US3547076A - Apparatus for increasing the contrast in liquid immersion developing of electrostatic image - Google Patents

Description

United States Patent [72] Inventor Arvhd R. Sakllltar Dork Forest, 111. [21] Appl. No. 625,405 [22] Filed March 23, 1967 [45] Patented Dec. 15, 1970 [73] Assignee TheShervvln-Willlams Company Cleveland, Ohio a corporation of Ohio [54] APPARATUS FOR INCREASING THE CONTRAST 1N LIQUID IMMERSION DEVELOPING OF ELECTROSTATIC IMAGE 5 Ch na, 2 Dram l ke.

[52] US. 118/637; 1 1 17/37 [51 Int. B$b /02 [50] Field Search 118/637; 117/175. 37; 96/1.3; 355/ [56] References Cled UNITED STATES PATENTS 3,068,l 12/1962 Gundlach 117/37 3,284,224 11/1966 Lehmann ll7/l7.5

5/1966 Ostensen Primary Examiner-Peter Feldman Attorneys-Thomas .1. Mc Dowell and Richard G. Smith ABSTRACT: This invention comprises an apparatus for improving the deposition of particles upon a photographic carrier having an electrostatic latent image on one surface, comprising the steps of passing the carrier into a tank of nonconducting liquid developer having electrostatically attractable colloidal particles therein, and between two electrodes submerged in the liquid developer, these electrodes being spaced from each other and being wide enough to embrace the entire width of the latent image on the carrier between the superim posed areas of the electrodes, while an alternating voltage of 001-20, preferably 0.01-10 volts, is applied to these electrodes at a frequency of cycles to 1000 kilocycles per second.

PATENTEDBEBISISYG 3547.076

' INVENTOR ARV/ND R. .SAKL IKAR BYwManW ATTORNEY APPARATUS FOR INCREASING THE CONTRAST IN LIQUID IMMERSION DEVELOPING OF ELECTROSTATIC IMAGE This invention relates to improvement in apparatus for developing photoconductive layers carrying latent electric images by a development method which is known as Liquid Immersion Development." In particular it relates to improvements whereby the image can be developed to give a more intense deposition of particles resulting in. a higher degree of definition and permanence.

In electrophotography, a plate or paper is used which has a layer of photoconductive material, such as light-sensitive zinc oxide which will accept and hold an electrostatic charge. This plate or paper is exposed in a manner to vary the charge thereon, according to the pattern or design to be reproduced and the plate or paper is subsequently developed by immersing it in an insulating liquid containing the colloidal particles, such that when it is immersed in the liquid, colloidal particles will be deposited in conformity with the charge on the various sections of the paper, thus producing a desired image.

The pigment or particle material'is preferably carried in a high resistance liquid which will not destroy the charge on the paper, at least during the development period. It is extremely important to obtain uniform coverage of the paper to insure a buildup of the charged particles in exact proportion to the image left by exposure of the charged plate. The colloidal particles must be uniformly and rapidly deposited in a density varying in proportion to the charge held by the various parts of the image, so as to accurately reproduce the original image as translated by the variations in charge applied to the paper. One of the problems in existing processes is to obtain deposition of particles in sufficient density in the relatively short time the image is being developed.

In the present method it is generally difficult to obtain uniform coverage of the paper with a deposition of the colloidal particles in exact proportion to the image left by the exposure of the charged plate. In other words, it is difficult to effect deposition of the colloidal particles in sufficient quantities and only in those areas where the colloidal particles have the correct polarity in relation to the charge on the paper.

In accordance with the present invention, it has surprisingly been found that the application of an alternating current on both sides of the paper, as more fullydescribed hereinafter, effects a marked improvement in the intensity of colloidal particle deposition in accordance with the variation in the charge on the paper. This effect of the'alternating current, or more truly alternating voltage, is entirely unexpected.- Since the process is based on the electrostatic charge on the paper'and the attraction of the charged paper for particles having more or less static charges, it might be expected that a static voltage or direct current application would be more appropriate. Nevertheless, it is now found that the application of an alternating current or alternating voltage is much more effective in increasing the intensity of particle deposition.

As described more fully hereinafter, the alternating current or alternating voltage is applied to two electrodes, one placed on each side of the coated paper as it is being passed through the developing liquid. In cases where the paper is passed below a guide or roller, the guide or roller can be made to serve as one electrode and the bottom of the developing liquid tray or reservoir can be used as the other electrode, provided they are spaced an appropriate distance from each other. Preferably the guide or roller and the bottom of the tray are made of metal so as to be conductive. However, if either or both of these are made of plastic or other nonconductive materials, they can be lined or coated with a thin metal foil or layer so as to serve as electrodes forthe purpose of this invention. As an illustration of the method'use'd in the practice of this invention, paper or other base material is coated with zinc oxide and an alkyd resin suspended in a drier and thinner so that after application of the solution the paper can be dried. Appropriate zinc oxide compositions for this purpose are known in the art.

After drying, the paper is charged electrostatically by subjecting it to a corona discharge. For'example, this is effected by placing the paper on a flat metal plate electrode beneath a metal point connected to the negative terminal of a 10 to 20 kilovolt DC source, the flat metal plate being connected to the positive electrode. Then the paper is subjected to the corona discharge for the an appropriate period such as 10 seconds, while the point is held 2 inches above the paper. In place of the point, a wire or group of wires, .or a sharp steel knife edge may be substituted. In such cases, the distance may be reduced between the wire or knife and the paper and the voltage reduced to give a corresponding electrical field in terms of volts per cm.

Alternatively a charge may be applied on the paper by positioning it between condenser plates. After such a charge is applied over the surface of the paper, the paper is exposed by subjecting it to the action of light in a camera or to any other method of causing the desired image to be produced on the electrostatically charged surface of the paper, the image being caused by the action of the light, etc: on the charged surface.

Subsequent development of the image is effected by simply immersing the paper in liquid developer which comprises a liquid insulating medium carrying colloidal particles. For example, the liquid can be a cyclohexane having a very high resistance and a low dielectric constant. In the liquid is dispersed carbon black or other suitable pigment and a control agent modifier to give to the pigment the required electrical charge. Thus, for imparting a positive charge to the pigment, an alkyd resin control agent and fixer may be included in the liquid. Preferably the liquid immersion developer (L.I.D.) may contain carbon black, crystal steep asphalt and a metallic salt of rosin as described in copending U.S. application, Ser. No. 485,549, filed Sept. 7, 1965 now U.S. Pat. No. 3,399,140. lfa negative charge is desired, linseed oil can be used as the con trol agent and fixer.

The liquid is applied by submerging the paper beneath the surface of the liquid or by pouring it on or spreading it on with a roller or other applicator. In most instances, the paper is fed under a guide submerged in the liquid so that as the paper passes under the guide, it is also submerged in the liquid and passes through for the desired submersion time.

In the drawings, FIG. I shows a cross-sectional view of a developing tank unit for electrostatic printing adapted for the practice of this invention by conversion of different portions of the equipment to electrodes between which the electrostatic paper is passed during the development process.

FIG. 2 shows a portion of a bottom of a developer reservoir and a portion of an applicator roller between which the sensitized or electrostatic paper passes during the developing process, the bottom of the tank serving as one electrode and the applicator roller serving as the other electrode.

The toner trays in most commercial LID electrostatic photocopiers are plastic. These can either be replaced by metal trays or the plastic trays can be'lined with aluminum foil to serve as one electrode. Moreover, in most commercial apparatus of this type, a piece of highly conductive metal such as copper or steel is submerged in the toner-bath and held above the paper as an aid in clarifying and intensifying the image. This piece of metal is generally called an intensifier or bias.

However, it is generally specified that this piece is to be free of electrical charge and is generally insulated from grounding. This intensifier or bias can be used as the second electrode. However it is desirable that the two electrodes cover the entire width of the paper and preferably a substantial part of the length of the paper as it is passed into the bath between these two electrodes.

The effectiveness of the alternating current field is dependent somewhat on the voltage and the frequency of the alternating current. Moreover, the range of operability depends somewhat on the material used on the paper, the toner itself, the electrodes and their spacing. In general, however, the variable frequency alternating current voltage is advantageously in the range of 0.01 volts to 20 volts, preferably 0.01-10 volts. and the frequency range is advantageously in the range of 20 cycles per second to 1000 kilocycles, preferably 40- 120 cycles.

Optimum results are obtained at about volts with a frequency of 90- l 20 cycles. With 10 volts and lOO cycles per second, the black density has been found to be 57 percent higher than is obtained without any electrical field applied. Except for the electrical field described herein and the elec trodes for the application thereof. the apparatus, the materials used and the method of preparation are generally similar to those previously used. However, the toner system must be nonconductive and the process for developing the images must be electrophoretic. Furthermore, the bias is designed and mounted in such a way that the paper does not come into actual contact with it.

The apparatus of this invention and the manner of its use are best described by reference to the drawings. FIG. 1 shows apparatus suitable for the immersion of carrier 1 having an electrostatic latent image in the upper surface thereof. This carrier, illustratively zinc oxide coated paper as described herein is fed by the ingress rolls 2 and 2 into liquid developer 3 which is contained in reservoir 4. Reservoir bottom 4 is made of metal so as to serve as one of the electrodes to which an alternating voltage is applied.

A metal bias 5, preferably copper, has a portion submerged in the liquid developer and serves also as a guide for carrier 1 in its path through the liquid developer and to egress rolls or squeezing rollers 6 and 7. These rolls remove all excess liquid from the carrier 1. Roller 6 is made up ofa resilient material such as rubber, while roller 7 is preferably steel or a similar hard material so that sufficient pressure can be applied to carrier 8 to remove the excess liquid. Wiper 8 is continuously pressed against the squeezing roll 7 to wipe it clean of any pigment particles left by contact of the roll with carrier 1.

The liquid developer from reservoir 4 flows slowly into the main tank 9 through the small opening 10 in the bottom of reservoir 4. The rate of flow from reservoir 4 through opening 10 into main tank 9 is much slower than the rate of pumping the liquid developer from the tank 9 into reservoir 4. The desired level in reservoir 4 is maintained by an overflow outlet, not shown, so that liquid will overflow from reservoir 4 into main tank 9.

While the reservoir 4 is made of metal or at least lines with metal, the main tank 9 can be made of plastic material or other nonconductive material. Reservoir bottom 4 is fastened to a supporting portion of main tank 9 by means of bolt 11. The liquid developer in reservoir 4 is continually refreshed by replacement with liquid from the main tank 9 from which liquid is withdrawn through pump system inlet 12 by pump 13 and fed to the reservoir 4 through pipe 14' and header 14.

Metal member or electrode 5 is held in position by support means not shown and is connected to an alternating current electrical source by conductor 15 with reservoir bottom 4 being connected to the other pole of the alternating current source by conductor 16.

As previously indicated, when an alternating voltage of l0 volts and cycles per second is applied to the two electrodes, the resultant black density has been found to be approximately 57 percent higher than is obtained when the electrical field is not applied.

FIG. 2 shows a cross-sectional view of a portion of apparatus adapted for the purpose of this invention by converting a metallic applicator roller 5 to serve as an electrode and the metallic bottom of a reservoir 4 containing liquid developer 3. Carrier 1 is shown in the liquid developer passing along the bottom of the reservoir between the electrode formed by applicator roller 5' and reservoir bottom 4 and out of contact with applicator roller 5'.

I claim:

1. Apparatus for developing by immersion in a liquid developer a photographic carrier having an electrostatic latent image in one surface thereof comprising:

a. a reservoir for a liquid developer ha'ving electrostatically attractable pigment particles therein;

b. a carrier'ingress means adjacent to one end of said reservoir;

. a carrier egress means adjacent to the opposite end of said reservoir, said carrier ingress and egress means being adapted to guide said carrier through said liquid developer;

d. a first electrode positioned below the path of said carrier in its passage through said liquid developer;

e. a second electrode positioned above the path of said car rier in its passage through said liquid developer, said second electrode being positioned so as to be immersed in said liquid developer contained in said tank, and appropriately positioned so that said carrier may be passed between said first electrode and said second electrode and so that said carrier is out of contact with said second electrode, the areas of said electrodes superimposed and spaced from each other being of sufficient width to cover that width of carrier which is desired to be developed and also covering at least a substantial portion of the length of said carrier; and

f. a means for supplying an alternating voltage to said electrodes.

2. The apparatus of claim 1 in which second electrode is also a conductive metal bias.

3. The apparatus of claim 1 in which said second electrode is also a conductive metal guide means.

4. The apparatus of claim 1 in which said second electrode is also a conductive metal roller.

5. The apparatus of claim 1 in which said first electrode comprises at least a portion of the bottom of said reservoir and said portion of said bottom is made ofa conductive metal.

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