US3010842A - Development of electrostatic images - Google Patents

Description

United States Patent 3,010,842 DEVELOPMENT OF ELECTROSTATIC IMAGES Eugene C. Richer, Columbus, Ohio, assiguor, by mesne assignments, to Xerox Corporation, a corporation of New York N0 Drawing. Filed Aug. 29, 1955, Ser. No. 531,280 1 Claim. (Cl. 117-37) This invention relates to a method for the development of electrostatic latent images.

In xerography it is usual to form :an electrostatic latent image on a surface. One method of doing this is to charge a photoconductive insulating surface and then dissipate the charge selectively by exposure to a pattern of activating radiation. Other means of forming electrostatic latent images are set forth in U.S. 2,647,464 to James P. Ebert. Whether formed by these means or any other, the resulting electrostatic charge pattern is conventionally utilized by the deposition of an electrostatic material thereon through electrostatic attraction whereby there is formed a visible image of electroscopic particles corresponding to the electrostatic latent image. Alternatively, the electrostatic charge pattern may be transferred to an insulating film and the electroscopic particles deposited thereon to form the visible image. In any case, this visible image, in turn, may be transferred to a second surface to form a xerographic print.

The process of depositing the electroscopic powder on the electrostatic image to render the electrostatic image visible is called the development step and is one of the most critical steps of the entire process. As the electrostatic latent image is developed by deposition of an electroscopic material thereon, it is obvious that the process offers freedom from dependence on the awkward and complicated liquid development processes of conventional photography. To emphasise this freedom the process has been named xerography as generally dry powders were used as the electroscopic materials. However, this is not necessarily the case as is shown in U.S. 2,551,582 to C. F. Carlson wherein the electroscopic material was applied in the form of a liquid spray.

It is evident that no picture can be better than the development step permits. About the coarsest type of image reproduced by a xerographic process requires a resolution of at least about 50 lines per inch, Commercial linecopying xerographic machines generally have a resolution power of about 125 to 250 lines per inch. The process used in obtaining this resolution is termed cascade development and is set forth in US. 2,618,552. This process involves the use of a finely-divided colored material colored called a toner deposited on a slightly more coarsely-divided material called a carrier. component developer is cascaded across the electrostatic image areas.

The control of the concentration of toner in the twocomponent developer becomes difficult on continued operation of a xerographic machine. When applied to continuous-tone development where resolutions greater than about 300 lines per inch are necessary it has been found impossible to obtain the desired quality of reproduction using. such a system.

xerographic development known as powder cloud development has been evolved. In this process a powder cloud is generated by agitating a mass of dry electroscopic material in a confined space or in the case of a liquid by spraying through an atomizer. The cloud so produced is charged and then contacted with the electrostatic latent image. This process makes possible the realization of extremely high resolutions suchas 1200 or more lines per inch. Furthermore, concentration of toner is more susceptible to control so that the process is easily adapt- I able for use on continuous machines.

This two- Accordingly, another system of 3,010,842 Patented Nov. 28, 1961 However, the process also has its defects. Thus, to realize the high resolutions possible with this system and to obtain solid-area coverage it is necessary to use a development electrode-that is a conductive electrode spaced close enough to the image-bearing surface to draw the lines of force externally above the image-bearing surface. Due to the low dielectric constant of air an extremely close spacing of the development electrode is desired for its effective use. This introduces a number of difficulties due to the extreme difficulty of obtaining adequate air flow in the very narrow and confined space between the development electrode and the image-bearing surface. Other difiicultie-s arise in attempting to obtain a constant flow for extended periods of time from the powder cloud generator and in flowinga uniform powder cloud over all areas of the image bearing surface. Moreover powder clouds are not uniformly charged to the same polaritya significant portion of the cloud will be of the opposite polarity to that desired for the development of the image. These oppositely charged particles in the cloud tend to deposit from the powder cloud on uncharged areas of the'image making difficult the obtaining of a clear white on such areas. Finally, the equipment forgenerating, charging and directing the flowing of the powder cloud becomes rather complicated.

The instant invention relates to a process which represents a substantial improvement in the art of xerographic development. In this process the electroscopic material is suspended in a liquid rather than a gas and the electrostatic latent image developed by contacting the imagebearing surface with the liquid suspension to produce a layer of liquid on the image-bearing surface whereby the electroscopic material is attracted out of the liquid to the image-bearing surface as by immersion of the image-bearing member in the suspension, by flowing the suspension over the image bearing surface or otherwise. This process has several advantages over the prior art: Thus, development is almost instantaneous. Contacting the liquid suspension of powder particles with the electrostatic image to form a layer of the liquid suspension thereon is easily and quickly accomplished without the necessity for complicated and cumbersome equipment. The particles are believed to be all charged with the same polarity (probably a zeta-potential) without the necessity for special charging equipment. The use of a liquid permits a higher concentration of useable developer particles than when gas is the suspending medium. The image defects due to irregularities in the air flow are obviated and, finally, the use of a liquid having a high dielectric constant permits more effective use of a development electrode.

The process of the invention will now be set forth in more detail. Y

According to the present invention a dispersion of electroscopic particles is formed in a suitable liquid for example as by stirring or otherwise as is well known to those skilled in the art. Suitable electroscopic particles include any pigment or similar material having a fine particle size and being insoluble in the suspending liquid. In general the finer the powder the better the grain in the developed image. It is preferred that the average particle I quality work it is preferred that the average particle size burnt sienna, ultramarine blue, Hausa yellow, rose bengale, Dupont Luxol blue, and methyl cellulose. As already mentioned the pigment should not be soluble in the liquid and it must also be so selectedrelative to the liquid used as to assume a charge of the desired polarity. The

electrostatic charge imparted to suspended particles is a function both of the nature of the particles and of the suspending liquid.

The liquid used must have a relatively high insulating value to avoid discharge of the latent electrostatic image. Suitable liquids include hydrocarbons such as benzene, xylene, hexane, naphtha, kerosene, etc.; halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene, chloroform, Genetrons (a trade name of Allied Chemica Co. for various fluorinated compounds), Freons (a trade name of E. I. du Pont de Nemours & Co. for various fluorinated compounds), etc.; miscellaneous liquids such as turpentine, etc.

Any method known to those skilled in the art may be used for dispersing the pigment in the liquid. In general simple stirring or agitation of the liquid is sufficient. Where a highly uniform and stable suspension is desired, the suspension formed by stirring may be passed through a colloid mill, but excellent suspensions may be attained without such treatment. where they do not impair the insulating properties of the liquid.

The density of pigment deposited on the xerographic plate is dependent on the charge of the electrostatic latent image, the pigment concentration in the liquid and the presence of a development electrode. As is the case in powder cloud development described above the development electrode consists of a conductive electrode, either of continuous conductive material as sheet metal or of a conductive foraminous grid, closely spaced to the i-magebearing surface. The function of the development electrode is to draw the lines of force of the electrostatic image externally above the image-bearing surface. This gives improved solid-area coverage and, by increasing the amount of the lines of force that are external to the imagebearing surface and hence available'to attract electrostatically charged marking particles, improves the overall efiiciency of the development process. Where the liquid suspension is contacted with the electrostatic image-bearing surface in a conductive dish or tank, the walls of the container will themselves constitute a development electrode. Moreover, the use of liquids having much higher dielectric constants than air permits the effective use of a development electrode at considerably greater spacings from the image-bearing surface than when air is the intervening dielectric.

The developed images are satisfactory for visual inspection on the plate while still wet. If desired, the wet image may be transferred by absorption by paper. If desired, the image may be dried and transferred to a desired base such as paper, plastic, etc. by the usual methods of electrostatic transfer as described in US. 2,576,047 to R. M. Schaffert or adhesive transfer as described in US.

2,661,289 to Mayo and Wolfe. The image, in turn, may 1 then be fixed by any means known to those skilled in the art such as those described, for example, in US. 2,297,691 to C. F. Carlson. Thus where either the pigment itself fusible by heat or solvent vapor, or where the sheet'to which it is transferred is coated or impregnated with a material fusible by heat or solvent vapor, then the use of either heat or solvent vapor will permanently aifix the image to the sheet. Alternatively, another method of fixing the image comprises spraying the surface of the sheet bearing the image with a fixative lacquer by atomizer. Other means of transferring and/or fixing the image known to those skilled in the art may be used.

.In the following examples a xerographic plate comprising a micron layer of selenium on an aluminum backing was charged and exposed to a pattern of light andshadow in'a XeroX Copier and XeroX Camera Unit Suspending agents may be used No. 1 commercially available from the Haloid Company v of Rochester, New York. In each case the electrostatic 4 Example I Examples 217 Liquid suspensions were prepared by dispersing 0.5 g.

of each of the following materials in cc. of naphtha and stirring with a steel rod. Development was as above.

Material Comments Sulphur No continent.

Poor image. Charcoal D Good reversal development.

Good positive development.

Exceptionally good positive image.

No comment.

Apparent reversal development.

Apparent reversal development.

Sharp and bright letters.

Positive image. Good solid area coverage.

Poor image.

Positive image. Good solid area coverage. Quite sharp.

No comment.

Reversal development.

Poor contrast and definition.

Too much deposition.

Fairly good continuous tone development.

Dupont Luxol Blue Rose Bengals Powdered Brass Cobalt blua. Yellow ochre American Vermilion.

Burnt umber Lithopone Burnt Sienna Cadmium red Chrome yellow (med) Powdered aluminum Methyl cellulose Example 18 A suspension was prepared by stirring 0.5 g. finelydivided charcoal into 100 cc. steam-distilled turpentine. An image was developed as in Example 1. Deposition was light.

Example 19 A liquid suspension was prepared by placing in a glass container 1.0' g. of finely-divided charcoal and 100 cc. of carbon tetrachloride. The container was then stoppered and dispersion completed by manually shaking a few times.

The suspension was then poured into a metal tray and the image-bearing surface contacted with the suspension by immersing the xerographic plate therein in the dark. The plate used in this example consisted of a 50 micron layer of selenium on a brass backing. After immersion the plate was withdrawn from the suspension, the lights turned on and the image transferred while wet by contact with dye-transfer paper. By this process a. completely developed xerographic image was obtained in just a few seconds and with only the simplest of equipment.

Examples 20 and 21 Suspensions were prepared as in Evample 1 using 0.1 g. finely-divided aluminum and 0.1 g. finely-divided copper, in each case dispersed in 100 cc. of naptha. Development was in Example 1. Both produced good images, the aluminum having slightly better resolution.

Example 22 Example 23 A suspension was prepared by stirring 4; teaspoon of an alcohol dag obtained from Acheson Colloids, Port Huron, Michigam into cc. of naphtha. The image was developed as in Example I. The image was thin due to light deposition.

Examples 24 and 25 Suspensions were prepared as in Example 1 using 0.5 g. of charcoal and 0.25 g. of aluminum powder, in each case dispersed in 100 cc. of xylene. The images were developed as in Example 1. Fair images were obtained.

Example 26 A suspension was prepared by stirring both 0.1 g. of charcoal and 0.1 g. of aluminum powder into 100 cc. of naphtha. The image was developed as in Example 1. The developed image was a mixture of both the charcoal and the aluminum.

The increased versatility of xerography due to the availability of such alternative development processes is obvious at once. Thus it is possible to develop an image either positively or negatively by selecting a liquid-pigment system wherein the charge on the particles is, respectively, either opposite to or the same as the polarity of the charge of the electrostatic image. Furthermore, it is possible to produce images of mixed colors by blending pigments of different color but similar charge polarity. It is also possible to simultaneously develop both the image and the background by using two pigments which take on the opposite polarity of charge when dispersed in the same liquid. While a. particular embodiment of the invention has been described, it is not intended to be limited thereto, but it is intended to cover the invention broadly within the spirit and scope of the appended claim.

I claim:

A process of developing an electrostatic latent image on an insulating surface which comprises contacting an electrically insulating layer bearing an electrostatic latent image with an electrically insulating liquid so that all of the image-bearing portions of said layer contact said liquid, said liquid having dispersed therein finely-divided particles of a first color and finely-divided particles of a second color, the particles of said first color and the particles of said second color having opposite polarities as a result of being dispersed in said liquid, and simultaneously developing both the image and background on said insulating layer.

References (Zited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Metcalfe: Liquid Developers for Xerography, Journal of Scientific Instruments, vol. 32, February 1955, pp. 74 and 75.

Atkinson et al.: Ferrography, Journal of the Franklin Institute, vol. 252, No. 5, November 1951 pp. 373-381.