US3756813A - Chargeless method of producing image on photoconductive surface - Google Patents

Abstract

AN IMAGE DEVELOPING TECHNIQUE ACCORDING TO WHICH AN IMAGE IS DEVELOPED UNDER DARKROOM CONDITIONS FIRST WITH A MEDIUM WHICH IS ADSORBED OR CHEMISORBED BY THE SURFACE AND WHICH EXTENDS THE IMAGE FORMING FIELD BY TRANSMITTING THE CHARGES FROM THE SURFACE TO A HIGHER LEVEL ABOVE THE SURFACE, AND THEN DEVELOPING THE RAISED IMAGE BY A DEVELOPER ATTRACTED TO THE FIELD, THE FIELD EXTENDER BEING CYCLOHEXANE, NITROBENZENE, FURAN, TOLUOL, ACETONE, KETONES, ALDEHYDES, ACIDS AND ESTERS.

Description

Patented Sept. 4, 1973 Int. Cl. 003 13/10, 13/22 US. Cl. 96--1 LY 6 Claims ABSTRACT OF THE DISCLOSURE An image developing technique according to which an image is developed under darkroom conditions first with a medium which is adsorbed or chemisorbed by the surface and which extends the image forming field by transmitting the charges from the surface to a higher level above the surface, and then developing the raised image by a developer attracted to the field, the field extender being cyclohexane, cycloheXanone, nitrobenzene, furan, toluol, acetone, ketones, aldehydes, acids and esters.

This application is a continuation-in-part of Ser. No. 870,079, filed Apr. 17, 1969, which in turn is a continuation-in-part of Ser. No. 449,882, filed Apr. 21, 1965, both earlier applications having been abandoned.

This invention relates to electrostatic photography and radiography and to the production of images on photoconductive surfaces, phosphor surfaces, metal and other surfaces by the impingement of light or X-rays on said surfaces. In particular this invention relates to a method of and means for producing images on such surfaces wherein no corona or electrically produced charge is applied to the surface before exposure, but also applies to charge methods particularly to low charge methods. For convenience images produced on a surface to which there has not been applied any corona or electrically produced charge will be termed herein as chargeless or light-induced images.

In prior art processes it has been claimed to produce images on photoconductor surfaces by producing in said photoconductive surfaces a change in conductivity wherein the conductivity of the layer is increased in the lightstruck areas following which the surface is charged by one component of the dry developer which is applied to the surface subsequent to the exposure. It is also known to produce on a surface a light-induced charge in the light-struck areas and to develop a visible image by means of sensitive liquid developers characterized in that the developers contain very fine particles of pigment and the like dispersed in an insulating liquid carrier. It is desirable to improve the density of chargeless images to match the charge methods, and to ensure that the polarity of the chargeless image is such as to give the desired attraction or repulsion development. Therefore it is one object of the present invention to increase the density of the image, and particularly though not only chargeless images, and to produce the image in a better manner. It is another object of the present invention to enable positive reproductions to be made by a process which in its preferred form involves no corona or electrical charging means to produce the surface charge.

The chargeless electrophotographic process as at pres ent known comprises the steps of exposing a dark-adapted electrophotographic surface or metal or phosphor surface or metallic salt or organic surface to a light or X-ray image and then developing the latent image produced by the impingement of the light or X-ray with developer particles, such as a pigment, dye, resin, oil or wax which are sensitive to the forces exerted by a low intensity electric field associated with the light-induced charge on the surface. Generally the density of the images is considerably less than that attained in the charge processes. Improvements have been made in the developed image and in the contrast of the images by the use of background cleaners which reduce background coloration but a disadvantage of present methods is the difiiculty of obtaining densities as high as those commonly associated with the charge methods.

According to the present invention there is provided a method of producing and developing an electrical image on a surface which comprises first allowing localized charges to be dissipated and while maintaining darkroom conditions exposing the surface to patterned radiation to produce image-forming fields thereon in accordance with said patterned radiation, and while still maintaining darkroom conditions developing the image-forming fields by applying particulate marking means from an electrically insulating carrier liquid, characterized by the added step of maintaining on the surface while effecting development a thin layer of a material which is absorbed, adsorbed or chemisorbed by the surface and which extends the imageforming fields by transmitting the charges from the surface to a higher level above the surface.

According to another aspect of the invention, there is provided a member for use as a support for an electrical image which is formed by exposing a surface of the member to patterned radiation to produce image-forming fields thereon, and developing the image-forming fields by applying particulate marking means from an electrically insulating carrier liquid, characterized by a layer of from 0.1 to 1 micron thick on the surface of a material which is absorbed, adsorbed or chemisorbed by the surface and Which extends the image-forming fields by transmitting the charges from the surface to a higher level above the surface.

The charge methods include the further step of charging an insulator on a surface and bleeding away the unwanted charge by a photoconductor through light or X-rays or other electromagnetic means, but even with the heavier charges then available the invention still improves development.

In one method of the present invention the following steps are used to attain the desired objects:

(1) The electrophotographic or metal surface or phosphor surface is dark-adapted by storing said surface in the dark for a period of about 24 hours or by subjecting said surface to a temperature of about F. for 1 hour and then exposing said surface to the activating radiation which is in question, whereby the radiation produces in and on the surface an electrostatic latent image or a change of state which results in a localization of charge or to produce an effect of polarization such as the Dember effect. The change of state produced varies in state according to the amount of light which has impinged on it and the result is one known as a continuous tone latent image.

(2) The surface is then treated with a field-extending material under safelight conditions whereby a thin film of said material is formed on or attached to said surface and the film or layer of sensitizer is activated according to the intensity of light which has previously impinged on the surface.

(3) The latent image the field of which is now made more readily available by the action of the absorbed or adsorbed or chemisorbed sensitizer is then developed by the application of an attractable liquid developer which would not normally develop a chargeless image but would develop a charged image of the prior art electrostatic printing processes.

In another method of the present invention the surface of the electrophotographic sheet or phosphor or metal sheet is first treated with the field-extending material before exposure of the dark-adapted surface. It should be appreciated that the field-extending material is not in the nature of a chemical photographic sensitizer but is an absorbed, adsorbed or chemisorbed film or layer of monomolecular or micellar layer which has the characteristic of making the surface charge more readily available by transmitting it to a higher level above the surface.

The field-extending material may comprise a liquid, a solution, a solid, a gas or an emulsion. The materials used cover a wide range of polar and non-polar substances with a wide range of values of dielectric constant and bulk specific resistivity. Optimum operation of the process depends on the formation of a thin layer of the fieldextending material on the electrophotographic surface in question, the layer being for example from 0.1 to 1 micron thick. This layer is applied by the following means:

(1) Depositing the layer from a solution by evaporating the solvent, for example by forced evaporation by air blast or by natural means under ambient conditions.

(2) Applying the material to the surface in a liquid and removing the excess material from the surface with a roller or other squeegee device which squeezes the excess away to leave a film, in which case development will occur whilst the film is still wet.

(3) Deposition of the required amount of field-extending material on the surface as a residue following evaporation of a carrier vehicle or solvent.

This invention does not depend on the use of sensitive fine-particle liquid developers as used in normal chargeless processes which we have heretofore described but functions successfully with developers designed for the charge processes.

It is believed that the developer particles which are not ordinarily retained by the chargeless image are attached to the surface by extending the field effect associated with the image by polarisation of the sensitizing material. The force retaining the deposited developer particles on the image forming surfaces increases as the distance between the particles and the attracting charged surface decreases. The minimum distance is limited particularly by developer particle size as demonstrated by the inability of the charge developers to retain material on a chargeless latent image. It is apparent that the thin layer of field-extending material serves virtually to decrease the distance between the developer particles and the developable surface charge by interposing dipoles to span the gap and consequently to provide a greater attracting force, or otherwise stated to provide an attractive bridge. Alternatively the action of the field-extending material is to desorb the gaseous barrier provided by the air or other insulating barrier and to provide a bridge of higher dielectric constant. Orientation of the field-extending material is possibly the governing factor, be this achieved by polar particles or by insulated particles which are polarized by the field to provide the dipole bridges at images areas, non-orientation at nonimage areas ensuring that no bridges are available and consequently a clean background results.

The field extending material may be applied to the photoconductor surface of electrophotographic paper or the like during manufacture and then forms a thin layer in which the polarising or dipole action takes place under an image field.

In further explanation of this invention it should perhaps be pointed out that according to the normal earlier used methods of producing and developing xerographic images it was necessary for the photoconductive surface to have an insulating layer or characteristic such that a charge could be applied to the surface by corona discharge means or the like which is held on the surface as long as dark conditions are maintained, the photoconductor however bleeding away this charge from the surface when it is struck by light so that, after charging, such a surface can then have a latent electrostatic image formed thereon by projecting a light image on to the surface, this latent electrostatic image being maintained so long as the image-bearing member is maintained under dark conditions. The image is rendered visible by applying particles of toner to the surface which will be either attracted to the image areas or in some cases will be forced to the non-image areas.

It is customary to provide means for fixing the image such as by including a fixing substance in a carrier liquid which will bond the image to the surface on evaporation of the carrier liquid or according to older methods it was customary to heat-fuse the image to the area.

At that stage it was generally believed that it was necessary to have a charge on the surface which could be bled away differentially, but more recently it has been shown that a charge can be produced on a surface by simply subjecting it to electromagnetic waves such as light waves or X-rays or the like, which charge, though very much less than the latent electrostatic image produced by the older methods, is nevertheless developable by a sensitive developer, but naturally as a lower order force exists on the electrostatic latent image, the densities attainable are not as good as with the methods where a charge is applied to the insulating surface and is then modified by the light image.

It must be clear at this stage therefore that according to the methods generally adopted heretofore a much heavier charge existed because the charge was applied to the insulative surface and then bled away where not required whereas in the later chargeless methods no charge whatever is applied and the effect is simply one of modifying the surface or interface materials themselves to give an image which can be developed electrically.

It is found that no insulating surface in the normal sense is necessary because the light-induced charges appear to be associated with the molecular structures of the material itself and for this reason the present invention applies to oxidizable or non-oxidizable surfaces.

Where there is no applied charge, it will be realised that the image which can be developed would be of a lower order in magnitude, and therefore the present invention is highly important in this field as it allows conditions to obtain which aid the development by providing bridging means or concentrating means whereby the relatively low order field can still give an image corresponding approximately to the image wihch can be obtained where charge methods are involved, and charge methods can naturally give better densities than previously.

It should be clear therefore that the invention is particularly suitable for chargeless development but it also applies to any method involving development of an electromagnetically induced image.

To illustrate the manner in which the invention is carried out the following examples are given relating to three modifications:

EXAMPLE 1 In this example the surface which was used to produce the latent chargeless image comprises a layer of photoconductive particles bound by an organic resin or other binder and coated on a backing such as paper, metal or plastic sheet. The sheet was dark-adapted by leaving the sheet in the dark for a period of 24 hours following which the sheet was removed to an exposing position where a contact print or a projection print was made in the following manner. The sheet was exposed to a light source for 5 seconds using a blue actinic watt lamp at a distance of 2 feet from the upper surface. After the exposure, the paper backed sheet was placed in a dish in which it was subjected to the action of a field-extending liquid for a period of 5 seconds, the field-extending liquid consisting of a technical grade of heptane, a nonpolar hydrocarbon liquid. After removal from the dish the Wet surface was allowed to dry in the atmosphere to the point where a change in the reflectance of the surface occurred giving a matt appearance indicating that the critical amount of material had been retained in an absorbed or adsorbed form. The sheet was then immersed in a developer, of which the particles were positively charged, contained in a dish. The composition of this developer has been described in prior patents of the present inventors wherein carbon black, reflux blue, and resins such as long oil alkyd resins are used to provide control and fixing actions. The concentration of the developer was limited to approximately 14% solids in the insulating carrier liquid. A development time of 5- seconds was required to produce a dense image. Upon removal from the developing bath the excess developer was removed by subjecting the surface to clean carrier liquid.

EXAMPLE 2 In the process of Example 1, the following liquid was substituted for the heptane: cyclohexane, commercial grade. The following liquids may also be employed: benzene, carbon tetrachloride, and carbon disulphide, other hydrocarbons and halogenated hydrocarbons.

EXAMPLE 3 In the process of Example 1, the following liquids of polar type may be substituted for the heptane: cyclohexanone, nitrobenzene, furan, toluol, and acetone, and other ketones, aldehydes, acids and esters.

EXAMPLE 4 In this embodiment of the present invention the expo sure step and the developing method were similar to those of Example 1 but the heptane was replaced with a mixture or solution of non-polar paraffin oil in heptane in the proportions 2 to percent of oil in the heptane, the heptane still constituting the field-extending material.

EXAMPLE 5 In the process of Example 4, the following liquids of low evaporation rate may be substituted for the paraffin oil added to the heptane; sunflower oil or linseed oil or animal fats, silicone oil, long oil or alkyd resin.

EXAMPLE 6 EXAMPLE 7 In this modification of the present invention, the electrophotographic sheet of Example 1 was replaced by metal sheet such as silver, lead or zinc.

EXAMPLE 8 In this modification of the present invention, the electrophotographic sheet of Example 1 was replaced by a selenium plate.

What we claim is:

117-37 LE; 250- R 1. A chargeless method of producing and developing an electrical image on a photoconducor surface which consists essentially of first allowing localized charges to be dissipated and while maintaining darkroom conditions exposing the photoconductor surface to patterned radiation to produce image-forming electrical fields thereon in accordance with said paterned radiation, and while still maintaining darkroom conditions developing the imageforming fields by applying particulate marking means from an electrically insulating carrier liquid, and maintaining on the surface while effecting development a thin layer of between 0.1 and 1 micron of a polar substance orientatable by the image forming fields, which substance is applied from a liquid and at least partially dried to be absorbed, adsorbed or chemisorbed by the surface and which extends the image-forming fields by transmitting the charges from the surface to a higher level above the surface, said substance being cyclohexane, cyclohexanone, nitrobenzene, furan, toluol, or acetone.

2. A method according to claim 1, wherein the fieldextending material is applied before imagewise exposure.

3. A method according to claim 1, wherein the fieldextending material is applied after imagewise exposure but before development.

4. A chargeless method of producing and developing an electrical image on a photoconductor surface which consists essentially of first allowing localized charges to be dissipated and while maintaining darkroom conditions exposing the photoconductor surface to patterned radiation to produce image-forming electrical fields thereon in accordance with said patterned radiation, and while still maintaining darkroom conditions developing the imageforming fields by applying particulate marking means from an electrically insulating carrier liquid, and maintaining on the surface while effecting development a thin layer of between 0.1 and 1 micron of a polarizable substance orientatable by the image forming fields, which substance is applied from a liquid and at least partially dried to be absorbed, adsorbed or chemisorbed by the surface and which extends the image-forming fields by transmitting the charges from the surface to a higher level above the surface, said substance being non-polar paraflin oil in heptane or a vegetable oil or animal fats, silicone oil or an alkyd resin or carbon tetrab-romide in a chlorinated fluorinated hydrocarbon liquid.

5. A method according to claim 4 wherein the fieldextending material is applied before imagewise exposure.

6. A method according to claim 4 wherein the fieldextending material is applied after imagewise exposure but before development.

References Cited UNITED STATES PATENTS 2,990,280 6/1961 Giaimo 96-1 3,068,115 12/1962 Gundlach 117-37 3,247,007 4/1966 Oliphant 117--37 3,251,688 5/1966 Mihajlon 96--1 3,032,432 5/1962 Metcalfe et a1. 11737 GEORGE F. LESMES, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R.