US 3856519 A
Dry, final copies are obtained in an electrostatographic imaging system employing liquid development wherein after development with the liquid developer, the imaging surface is contacted with a transfer surface which has previously been supplied with a highly volatile insulating liquid and the toner image on the imaging surface is electrostatically transferred to the transfer sheet. Following separation of the transfer sheet from the imaging surface, the highly volatile insulating liquid available on the transferred sheet is readily removed to provide a dry, final print.
Description (OCR text may contain errors)
United States Patent 11 1 Honjo et al.
[ Dec. 24, 1974 TRANSFER OF TOWER USING A VOLATILE INSULATING LIQUID Inventors: Satoru Honjo, Tokyo; Masamichi Sato, Asaka, both of Japan Assignee: Xerox Corporation, Stamford,
Filed: Jan. 29, 1971 Appl. N0.: 111,126
Foreign Application Priority Data June 4, 1970 Japan 45-48234 US. Cl. 96/1.4, 96/1 LY, 117/37 LE, 252/621 Int. Cl. G03q 13/10, G03q 13/16 Field 61 Search 117/37 LX; 96/1 R, 1 LY, 96/l.4; 252/621; 101/426 References Cited UNITED STATES PATENTS 10/1959 Metcalfe et al 117/37 Primary Examiner-Roland E. Martin, Jr.
 ABSTRACT Dry, final copies are obtained in an electrostatographic imaging system employing liquid development wherein after development with the liquid developer, the imaging surface is contacted with a transfer surface which has previously been supplied with a highly volatile insulating liquid and the toner image on the imaging surface is electrostatically transferred to the transfer sheet. Following separation of the transfer sheet from the imaging surface, the highly volatile insulating liquid available on the transferred sheet is readily removed to provide a dry, final print.
6 Claims, No Drawings TRANSFER OF TOWER USING A VOLATILE INSULATING LIQUID BACKGROUND OF THE INVENTION This invention relates to imaging systems and more particularly to improved methods for the transfer of toner images obtained in liquid development tech niques from the imaging surface to a receiving surface.
The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic electrostatographic process as taught by C. F. Carlson in US. Pat. No. 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 marking 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. The powder image may then be transferred to a support surface such as paper and permanently affixed to the support by any suitable means such as heat fixing or solvent fixing. Alternatively, the powder image may be fixed to the photoconductive layer if elimination of the powder transfer step is desired. In addition, instead of latent image formation by uniform charging followed by imagewise exposure, the latent image may be formed by directly charging the layer in image configuration. Other methods are known for applying electroscopic particles to the imaging surface. Included within this group are the *cascade" development technique disclosed by E. N. Wise in US. Pat. No. 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in US. Pat. No. 2,221,776; and the magnetic brush process disclosed, for example. in US. Pat. No. 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 a charged image pattern, the suspended particles migrate toward the charged portions of the imaging surface separating out of the insulating liquid. This electrophoretic migration of charged particles results in the deposition of the charged particles on the imaging surface in image configuration. Electrophoretic development of an electrostatic latent image may, for example. be obtained by pouring the developer over the image bearing surface, by immersing the imaging surface in a pool of the developer or by presenting the liquid developer on a smooth surface roller and moving the roller against the imaging surface. The liquid development technique has been shown to provide developed images of excellent quality and to provide particular advantages over other development methods in offering ease in handling.
In a typical conventional method employing a liquid development technique and a photoconductive insulating layer as the electrostatographic imaging surface, after formation of the developed image on the photoconductive insulating layer by the developer, the developer is transferred to a receiver sheet to provide a copy, for example, on ordinary paper. This transfer may be accomplished with the assistance of electrostatic transfer such as by supplying thereupon a corona ion formed by means of corona discharge in a manner similar to that which is widely used in the transfer of dry toner to a toner receiving sheet. In this method, an electrophotographic material comprising a photoconductive insulating layer of, for example, photoconductive selenium, zinc oxide or an organic photoconductive material in an insulating resin present on a conductive substrate such as a sheet of conductive paper or plastic film may be electrostatically charged in the absence of light and exposed to a light and shadow pattern to be reproduced to dissipate the electrostatic charge in the nonimage or background areas. The electrostatic latent image present on electrophotographic materials is developed, for example, by immersing the electrophotographic material in liquid developer comprised of minute charged particles hereinafter referred to as toner suspended in an insulating liquid. After development, the electrophotographic material is removed from the liquid developer and the surface holding the toner formed during development is contacted with a transfer sheet composed, for example, of ordinary paper or plastic film and subjected to corona discharge to electrostatically transfer the toner image from the electrophotographic paper to the transfer sheet.
Typically, the liquid developers are composed of insulating liquids which have relatively high boiling points in order to minimize evaporation during idle periods and to maintain constant toner concentration in the liquid developer. Accordingly, it is difficult to evaporate these liquids from the imaging surface and when transfer of the liquid developer is to be made to a receiving surface such as ordinary paper, the liquid remaining on the imaging surface holding the toner image penetrates into the transfer sheet and thereby wets the paper. The wetting of the paper not only increases the weight of the sheet, but since the liquid gradually evaporates from the sheet it results in an undesirable effect on printed material or other articles eventually brought into contact with the transfer sheet. It is very important, however, that the toner image obtained'by liquid development on the imaging surface be moistened with a liquid employed in the liquid developer or other similar liquid since if the imaging surface bearing the toner and developer liquid is completely dried, the attractive force between the toner particles or between the particles and the electrostatographic imaging surface increases to such a large extent that electrostatic transfer of the toner from the imaging surface to the transfer surface is seriously inhibited. Generally, electrostatic transfer is rendered possible only when the toner image on the imaging surface is moistened with a liquid present in the liquid developer or other similar liquid which penetrates into the transfer sheet leading to the undesirable drawbacks mentioned above. On the other hand, after development on the imaging surface and while the toner image remains wet, it is possible to electrostatically transfer the toner to the transfer surface since the attractive force between the tonerparticles and the photoconductive layer is not excessively strong.
SUMMARY OF THE INVENTION It is therefore an object of this invention to'provide a liquid development system which overcomes the above noted deficiencies.
It is another object of this invention to provide a method of transferring toner images obtained from liquid development to a receiving surface.
It is another object of this invention to provide a liquid development system capable of producing dry copies on a transfer sheet.
It is another object of this invention to provide a liquid development system wherein the toner image is electrostatically transferred from the imaging surface to a receiver surface.
it is another object of this invention to provide a method of transferring liquid developer from an imaging surface to a transfer surface without previously drying the imaging surface.
The above objects and others are accomplished generally speaking, by providing an electrostatographic imaging system of the liquid development type wherein the liquid developer is transferred from the imaging surface which remains wet by the liquid developer to a transfer surface which has previously been moistened with ahighly volatile liquid and applying an electrostatic field to transfer the toner image from the imaging surface to the transfer surface thereby preventing the penetration of the relatively high boiling liquid of the liquid developer into the transfer sheet. 7
More specifically, while the high boiling insulating liquid present in the liquid developer and therefore present on the imaging surface immediately following development would penetrate and wet a transfer sheet such as one composed of ordinary paper, after the transfer sheet is moistened-with a low boiling liquid of this invention, the high boiling insulating liquid of the liquid developer is no longer capable of penetrating into the transfer sheet during the transfer operation. In addition, after completion of the transfer of the toner in image configuration, the low boiling liquid with which the transfer sheet is contacted prior to transfer can be readily and speedily removed from the transfer sheet by drying in ambient air or a warm air stream. The final print therefore is not moist and does not contain liquid which will evaporate at a relatively slow rate and produce the above mentioned undesirable effects on printed material or other articles withwhich the transfer sheet eventually comes in contact. In essence, the technique of this invention provides a thin, readily removable barrier layer of a low boiling insulating liquid on the transfer sheet which precludes transfer of significant quantities of high boiling liquids from the imaging surface to the transfer surface and which therefore rapidly produces a dry print of lasting quality.
Any suitable low boiling liquid may be employed as the transfer liquid in the practice of this invention. Typically, the low boiling liquids are nonpolar, highly electrically insulating materials having resistivities generally greater than about 10' ohm-cm in order to minimize any possible dissipation of charge on the toner particles during the transfer step. In addition, the low boiling transfer liquid should generally have a relatively low dielectric constant. generally less than about 3.5 since materials of high dielectric constant and high conductivity render it difficult to provide a strong attractive force on the toner particles to transfer them from the imaging surface to the transfer sheet during the transfer process. Typically, the low boiling liquids with which the transfer surface is contacted have boiling points of less than about 150C. and have high evaporating rates. Typically, the low boiling liquids are highly volatile compared with the insulating liquid in the liquid developer and have evaporating rates for temperatures between about room temperature and 50C. generally greater than about two times that of nbutyl acetate which is a generally recognized standard upon which evaporation rates may be compared. See, for example, Chapter 1 1, Table l 1.1 in The Technology of Solvents and Plasticizers by A. K. Doolittle published by John Wiley and Sons, Incorporated, 1954. It is also desirable to employ low boilingtransfe'r liquids which are noncombustible and to this end, the chlorinated hydrocarbons and chlorofluorinated hydrocarbons of low boiling points such as CCl FCCl F or CCl F CClF The above materials are commercially available from Daikin Industries Company, Ltd. under the names Daiflon Solvent S-2 and Daiflon 8-3 respectively. In addit ion, it is possible to employ a mixture of methanol or other hydrocarbons with Daiflon.
Additional typical low boiling insulating transfer liquids include low boiling hydrocarbons, such as, npentane, hexane, cyclohexane, toluene, methyl cyclohexane and benzene; low boiling halogenated hydrocarbons, such as, ethylene chloride, trichloroethylene, tetrachlorodifluoroethane and trichlorotrifluoroethane. Since the toner particles which are transferred from the imaging surface to the receiving surface are generally covered with a layer of the carrier liquid of the liquid developer which remains on the final print until drying, it is generally preferred to provide a low boiling transfer liquid which is miscible with the carrier liquid so as to dilute the carrier liquid with the transfer liquid to enable more rapid drying. However, any resin component present in the liquid developer need not be soluble in the low boiling transfer liquid since the toner particles are transferred by means of the applied electric field and are covered with a layer of the carrier liquid.
Any suitable transfer surface may be employed. Typically, the transfer surface will take the form of ordinary paper or other fibrous material. The low boiling insulating transfer liquid may be applied to the transfer sheet by any suitable means. It may be applied, for example, merely by immersing the sheet in a bath of the low boiling liquid to impregnate the sheet with the low boiling liquid. Alternatively, it may be applied by bringing the transfer sheet into contact with a cloth or sponge which has previously been impregnated with the low boiling insulating transfer liquid or it may be applied by means of spraying. To provide uniformity of drying, it is generally preferred to provide a substantially uniform amount of liquid on the transfer surface. Excessive liquid may, therefore, be removed from the transfer surface by means of squeeze rollers in order to prevent excessive impregnation. The electrostatographic imaging surface bearing the wet liquid developer on its surface may be placed in contact with the transfer surface prewetted with the transfer liquid in any suitable manner. These two surfaces may, for example, be contacted by contacting their entire surfaces at once in face to face contact. Following or substantially simultaneously with contact the toner is electrostatically transferred to the transfer paper by any suitable technique. Typically,
a corona charging device such as is used in charging the imaging surface may be employed to charge the transfer surface producing an electric field which draws the toner to the transfer surface. The transfer surface is generally charged to a polarity opposite the polarity of 5 charge on the toner particles. Following transfer of the toner image to the transfer surface, the volatile transfer liquid may be removed in any suitable manner such by a stream of warm air and the imaging surface may be cleaned of residual liquid developer by any suitable cleaning process.
From the above description of the invention, the choice of specific materials and operating conditions is deemed to be well within the scope of those skilled in the art, and therefore, the scope of the invention is not limited by the hereinabove mentioned illustrative materials and techniques. For example, while photosensitive paper comprising zinc oxide in an insulating binder layer coated on ordinary fibrous paper can be impregnated as mentioned above, it is to be understood that other imaging members may be employed and that the choice of particular imaging member and particular development system may be readily determined by one skilled in the art. For example, cadmium sulfide, zinc sulfide, zinc selenide, cadmium selenide, titanium dioxide, phthalocyanine and polyvinyl carbazole may be employed as a photoconductive material. In addition, other suitable electrostatic imaging members may be employed.
Development of the electrostatic latent image may be obtained with any suitable liquid developer. Typically liquid developers well known in the art contain electroscopic marking particles dispersed in an insulating liquid vehicle and may also contain charge control agents and suspending agents for their well known functions. Generally, the high boiling point liquid vehicles employed have relatively high insulating values having volume resistivities greater than about ohm-cm so as not to affect the electrostatic charge pattern on the insulating layer and generally also have low dielectric constants of less than about 3.5. Typically specific vehicles include hydrocarbons, such as kerosene; and halodispersed or suspended in the liquid by stirring or agitation and where a highly uniform or stable suspension is desired, the suspension may be passed through a colloid mill.
DESCRIPTION OF PREFERRED EMBODIMENTS The following preferred examples further define and describe the preferred materials, methods and techniques of the present invention. Unless otherwise indicated, all parts and percentages in the examples are by weight.
EXAMPLE I An electrophotographic material comprising a vacuum evaporated amorphous photoconductive selenium layer about 25 microns thick on an aluminum plate is electrostatically charged to a surface potential of about 400 volts by positive corona discharge in the dark. The photoconductive insulating layer is imagewise exposed to a light image through a positive transparency and immersed in a liquid developer prepared by dispersing one gram of carbon black, which has previously been treated with a methyl ethyl ketone solution of vinyl chloride-vinyl acetate copolymer, in a'mixture of 1,000 milliliters of kerosene and 50 milliliters of linseed oil to obtain a toner particle size of about one to five microns. The photoconductive insulating plate is immersed in the liquid developer for about 1 minute, removed and placed in a horizontal position. A transfer sheet of ordinary white paper, about 150 microns in thickness is immersed in Isopar E (an isoparaffinic component rich fraction of petroleum distilling within the range of l 16C to about 143C. and with an evaporating speed of 2 times that of butyl acetate at about 20C. available from Standard Oil Compnay of New Jersey) for several seconds. Thereafter, the transfer sheet is squeezed by means of rubber rollers to remove liquid and overlayed on the photoconductive insulating layer. Positive corona discharge at. a distance of about 2 centimeters above the transfer sheet effects transfer of the toner image to the transfer sheet. The liquid contained in the transfer sheet is completely removed by placing the sheet in warm air at a temperature of about 50C. for a few seconds.
EXAMPLE II The procedure of Example I is repeated except that the paper is immersed in a bath of a mixture of about parts by volume Daiflon S-3and 40 parts by volume Isopar E. Daiflon S-3 has a boiling point of about 47C. and an extremely high evaporating rate. Results similar to those obtained in Example I are achieved.
While the above two examples have been described with respect to the use of ordinary paper as the transfer sheet, the method of the present invention is also effective when a plastic film which is penetratable only with difficulty is employed since the amount of carrier liquid carried away in the plastic film is also decreased. Furthermore, instead of employing corona discharge for electrostatic transfer, it is also possible to place an electrode over the transfer sheet and to apply an electric potential between the electrode and the electroconductive support material of the photosensitive material.
Although specific materials and operational techniques are set forth in the above exemplary embodiments using the transfer materials and techniques of this invention, they are merely intended as illustrations of the present invention. There are other developer materials and techniques than 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:
I. An imaging method comprising the steps of forming an electrostatic charge pattern on an electrostatographic imaging surface, developing the electrostatic latent image with a liquid developer comprising a high boiling point insulating liquid having electroscopic marking particles dispersed therein, contacting said imaging surface with said liquid developer thereon with a transfer sheet having substantially uniformly available at the contacting surface a volatile insulating liquid having a lower boiling point and a greater evaporation rate than said high boiling point liquid, and having an evaporation rate greater than about two times that of n-butyl acetate, transferring said particles from said imaging surface to said transfer sheet while said transfer sheet and said imaging surface are in contact, and drying said transfer sheet.
2. The method of claim 1 wherein said transfer sheet is uniformly impregnated with said volatile transfer liquid.
3. The method of claim 1 wherein said electrostatographic imaging surface comprises a photoconductive insulating layer.
4. The method of claim 1 wherein said particles are electrostatically transferred to said transfer sheet by charging said transfer sheet by-corona discharge.
5. The method of transferring from an electrostatographic imaging surface to a transfer sheet a developed toner image formed from a liquid developer comprising a high boiling insulating liquid and charged toner particles, said method comprising contacting said transfer sheet with a low boiling highly volatile insulating liquid having a lower boiling point and a greater evaporation rate than said high boiling liquid and having an evaporation rate greater than about two times that of n-butyl acetate, placing said transfer sheet in contact with said imaging surface while said toner image remains wet and electrostatically transferring said toner to said transfer sheet.
6. The method of claim 5 wherein said low boiling volatile insulating liquid is miscible with said high boiling insulating liquid.