=70:30, degree of polymerization-260) was poured into kerosene containing linseed oil under agitation by ultrasonic energy. The resulting uniform despersion was used as the coating dispersion 21. For developer 30, a dispersion comprising the following ingredients was first formed:
Microlith Blue 4G-K (Ciba Ltd.) 203.
Acetone 500ml. Toluol 300ml. Methylethyl Ketone 200ml.
(Microlith Blue 4G-K is composed of a blue chloride/vinyl acetate copolymer 85:15). 100 ml. of this dispersion was dispersed by means of an ultrasonic blender into the following mixed solvent:
pigment and vinyl Kerosene 2500ml. Cyclohexane 7000ml. Linseed oil 500ml.
The particles in both dispersions were charged negatively. An electrostatic latent image of positive polarity was formed on an organic photoconductive coating mainly comprising polyvinylcarbazol, and it was processed in the manner described hereinabove. An excellent image free of background coloration was obtained.
What is claimed' is: 1. A method of developing an electrostatic latent image formed on an electrophotographic surface comprising applying over the image surface a substantially uniform coating of particles having a coloration which does not contrast with the image background coloration, said particles carrying a charge of opposite polarity from that of the image charge, the density of said particles in said coating being selected substantially just to neutralize the residual potential on said surface in background and image highlight areas, and thereafter developing said latent image with a developer having colored toner particles.
2. A method as set forth in claim 1, wherein the step of applying said coating particles to said image surface includes usinga thin film of a dispersion of said particles in an electrically insulating liquid vehicle.
3. A method as set forth in claim 2, wherein said coating particles are colorless.
.4. A method as set forth in claim 3, wherein said developer is a dispersion of said toner particles in an electrically insulating liquid vehicle.
5. A method as set forth in claim 1, wherein said developer is a dispersion of said toner particles in an electrically insulating liquid vehicle.
6. A method as set forth in claim 5, wherein said coating particles .are colorless.
7. A method as set forth in claim 5, wherein the step of applying said coating particles to said image surface includes using a thin film of a dispersion of 'said particles in an electrically insulating liquid vehicle.