US3642471A

US3642471A - Liquid developing process in an electrostatographic imaging system

Info

Publication number: US3642471A
Application number: US70551A
Authority: US
Inventors: Masamichi Sato; Osamu Fukushima
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1969-09-19
Filing date: 1970-09-08
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15
Also published as: CA936034A; JPS4912429B1; DE2046219A1

Abstract

An electrostatographic imaging system employing liquid development of an electrostatic latent image present on an electrophotographic sheet wherein prior to development at least the nonimage bearing surface of said sheet is contacted with an insulating liquid having a boiling point not exceeding 100* C. and subsequently both surfaces of said sheet are contacted with an insulating liquid having a boiling point greater than about 150* C. The electrophotographic sheet is dried immediately after immersion in the liquid developer.

Description

United States Patent Sato et al.

[54] LIQUID DEVELOPING PROCESS IN AN ELECTROSTATOGRAPI-IIC IMAGING SYSTEM [72] inventors: Masamichi Sato; Osamu Fukushima, both of Asaka, Japan [73] Assignee: Xerox Corporation, Stamford, Conn.

[22] Filed: Sept. 8, 1970 [211 App]. No.: 70,551

[30] Foreign Application Priority Data Sept. 19, 1969 Japan ..44/74406 [52] U.S. Cl. ..96/l.8, 96/1 R, 96/1 LY, 117/37 LE [51] Int. Cl. ..G03g 5/00, 603g 7/00 [58] Field of Search ..96/1, 1 LY; 117/37 LE [56] References Cited UNITED STATES PATENTS 3,512,965 4/1970 Matkam..... .g

[451 Feb. 15, 1972 3,540,885 11/1970 Honjo et al. ..96/l

Primary Examiner-George F. Lesmes Assistant Examiner-John R. Miller Att0rney.lames .l. Ralabate, Albert A. Mahassel and Samuel E. Mott ABSTRACT 9 Claims, N0 Drawings LIQUID DEVELOPING PROCESS IN AN ELECTROSTATOGRAPHIC IMAGING SYSTEM BACKGROUND OF THE lNVENTlON This invention relates to imaging systems and more particularly, to liquid development systems for developing electrostatic latent images present on electrophotographic layers.

The formation and development of images on the surface of photoconductor material by electrostatic means is well known. The basic xerographic process as taught by C. F. Carlson in U.S. 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. Ineluded within this group are the cascade development technique disclosed by E. N. Wise in U.S. Pat. No. 2,618,552; the powder cloud development technique disclosed by C. F. Carlson in U.S. Pat. No. 2,221,776; and the magnetic brush process disclosed for example, in U.S. 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.

Automatic copying machines employing liquid development techniques particularly with electrophotographic sheets as the imaging surface are in widespread use today. In a particularly successful machine concept, the electrophotographic sheet bearing an electrostatic latent image is immersed in the liquid developer so that both the image bearing surface and the rear surface are contacted with the liquid developer. Toner particles present in the developer liquid which are very close to the surface of the electrophotographic sheet deposit on the sheet irrespective of the presence or absence of electrical charge on both the image bearing surface and the rear surface and may remain permanently on the surfaces. This small deposition of toner particles in the background areas on the electrophotographic sheet is referred to in the art as fogging. Fogging also occurs by the deposition of toner particles from that portion of the liquid developer which penetrates into the photoconductive layer and the base of the electrophotographic sheet. Fogging due to penetration by this mechanism is particularly conspicuous when employing an electrophotographic sheet such as Electrofax paper, which is a paper base coated with a mixture of a photoconductive powder in an insulating resin.

To minimize or eliminate the occurrence of fogging, prior to immersing the electrophotographic sheet in the liquid developer, the electrophotographic sheet may be prebathed or presoaked in a prebathing liquid which is retained on both sides of the sheet and in the fibrous construction of the base paper. The prebathing liquid is generally insulating, inert to the photoconductive material and miscible with the liquid developer in order to minimize any possibility of destruction of the imaging surface or contamination of the liquid developer. in order to provide these requirements, the prebathing liquid is typically very similar or identical to the carrier liquid employed in the liquid developer. Typically, these materials are nonpolar hydrocarbons and mineral oils. ln automatic copying machines, insulating, high-boiling liquids such as kerosene and Decalin have proved effective because they are less susceptible to loss due to vaporization within the copying machine and consequently provide reduced loss over a period of time and also provide little hazard due to fire.

Use of high-boiling materials as the prebathing liquid, however, has several shortcomings. For example, when the electrophotographic sheet has a paper base, the prebathing liquid wets both the surface bearing the electrostatic latent image and simultaneously penetrates into the fibrous construction of the paper base through the opposite surface of the sheet. Since the openings in the fibrous construction of the paper are extremely small, the prebathing liquid which has penetrated into these openings cannot be very easily forced out. Thus, throughout the stage of development, the prebathing liquid, which is retained in the fibrous construction of the paper base intercepts the liquid developing agent and consequently prevents it from penetrating into the openings in the fiber and smearing the base of the sheet. After the development step, the eleetrophotographic sheet is passed through a pair of squeezing rollers to remove any liquid developer that may adhere to the image bearing surface and the reverse surface of the sheet. However, this squeezing operation cannot remove the prebathing liquid which has been lodged deep within the fibrous construction of the paper. Consequently, the sheet thus obtained has a large volume of prebathing liquid penetrated inside the paper serving as the base for the electrophotographic sheet and therefore has a heavy, moist feeling. As a result, it is not desirable to have this sheet placed on top of another sheet because the prebath which has been retained within the sheet, gradually finds its way to the surface and wets the paper placed nearby or may dissolve the printing ink on the printed sheet of paper when placed adjacent to it. To prevent this undesirable contamination of adjacent surface, the prebathing liquid lodged deep within the construction of the paper base may be vaporized by means of a current of warm air. However, since the prebathing liquid has a high boiling point, this drying operation involving the use of a current of warrn air, which must be of such a temperature as to have no adverse affect on the paper, becomes excessively time consuming.

The practice of washing the electrophotographic sheet after development with a cleaning liquid to remove liquid developer from the electrophotographic sheet, prior to the removal of liquid developer through squeezing rollers is also frequently employed. It is generally preferred to employ a cleaning liquid which is insulating and has a low boiling point to take advantage of the comparative ease in drying. This is necessary because the cleaning liquid partially replaces the prebathing liquid which has been lodged deep within the construction of the paper and the low boiling cleaning liquid which has penetrated into the paper construction may be readily vaporized with a current of warm air. If, however, the prebathing liquid which is lodged deep in the paper is to be completely replaced with the cleaning liquid by this washing operation a considerable effort in time is necessary to complete this replacement. Therefore, any advantage that one would expect to be derived from the use of such a cleaning liquid is impractical in the case of a treatment which necessarily must be carried out by an automatic device requiring expeditious processing.

When a liquid of a relatively low boiling point is employed as the prebathing liquid, drying of the electrophotographic sheet after development may be more rapidly accomplished. However, as already discussed above, the use of such low-boiling liquids provides various difficulties.

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 liquid development system capable of producing dry copies at increased processing speed.

It is another object of this invention to provide a liquid development technique wherein the final copies are substantially free of prebathing liquid.

It is another object of this invention to provide an improved technique for prebathing an electrophotographic sheet prior to development.

It is another object of this invention to provide a liquid development technique producing developed images with reduced fogging in background areas.

The above objects and others are accomplished, generally speaking, by providing an electrostatographic imaging system of the liquid development type wherein prior to development of an electrostatic latent image present on electrophotographic sheet with a liquid developer having charged particles suspended in an insulating liquid at least the reverse or nonimage bearing surface of the electrophotographic sheet is contacted with a low-boiling, insulating liquid and subsequently, both surfaces of the electrophotographic sheet are brought into contact with a high-boiling insulating liquid.

More specifically, the present invention is based upon the principle that the electrophotographic sheet is first wetted with a low-boiling insulating liquid, hereinafter referred to as the first prebathing liquid, to allow it to penetrate sufiiciently into the paper serving as the base of the electrophotographic sheet, and the electrophotographic sheet is subsequently contacted with a high-boiling insulating liquid, hereinafter referred to as a second prebathing liquid to permit the formation of a film of the second prebathing liquid on the imaging surface. Thereafter, the electrophotographic sheet is immersed in the liquid developer to provide development of the electrostatic latent image. Since the first prebathing liquid sufficiently penetrates the fibrous construction of the paper, the second prebathing liquid cannot penetrate the paper. Furthermore, after application ofthe film of the second bathing liquid has been formed on the first prebathing liquid, the first prebathing liquid is prevented from mingling with the liquid developer and thus, deterioration by contamination of the liquid developer bath is prevented.

The first prebathing liquid may be applied only to the opposite or nonimage bearing surface of the electrophotographic sheet instead of being applied to both surfaces.

After treatment with the first prebathing liquid, the electrophotographic sheet may be subjected to the second prebathing treatment immediately. This practice, however, is not desirable because the first prebathing liquid intermingles with the second prebathing liquid and eventually finds its way gradually into the liquid developer. No serious problem, however, is posed when the volume of the second prebathing liquid is so large that introduction of a relatively small volume of the first prebathing liquid would provide no noticable change in the composition. However, large differences in the relative volumes of the liquids are not normally encountered. Therefore, to insure prevention of mixing of the first prebathing liquid with the liquid developer, it is preferred that subsequent to treatment of the electrophotographic sheet with a first prebathing liquid that the sheet be passed between insulating squeezing rollers to remove the portion of the first prebathing liquid which may adhere to the surfaces of the sheet. Thereafter, the electrophotographic sheet may be treated with the second prebathing liquid.

According to this invention, after development of the electrostatic latent image present on the electrophotographic sheet with a liquid developer, the sheet may be passed through squeezing rollers to remove the residual liquid developer and may thereafter be easily dried because only the low-boiling liquid remains lodged inside the construction of the paper. However, where the developing time is long, partial replacement may occur between the first prebathing liquid lodged in the paper and the liquid developer. It is, therefore, desirable to wash off the liquid developer with a low-boiling cleaning liquid after development and at the same time cause the cleaning liquid to replace the liquid developer which has penetrated deep into the paper. Any suitable cleaning liquid may be employed. Typically, the cleaning liquid may be one or more of the liquids used in the first prebathing treatment.

Any suitable material may be employed as the first prebathing liquid. Typically, the first prebathing liquid is selected to have such properties that it does not destroy the electrostatic latent image present on the electrophotographic layer, nor does it attack the structure of the electrophotographic sheet. Typically, these materials are selected from the group of nonpolar, highly insulating organic liquids having boiling points below about C. To prevent destruction of the electrostatic latent image it is generally preferred to use liquids having dielectric constants below about 3.5, resistivities greater than about 10 ohm-cm. and solubility parameters below about 8. Typical specific materials useful as the first prebathing liquid include cyclohexane, n-hexane, n-heptane, isopentane, CCl FCCl F, CCl FCClF Any suitable liquid may be employed as the second prebathing liquid. Typically, the second prebathing liquids have boiling points in the range of from about to about 300 C., dielectric constants less than about 3.5, volume resistivities greater than 10 ohm-cm. and solubility parameters below 8. Typical specific materials include kerosene and lsopar H, an isoparaffinic hydrocarbon available from Humble Oil and Refining Company.

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 hereinabovementioned illustrative materials. For example, while photosensitive paper comprising zinc oxide in an insulating binder layer coated on ordinary paper which is fibrous and can be impregnated are discussed 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 electrostatographic imaging members may be employed.

Development of the electrostatic latent image may be obtained with any suitable liquid developer. Typical liquid developers contain electroscopic marking particles dispersed in an insulating liquid vehicle and may also contain control agents and suspending agents for their well-known functions. The liquid employed must have a relatively high insulating value, generally having a volume resistivity greater than about 10" ohm-cm. so as not to affect the electrostatic charge pattern on the insulating layer and low dielectric constants of less than about 3.5. Typical specific vehicles include hydrocarbons such as benzene, xylene, hexane, naptha, kerosene, halogenated hydrocarbons such as carbon tetrachloride, trichloroethylene and chloroform. Typical electroscopic marking particles include among others, charcoal, carbon black, magnesium oxide, lithopone, cadmium yellow, chrome yellow, cobalt blue, cadmium red, burnt siena, Hansa yellow, rose bengal and phthalocyanine. Typically, the electroscopic marking particles are present in an amount of from about 4 to about grams per liter. The electroscopic marking particles are conventionally dispersed and suspended in the liquid by stirring or agitation and where a highly uniform and stable suspension is desired, the suspension may be passed through a colloid mill.

The electrophotographic sheet may be treated with the first or second prebathing liquid at any suitable time. It may, for example, be treated sequentially immediately following formation of the electrostatic latent image. Alternatively, the electrophotographic sheet may be treated first with the first prebathing liquid and may even be treated with a second prebathing liquid prior to the formation of the electrostatic latent image. The prebathing liquid may be applied to the electrophotographic sheet in any suitable manner. The sheet may, for example, be immersed in the prebath or may be brought into contact with a sponge or felt impregnated with a prebathing liquid. The sheet may also be sprayed with the prebathing liquid. A particularly preferred means of applying the prebathing liquid is by means of a spongy roller impregnated with the liquid which is rolled across the electrophotographic sheet and transfers theliquid to the sheet on contact.

DESCRIPTION OF PREFERRED EMBODIMENTS The following preferred examples further define, describe and compare preferred materials, methods and techniques of the present invention. In the examples, all parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 An electrophotographic sheet is prepared by coating a sheet of art paper about 150 microns in thickness which is undercoated with a conductive material such as conductive polymer 261 (available from Calgon Corporation) with a mixture of photoconductive zinc oxide powder in an insulating resin which when dry has a thickness of about 7 microns. The electrophotographic sheet is charged negatively in the dark to a surface potential of about 200 volts. The charged sheet is exposed to light projected through an original to form an electrostatic latent image. The image bearing sheet is then immersed for about 5 seconds in lsopar E, a low-boiling, insulating, isoparaffinic hydrocarbon availablefrom Humble Oil and Refining Company and subsequently passes through a pair of squeezing rollers made of silicon rubber to remove the lsopar E adhering to both surfaces of the sheet. The pores in the fibrous construction of the base paper are impregnatedwith lsopar E. Thereafter, the electrophotographic sheet is soaked for about 5 seconds in the second prebathing liquid or lsopar H, a high-boiling, insulating, iosparaffinic hydrocarbon also available from Humble Oil and Refining Company, removed from the bath, held up to allow the second prebath to trickle down and immersed in the liquid developer. The liquid developer is prepared by dispersing carbon black, ranging in particle size of from about 0.1 to about 1 micron in lsopar H with a small quantity of varnish dispersion stabilizer, also being present. After about 1 minute immersion in the liquid developer, the sheet is removed and passed through a pair of squeezing rollers to remove liquid developer adhering to both surfaces of the sheet. The squeezing roller which contacts the surface carrying the toner image is a hard, smooth metallic roller with a mirror finish surface and the squeezing roller that contacts the reverse soft surface is a soft, resilient rubber roller. Thereafter, the sheet is exposed to a current of warm air at about 50 C. for about 20 seconds to completely vaporize the liquid lodged deep in the paper. A dry print with substantially no fogging in the background areas is produced.

EXAMPLE ll The procedure of Example 1 is repeated except that immediately following development of the electrostatic latent image on the electrophotographic layer, both surfaces of the electrophotographic sheet are sprayed with a cleaning liquid of lsopar E to remove the liquid developer which has adhered to both surfaces. The electrophotographic sheet is then passed through the squeezing rollers to remove the cleaning liquid followed by drying. Complete drying is obtained in several tens of seconds when the sheet is allowed to stand at normal room temperature. When exposed to a current of warm air at 50 C., drying is obtained in less than about 10 seconds. Dry prints of similar quality to those obtained in Example I are produced.

' EXAMPLE in The procedure of Example I is repeated except that prior to formation of the electrostatic latent image on the electrophotographic sheet, the sheet is first contacted with a sponge roller which is impregnated with the first prebathing liquid, lsopar E. Thereafter, the formation of the electrostatic latent image, the second prebathing treatment and the remaining procedure of Example 1 are repeated. Prints of quality similar to those obtained in Example I are produced.

Although specific materials and operational techniques are set forth in the above exemplary embodiments using the development techniques of this invention, these 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:

1. In an electrostatographic imaging method comprising forming an electrostatic latent image on an electrostatographic imaging member comprising an image layer capable of receiving an electrostatic latent image coated on a fibrous substrate, developing said electrostatic latent image by contacting said imaging layer with a liquid developer comprising marking particles dispersed in an insulating liquid and drying said imaging member, the improvement comprising prior to development of the electrostatic latent image, contacting at least the side of said imaging member opposite and imaging layer with a first prebathing liquid comprising an insulating nonpolar organic liquid having a boiling point no greater than about C. and subsequently, contacting both surfaces of said imaging member with a second prebathing liquid comprising an insulating organic liquid having a boiling point between about and 300 C.

2. The method of claim 1 wherein both surfaces of said imaging member are contacted with said first prebathing liquid.

3. The method of claim 1 wherein said imaging member is contacted with said first prebathing liquid prior to formation of said electrostatic latent image.

4. The method of claim 1 wherein said imaging member is contacted with said first prebathing liquid after formation of said electrostatic latent image.

5. The method of claim 1 wherein following development of said electrostatic latent image, said imaging layer is contacted with a cleaning liquid comprising an insulating liquid having a boiling point less than about 100 C.

6. The method of claim 1 wherein said imaging layer comprises photoconductive pigment particles substantially unifonnly dispersed in an insulating material.

7. The method of claim 6 wherein said photoconductive pigment particles comprise zinc oxide and said insulating material is a resin.

- 8. The method of claim 1 wherein excess first prebathing liquid is removed from the surface of said imaging member before contact with said second prebathing liquid.

9. The method of claim 1 wherein said fibrous substrate is paper.

Claims

2. The method of claim 1 wherein both surfaces of said imaging member are contacted with said first prebathing liquid.
3. The method of claim 1 wherein said imaging member is contacted with said first prebathing liquid prior to formation of said electrostatic latent image.
4. The method of claim 1 wherein said imaging member is contacted with said first prebathing liquid after formation of said electrostatic latent image.
5. The method of claim 1 wherein following development of said electrostatic latent image, said imaging layer is contacted with a cleaning liquid comprising an insulating liquid having a boiling point less than about 100* C.
6. The method of claim 1 wherein said imaging layer comprises photoconductive pigment particles substantially uniformly dispersed in an insulating material.
7. The method of claim 6 wherein said photoconductive pigment particles comprise zinc oxide and said insulating material is a resin.
8. The method of claim 1 wherein excess first prebathing liquid is removed from the surface of said imaging member before contact with said second prebathing liquid.
9. The method of claim 1 wherein said fibrous substrate is paper.