US3343956A - Electrostatic printing process wherein development is achieved by sequenctial application of carrier liquid and developer particles - Google Patents

Description

United States Patent ELECTROSTATIC PRINTING PROCESS WHEREIN DEVELOPMENT IS ACHIEVED BY SEQUENTIAL APPLICATION OF CARRIER LIQUID AND DE- VELOPER PARTICLES Paul E. Wright, Haddonfield, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Sept. 29, 1961, Ser. No. 141,711 2 Claims. (Cl. 96-12) This invention relates generally to electrostatic printing and more specifically to improved methods and means for developing electrostatic images.

In the art of electrostatic printing, electrostatic images are produced on an insulating surface and are then developed into visible images with electroscopic developer particles. The electrostatic images may be produced by direct charge deposition as by selectively energizing pin electrodes to deposit charges on an insulating plastic surface in a dot pattern. Electrostatic images may also be produced electrophotographically as by subjecting a photoconductive insulating surface to substantially uniform discharge from a corona generating device and then exposing the surface to a light image to discharge the illuminated areas thereon to thereby produce the electrostatic image. In either case, the electrostatic image can be developed into a visible image by various methods such as cascade development, magnetic brush development, or powder cloud development, all well known in the art. In recent times, methods of developing electrostatic images have been devised wherein a dispersion of electroscopic particles in an insulating liquid is employed and the electrostatic image developed by cataphoretic deposition of particles from the liquid. The developer dispersions may be applied to electrostatic images on a surface by flowing the dispersion thereover, by immersing the surface in the dispersion or by passing a roller coated with dispersion over the surface.

While developing methods employing liquid dispersions provide many advantages over those achieved with methods employing dry powders, there are, often times, attendant disadvantages. For example, the aforementioned flowing and immersion techniques frequently require more time for development than methods employing dry powders. Liquid development using rollers can introduce a problem with respect to offset printing of ghost images. While liquid developing techniques gener ally produce higher resolution images, difficulties can be experienced in filling in large, solid-colored image areas.

Some liquid developer dispersions possess a self-fixing property such that, when an electrostatic image is developed therewith, the developed image need not be heated or otherwise treated to render the image permanent. Such dispersions include a binder material dissolved in a highly volatile carrier liquid which, while it is a solvent for the binder, is not a solvent for the developer particles dispersed therein. Self-fixing liquid developers are highly desirable but may introduce vexatious problems. Due to the volatility of the carrier liquid, evaporation thereof rapidly changes the proportions of the developer particles and the binder material in the carrier liquid. Such a change in proportions can result in the production of prints varying in contrast and can even completely negate the effectiveness of liquid development. Rapid evaporation of carrier liquid in developer apparatus such as, for example, roller developing apparatus presents another problem in that water condensation results on the rollers and contaminates the developer composition. Any apparatus employing a liquid developer containing a binder will require frequent cleaning to prevent binder material from gumrning up in the apparatus.

Accordingly, it is a general object of this invention to provide improved methods, devices, and apparatus for applying liquid developer composition to electrostatic images.

It is another object of this invention to provide improved methods, devices and apparatus for developing electrostatic images at high speeds.

Yet another object is to provide improved methods, devices and apparatus for developing electrostatic images with improved fill-in of large, solid-colored image areas.

It is also an object of this invention to provide improved methods, devices and apparatus for developing electrostatic images with a liquid developer composition without requiring either a binder material or a highly volatile carrier liquid for producing self-fixing visible images.

These and other objects and advantages are achieved in accordance with the methods of this invention by contacting across an electrostatic image an absorbent pad having dispersed therein a quantity of electroscopic developer particles. At least that portion of the absorbent pad which is cont acted to the electrostatic image is wetted with an insulating carrier liquid. This may be accomplished in any one of several ways. For example, an absorbent pad, containing a dry dipersion of developer particles, can be contacted across an electrostatic image to which there has been previously applied a thin layer of insulating liquid. An absorbent pad,- containing a dry dispersion of developer particles, can be dipped in an insulating liquid and then contacted across an electrostatic image. A quantity of liquid developer composition comprising developer particles di-psersed in an insulating liquid can be absorbed into the pad and thereafter contacted across an electrostatic image. In any oase, as the pad is moved across a surface bearing an electrostatic charge image, the electroscopic particles are electrostatically attracted from the pad and deposit on the surface in configuration with the charge image, Images can be developed on a surface substantially at the maximum speed that the pad can be manually wiped over the surface. I

Development of electrostatic images with absorbent pads, as aforesaid, also provides novel methods of producing visible images in a plurality of colors by successively developing different electrostatic images on a single insulating or photo-conductive surface with different colored developer particles. When a volatile carrier liquid is used in a method of this invention, a developed image can be produced which is self-fixing and has more resistance to smearing than does commercial newsprint.

This invention also includes a disposable article of manufacture. Such an article comprises an absorbent pad, such as for example, absorbent cotton having a quantity of developer particles dispersed therein or one which has been soaked with, but not saturated with, a developer composition comprising electroscopic developer particles dispersed in an insulating liquid. Such an article, containing a liquid developer composition, may be supplied to an ultimate user in a sealed container. When the development of images with the pad results in depletion of the dispersion contained therein, it can be discarded. Means can also be provided for replenishing the dispersion contained in the pad and thereby extend the useful life thereof.

Improved apparatus for use in electrostatic printing equipment is also disclosed herein. Such apparatus includes an absorbent pad which is contacted across an electrostatic image bearing surface. Means are provided for feeding developer composition to the pad while it is in operation to maintain a sufficient amount of developer composition in the pad. With such apparatus, depletion of developer composition presents no problem and operating life is limited only by normal Wear.

Additional objects and advantages will be evident from the following detailed description which is illustrated in the accompanying drawings wherein:

FIGURE 1 is a perspective view of one embodiment of a developer pad encased in a sealed container in accordance with this invention;

FIGURE 2 is a perspective view illustrating a method of using the developer pad of FIG. 1 to develop an electrostatic image;

FIGURE 3 is a perspective view, partly in cross section, of another embodiment of a developer device in accordance with this invention;

FIGURE 4 is a view in elevation and partly in cross section of a third embodiment of a developer device in accordance with this invention in which developer fluid is added to a developer pad continuously during use.

Similar reference characters are applied to similar elements throughout the drawings.

As shown in FIGURE 1, a developer device, in accordance with one embodiment of this invention, comprises a fabric pad of absorbent material containing a dispersion of electroscopic particles dispersed in an insulating liquid. The fabric pad 11 preferably comprises a cotton material such as, for example, a cotton wicking material. Almost equally effective are other absorbent materials such as felt padding and fiber glass padding materials. As illustrated, the pad is enclosed in an airtight, or nearly so, container 12 in order to prevent evaporation of the insulating liquid before the pad is used for developing electrostatic images.

In fabricating a suitable developer device, a liquid de veloper composition is first formulated. Such a composition may include, for example, an insulating carrier liquid consisting of about equal parts of trichlorotrifiuoroethane and a dimethyl polysiloxane having a viscosity of about 3 centistokes or less. Many well-known developer powders may be dispersed in the carrier liquid to provide the liquid developer composition. For example, a suitable black powder may comprise 200 parts by weight of thermoplastic resin such as, for example, Piccolastic Resin 4358A (Pennsylvania Industrial Chemical Corp., Clairton, Pa.); 12 parts by weight of carbon black; 12 parts by weight of Nigrosine SSB (Color Index No. 50415) and 8 parts by weight of Isosol Black (Color Index Solvent Black 13). The developer powder is prepared by melting the resin and mixing in the other materials. When a uniform mix is obtained, it is cooled, ground to a fine powder and classified as to patricle size. A convenient particle size is one obtained by screening through a 200 mesh or finer screen to provide a maximum particle diameter about equal to or less than 74 microns. Up to about 6 parts by weight of developer powder are stirred into about 100 par-ts by weight of the carrier liquid to provide a liquid developer composition.

A developer pad about 3: to 4 inches long and to /2 inch wide, which has been dipped once in a developer composition, will provide a developer device capable of developing 50 or more electrostatic images on 8 /2 by 11 inch surfaces.

FIGURE 2 illustrates a method of manually developing an electrostatic image on an insulating surface 14. A developer pad 11 containing developer composition is held in an ordinary spring clip 16 to provide convenient means for handling the pad 11. The pad is then contacted to and swept across the insulating surface 14. As it is swept across the surface 14 developer powder particles are electrostatically exrtacted from the pad 11 and from the carrier liquid in the pad. The extracted particles are electrostatically held in the charged areas on the surface 14 to form thereon a visible powder image 18. Images can be developed in this manner as fast as the pad can be swept across the surface. When an electrostatic image is developed in this manner, using the developer composition described heretofore, the carrier liquid extracted from the composition evaporates almost immediately from the insulating surface 14 providing a fixed black image 18 on the surface 14. This image 18 is at least as resistant to smearing as is ordinary newsprint.

By using a plurality of pads such as the pad 11 of FIGURE 1, the preparation of color reproductions is made possible. Each pad is used to develop a different electrostatic image in a different color. For example, an absorbent pad is prepared for developing an electrostatic image into a black image using the developer composition described heretofore. Developer compositions for producing other colors can be prepared by dispersing color pigments or combinations thereof in a carrier liquid comprising about equal parts by volume of dimethyl polysiloxane and trichlorotrifluoroethane. It is preferred that the compositions contain about 0.005 to 6% by weight of color pigment and that the pigment or pigments be substantially insoluble in the carrier liquid.

Many color pigments can be incorporated into developer compositions to be absorbed into the pad 1 1 of FIGURE 1. The following comprises a partial list of those which have been found to be suitable:

(1) Cyan blue toner GT (described in U. S. Patent 2,486,-

351 to R. H. Wiswall, Jr.)

(2) Benzidine Yellow (3) Brilliant Oil Blue BMA (Color Index No. 61555) (4) Sudan III Red (Color Index No. 26100) (5) Oil Yellow 2G (Color Index No. 11020) (6) Hansa Yellow 2G (Color Index No. 11680) (7) Pyrazalone Pigment Red 39 (Color Index No. 21080) Color reproductions can be very conveniently produced using a plurality of absorbent pads each containing a different color developer dispersion. An electrophotographic method of color reproduction can be practiced in the following manner. A dye-sensitized electrophotographic sheet is charged to a substantially uniform surface potential. Dye-sensitized electrophotographic sheets and the charging thereof are described in Electrofax, Direct Electrophotographic Printing on Paper by C. J. Young and H. G. Greig RCA Review, December 1954, vol. XV, No. 4. The charged sheet is exposed through filters to an original color transparency, the filters being selected to mask that light which corresponds to the one color component which it is desired to reproduce. A latent electrostatic image is thus produced which is representative of the one color component of the original. An absorbent developer pad containing an appropriate colored developer composition is then used to develop the electrostatic image. Repeating these steps, each time with difierent color filters and different color developer compositions results in the superimposition of one color over another on the electrophotographic sheet to provide a substantially faithful reproduction of the original color image.

Because each color, as it is reproduced on the electrophotographic sheet, is self-fixing, no smearing of one color results when the developer pad is wiped across that color during a subsequent step. Colors reproduced in this manner are, in fact, substantially more resistant to smearing than is ordinary newsprint. Also, because no heat is required to fix a color to an electrophotographic sheet such as paper, there is no danger of producing dimensional changes in a paper sheet during processing as would be the case if each color had to be heat fixed thereto. Because of this fact, registration of superimposed colors is greatly simplified.

The developer pad 11 can also be provided with a dry dispersion of developer particles. This can be accomplished, for example, by dispersing developer particles such as carbon black or the color pigments mentioned heretofore in a volatile non-solvent liquid such as trichlorotrifiuoroethane. The pad 11 is soaked in or saturated with this dispersion and the liquid allowed to evaporate therefrom. A dry dispersion of developer particles in the absorbent pad 11 results. An electrostatic image is developed with this pad 11 by merely wetting at least that portion of it which is contacted across the image. For example, the developer pad containing the dry dispersion can be dipped in a tray containing an insulating liquid such as dimethyl polysiloxane and thereafter wiped across an electrostatic image to develop it.

Another novel method of color reproduction can be realized employing developer pads each containing a dry dispersion of a different colored pigment. A thin film of a slow evaporating carrier liquid such as dimethyl polysiloxane is applied to an electrophotographic sheet by wiping thereacross a pad containing such liquid. Electrostatic images corresponding to color components of an original color image are thereafter reproduced and developed as described heretofore. During each development step, a pad containing a different colored pigment is employed, the siloxane liquid on the sheet acts as the carrier for each pigment and remains on the sheet for a time sufficient to produce all colors in the reproduction. An unusual feature of this method of color reproduction resides in the fact that, although the sheet is wetted with carrier liquid throughout the time required to produce all colors, no smearing results when each successive color is developed on the sheet. Once a color reproduction has been made, the sheet can be set aside to dry or, if desired, drying can be accelerated by heating.

Another embodiment of this invention, illustrated in FIGURE 3, comprises a rectangular fabric pad 11 carried by a holder 20, the handle portion of which indludes a cavity 22 filled with liquid developer composition. To prevent evaportion of liquid from the composition during shipment and when the device is not in use, a removable rectangular cap 24 is provided which fits over the fabric pad 12 and the lower end of the holder 20. The holder 20 is preferably made of a flexible material such as polyethylene so that developer composition extracted from the pad 12 can be replaced by applying 'sligh-t pressure to the holder 20 adjacent the cavity 22 therein.

Electrostatic printers have been devised for recording on continuous webs at high speeds. One such printer records on 8 /2 inch wide electrophotographic paper from a cathode ray tube at rates of more than 10,000 characters per second and paper speeds of 18 inches or more per second. More details with respect to such a machine are included in an article entitled A Thin-Window Cathode- Ray Tube for High Speed Printing with Electrofax by R. G. Olden, RCA Review, September 1957, vol. XVIII, No. 3. It can be readily understood that development of electrostatic images at such high speeds introduces problems with respect to proper formation of visible images and with respect to rapid depletion of developer material.

Many problems in high speed development of electrostatic images are overcome by providing a developer apparatus such as that illustrated in FIGURE 4. In this apparatus, a continuous web 30 of Electrofax paper is drawn over a roller 32 and in contact with a developer pad 11. The developer pad 11 is supported in a substantially rectangular housing 34 which contains a reservoir 36 of liquid developer composition. Developer compositions are fed into one end of the reservoir 36 through a conduit 38. A centrifugal pump 42 is provided to pump developer composition from a supply tank 44 through the conduit 38 and into the reservoir 36. A return path for the developer composition is provided by another conduit 46 communicating with the opposite end of the reservoir 36 and with the supply tank 44. In this latter conduit 46 a check valve 48 is provided so that the pressure within the reservoir 36 may be adjusted to insure that the proper amount of developer composition is fed to the developer pad 11.

In high speed recording equipment it may be desirable to develop images on continuous webs at different speeds. Any change in development speed will, of course, result in a change in the rate at which developer composition is used up. In the apparatus of FIGURE 4, means are provided to vary the rate at which developer composition is supplied to reservoir 36 and the developer pad 11 in accordance with the speed at which the web 30 is drawn between the developer pad 11 and the roller 32. At one end of the roller 32, the shaft 50 thereof is provided with a spur gear 52 which in turn meshes with another spur gear 54 coupled to the pump 42. Thus any change in web speed will automaticaly produce a corresponding change in pump speed to thereby vary the pressure in the reservoir 36 and the rate of feeding developer composition to the developer pad 11.

What is claimed is:

1. In a method of printing including the step of producing an eletrostatic charge image on an insulating surface, the improvement comprising developing said charge image into a visible image by first applying a thin film of insulating carrier liquid to said surface and thereafter contacting thereacross absorbent material having dispersed therein a quantity of dry electroscopic developer particles.

2. A method of color electrophotography comprising the steps of applying a thin film of insulating carrier liquid to a surface of photoconductive insulating material substantially uniformly charging a surface of photoconductive insulating material; exposing said surface to a light image containing all color components except one color component of an original color image to produce on said surface a latent electrostatic image representative of said one color component of said original image; developing said electrostatic image into a visible color image by contacting across said surface absorbent material having dispersed therein a quantity of electroscopic developer particles of said one color, at least that portion of said material in contact with said surface being wetted by an insulating carrier liquid; and repeating said steps at least once to produce 'an electrostatic image corresponding to a different color component of said original image and to develop said last mentioned electrostatic image with electroscopic developer particles of said different color dispersed in a different absorbent material.

References Cited UNITED STATES PATENTS 1,769,250 7/1930 Arvidson 118-266 1,784,913 12/1930 Scott 96-1 2,463,711 3/ 1949 Nagle 204-224 2,540,602 2/ 1 Thomas et a1 204-224 2,590,927 4/ 1952 Brandt et a1 204-224 2,713,176 7/1955 Rosenthal (I) 15-134 2,726,200 12/ 1955 Holsapple 204-224 2,750,332 6/1956 Miller 204-224 2,805,641 9/ 1957 Rosenthal (H) 118-265 2,811,465 10/ 1957 Greig 96-1 2,833,702 5/ 1958 Elfers 204-224 2,961,395 11/1960 Icxi 204-224 2,986,521 5/ 1961 Wielicki 96-1 3,010,883 11/1961 Johnson et a1. 204-224 3,038,799 6/ 1962 Metcalfe et a1 96-1 3,050,768 8/1962 Rosenthal (III) 15-563 3,053,688 9/1962 Greig 96-1 3,081,263 3/ 1963 Metcalfe et a1 96-1 X 3,102,045 8/1963 Metcalfe et al 96-1 X 3,150,976 9/ 1964 Johnson 96-1 NORMAN G. TORCHIN, Primary Examiner.

ALEXANDER D. RICCI, Examiner.

C. E. VAN HORN, Assistant Examiner.

Claims (1)

1. IN A METHOD OF PRINTING INCLUDING THE STEP OF PRODUCING AN ELETROSTATIC CHARGE IMAGE ON AN INSULATING SURFACE, THE IMPROVEMENT COMPRISING DEVELOPING SAID CHARGE IMAGE INTO A VISIBLE IMAGE BY FIRST APPLYING A THIN FILM OF INSULATING CARRIER LIQUID TO SAID SURFACE AND THEREAFTER CONTACTING THEREACROSS ABSORBENT MATERIAL HAVING DISPERSED THEREIN A QUANTITY OF DRY ELECTROSCOPIC DEVELOPER PARTICLES.