US3814599A - Electrophotographic transfer process - Google Patents

Abstract

IMAGE RECEIVING ELEMENTS ARE PROVIDED FOR RECEIVING PATTERNS OF ELECTOSCOPIC TONER PARTICLES. THESE ELEMENTS ARE COMPRISED OF A FIBROUS SUPPORT BEARING ON ONE SURFACE THEREOF, A LAYER OF AN ELECTRICALLY CONDUCTIVE POLYMER COMPRISING A SALT OF A CARBOXY ESTER LACTONE RESIN. THE FIBROUS SUPPORT SHOULD HAVE A VOLUME RESISTIVITY OF AT LEAST ABOUT 10**9 OHM-CM., AND SAID LAYER SHOULD HAVE A SURFACE RESISTIVITY LESS THAN 10**9 OHMS PER SQUARE. AN ADDITIONAL LAYER CAN BE PROVIDED BETWEEN THE SUPPORT AND THE FIRST NAMED LAYER TO RESIST PENETRATION OF LIQUID DEVELOPER. AN EXAMPLE OF RESIN FOR THE FIRST NAMED LAYER IS A SODIUM SALT OF A HYDRATED INTERPOLYMER OF MALEIC ANHYDRIDE AND VINYL ACETATE. THE INVENTION ALSO CONCERNS AN ELECTROPHOTOGRAPHIT PROCESS UTILIZING THE NOVEL IMAGE RECEIVING ELEMENT TO RECEIVE A TONER IMAGE FROM AN IMAGECARRING ELEMENT.

Description

United States Patent 3,814,599 ELECTROPHOTOGRAPHIC TRANSFER PROCESS David A. Cree, Webster, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y.

N0 Drawing. Filed July 6, 1971, Ser. No. 160,065

Int. Cl. G03g 13/16 U.S. Cl. 961.4 7 Claims ABSTRACT OF THE DISCLOSURE Image receiving elements are provided for receiving patterns of electroscopic toner particles. These elements are comprised of a fibrous support bearing on one surface thereof, a layer of an electrically conductive polymer comprisng a salt of a carboxy ester lactone resin. The fibrous support should have a volume resistivity of at least about ohm-cm, and said layer should have a surface resistivity less than about 10 ohms per square. An additional layer can be provided between the support and the first named layer to resist penetration of liquid developer. An example of resin for the first named layer is a sodium salt of a hydrated interpolymer of maleic anhydride and vinyl acetate. The invention also concerns an electrophotographic process utilizing the novel image receiving element to receive a toner image from an imagecarrying element.

This invention relates to electrography and more particularly to elements adapted for receiving liquid developed images formed from electrostatic charge patterns.

Electrographic imaging processes and particularly electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature. In electrophotography, it is known to use an element comprised of a support bearing a layer of a photoconductive composition typically comprising a resinous binder and a photoconductor which can be organic or inorganic. The photoconductive layer, while in the dark, will accept and retain an electrostatic charge. After charging, such an element is exposed in a suitable manner to an imagewise pattern of electromagnetic radiation. This exposure causes the charge on the surface of the element to be reduced in accordance with the amount of actinic radiation received by each area of the surface. After exposure, the resultant electrostatic charge pattern is developed by contacting it with electroscopic marking particles which adhere to the element in accordance with the charge pattern. The marking particles may be in the form of a dry powder or they may be carried in an electrically insulating organic liquid, as is well known in the art. In those cases in which high resolution is desired, liquid development is preferred, as the size of the toner particles contained in liquid developers is much smaller than can be readily obtained with dry developers, and the distribution of toner on the electrostatic image is much more uniform.

If it is desired to reuse the electrophotographic element, the developed image must be transferred to a suitable receiving element. A developed toner image obtained through the use of a dry developer can be transferred to ordinary paper, for example, with the assistance of a corona charging device. It is found, however, that a liquid developed image does not transfer well to ordinary paper in this manner. In order to achieve good transfer, especially of liquid developed images, it is necessary to the various prior art receiver sheets that they be specially treated to obtain maximum electric conductivity therein. Then, during transfer, an electrical field can be established between an electrode on the rear of such element and the conducting layer of a photoconductive element bearing a developed image. The receiving elements of the prior art involve a necessary treatment of the paper support with 3,814,599 Patented June 4, 1974 a variety of expensive chemicals often involving a number of costly manufacturing steps. These prior elements have also been found to be extremely sensitive to environmental conditions which tend to produce undesirable curl, etc. Such prior elements also exhibit a lowering of the electrical conductivity thereof under low relative humidity conditions as often encountered in electrostatic copying apparatus. A lowering of the conductivity will inhibit good image transfer. Accordingly, there is a need for improved and less expensive transfer elements for receiving toner images which overcome these difiiculties, and for an improved toner transfer process utilizing the novel elements.

It is, therefore, an object of this invention to provide novel receiving elements for use in electrography.

It is another object of this invention to provide improved receiver sheets for use in the transfer of liquid developed images, Which receiver sheets enable the production of transferred images having increased sharpness and density.

It is yet another object of this invention to provide such receiver elements having improved liquid carrier holdout.

It is a further object of this invention to provide novel receiver elements which can be used with an improved toner transfer process.

It is still a further object of this invention to provide improved processes for the production of transferred liquid developed images using the improved receiver elements described herein.

These and other objects and advantages are achieved through the production of receiving elements comprising a porous, fibrous Web support bearing on at least one surface thereof a layer of an electrically conductive filmforming polymeric material, said layer having a surface resistivity less than about 10 ohms per square. Although conductive layers of many kinds, such as cuprous iodide, Calgon 261 Conductive Polymer, polystyrene sulfonic acid sodium salt in binders such as vinyl polymers, gelatin, etc., could be used to form conductive layers, the present elements preferably utilize conductive layers comprised of salts of carboxy ester lactones prepared from resinous interpolymers derived from the polymerization product of any of several unsaturated a,B-dicarboxylic acid anhydrides with the vinyl ester of an organic acid. When the above polymerization product is modified by reaction with a monohydric alcohol and an acid catalyst, it is deacylated and simultaneously esterified with the monohydric alcohol and converted into the lactone. These compounds and their preparation have been described in McNally and Van Dyke U.S. Pat. 2,306,071, dated Dec. 22, 1942. The compounds of McNally et al. are insoluble in water, and are insoluble in alkali unless hydrolyzed. The hydrolysis treatment comprises heating a solution of the interpolymer with a mixture of a monohydric hydroxy acid and a monohydric alkanol under acid conditions, as further described in Minsk U.S. Pat. 3,007,901, dated Nov. 7, 1961. They are then converted to a Water-soluble form by dissolving in an appropriate solvent and adding just suflicient alkaline reagent thereto to fix the pH of the solution at from about 5 to about 8 whereby the inherently hydrophobic, unneutralized resinous carboxy ester lactone is converted to the inherently hydrophilic alkali salt of the lactone, maintaining the original lactone rings substantially intact. The conversion to the water-soluble form is further set forth in detail in Minsk U.S. Pat. 3,169,946, dated Feb. 16, 1965. Other suitable polymers for use in the conductive layers are prepared similarly to the method set forth above, except that the interpolymer of McNally et al. is instead treated with a mixture of water, a monohydric alcohol and an acid catalyst in which the water is present in a molar ratio of at least 1.8;1

to the monohydric alcohol or without any alcohol present, wherein the starting polymer has an inherent viscosity of no more than about 0.25 measured in acetone solution. This procedure is set forth in more detail in Minsk et al. US. Pat. 3,260,706, dated July 12, 1966. The resulting polymer can readily be convertel to the water-soluble form by the method of the Minsk US. Pat. 3,169,946 already referred to. These polymers may contain conventional coating addenda such as matting agents, etc., if desired.

A particularly advantageous embodiment is obtained when the carboxy ester lactone resin is coated from an aqueous solution having a pH between about 5 and 6. This can be accomplished by precipitating the lactone in water followed by acid washing as described in the above patents. Next, the polymeric lactone is converted to a salt form by the addition of sodium bicarbonate, for example. The pH can thus be adjusted by varying the amount of sodium bicarbonate added, with lower amounts of bicarbonate giving a lower pH value.

The elements of this invention are typically formed on a fibrous support of relatively low conductivity. Useful materials include any of a variety of matted fibrous materials, which may be natural or synthetic, such as polyesters, polyamides, and cellulose fibers and the like. Conventional paper which has received no treatment to increase its conductivity at normal humidities is particularly useful. Generally, the volume resistivity of the support is in excess of about to 10 ohm-cm. The present invention thus provides receiving sheets which can use ordinary, inexpensive paper supports with a conducting overlayer, rather than expensive conductive papers having an insulating layer thereon as referred to in the prior art.

The paper or other support is then coated on one surface with a solution containing the conductive polymer herein described. The concentration of polymer in the solution may be from about 10 gm./l. to about 150 gm./l.

- as desired to produce optimum coating conditions. The

concentration is not critical to the operation of the invention, and any concentration found to be convenient can be used. Similarly, coating thickness can be varied over a wide range, so that there results a dried layer of conductive polymer having a surface resistivity less than about 10 ohm/sq. It is found that typically this level of conductivity can be obtained by coating the polymer at a dry weight of from about 2 to about 10 g./m. of support. Similarly, coating temperature can be varied over a rather wide range. Useful results are obtained when the solution is coated at a temperature between about and about 75 C., with a particularly preferred temperature being between about and 40 C.

If desired, there may be provided an interlayer between the support and the conductive layer. This interlayer is preferably formed of a polymeric material resistant to the penetration of the carrier liquid with which the element will be brought into contact during image transfer. It may optionally contain brighteners or reflecting means in order to enhance the contrast and brilliance of the image which will ultimately reside on the element. Suitable polymeric materials include, for example, polystyrene, gelatin, polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral. A preferred composition for the interlayer comprises polyethylene which contains a pigment such as, for example, titanium dioxide, barium sulfate or other highly reflective, particulate material.

Preferred receiving elements according to the invention also hear a backing layer on the side of the support opposite to that bearing the image-receiving layer. The backing layer functions to reduce curl and to prevent any carrier liquid from penetrating into the support from the reverse side. In general, the electrical and solventresistance properties of this layer are the same as are required for the optional intermediate layer, so that it may be formed of any material suitable for that layer. Optical brighteners generally are not necessary in the backing layer, as the light necessary to view the image is that light reflected from the interlayer and from the support. Similarly, reflecting pigments are not required, for the same reason. The contribution of reflecting means in the backing layer is negligible compared to that of the support and any layers between the support and the image-receiving layers.

The image-receiving sheets of the invention are used in an electrophotographic process involving toner transfer. An electrophotographic recording element is first used to produce an electrostatic charge pattern corresponding to a pattern of actinic radiation to be recorded. Particularly useful recording elements comprise a conductive support bearing a layer of an organic photoconductive composition of the type well known in the art. The photosensitive surface of the element is first charged electrostatically to a convenient predetermined potential, for example, about500-60O volts, in the absence of activating radiation, and is then exposed to a pattern of such radiation which is to be recorded, thereby producing a pattern of electrostatic charge corresponding to the radiation pattern. The surface bearing the charge pattern is then contacted with electroscopic marking particles, preferably carried in the form of a dispersion in an electrically insulating liquid. The particles adhere to the surface of the element in accordance with the charge pattern to form a reproduction of the original from which the charge pattern is derived. The image-receiving surface of the image-receiving element is then brought into contact with the toner-bearing surface of the recording element, and simultaneously therewith, a bias potential is applied between the conductive support of the recording element and the conductive layer of the image-receiving element. The two elements are then carefully peeled apart, whereupon a sharp, high-quality transferred image is found on the image-receiving element. After separation, the bias voltage is disconnected. The image on the receiving element typically needs no further treatment to be smudge-proof and permanent.

Liquid developers suitable for forming the image to be transferred may be chosen from a wide variety of materials. These developers typically comprise suitable marking particles dispersed in an insulating carrier liquid. Preferably, the carrier liquid has a low dielectric con stant and a very high electrical resistance such that it will not disturb or destroy the electrostatic charge pattern being developed. In general, useful carrier liquids should have a dielectric constant of less than about 3, should have a volume resistivity greater than about 10 ohm/ cm. and should be stable under a variety of conditions. Suitable carrier liquids include halogenated hydrocarbon solvents such as, for example, fluorinated lower alkanes, such as trichloromonofluoromethane, trichlorotrifluoroethane, etc., having a boiling range typically from about 2 C. to about 55 C. Other hydrocarbon solvents are useful, such as isoparaffinic hydrocarbons having a boiling range of from about C. to about C., such as Isopar G (Humble Oil & Refining Co.) or cyclohydrocarbons such as cyclohexane. Additional carrier liquids which may be useful in certain situations include polysiloxanes, odorless mineral spirits, octane, etc. The marking particles dispersed in the carrier liquid typically comprise a resinous binder and optionally contains various colorants and/or charge control agents as is well known in the art.

The following examples are included for a further understanding of the invention.

EXAMPLE 1 A receiver sheet is prepared by coating one surface of an untreated paper support with an electrically insulating layer comprising clear polyethylene having a thickness of about 10 microns. The other surface bears a 25-micron thick polyethylene layer containing titanium dioxide, over which is a conductive layer comprising the sodium salt of a resinous carboxy ester lactone, prepared by the method of Examples 2 and 17 of the aforementioned Minsk US. Pat. 3,169,946. The dried weight of the conductive coating is 9' grams per square meter. An electrostatic charge pattern is formed on the surface of an electrophotographic element comprising a conductive support bearing a layer comprising an organic photoconductor which has been optically sensitized so as to give it essentially panchromatic response. The charge pattern is formed by uniformly charging the surface of the photoconductive layer to about +1000 volts and exposing it to the light from a 3000" K. tungsten source which has passed through a test color negative and a Corning 5-60 Blue filter. The test negative bears the image of an archery target containing concentric black, blue, red and yellow annuli. The charge pattern is rendered visible by contacting it with a yellow pigmented positive liquid developer comprising an alkyd resin, a phenol-formaldehyde resin, a hydrocarbon solvent and a charge control agent, all dispersed in an isoparaffinic hydrocarbon liquid carrier (Isopar G). A development electrode biased at +900 volts was also utilized. The toner image is then transferred to the receiver sheet of the invention by placing the conductive layer of the receiver sheet in contact with the image-bearing layer of the photoconductive element and applying a 900 volt DC bias potential between the conductive layer of the receiver sheet and the conductive support of the electro photographic element, the receiver sheet being connected to the negative terminal of the bias supply. The element and the sheet are then separated and the bias removed. The image is found to have transferred cleanly to the receiver sheet. A second and a third image are formed in a similar manner by exposing through a Kodak Wratten No. 61 green filter and a Kodak Wratten No. 70 red filter, respectively, followed by development of the resultant charge patterns with magenta and cyan liquid developers, respectively. The resulting images are each transferred in register to the same rceiver sheet. A second print is made in the same manner from a test color negative bearing the image of a girl sitting on a ladder in front of a standard archery target. As a control, a receiver sheet containing Calgon Conductive Polymer 261 is overcoated with a conductive baryta layer at 30 g./m. followed by the appli cation of a layer comprising the sodium salt of a resinous carboxy ester lactone coated at 9 g./rn. Transfers are made in the above manner to this receiver sheet. Reflection density measurements are made on a reflection densitometer at various locations on the two subjects reproduced in the prints. The densities measured are given on It is seen that the receiver sheet prepared according to the invention gives appreciably greater image density than the receiver sheet with Calgon 261. In a further experiment to demonstrate the improved results that can be obtained with the receiver sheets of the present invention, gloss measurements are made using a goniophotometer adjusted so that the incident light strikes the surface at 75 from the normal, and reflected light is collected at 75 on the opposite side of the normal, the incident light, the reflected light and the normal all being contained in the same plane. The results using the same two subjects referred to above are given in Table II.

Several polyethylene coated paper supports as described in Example 1 having a back coating of unpigmented polyethylene are each overcoated with a conductive layer as described in Example 1 only each conductive layer is formed from an aqueous solution of polymer having a different pH value. The resin lactone is prepared as described in Example 2 of Minsk US. Pat. No. 3,169,946 to give a solution of product having a pH of about 5.2. Sodium bicarbonate is then added in varying amounts to give greater or lesser conversion to the sodium salt and to give a higher or lower pH value. The following pH values are obtained when the designated amount of sodium bicarbonate is added to 1000 ml. of a 7% solution of the lactone as prepared above.

Solution number:

The resultant solutions are then used to prepare receiver elements 1 through 6, respectively, having a conductive coating weight of 6 g./m. Elements 1 through 6 all have a surface resistivity of about 7 10 ohms/ sq. An electrostatic charge pattern is then formed on the surface of an electrophotographic element comprising a conductive support bearing a layer comprising a sensitized organic photoconductor in a resin binder. The surface of the photoconductive layer is uniformly charged to about 600 volts and imagewise exposed to a 3000 K. tungsten source. The charge pattern is rendered visible by contacting it with a liquid developer as in Example 1. The toner image is then transferred to one of the receiver sheet by placing the conductive layer of the receiver sheet in contact with the image-bearing layer of the photoconductive element and applying a 900 volt bias potential. This potential is applied between the conductive layer of the receiver sheet and the conductive support of the electrophotographic element with the receiver sheet being connected to the negative terminal of the bias supply. The element and the sheet are then separated and the bias removed. The image is found to have transferred to the receiver sheet. This process is repeated for each of the several receiver sheets prepared as described earlier in this example. A visible image is obtained with all six receiver sheets; however, the quality of the transfer print, as judged on the basis of the uniformity and degree of image transfer, is better with elements 1 through 4.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. In an electrophotographic process comprising the steps of forming an electroscopic toner image on an imagecarrying element and transferring said toner image to an image-receiving member, the improvement comprising using as said member a fibrous web support having a volume resistivity of at least about 10 ohm-cm, said support bearing on at least one surface thereof an imagereceiving layer comprising an electrically conductive salt of a film-forming polymeric resin lactone, said layer having a surface resistivity less than about 10? ohms per square.

2. In an electrophotographic process comprising the steps of forming an electrostatic charge pattern on an imaging surface of an image-carrying element, contacting said surface with a liquid developer comprising electroscopic mar-king particles carried in an electrically insulating liquid having a dielectric constant less than about 3 and a volume resistivity greater than about 10 ohm-cm, whereby said particles are deposited on said surface in accordance with said charge pattern to form a reproduction thereof, and transferring said particles to an imagereceiving member, the improvement comprising using as said memer a fibrous cellulosic web support having a volume resistivity of at least about 10 ohm-cm., said support bearing on one surface thereof an outermost image-receiving layer comprising an electrically conductive salt of a polymeric carboxy ester lactone, said receiving layer having a surface resistivity less than about 10 ohms per square.

3. The process described in claim 2 wherein said web bears an electrically insulating liquid developer resistant layer on the surface opposite to that bearing said electrically conductive layer.

4. The process as described in claim 2 wherein said polymeric lactone is selected from the group consisting of alkali metal or ammonium salts of a hydrated interpolymer of maleic anhydride and vinyl acetate.

5. The process as described in claim 4 wherein the alkali metal forming said salt is sodium.

6. A process for forming a visible image comprising the steps of:

(a) forming a charge pattern on the insulating surface of an element comprising a normally insulating layer carried on a conductive support;

(b) contacting said surface with a liquid developer comprising electroscopic marking particles carried in an electrically insulating liquid having a dielectric constant less than about 3 and a volume resistivity greater than about 10 ohm-cm; (c) bringing said surface into intimate contact with the conductive surface of an image-receiving member comprising a fibrous cellulosic web support having a volume resistivity of at least about ohm-cm, said support bearing on one surface thereof an electrically conductive image-receiving layer comprising an electrically conductive salt of a polymeric resin lactone, said receiving layer having a surface resistivity less than about 10 ohms per square; and (d) simultaneously with (c), applying a potential difference between said conductive support and said imagereceiving layer, said potential difference being in such a direction as to force toner particles to said receiving layer; whereby said toner particles form a visible image on said image-receiving member corresponding to said charge pattern.

7. The process described in claim 6 additionally comprising the steps of separating said layers and removing said potential difference.

References Cited UNITED STATES PATENTS 3,672,930 6/1972 Trachtenberg 11737 LE 3,565,614 2/1971 Carreira et a1. 11737 LE 3,169,946 2/1965 Minsk 26078.3 3,260,706 7/1966 Minsk et a1. 260-78.3

FOREIGN PATENTS 1,136,457 12/1968 Great Britain 11737 MICHAEL SOFOCLEOUS, Primary Examiner US. Cl. X.R..

9 6-1 LY; 117-37 LE, 72 FP, -P, UA