US3346475A - Electrophotographic method using an unsymmetrical ac current during development - Google Patents

Description

Oct. 10, 1967 J. MATKAN ETAL 3,346,475 ELECTROPHOTOGRAPHIG METHOD USING AN UNSYMMETRICAL AC CURRENT DURING DEVELOPMENT Filed Feb. 20, 1964 5 Sheets-Sheet 1 GENERHTOR GEN/:RnroR Oct. 10, 1967 J. MATKAN ETAL ELECTROPHOTOGRAPHIC METHOD USING AN UNSYMMETRIC AC CURRENT DURING DEVELOPMENT 5 Sheets-Sheet 2 Filed Feb. 20, 1964 1967 J. MATKAN ETAL 3,346,475

ELECTROPHOTOGRAPHIC METHOD USING AN UNSYMMETRICAL AC CURRENT DURING DEVELOPMENT Filed Feb. 20, 1964 I a Sheets-Sheet 5 IIE I0 United States Patent 3,346,475 ELECTROPHOTOGRAPHIC METHOD USING AN UNSYMMETRICAL AC CURRENT DURING DE- VELOPMENT Josef Matkau, Malvern, South Australia, and Robert A.

Chapman, Dernaucourt, South Australia, Australia, assignors to Research Laboratories of Australia Limited, North Adelaide, South Australia, Australia Filed Feb. 20, 1964, Ser. No. 346,184 Claims priority, application Australia, Feb. 25,

6 Claims. 101. 204-181) ABSTRACT OF THE DISCLOSURE This invention relates to the process of reproducing images and in particular it relates to a process wherein images are obtained by rendering visible memory effects produced in semiconductors.

Semiconductors are substances which are generally characterised by their electrical conductivity being variable by external factors such as for instance electromagnetic radiation. One particular type of semiconductors is known as photoconductors. When a photoconductive substance is irradiated by a source of lightor X-rays, the electrical conductivity of the photoconductive substance increases Within certain limits in proportion with the intensity of such incident radiation. Because of this property photoconductors are used in the art of printing and in particular in electrophotography and xerography, both of which are generally within the scope of the art of elec-. trostatic printing. v

Electrostatic printing in its widest form can be (let scribed as a process wherein an insulating surface is provided with an electrostatic charge pattern in selective areas conforming to an image and wherein such charge pattern is subsequently rendered visible by applying a dry developer powder or a developer material suspended in an insulating liquid dispersant in order that such developer may be attracted and held electrostatically to the charged areas contained on the insulating surface. The developed image may be fixed to the surface of the insulator or transferred to another surf-ace and fixed thereon, if desired. v v i g Electrophotography is a special case of electrostatic printing and usually it involves the following process: A relatively conductive backing member such as metal'or paper sheet having deposited thereon a photoconductive material, such as for instance vitreous selenium or a particulate photoconductor such as for instance zinc oxide or lead monoxide and the like in a resin matrix, is subjected to a corona discharge whereby a uniform electro static charge is deposited onto the photoconductive layer. Such charged photoconductive layer is then exposed to a light or X-ray pattern whereby the irradiated areas he: come discharged whereas the shielded areas remain 3,346,475 Patented Oct. 10, 1967 ice charged and thus form a latent electrostatic image. Such latent electrostatic image is then rendered visible by applying a dry developer powder or a developer material suspended in an insulating liquid dispersant in order that such developer may be attracted and held electrostatically to the charged areas. The developed image may be fixed to the surface of the photoconductor or it may be transferred to another surface and fixed thereon, if desired.

The term xerography is generally used in connection With selenium as the photoconductor and dry powder developers in an electrophotographic process as hereinbefore described.

It will be noted that electrostatic printing and in particular electrophotography and xerography are based es-' sentially on the existence of an electrostatic charge in imagewise configuration upon an insulating surface and therefore all these processes involve in the first instance the step of depositing an electrostatic charge onto such insulating surface and then, as in the cases of electrophotography and xerography, modifying such electrostatic charge imagewise. It will be also noted that in electrostatic printing, in electrophotography and in xerography a latent electrostatic image is rendered visible by the application of particulate developer material which is attractable by the electrostatic charge existing on the insulating surface.

The present invention relates to a method wherein an image is formed on a photoconductor or semiconductor surface by selective deposition onto such surface of toner material or other medium in accordance with a conductivity pattern or memory effect contained by such surface, and wherein such deposition of toner material takes place preferentially in a dielectrophoretic cell in which however the potentials applied are insufficient to impress a charge on the photoconductor or semiconductor held in such cell.

By the present method therefore the image is formed by the deposition of visually detectable matter under the influence of a low voltage field such field itself being controlled in intensity by the variance in conductivity of a pattern producing member. Consequently, the visible image produced corresponds to the pattern of conduc, tivity variance or memory effect existing on the pattern producing member.

The pattern controlling medium can conveniently be formed by exposure to light or X-rays of a photoconductor which exposure changes the resisitivityof such photoconductor in proportion to the intensity of the incident radiation and whereby such change in the resistivity of the photoconductor persists at least for some finite time after cessation of such exposure'as is known as a memory effect in relation to semiconductors. The photoconductor then forms the controlling means for the deposition of the toner or the like. Alternatively, X-ray patterns may be projected on to-the cell while visual image formation is effected or one of the electrodes may be transparent to allow a light image to be produced on the photoconductor sheet simultaneously with such imaging action. l I

The photoconductive element can comprise a photoconductive film deposited on a relatively conducting backing such as metal plate or foil or paper and the like. The photoconductive film can be a layer of selenium or of an organic photoconductor, or it can be also a film comprising a particulate photoconductor such 'as for instance zinc oxide, cadmium selenide, lead monoxide and the like dispersed in an insulating medium such as a resin binder. Such photoconductive film can also contain sensitising dyes for the purpose of increasing the response of such film to certain desired parts of the electromag netic spectrum. It is also possible to deposit such photo conductive film directly onto one of the electrodes contained in the dielectrophoretic cell.

The actual method of forming the image will thus be seen to consist in either using the memory effect of a photoconductor medium which has previously had a pattern exposed thereon, or to expose the photoconductor medium to a light pattern or an X-ray pattern or other condition at the time that imaging is being effected, such imaging being in each case effected by placing the photoconductive element into a dielectrophoretic cell containing two electrodes which are spaced a small distance apart, the photoconductive element being held against one of the electrodes and having the photoconductive layer exposed towards the other electrode, both such electrodes and the photoconductive element being immersed in an electrically insulating liquid in which are suspended toner particles or other means which can be moved under the influence of an electrostatic field, the image formation being effected by connecting the electrodes to a supply of electrical current of the type as will be described in the following disclosure.

The insulating liquid contained in the dielectrophoretic cell as referred to in the foregoing can be for instance a hydrocarbon solvent of high electrical volume resistivity such as for instance not less than ohm centimeters and of low dielectric constant such as less than 3. In such insulating liquid are suspended toner particles which may comprise pigments or other means which can be moved under the influence of an electrostatic field and such toner suspension may also contain other matter such' as for instance oils or resins or varnishes the presence of which may be formed to be desirable for instance to aid the dispersion of the toner material. Such suspended matter or such toner particles in accordance with the present invention are characterised by being polarity sensitive, that is such particles have a tendency when in suspension to become attracted or repelled preferentially by electrostatic charges or an electrode of a certain polarity. Toner or particle suspensions of this nature are substantially similar to liquid developers as known in the art of electrophotography.

As disclosed in the foregoing, the toner deposition is effected by the influence of an electrostatic field which is established by connecting the said electrodes to a power supply and the output of such power supply in one embodiment of this invention can be a direct current or a pulsating direct current whereas in another embodiment of this invention such output can be a symmetrical alternating current, in the case of a photoconductor of particular rectifying properties, or an unsymmetrical pulsating current of unequal magnitude, that is to say the pulsation may have a bias of either a positive or negative potential applied either in magnitude or in time.

To explain this second form of the invention further it may be stated that a square wave can for instance be generated which has its datum line displaced from the central position of the wave so that one pulse will be stronger than the other pulse by reason of the greater amount of displacement, or alternatively the pulse can have an unequal time element so that the one crest is relatively long and the opposite remaining crest of that wave is relatively short, in both cases the effect being to favour either the positive or the negative pulse in amplitude or time.

To enable the present invention to be fully understood the following description will be made with reference to the accompanying drawings in which:

FIG. 1 is a view showing the exposure of the photoconductor sheet to light to produce the memory image,

FIG. 2 shows how the electrical cell in which development then takes place is made up, the generator producing either direct current or biased alternating current,

FIG. 3 shows how exposure and development can be effected simultaneously,

In 'FIG. 1 a light source 1 projects an image from a" film 2 through a lense 3 on to a photoconductor film 4 on a backing member Ssuch as a sheet of paper.

In FIG. 2 the generator 7 is coupled to the back electrade 8 and to the front electrode 9 which are spaced apart and are immersed in a liquid 10 in tank 11, the image to be developed being in the form of a latent memory effect that is to sayin the form of a conductivity pattern on the photoconductor film 4 of the backing 5 resting on the back electrode 8.

It will be realised from the illustration that the invention applies to that type of cell in which there. is a gap between the front electrode 9 and the photoconductive film 4, which gap is filled with an insulating liquid 10 in which the toner particles, which are to be moved for developmental purposes, are suspended.

After a memory effect has been produced in the photoconductive film 4 by means of the light or other electromagnetic waves as shown in 'FIG. 1 that is to say after a Variance in the relative conductivity of the exposed and unexposed areas of the photoconductive film has been effected such variance in conductivity or memory effect con be rendered visible by toner deposition in the dielectrophoretic cell as shown in FIG. 2. In FIG. 2 the conductive backing Sis held against the back electrode 8 whereas the photoconductive film 4 faces the front electrode 9 and thereby the field between the said two electrodes becomes modified in intensity by the variance in conductivity due to the memory effect contained 'in the photoconductive layer 4. Such modified field between the electrodes causes the toner particles to deposit onto the photoconductive layer substantially in conformity with the field intensity whereby an image is formed on such layer corresponding in density to the strength of such electrical field, such deposition of toner or other particulate means being substantially a dielectrophoretic process.

It might be mentioned herein that the dielectrophoretic cell in acordance with this invention is not to be mistaken for a type of cell which has been proposed heretofore in which an essentially conductive dry toner dust was used which itself formed the electrode, the toner dust being in that case applied at least at the front of the photoconductor sheet and itself forming as it were the front electrode, there being no insulating liquid between the face of the photoconductor film on which the latent image is contained and the electrode which effects development.

In that case development takes place from a conductive powder which is held in direct contact with the face of the photoconductor film and adhesion of the developer only to the required areas takes place due to a modifying action of the photoconductor film in the non-image areas, that is when a powder is positive and the ground plate negative, available electrons in the exposed areas are extracted from the photoconductor layer to neutralize the conductive powder which then falls off. In our present invention the effect is that as shown in FIGS. 6- to 9 the field as represented by field lines 25 extends through the insulating liquid 26 between the back electrode 27 and the front electrode 28 but the intensity of such field is modified as shown in FIGS. 7 and 8 by the latent memory effect that is to say by the incresed conductivity in the exposed areas 29 on the photoconductive element 30 so that dielectrophoretic deposition of the toner particle by direct movement due to the field in the insulating liquid takes place, this deposition of course being in accordance with the electrostatic field existing on the photoconductor film and thus allowing the latent image to be rendered visible by deposition of toner particles.

As the photoconductor has a memory effect the preferred method is to use that contained in FIGS. 1 and 2 but if it is desired the procedure can be altered by combiuing the process of FIGS. 1 and 2 into that shown in FIG. 3 in which the various integers bear the same reference numerals as in FIGS. 1 and 2 but the light acts simultaneously with the generation of the depositing voltage, in this case the top electrode 9 of course being transparent to allow the image to be projected therethroug-h on to the photoconductor film 4, the top electrode being a grid which oscillates or as said being otherwise arranged in such a manner that the light or electromagnetic radiation image can pass therethrough.

It will be obvious that the invention will operate if a direct current potential is applied which has the required polarity to move the particles under the influence of the induced field to those areas of higher conductivity where the field current can more readily flow through the photoconductor and thus it Will 'be noted that the rate of toner deposition and consequently the image density on the photoconductive layer is proportional to the conductivity of such layer, that is the image density is proportional to the intensity of radiation to which the photoconductive layer was previously exposed, the resulting image being therefore a reversal of such as for instance a positive to negative reproduction of the original radiation pattern. Whilst excellent results can be obtained by this process on direct current operation on certain photoconductor films characterised by a relatively high difference in conductivity effected by exposure, in the case of photoconductor films characterised by a relatively small difference in conductivity elfected by exposure direct current operation results in a relatively low image contrast because the small difference in conductivity between the image and non-image areas results in excessive toner deposition onto the non-image areas forming undesirable background Which is substantially proportional to the voltage applied to the electrodes in the dielectrophoretic cell, and it will be realised that if such voltage is increased for the purpose of obtaining higher image density, the background density will also increase proportionally.

The foregoing disadvantages of photoconductor films characterised by a relatively small difference in conductivity effected by exposure are overcome by the application of an alternating current of the bias type as proposed in the second embodiment of this invention wherein very much improved results can be obtained with respect to high image contrast and reduction of undesirable background contamination and wherein it is also possible to reproduce an image either in facsimile or in reversed form as may be found desirable from case to case by appropriately selecting the toner ma terial with respect to its polarity sensitivity and by moving such toner material in the modified electrostatic field more definitely in one direction than the other.

The types of alternating Waves which can be elfectively used in this invention are shown in FIGS. 4 and 5, FIG. 4 showing how an ordinary square Wave can have its datum line displaced from the centre of the wave so as to give a lower amplitude 14 on the one side of the datum line 15 and a high amplitude 16 on the other side thereof.

In FIG. 5 is shown a square wave which is symmetrical about the datum line 17, and in which the complete cycle 18 is shown broken up into a longer portion 19 and a shorter portion 20 so that there is a variation in time of the positive and negative pulses.

'ment, such areas having been rendered more conductive during the preceding exposure, whereas the image areas have remained relatively less conductive due to the shielding action of the image contained in the original radiation pattern. The properties of the toner material to be used in combination with the intensity and type of the pulsating current to be applied to the electrode depends on the electrical characteristics of the photoconductive element preferred. If for instance the photoconductive element contains inc oxide which is an n-type photoconductor and'is characterized by rectifying properties in that its resistivity is substantially higher when biased in one direction than in the other direction, the toner material preferred in this embodiment is one which is capable of being repelled by negative electrostatic charges or by a negative electrode or consequently one which can be attractive by positive electrostatic charges or by a positive electrode. In this case the pulsating current is applied to the electrode in such manner that the total energy of negative pulses appearing on the front electrode facing the photosensitive layer is higher than the total energy of the negative pulses appearing on the rear electrode located behind the photoconductive element. Thus at a time when a high energy negative pulse appears on the front electrode concurrently with a high energy positive pulse on the rear electrode, an electrostatic field through the photoconductive element is established between the two electrodes and in eifect the photoconductive element is in this case in a reverse biased position. The intensity of such electrostatic field is controlled locally by the conductivity pattern on the photoconductive element in that highest field intensity exists in the higher conductive exposed areas. Deposition of the polarity sensitive toner material takes place in the high field intensity regions onto the conductive exposed areas of the photoconductive element by repulsion from the front electrode and/or by attraction from the rear electrode. However, due to the relatively small difference in resistivity between the exposed and unexposed areas of the photoconductive element, toner material will also deposit to some degree onto the unexposed areas causing objectionable background and diminishing contrast. Upon cycle reversal of the pulsating current, at a moment when a low energy positive pulse appears on the front electrode concurrently with a low energy negative pulse on the rear electrode, the electrostatic field through the photoconductive element becomes reversed between the electrodes. The overall intensity of such reversed electrostatic field is lower than that of the preceding electrostatic field due to the lower energy pulses appearing on both electrodes effecting such reversed field.

Toner material deposition at a lower rate takes place in presence of such lower intensity reversed field away from the photoconductive element and onto the positive front electrode and since the photoconductive element is in this case in a forward biased position so that the conductivity pattern has no influence on the field intensity, such deposition of toner material onto the front electrode is essentially uniform over the whole area facing the photoconductive element and the effect of such toner deposition onto the front electrode is uniform removal of toner material from the photoconductive element.

It will be realised that during subsequent cycles of high energy negative pulses on the front electrode more toner material will be deposited onto the conductive areas of the photoconductive element concurrently with some deposition onto the non-conductive areas whereas, during subsequent cycles of low energy positive pulses on the front electrode, toner material will be constantly removed at a lower rate from the whole surface of the photoconductive element. It is" possible to adjust the magnitude of the high pulses to that of the low energy pulses in such manner that the quantity of toner material removed from the photoconductive element during the cycles wherein the front electrode is supplied with low energy positive pulses is such that substantially all toner material becomes removed from the-less conductive unexposed areas of the photoconductive element whereas high image density is retained in the more conductive exposed areas with theresult that a reversed image reproduction of the original radiation pattern with high image density and contrast and with clean background can be obtained. To further understand this efiect reference may be had to FIGS. 6, 7, 8 and 9 which show how development is effected according to this invention, FIG. 6 showing the eifect when a negative backing electrode 27 is used for the zinc oxide photoconductor paper 30 and a positive developer 31 is applied from the insulating liquid 26 under which conditions the developer is deposited uniformly over the whole of the photoconductor sheet due to the attraction of the positive toner by the negative plate, the photoconductor film being here in forward biased position and the higher conductive exposed area 29 therefore in this instance not significantly effecting the deposition. When however the positive cycle is applied to the back plate, the condition of FIG. 7 exists in that during this part of the cycle the toner 31 which has previously been deposited on the image area 29 will be pushed ofi preferentially from this area due to the toner 31 and the electrode 27 having the same sign, the intensity of field lines being highest in the more conductive image area 29 when the photoconductor film is in reverse biased position as in this instance. a In FIGS. 8 and 9 are shown the effects when a negative toner 32 is used. In FIG. 8 is shown the attraction during the positive cycle, that is when the backing plate 27 is positive and the photoconductor paper 30 is in reverse biased position. During the opposite cycle as shown in FIG. 9 there is a pushing away of the toner particles 32 from the whole of the photoconductive layer of the member 30 so that if any toner has attached itself to the non-image areas it will be released and moved away. The photoconductor paper 30 is in this instance in forward biased position.

In the first case therefore, that is FIGS. 6 and 7, a

facsimile copy results whereas in the second case, that is FIGS. 8 and 9, a reverse image is the result.

In order to illustrate the relation between the characteristics of the photoconductor and the form and magnitude of the current the process in accordance with this invention will be now described in the following examples as applied to two types of commercially available electrophotographic papers. Both types of such papers comprised a relatively conducting paper backing having deposited thereon a photoconductive film containing zinc oxide dispersed in a resin binder. Samples of the two types of papers were first dark rested for 48 hours and then a separate sample of each type was exposed in part to visible light emitted from a 100 watt incandescent globe, to ultraviolet radiation and the X-rays generated by a tube operating in the range between 50-100 kvp. Numerous exposures of varying duration were made and also the current in the X-ray tube was varied. One separate sample Was used for each exposure and after the exposure the bulk resistivity of the sample both in the dark rested and in the exposed portion was measured with a gold contact electrode. Disregarding the speed of response or sensitivity of the samples to the types of radiation employed, which is a matter of the duration of the exposure or .a matter of the intensity of radiation only, the maximum measurable difierences in resistivity of the two types of papers were found to be as follows:

Resistivity in ohms per sq. cm. TYPE OF PAPER It-will be noted that the change in resistivity of paper type A is relatively higher than that of paper type B.

In all the following examples the papers were exposed to a light or radiation pattern for the purpose of forming a memory effect or a corresponding pattern of conductivity variance and the papers were processed in'the arrangements as shown in FIGS. 1, 2 and 3. The image formation was effected by employing toner dispersions of the required polarity, and formulations of such toner materials are given following the examples. The toner concentration was approximately that given in the formulations in which case the bulk resistivity between the electrodes as measured across the liquid toner dispersion was 10 ohms per square centimeter, which, as will be noted, is below the resistivity of either of the papers so that the conductivity pattern in the paper was capable of controlling the field between the electrodes. The electrodes were spaced 3.5 mm. apart.

Example 1 A reversed image was formed on the type A paper employing direct current. The potential was 36 volts negative on the front electrode, 40 seconds developing time. A high intensity image Was obtained with no objectionable background contamination.

Example 2 In Example 1 thepotential was raised to 250 volts and the developing time was reduced to 5 seconds. The resulting image was substantially similar to that of Example 1.

Example 3 A reversed image was formed on the type B paper employing direct current. The optimum result was obtained with a potential of 150 volts negative on the front electrode and a developing time of 15 seconds. Due to the relatively small difference in the conductivity of the exposed and unexposed areas the image contrast was poor and heavy background staining occurred.

Example 4 Example 5 A high contrast reversed image with clean background was obtained on the type B paper employing anunsymmetrical alternating current as shown in FIG. 4 with a pulse duration of 150 milliseconds. E1 and E2 were 525 volts and 175 volts, respectively, E1 being the depositing pulse of negative polarity on the front electrode. Developing time was 30 seconds.

Example 6 A high contrast facsimile image with clean background was formed on the type B paper employing an unsymmetrical alternating current as shown in FIG. 4 with a pulse duration of 150 milliseconds. E1 and E2 were 540 and volts, respectively, E2 being .the depositing pulse of positive polarity on the front electrode. Developing time was 45 seconds.

Example 7 A high contrast reversed image with clean background 'was formed on the type A paper employing an unsymmetrical alternating current as shown in FIG. 4 with a pulse duration of milliseconds. E1 and E2 were 450 volts and 250 volts, respectively. E1 being the depositing pulse of negative polarity on the front electrode. Developing time was 45 seconds.

9 Example 8 A high contrast facsimile image with clean background was formed on the type A paper employing an unsymmetrical alternating current as shown in FIG. With a peak potential of 1400 volts and 30 seconds developing time, T1 and T2 were in the ratio of 400: 100 milliseconds with T2 as the depositing positive pulse on the front electrode.

The power supply for the purposes of this invention may comprise a simple set of dry cells arranged as a battery to supply the voltage required in case it is desired to operate on direct current and it is also possible to obtain direct current by rectifying alternating current or from any other source commonly known. The unsymmetrical alternating current may be obtained from the commonly known multivibrator circuit as shown in the following but it is also possible to obtain such current from any other source of low frequency alternatingcurent wherein it is possible to vary either the datum line of the wave or the duration of each of the positive and negative parts of the wave.

As said in the foregoing the unsymmetrical alternating current may be obtained as the output from the anodes of a multivibrator or similar circuit. A suitable amplifier circuit which has a low output impedance and is economical of current and which can be combined as a symmetrical pair connected to form a multivibrator may comprise a pair of tubes connected in series so far as their operation is concerned, but a resistance is placed between the cathode of the one valve and the anode of the other, the latter valve having a signal applied to its grid and the grid of the second valve being fed from the anode of this first valve, the output being taken from the cathode of the second valve.

In the push pull circuit shown in FIG. for producing a suitable wave form, V1 and V3 are grounded cathode amplifiers with their static loads formed by V2 and V4. The dynamic load of V1 and V3 is the input impedance of V2 and V4 and as such impedance is very high, the actual gain obtained from V1 and V3 approaches the maximum theoretical value for the particular valve type used. The valves of each pair need not be identical valve types.

In the form shown in FIG. 10 the switching speed is very high and the output impedance is low and the average current drawn is also low. The usual analysis of multivibrator operation applies, pulse length being controlled by the values of C1 and C2 and also by the potential to which the grid resistors Rgl and Rg2 are returned. The pulse output is very nearly the rail potential.

If desired a transistorised pulse generator can be used. Development time however will be in such case substantially longer as the maximum pulse voltage obtainable from commercially available transistors is considerably lower than that from valves.

The following are examples of toners which are polarity sensitive and can be used in accordance with this invention.

(1) Black toner.This is a so called negative toner which is used for reversal reproduction, attracted by positive polarity charges or by a positive electrode.

Grams Microlith Black CT pigment 50 Rhodene 1.42/70 resin 160 The ingredients are milled to form a paste. The toner dispersion is prepared by stirring approximately 10 grams of this paste into 500 cc. of any one of the following liquids or into the mixture of such: Shell Solvents X4 and X55, Shellsol T and trichlorotrifiuoroethane.

(2) Blue toner.-This is also a so called negative toner which is used for reversal reproduction. The toner particles are attracted by positive polarity charges or by a positive electrode.

Grams Prussian -Blue pigment 30 Rhodene L42/ 70 resin electrode.

Grams Fastel Pink B Supra pigment 20 Varnish 30 The ingredients are milled to form a paste. The varnish is prepared by heating 800 grams of polymerised linseed oil with 300 grams of Staybelite resin at 260 C. The toner dispersion is prepared by stirring 10 grams of this paste into 500 cc. of Shell solvent X55.

Although in this specification reference has been had to a pair of electrodes of the plate type, it is quite obvious that the invention can similarly be applied in continuous types of machines utilizing roller electrodes or similar means. One roller electrode for instance could carry the photoconductor sheet or the like while another, or a simple fixed electrode, could be positioned in relation thereto to give the necessary field for image deposition on the rotating roller.

It will be also apparent that although reference was made in the foregoing to an unsymmetrical waveform of alternating current it is also possible to employ symmetrical alternating current which may be found to be most suitable in view of the characteristics of certain photoconductors.

The invention can also be used by producing an image in such a way that it can then be transferred, that is to say the image can be formed by an imaging material or coating medium which remains in a mobile state for a sufficient time to allow the image to be transferred after formation and subsequently fixed to another base. For the purpose of transfer the image can be formed on a sheet containing a film of semiconductor or photoconductor such sheet being placed between the electrodes in a manner as hereinbefore described, or alternatively such image can be formed directly on one of the electrodes of the cell in which case such electrode can itself carry a film of semiconductor or photoconductor substance at least on the side adjacent to the electrode of opposite polarity.

Since it is apparent that one skilled in the art may select according to the teachings of this invention appropriate waveforms and potentials in conjunction with suitable toner material other than that mentioned in the foregoing to obtain optimum results on a particular type of ph-otoconductor surface and since many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all the matter contained in the foregoing examples is to be interpreted as illustrative and not in a limiting sense.

There has been described a process for the reproduction of images and it will be apparent from the foregoing disclosure that this invention as distinct from electrostatic printing and in particular from electrophotography and xerography does not rely for its operation on the existence of an electrostatic charge on the surface of a photoconductor and the image formation in accordance with this invention is not dependent on the attraction of toner material to such existing electrostatic charges in that this present invention utilises a continuing electrostatic field which is modified in intensity by semiconductor memory effects for the purpose of depositing toner material by dielectrophoretic movement caused by such field.

The materials mentioned by trade names in the foregoing may be described as follows:

Microlith Black CT is a resinated carbon black pigment produced by Ciba.

Rhoden'e'L42/70is a' long oil safilower alkyd resin, sp. gr. I9650.975, oil length 64%, acid value 6-10, produced by Polymer Corp Australia.

Fastel Pink B Supra is an organic red pigment produced Staybelite Resin is a hydrogenated rosin produced by Hercules Powder C0.

Shell X4 is a substantially aliphatic hydrocarbon solvent, sp. gr. 0.67, boiling range 5870 C., KB value 30,

I produced by Shell Chemicals.

Shell X55 is a. substantially aliphatic hydrocarbon solvent, -sp. gr. 0.72, boiling range 58-140 C., KB value 40, produced by Shell Chemicals. e

Shellsol T is 'an aliphatic hydrocarbon solvent, sp. gr.

0.76, boiling range 180-207 C., KB value 26, produced by Shell Chemicals.

What we claim is:

1. The method of producing an image which comprises dark resting a photoconductor surface, subjecting the dark rested photoconductor surface to an electromagnetic Wave pattern whereby an image is produced thereon which has a varying conductivity according to the intensity of the electromagnetic wave striking any specific area, and while immersing the photoconductor surface in an electrically insulating liquid which carries electrically responsive toner particles, passing an alternating electrical current of unsymmetrical wave form, with a bias to move toner particles more definitely toward the photoconductor surface than away from the said surface, between electrodes disposed one on either side of the said surface but spaced from the surface on the image side whereby to cause toner particles to be deposited onto the said surface in proportion to the strength of the depositing electrical field at any specific area.

2. The method according to claim 1 wherein the bias 'is an amplitude bias.

3. The method according to claim 1 wherein the bias is a time bias.

4. The method of producing an image which comprises dark resting a photoconductor surface, subjecting the dark rested photoconductor surface to a light pattern whereby an image is produced thereon which has a vary ing conductivity according to the intensity of the light striking any specific area, and while immersing the photoconductor surface in an electrically insulating liquid which carries electrically responsive toner particles, passing an alternating electrical current of unsymmetrical wave form, with a bias to move toner particles more definitely toward the photoconductor surface than away from the said surface, between electrodes disposed one on either side of the said surface but spaced from the surface on the image side whereby to cause toner particles to be deposited onto the said surface in proportion to the strength of the depositing electrical field at any specific area.

5. The method according to claim 4 wherein the bias is an amplitude bias.

6. The method according to claim 4 wherein the bias is a time bias.

References Cited UNITED STATES PATENTS 2,892,709 6/1959 Mayer 961 2,976,144 3/1961 Rose 961 2,978,968 4/1961 Schwartz 96-1 X 3,043,684 7/1962 Mayer 961 3,084,043 4/ 1963 Gundlach 961 3,100,426 8/ 1963 Kaprelian 1.7 3,212,890 10/1965 Kimble et al. 961.8 X 3,245,381 4/ 1966 Brenneisen et al. 11737 X OTHER REFERENCES Amick: A review of Electrofax Behavior, RCA Review, December 1959, pp. 763-7 67.

NORMAN G. TORCHIN, Primary Examiner.

C. E. VANHORN, Assistant Examiner.

Claims (1)

1. THE METHOD OF PRODUCING AN IMAGE WHICH COMPRISES DARK RESTING A PHOTOCONDUCTOR SURFACE, SUBJECTING THE DARK RESTED PHOTOCONDUCTOR SURFACE TO AN ELECTROMAGNETIC WAVE PATTERN WHEREBY AN IMAGE IS PRODUCED THEREON WHICH HAS A VARYING CONDUCTIVITY ACCORDING TO THE INTENSITY OF THE ELECTROMAGNETIC WAVE STRIKING ANY SPECIFIC AREA, AND WHILE IMMERSING THE PHOTOCONDUCTOR SURFACE IN AN ELECTRICALLY INSULATING LIQUID WHICH CARRIES ELECTRICALLY RESPONSIVE TONER PARTICLES, PASSING AN ALTERNATING ELECTRICAL CURRENT OF UNSYMMETRICAL WAVE FORM, WITH A BIAS TO MOVE TONER PARTICLES MORE DEFINITELY TOWARD THE PHOTOCONDUCTOR SURFACE THAN AWAY FROM THE SAID SURFACE, BETWEEN ELECTRODES DISPOSED ONE ON EITHER SIDE OF THE SAID SURFACE BUT SPACED FROM THE SURFACE ON THE IMAGE SIDE WHEREBY TO CAUSE TONER PARTICLES TO BE DEPOSITED ONTO THE SAID SURFACE IN PROPORTION TO THE STRENGTH OF THE DEPOSITING ELECTRICAL FIELD AT ANY SPECIFIC AREA.

Images