US3486922A - Development of electrostatic patterns with aqueous conductive developing liquid - Google Patents

Description

Dec. 30. 1969 5 ET AL 3,486,922

DEVELOPMENT OF ELECTROSTATIC PATTERNS WITH AQUEOUS CONDUOTIVE DEVELOPING LIQUID Original Filed Nov. 18, 1965 2 Sheets-Sheet 1 FIG. 70 FIG. 7b

9 eay" V/cm F/G.2G

AQUEOUS DROPLET INSULATING LAYER g BEFORE w CHARGING n M comoucrwa SUPPORT F/OZD AQUEOUS DROPLET INSULATING LAYER BEFORE CHARGING ffi CONDUCTIVE 5 SUPPORT 2c INVENTORS PAUL MAR/A CASS/ERS ROBERT JOSEPH NOE JOZEF LEONARD VAN ENGELAND WATSON, COLE GRINDLE & WA TSON ATTORNEYS Dec. 30. 1969 P, s ET AL 3,486,922

DEVELOPMENT OF ELECTROSTATIC PATTERNS WITH AQUEOUS CONDUCTIVE DEVELOPING LIQUID Original Filed Nov. 18, 1965 2 Sheets-Sheet 2 AC OR 0c v0 LTA G E SOURCE ELECTROSTATIC DEVELOPMENT WITH HYDROPHOBIC RESIDUE CHARGE IMAGE AQUEOUS DEVELOPER FROM DEVELOPMENT CHARGE uoum' CARR'iER") 1 5 1 1 i Z 1 I 2 i F/G.4Cl F/G.4b F/G.4C

CHARGE PATTERN RE'DEVELOPMENT WITH AFTER RE'CHAR'GING AQUEOUS DYESTUFF SOLUTION r-" -"s T I i I F/G.4d F/G.4

INVENTORS PAUL MARIA cAss/E s ROBERT JOSEPH NOE JOZEF LEONARD VAN ENGELAND WATSON, COLE, GR/NDLE 8 WA TSON ATTORNEYS United States Patent Int. Cl. G03g 13/02 US. Cl. 117-37 12 Claims ABSTRACT OF THE DISCLOSURE Electrostatic charge patterns on normally insulating electrostatic charge carriers having a hydrophoblic surface are developed by means of surface wetting with a liquid. An aqueous conductive developing liquid is brought into substantially uniform contact with the hydrophobic surface of the carrier in the presence of a superimposed direct current electric field extending generally at right angles to the surface. The hydrophobic surface is normally unwettable by the aqueous developing liquid, but under the influence of attractive electrostatic field forces provided by the charge pattern and the superimposed field, the liquid is selectively attracted into wetting relation therewith. The period of development contact is less than the discharge time of the charge pattern upon contact of the carrier with the liquid. The contact may occur over the entire surface or progressively across the surface, and where the charge carrier has rectifying characteristics, the field can be created by alternating current.

This application is a continuation of US. patent application Ser. No. 508,486 filed Nov. 18, 1965, now abandoned, which was a continuation-in-part of Us. patent application Ser. No. 150,820 filed Nov. 7, 1961, now abandoned.

Many image and signal recording processes are known which involve the creation of an electrostatic record from or by means of which a visible record can be subsequently be formed. We refer by way of example of electrostatic printing processes (see e.g. United Kingdom patent specification 618,512), electrostatic recording processes, xerography (see e.g., United Kingdom patent specification 672,767), the Electrofax process as described by C. J. Young and H. C. Greig in RCA Rev. (1954) 469, electrothermography as described by P. M. Cassiers, J. Soc. Phot. Sci. Eng. 4 (1960) 199.

Many methods of developing latent electrostatic records have been proposed. We mention in particular, methods in which the electrostatic record is developed to a powder image by the electrostatic attraction of powder dusted over the electrostatic record or by the deposition of powder by electrophoresis from dispersion in a liquid with a high electric resistivity. For the latter method reference is made e.g. to the United States Patent 2,913,353.

Although powder development gives good results, it is open to various objections. These arise in part from the inconvenience of working up suitable dry powders and the ice necessity for a special fixing step after deposit of the powder, e.g., heating or application of solvents, so as to make the image permanent. Moreover, in order to obtain uniform and equivalent image reproduction with powder development, a rather extensive and expensive installation has to be used.

When developing with dispersions in liquids with high electrical resistivity, the choice of suitable liquid media is limited. Electrically insulating liquids e.g. chlorinated hydrocarbons or high purity water have to be used.

The present invention, which will be defined hereinafter, is based in part, on the discovery that if an electrostatic charge carrier having a hydrophobic surface and bearing an electrostatic charge pattern detectable at such surface is contacted non-differentially (as will hereafter be defined), with an appropriately chosen liquid medium, the electrostatic charges influence the interfacial tension between the hydrophobic surface and the liquid medium so that the liquid wets the surface according to the charge pattern. In consequence a record in terms of surface wetting and indicative of an electrostatic record in the form of image-wise or signal-wise deposited or induced electrostatic charges borne by a support can be formed by simply flooding or otherwise supplying the whole support with liquid.

The expression hydrophobic where used in this specification in relation to a charge carrier surface means that water forms on the said surface (in the absence of attractive electrostatic charges) a contact angle of at least (For a definition of contact angle see J. Alexander, Colloid Chemistry, vol I, Principles and Applications, 4th ed., D. van Nostrand Company, Inc., New York, pp. 79-80.)

For recording the electrostatic charge pattern a liquid medium is used which does not normally wet the hydrophobic surface of the charge carrier, or not to any appreciable extent, but which will wet such surface under the influence of the encountered electrostatic attractive charges so as to record the electrostatic charge pattern in terms of surface wetting. In the interest of forming good quality records, it is preferably for the electrostatic charges to be capable of reducing the contact angle between the liquid and the hydrophobic surface to such an extent that complete or spreading wetting occurs (contact angle of 0 or approximately so). Water together with one or more other ingredients (e.g., an organic polar liquid), can be used provided that the composition as a whole is sufiiciently electrically polarizable, so preferably electrically conductive. When using a mixture of liquids, the minimum proportion of water required will depend upon the properties of the other ingredient or ingredients. If an organic liquid with high dielectric constant such as formamide is incorporated, this may be present in sub stantial amount, e.g., 60% by weight, but in general it is preferred in all cases to use a liquid comprising at least 60% by weight of water and with a specific conductivity of at least 10 mho/cm. As will hereafter be illustrated, the liquid used in the process can be a solution. It is also possible to use a liquid which contains dispersed material, e.g., to use an aqueous liquid in which a finely divided material is dispersed, forming a stable suspension or emulsion. However, in view of the known electrophoretic development technique already referred to herein it is to be understood that we lay no claim in this specification to the use of insulating liquid being a suspension or emulsion of such constitution that the electrostatic charges cause dispersed solid or liquid to move through the continuous phase and thus to increase in concentration at the areas where this continuous phase liquid Wets the charge carrier. It has already been mentioned that in the electrophoretic development the choice of suitable liquid media is limited. In electrophoretic development the objective is always to bring about displacement of the dispersed material (which constitutes the developer) through the continuous liquid phase under the influence of electrostatic charges constituting a latent image. Taking into account a rather long sedimentation time of the dispersed particles it is essential in electrophoretic development to use as carrier for the dispersed developer, a liquid which is electrically insulating thus preventing the carrying off of the electric charges of the electrostatic image. In British patent specification 755,486 for example, which describes electrophoretic development, it is stated that apart from elec trically insulating organic liquids, Water can be used on the understanding that it is of high purity.

It has now been found that record-wise wetting of an electrostatically charge containing hydrophobic dielectric medium, preferably a photoconductive layer containing an electrostatic charge image, with a conductive aqueous liquid can be obtained on the electrostatically liquid attracting areas of that medium by non-differentially contacting said medium with said liquid for a period not succeeding the discharge time of the charge which will be dissipated through that liquid, preferably within a time interval not greater than the time interval in seconds at which the original charge is diminished to the l/e value, e being the base number of Napierian logarithms.

A second discovery on which the present invention is based is that the quality of a record in terms of surface wetting formed as above described and the relationship between the record and the electrostatic charge pattern, i.e., whether these are in positive to positive or positive to negative relationship, is influenced if a biassing electrical potential of suitable magnitude is applied between electrodes extending over the area of the liquid/surface contact zone, on opposite sides thereof, so as to superimpose an electric field which traverses the surface normally thereto.

The superimposed field may be a direct current electric field or, if the charge carrier has rectifying properties, an alternating current field. If a direct current field of appropriate strength with a direction opposite to that of the field due to the electrostatic charge pattern is superimposed, the effect is to improve the quality of the record, and in particular of giving records with better contrast, whereas by superimposing a direct current electric field of appropriate voltage in the same direction as the field due to the electrostatic charge pattern the effect is that the liquid is inhibited from or restrained in wetting the electrostatically charged or image areas of the surface and is caused to wet such surface in the other areas where there is no or negligible electrostatic charge thereby forming a reversed or counter image. In the event that the charge carrier has rectifying properties and an alternating current field of appropriate strength and frequency is superimposed, an effective bias is created which assists or retards wetting in a given area of the charge carrier depending on the composition of the carrier. When using a carrier comprising photoconductive zinc oxide, a counter image is obtained while with materials causing rectification in the reversed sense a non-reversed image of improved quality (sharper contrast) is surprisingly obtained. It is preferable to use an alternating current field with a frequency substantially above the normal 50 cycles, say a frequency of 100 cycles or more, and we have so far obtained the best results with frequencies above 500 cycles, e.g., 1000 cycles, all frequencies mentioned herein being in cycles per second.

Accordingly the present invention includes any process in which a hydropholic surface of an electrostatic charge carrier bearing an electrostatic charge pattern which may be a persistent internal polarization image (see Advances in Xerography 8l962) Photographic Science and Engineering, vol. 7, 1963 pp. 9 and 10) detectable at such surface is wetted with liquid according to such charge pattern thereby to record such pattern in terms of surface wetting, by contacting said surface nondifferentially (as herein defined), and either or not within a superimposed direct or alternating current electric field extending normally such surface, with an aqueou conductive liquid with which such surface is normally substantially unwettable but by which such surface iS wettable under the influence of attractive electrostatic charges.

The expression electrostatic charge pattern is used broadly to include any distribution of charges within a support area such that different parts of this area are charged to different extents or such that some parts only are charged. Thus the expression includes electrostatic images of reading matter, diagrams, pictures etc., and charges constituting the electrostatic record of wireless or other signals.

By the expression non-differential as used herein in relation to the contact of the charge carrier surface by the liquid we mean that said surface is contacted by the liquid alike in the charged and uncharged areas, or, if the electrostatic charge pattern results from overall but differential charging of the carrier, then alike in the differentially charged areas. This non-differential contact may be brought about by supplying liquid onto the whole surface of the charge carrier or by contacting the surface with the liquid at a multiplicity of closely spaced points or lines uniformly distributed over the carrier surface. The aqueous liquid medium for the development may so be supplied to the charge carrier by a liquid supply means wherein such medium is held in a capillary recess or passage or a plurality of capillary recesses or passages.

From what has already been said as to the different relationships in which the electrostatic charge pattern and the record in terms of surface wetting may stand to each other, it will be understood that when in this specification reference is made to the wetting of a surface according to an electrostatic charge pattern, it is meant that the deposited liquid pattern visibly or detachably records the subject of the electrostatic record. The liquid and electrostatic records need not correspond in the sense that two positive records of a text correspond; on the contrary one may be positive and the other negative.

The expression counter image is used herein to devote an image which is reversed in this sense with respect to electrostatic image.

By way of further explanation, it is not necessary, as will be clear from the examples of the invention to be given hereafter, for the whole charge carrier surface to be non-differentially contacted with the liquid at one and the same time. For example the liquid can be applied to the surface by means of a roller which moves progressively thereover. In such cases, the superimposed electric field need not extend across the whole of the charge carrier surface during the whole of the period in which liquid is being applied to such surface but can at any given moment merely extend substantially uniformly across the area of the charge carrier surface in which contact with liquid is at that moment taking place and the said field may move progressively along the said surface with the liquid applicator. The claims hereof are to be construed accordingly.

The new technique of depositing liquid in dependence on an electrostatic charge pattern in a superimposed electric field may be applied to give a one-step development process in which, by the application of developing liquid, a visible image which does not require a special after treatment for fixing purposes is created. Thus, for example, the aqueous medium used in a process acc0rding to the invention as above defined may be an ink or dye or a medium which contains an ingredient which reacts, e.g., with a component present in the charge carrier or with the atmosphere, to form a dye. The invention is not however limited to the direct production of dye or other visible or final records from an electrostatic record. The new techniques opens the way to a variety of alternative ways of using such a record. As one example, after depositing an uncoloured hydrophilic composition on the hydrophobic charge carrier surface under the influence and according to the pattern of electrostatic charges thereon the resulting liquid deposit can be converted into a readily visible image by passing over the surface a roller charged with an aqueous dyestuif solution. Various other ways of indirectly forming a visible record starting with an electrostatic record will be described hereinafter. It is also to be understood that aque ous medium deposited under the influence of the electrostatic charge pattern can be transferred to another support for the purpose of producing a visible record on that support in the same or by a subsequent step.

The particular method by which the electrostatic charge pattern is formed is not in any way crucial. By way of example the techniques for producing electrostaitc records as summarised in the introductory paragraphs of this specification can be used, or any method, e.g., electrical polarization of an insulating material. The invention has so far however been primarily developed in connection wiih electrophotographic processes in which the electrostatic record is created in a photoconductive layer as a result of the application of an electric charge or field on record-wise exposure of the layer to light or other radiation which raises the electrical conductivity of the layer.

In a process according to the invention non-differential supply of liquid to the charge carrier may be effected in a variety of ways. A method will later be illustrated, which involves the progressive contact of the whole electrostatic record area with a continuous liquid face.

As a carrier for an electrostatic latent record to be used in a process according to the invention insulating layers or sheet containing photoconductive or thermoconductive substances may be used. These materials preferably comprise a back-layer or a support, which possesses higher conductivity than the insulating layer.

According to a preferred feature in a process according to the present invention, a photoconductive layer with hydrophobic properties is used as carrier for the electrostatic charge pattern. Preferably the photoconductive layer comprises a photoconductor dispersed in an insulating binding agent. The usual photoconductive layers which comprise organic or mineral photoconductive substances incorporated into insulating polymeric binding agent, and the photoconductive layers which comprise organic photoconductive polymers, possess a hydrophobic character. Examples of photoconductive layers comprising photoconductive polymers are described e.g. in United Kingdom patent specifications 964,877 and 964,875. Examples of photoconductive binding agents more especially for photoconductive zinc oxide are described in United Kingdom Patent 964,885.

As an illustration, a nonlimiting list of hydrophobic polymeric binding agents for photoconductive materials is niven here:

Polyvinyl acetate Copolymer of vinyl acetate and an ester of vinyl alcohol and a higher aliphatic carboxylic acid such as lauric acid, stearic acid, palmitic acid, e.g. co[vinyl acetate/ vinyl stearate] (85/15) Polyalkylmethacrylate, e.g. Plexigum P26 (trademark for an acrylic resin marketed by Rohm Haas G.m.b.I-l., Darmstadt, Germany) Kunstharz EM (trade name for a ketone resin prepared by the condensation of an aliphatic ketone with formaldehyde, marketed by Rheinpreussen G.m.b.H., Homberg, Germany Esterified colophony Syntex 800 trademark for a cyclic rubber from the N.V. Chemische Industrie Syrnes, Hoek van Holland, Netherlands Polyol X-450 (trademark for a copolymer of the following formula:

marketed by lShell Chemical Corporation, New York, N.Y., U.S.A.)

Co[N-vinyl car-bazole/ethyl acrylate] prepared as described in United Kingdom patent specification 964,875

Polyvinylchloride e.g. Hostalit C 270 (trademark of Farbwerke Hoechst A.G., Frankfurt and Main-Hochst, W. Germany) Piccolastic D-100 (trade name for a thermoplastic styrene polymer marketed by Pennsylvania Industrial Chemical Corporation, Clairton, Pa., U.S.A.).

The ratio of insulating binding agent and photoconductor depends on the required quality of the photoconductive layer with respect to the photoconductive properties, mechanical strength capacity and insulating power. Good results are obtained with a ratio of binding agent and photoconductor of 1:3 to 1:9. When using layers with a relatively high content of binding agent, the image sharpness diminishes and when using layers with a much lower content of binding agent, the relaxation time of the charge diminishes quickly.

To layers containing binding agents which are not sufficiently hydrophobic by themselves, the desired waterrepellent properties can be conferred as known per se by special additives or by an after-treatment. Additives enhancing the hydrophobicity e.g. stearic acid as described in the United Kingdom patent specification 883,783, or binding agents bearing active hydroxyl groups which react with diisocyanates, as described in the United Kingdom patent specification 896,610, can be incorporated in the photoconductive layer.

If polystyrene-butadiene latexes are used as binding agents, a thermal after-treatment can be applied as described in the United Kingdom patent specification 766,- 979.

The surface of the photoconductive layer can also be made more hydrophobic by adsorption of an appropriate substance on the photoconductive layer.

It is also possible to make the surface of a photoconductive layer more hydrophobic by applying a coating layer comprising a hydrophobic layer-forming material such as a hydrophobic polymer, varnish or wax.

If the photoconductive layer is more hydrophobic than is required, a layer (which may be very thin) of a less hydrophobic substance can be applied thereon or the surface can be modified by applying a sufficiently small quantity of a hydrophilic colloid, e.g., gelatin, polyvinylalcohol, a cellulose derivative or an alginic acid derivative. The thickness of such hydrophilic layer preferably varies between 0.2 and 2 This layer can also be applied from a strongly diluted solution of a wetting agent, but should not enable the surface charge to be carried off.

The photoconductive layer and/or a top layer for increasing the hydrophobic properties may includes other known additives such as plasticisers, dispersing agents, optical bleaching agents, substances counteracting oxidation and aging, agents improving the gloss, matting agents, sensitizing dyes and chlorine containing polymers which increase the sensitivity as described in our United Kingdom patent specification 964,878. The amount and the nature of these substances are chosen in such a way that the chargeability of the layer is not markedly diminished.

In the course of our experiments it has been found that particularly good results can be obtained in the formation of a liquid record by means of an electrostatic charge record if the electrostatic charge carrier is a photoconductive layer comprising photoconductive zinc oxide as a photoconductive component. The photoconductive zinc oxide/binder composition preferably consists for at least 50% of zinc oxide in a hydrophobic binder. Specially good results are obtainable with zinc oxide having been prefer-ably treated with a suitable acid compound or salt which increases the dark-resistivity of the zinc oxide. The increase of the dark-resistivity of the photoconductive zinc oxide permits the formation of a photoconductive layer, which comprises the zinc oxide dispersed in a binder and in which the specific resistivity of the binder is not markedly higher or is even the same as or lower than that of the acid-treated zinc oxide.

By usual photoconductive zinc oxide is understood any commercially available type of zinc oxide which has been prepared according to the French process, in this process zinc oxide is prepared by oxidation of zinc vapour.

Zinc oxide types which appear from our experiments to give particularly good results are e.g.:

Blanc de Zinc, Neige Extra Pur, types A, B and C, marketed by Vieille Montague S.A., Liege, Belgium,

Zinkoxd (reinst) marketed by E. Merck A.G., Darmstadt,

Germany,

Florence Green Seal Lead-Free Zinc Oxide marketed by the New Jersey Zinc Company, New York, N.Y., U.S.A.

Zinc Oxide Analytical Reagent, marketed by Mallinckrodt Chemical Works, St. Louis, Mo., U.S.A.

The preferred compounds with acid properties for treating the photoconductive zinc oxide are:

Aliphatic non-substituted monoand dibasic carboxylic acids or aliphatic monoand dibasic acids which may contain one or more hydroxyl groups such as lactic acid and tartaric acid, and Organic phosphorus compounds corresponding to the general formulae:

wherein:

R represents a hydrogen atom, a hydroxyl group or a chlorine atom,

R represents a hydroxyl group, a chlorine atom, an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an alkoxy group, a substituted alkoxy group, an aryloxy group or a substituted aryloxy group, and

R represents an alkyl group, a substituted alkyl group, an aryl group, a substituted aryl group, an alkoxy group, a substituted alkoxy group, an aryloxy group or a substituted aryloxy group.

The following further elucidation of the invention includes a description of further factors which can usefully be taken into account in preparing the electrostatic charge carrier and in selecting the composition of this carrier and of the liquid.

As described on pages 394-395 of the aforesaid book of Schwartz, Perry and Berch, it is explained that there is a relation between the roughness of the surface of a solid material and the contact angle with a particular liquid. It appears therefrom that the size of the contact angle, with other words, the wetting power, may be influenced by adapting the roughness of the surface.

The degree of roughness in the structure of the surface of a charge carrier in the form of a photoconductive layer comprising a dispersed photoconductor may be influenced by suitably chosing the grain size and the degree of dispersion of the photoconductive material or by the controlled flocculation of the binding agent in some solvents. Further, a granular structure of the surface can be obtained by adding compounds to the photoconductive layer.

Further a certain rough surface structure can arise by applying the photoconductive layer onto a screened support. Surface roughness can also be obtained by applying the photoconductive layer in a regular or irregular screen form as e.g. by applying with a screened roller or by pressing a screen profile into the half-dry or plasticized thermoplastic material of the photoconductive layer or a superimposed layer.

The surface roughness of a layer, i.e. the degree of the unevennesscs, may be determnied with a Perth-O-Meter (ref. Dr. Perthen, Hannover/Hommel-Werke, Mannheim). It is expressed by the sum of two terms, namely W +Rt wherein W (Welligkeit is the depth measure for the macro-unevennesses of the surface and Rt is the depth measure of the micro-unevennesses of the surface. Macrounevenness here refers to the deviations of the general surface contour from a plane and Micro-unevenness refers to the surface irregularities which are present locally in successive parts of the surface contour.

The total value W+Rt of the surface roughness is preferably comprised between 2 and 15 The degree and kind of surface unevenness which is permissible depends in part upon the way in which liquid is to be made available for wetting the surface. The liquid must have proper access to all parts of the surface. When the liquid is applied from a roller onto the charge carrier, attention should be paid to the distance between the free liquid surface on the wetting device and the deepest point of any recesses or depressions or reticulations in the surface to be developed. Good results have been obtained with distances comprised between 1 and IOU/1., preferably between 3 and 30 If the distance is taken too small, e.g. by exerting too strong a pressure, the charge image is destroyed. If too big a distance is taken, the fine image details are lost. The adjustment of the suitable distance between the liquid surface and the profile of the material depends on the surface structure of the material and the kind of the developing liquid.

If the developing liquid is brought near the surface to be developed at normal atmospheric pressure, the surface roughness will lie preferably in the average of the above given extremes. The optimum value of surface roughness for obtaining a favourable contact angle is shifted to higher or lower values, according whether the wetting with liquid is facilitated or inhibited by one or more of the following factors: viscosity and surface tension of the liquid, surface roughness and surface tension of the material to be developed, interfacial tension of the insulating material/liquid system, the quantity of the conveyed liquid, the pressure exerted upon the liquid, gravity, capillary forces, streaming potential (see Electrical Phenomena at Interfaces in Chemistry Physics and Biology by J. A. I. Butler Co. Ltd. London (1951) pp. 77- 82) and magnetic and electrical forces which arise between the material to be wetted and the wetting liquid.

As aforesaid, the contact angle formed by the developing liquid with the uncharged insulating material, has to be such that the liquid on or opposite to the uncharged areas or on or opposite to the image areas with insufficient field strength does not or does not sufficiently under go electrostatic influence which causes spreading out or wetting of the surface. On the other areas, on the other hand, the electrostatic field at the surface of the insulating material performs the work necessary for wetting.

The work required for Wetting can be increased or decreased by modifying the physical properties of the liquid and of the insulating material to be wetted, e.g. by modifying surface tension, electric conductivity, polarizability, capacity or viscosity.

In order to obtain a visible image, the conductive aqueous medium may contain preferably in solution or in dispersion dyestuffs which are fast to light. Agents for improving the fastness to light of the dyestuffs and mordanting agents may be added. Further, compounds may be added to the liquid which improve dissolving, emulsifying and dispersing of substances which influence the physical and chemical behaviour of the liquid. These substances comprise organic or mineral dyestuffs, substances which increase or decrease the surface tension and/ or the conductivity and/or the polarizability and/ or the capacity and/or the viscosity, binding agents e.g. colloids and latexes, macromolecular compounds, substances improving the fastness to light of the dyestuffs, substances preventing the ink image from drying quickly. For the latter purpose, a hygroscopic substance is preferably used if the liquid medium is water.

In another embodiment an aqueous developing liquid is used which contains no dyestuff but a colourless or little coloured component, which is converted into a dyestulf only during or after the development, by oxidation with atmospheric oxygen, by exposing to light or heat, or by reaction with a reaction partner which is present in the support of the electrostatic image or (in case of transfer) in the ultimate support.

As we have already explained, processes according to the invention are preferably performed using an aqueous medium consisting for at least 60% by weight of water. Very suitable developing liquids comprise water and one or more compounds of the following categories in the proportions stated:

(1) 03-20% of a dyestuff soluble or dispersible in water.-Suitable organic dyestuffs are e.g.:

Crystal Violet C.I. 42,555 Malachite Green C.I. 42,000 Methylene Blue C.I. 52,015 Victoria Blue 0.1. 42,595 and CI. 44,045 Carmine Red -C.I. 75,470 Nigrosine C 140 powder C.I. 50,420 Chloramine Black EX (dark) C.I. 30,235 Rayon Black C (double cone.) (3.1. 35,255 Chris Cuprofles 3 LB C.I. Direct Black 63 Suitable inorganic pigments are any structural form of carbon e.g. graphite, carbon black, lamp black, bone black, charcoal, Ultramarine blue, cadmium sulfide, titanium dioxide, zinc oxide, iron oxide, magnetic iron oxide, aluminum powder, and bronze powder.

(2) Colourless or little coloured compounds conversable to a coloured compound.-Such a colourless or little coloured compound can be converted into a coloured compound, during or after wetting the charge carrier, with a substance present in or on the material to be developed. Suitable reaction partners the reactivity of which is promoted by the presence of a liquid phase are e.g. those described in United Kingdom patent specification 898,354.

Suitable known colour reactions in aqueous medium are e.g. coupling reactions of diazonium compounds with known coupler compounds such as [i-naphthol.

For the colour formation in situ ferric salts are suitable which react with aromatic compounds containing hydroxyl groups e.g, pyrogallol and dodecyl gallate.

Suitable colourless reaction partners are the colourless triazolium and tetrazolium compounds such as those described in the British patent specification 670,883. These compounds are converted into a coloured compound by a reducing reaction partner.

The classical colour coupling reactions between oxidiz' able aromatic amino developers and colour couplers are also applicable. Classical colour coupling reactions are described e.g. in The theory of the Photographic Process by C. E. Kenneth Mees, revised ed., 1954, The Macmillan Company, New York, pages 584-589.

Many other colour reactions and bleaching-out reactions of colours, which also may be used for forming an outlined image pattern, are known to those skilled in the reproduction art. Further, an extensive list of colour reactions is set out by Feigl in Spot Tests 1954, Elsevier Publish. Corp, Amsterdam.

It is also possible, provided a suitable solvent or dispersing agent for polyvinyl chloride is used, to cause a colour reaction with zinc oxide, after the deposition of a polyvinyl chloride residue on the charged areas of a layer containing zinc oxide by Warming the polyvinyl chloride.

It is evident that an applied colour reaction may be activated or accelerated by heat and light, and that in the layer with the electrostatic pattern as well as in the developing liquid catalyst known per se can be incorporated which promote the colour reaction, if necessary.

The formation of a visible image by reacting of one or more reaction partners in the material surface to be developed, and which is attainable for the reaction with one or more reaction partners in a liquid phase, has the advantage that the formed image is well anchored in the surface of the material so as to be very resistant to mechanical erasure.

(3) 0.220% of a substance influencing the surface tension-The surface tension may be increased by adding water-soluble substances e.g. potassium carbonate, aluminium sulfate, iron sulfate, cadmium chloride and magnesium sulfate. A list of other such substances as given e.g. in Taschenbuch fiir Chemiker und Physiker by J. DAns and E. Lax, edition 1949, Springer Verlag, p. 1008.

The surface tension may be decreased by adding Watermiscible organic substances e.g. methanol, ethanol, acetone, methyl ethyl ketone, acetic acid, hydroquinone, lauryl sulfonate, dodecyl sulfonates, saponine and polyglycol derivatives.

Other useful surface-active substances are given in Textilhilfsmittel und Waschrohstoffe by K. Lindner, Wiss. verlagsgesellsch. m.b.H., Stuttgart 1954.

(4) Up to 10% of a substance which delays drying up of the ink.e.g. glycerol, glycol and sorbitol.

(5) Up to 10% of a water-soluble or water-dispersible blinding agente.g. gum arabic, carboxymethyl cellulose, casein, polyvinylpyrrolidine, polyacrylates, polystyrene, polyvinylacetate, waxes, silicates and colloidal silicic acid. These substances decrease ability of the dry ink of being washed away by water.

(6) Up to 35% of an organic polar liquid with high dielectric constant and miscible with water.- e.g. formamide. According to the choice of the used dyestuffs (if any) in the insulating layer and in the ink, different colour contrasts may be obtained. If the colour of the ink is darker than that of the insulating layer, a positive image is obtained. If the colour of the ink is lighter than that of the insulating layer, counter image (positive print of a negative original) is obtained. In the latter case, a photoconductor which is dark-coloured in itself is used e.g. selenium, cadmium sulfide, cadmium selenide and antimony sulfide, or a white photoconductor e.g. zinc oxide, to which sensitizing or other dyestuffs are added. Development can than be carried out with a dispersion of a white or clear-coloured pigment e.g. titanium oxide, zinc oxide, zinc sulfide, barium sulfate, antimony oxide, china clay and calcium carbonate. Counter images can also be formed by using as charge carrier an insulating layer comprising at least at its surface a dark dyestuff and by using an aqueous solution of a bleaching agent for this dyestuff, for the development.

With reference to embodiments of the invention in which the electrostatic charge carrier is a photoconductive layer which has a relaxation time of preferably at least 0.1 second, and which is united with a conductive support. Suitable conductive supports are e.g. plates or foils of metal such as aluminum, copper, bronze, lead and zinc, or glass-plates provided with a thin layer of tin oxide having a specific resistivity of 10 to 10 ohm.cm., foils or fabrics of plastic substances, provided with a thin conductive layer such as described in United Kingdom patent specification 964,873 and finally paper. Suitable kinds of paper are those which show a resistivity lower than 10 ohm-cm. at a relative humidity of 50% e.g. the kinds of paper described in United Kingdom Patent 995,491. Other suitable kinds of paper are those containing at least 2% of conductive filing materials e.g. carbon. Other suitable papers are those of which the surface, which is turned to the photoconductive layer, is provided with a conductive layer e.g. a thin lead or aluminium foil, or a dispersion of a. metal powder or of carbon powder in a binding agent. Finally the fabrics consisting preponderatingly of carbon e.g. those marketed by Union Carbide Corporation, New York, N.Y., U.S.A. under the name Graphite Cloth Grade WCA, WCB and WCC, may successfully be used as conductive supports.

Fabrics or kinds of paper which show insufiicient conductivity at the prevailing air humidity, may also be used successfully, provided that the rear side of the support is wetted with Water before or during the development.

Printing plates can also be manufactured according to the invention. For example an electrostatic latent image can be developed with an aqueous dispersion of a hydrophobic binding agent and the moist image then transferred to a gelatin foil and dried. After wetting the gelatin and rubbing with printing ink, only the areas covered with binding agents accept ink. Suitable dispersions of hydrophobic binding agents are latexes e.g., polystyrene latex, latex of cobutadiene/acrylonitrile and Lytron S-lO (trademark for a thermoplastic copolymer of styrene marketed by Monsanto Chemical Company, Springfield, Mass., United States of America).

Reference is now made to the accompanying drawings by way of explanation and illustration of the invention. In these drawings:

FIGS. la and lb and FIGS. 2a, 2b and 2c are explanatory of the phenomenon of surface wetting and the way in which the invention utilises this phenomenon;

FIG. 3 is a diagrammatic representation of a device for use in carrying out the invention. In this figure however the means for applying the superimposed electric field is not represented;

FIG. 4 represents stages in the formation of a reversed record (counter image).

Referring firstly to FIGS. 10, 1b, 2a, 2b and 2c, the first two of these figures show respectively a drop deposited on a surface which is wetted by the liquid and a drop deposited on a surface which is substantially unwetted by the liquid. The so-called contact angle is the angle between the surface on which the drop is deposited and a tangent to the curve of the liquid drop drawn through a point where the drop curve meets the surface. In the case of the first drop the contact angle is much less than 90 while in the case of the second drop the contact angle is substantially greater than 90.

FIG. 2a shows graphically how the contact angle of an aqueous developing liquid on the hydrophobic surface of an electrophotographic layer is influenced by an electric charge. After corona-charging the layer has, e.g., at its surface, a field strength of 100 to 600 volt/cm. and after exposure a field strength between and 100 volt/cm.

FIGS. 2b and 2c diagrammatically represent the behaviour of a drop of aqueous medium 3 on the hydrophobic surface of an insulating layer 2 borne by a conductive support 1, before and after charging. FIG. 2b corresponds with point b of the curve of FIG. 2a and FIG. 20 corresponds with point c of such curve. At the charge level represented by point c the liquid drop which is normally repelled by the surface and leaves this substantially unwetted, becomes practically completely spread out.

Referring now to FIG. 3, the apparatus here represented comprises a roller 10 which rotates in a container 9 filled with aqueous ink 6. A photoconductive layer bearing an electrostatic charge pattern is advanced between a smooth surface roller 10 and a pressure roller 11. The output of liquid can be regulated by means of a roller or by means of a doctor knife 17 as shown.

The liquid development is influenced by applying during the passage of the foil 5 between the rollers, an electric field perpendicularly to the foil. The means which applies the electric field is not shown but in fact a source of is connected to the rollers 10 and 11. A tension of 10100 volts, the field direction of which is opposite to that of the charged photoconductive layers, increases the contrast. A tension of -150 volts with a field of the same direction as that of the charged photoconductive layer reverses the image, i.e., liquid is deposited onto the exposed areas.

By applying an alternating field perpendicular to the foil, e.g., by connecting rollers 10 and 11 to the terminals of an alternating voltage source, the image quality can be improved. Both the magnitude of the alternating voltage and the frequency affect the results. If at a relatively low frequency, e.g., 50 cycles, a little ink is found to deposit on the unexposed areas, this soiling of the image background can be avoided or reduced by increasing the frequency.

The rate of conveying the photoconductive material between the rollers can vary between wide limits. Good results have been obtained with speeds of the photoconductive material between 1 cm./ sec. and 1 m./sec.

Reference is now made to FIG. 4. According to the process here represented, a latent electrostatic image on a charge carrier (FIG. 4a) is developed to the invention with an aqueous conductive solution, emulsion or dispersion (FIG. 4b) which after drying leaves a hydrophobic residue (FIG. 4c) which is electrically conductive. For this purpose, an aqueous dispersion of modified paraflin as marketed by Mobil Oil A.G., of Hamburg, W. Germany under the trademark Mobilcer A is suitable. After drying, charging is repeated once again (FIG. 4d). Only the uncovered areas of the carrier retain electrostatic charges. Development now takes place with an aqueous dyestutf solution (FIG. 40) which wets only the areas made hydrophilic by the previous charging.

Ink images obtained according to the invention may be transferred onto another support in a simple way, by bringing the still wet ink images in contact with an inkaccepting surface e.g. a porous surface such as paper or a transparent foil covered with gelatin. This process permits e.g. to reproduce documents according to the reflectographic exposure method, whereby a positive mirror image is obtained which is transferred onto another support so as to form a positive legible image.

Finally it has to be mentioned that the still wet ink image may be transferred onto the back-side of the support by rolling up the developed image still wet around a cylindrical rod of small diameter. In this way an ink print is transferred onto the back-side of the wetted material. This method is especially interesting to obtain a mirror image of a text. In this method good results have been obtained, especially when using those electrophotographic materials according to the present invention which comprise a thin paper support with a certain sur face roughness.

The transfer of a colour image to a transparent support in the presence of a mordanting agent for the dyestuff is specially suitable for producing images in more than one colour. Suitable dyestuffs and mordanting agents for this process are described e.g. in the -United States patent specification 1,121,187. The procedure is the following: Thre colour separation records are formed by exposing three sheets of an electrophotographic material to a coloured original, one through a cyan, one through a magenta and the other through a yellow filter.

Photoconductive substances the spectral sensitivity of which is suited for the manufacture of electrophotographic materials used for the reproduction of colour images are e.g. selenium to which arsenic or tellurium are added or zinc oxide to which sensitizing agents are added as described in the United Kingdom patent specifications 885,718; 895,723; 954,017; 954,018; 885,715; 885,716 and 885,717.

13 The three selection images are developed according to the invention with an ink which has the same absorption spectrum as the used filter. These ink images are then successively transferred in exact register to the same foil, so as to obtain a true colour reproduction of the original.

The invention may further be applied successfully in different printing techniques e.g. in hectography, offset printing and for obtaining etch reliefs. In a hectographic process, a latent electrophotographic image is developed with a concentrated dispersion of a dyest-uif e.g. an aqueous dispersion of crystal violet or malachite green. After drying, the developed image is employed in an alcohol duplicating machine to make the desired number of copies.

For manufacturing offset printing plates, a latent electrophotographic image is developed with a dispersion which after drying results in a hydrophobic residue. Then the material is braced on an offset machine and the image background is made hydrophilic e.g. with ferricyanide and organic acids.

For manufacturing etch reliefs, an electrophotographic material is used which in its photoconductive layer contains a polymer with free hydroxyl groups (e.g. the copolymer available under the trademark Polyol X-450 hereinbefore referred to) as a binding agent. The electrostatic latent image is developed with an aqueous dispersion of a substrate (hardening agent) which is able to react with these hydroxyl groups. The hardening agent either can be incorporated into the ink or dispersed in the binding agent and a catalyst for promoting the hardening reaction may be dissolved in the ink.,. Suitable hardening agents for these polymers are e.g. urea derivatives such as dimethylol urea described in the British Patent specification 580,275, or monocyclic ureas, described in the United Kingdom patent specification 575,260. After drying, the electrophotographic material is heated for a short while at 100-150 C. Hereby a reaction takes place on the image areas between the binding agent and the hardener, whereby on these areas the solubility of the photoconductive layer is diminished markedly. Now the non hardened image parts can be removed with a suitable solvent whereupon the metal support can be etched through the etch resist in a known way.

The following examples illustrate the present invention.

EXAMPLE 1 To 500 cos. of a 4% solution of Flexbond D-13 (trademark) in ethanol, 225 g. of photoconductive zinc oxide (neige extra pure, type A, marketed by Vieille Montagne S.A., Liege Belgium) are added. The mixture is ground in a ball-mill for 48 h., whereupon the following composition is added whilst thoroughly stirring:

Ccs. 2% solution of Flexbond D13 (trademark) in ethanol 500 solution of monobutyl phosphate in ethanol 10 10% solution of succinic acid in dimethyl form amide 10 1% solution of fiuoresceine (C.I. 45,350) in ethanol 10 This photoconductive dispersion is applied by knifecoating onto a paper support provided with an aluminium foil at a rate of 10 sq. m. per liter, and dried.

The obtained photoconductive layer is charged up to a tension of --300 v. by a tension of --7000 v. at the corona wires, and exposed for 0.7 sec. to a diapositive with a 75 watt bulb placed at a distance of 10 cm. Then the latent image is developed with a developing device as shown in FIG. 3 which is explained below, by means of a developing ink containing:

A 15% colloidal graphite dispersion having a specific conductivity of 8.2x 10- mho/cm g 5 Ludox (trademark for a 30% aqueous dispersion of colloidal silicic acid marketed by E. I. du Pont de Nemours & Co. Wilmington, Del., U.S.A.) g 15 Water ccs 10 The apparatus shown in FIGURE 3 comprises two rollers and an ink container. Roller 11 is an aluminum cylinder with a diameter of 15 mm. and a length of 25 cm., which accomplishes the function of guiding roller for the material to be developed. Roller 10 is a smooth chromiumnickel steel 18/8 cylinder with a diameter which is twice as large as that of roller 11. The rollers 10 and 11 are connected to a v. direct current source so that during development a direct current field is superimposed normally to the foil 5, the direction of the field being opposite to that due to the electrostatic latent image.

The photoconductive layer of the exposed material is contacted with the ink roller and held at a distance of 0.5 mm. so as to form an ink meniscus of 3 mm. The photoconductive material is moved at a speed which is equal to that of the ink roller. The roller 10 rotates at a speed of 2 m./ min. and is partly immersed in the developing ink.

The ink settles onto the nondischarged image areas and forms after drying a positive non-washable image.

EXAMPLE 2 Onto a photographic paper support the following conductive precoating layer is applied:

A 30% aqueous dispersion of colloidal carbon, the

carbon particles of which have an average diameter of 0.1 and which possesses a specific conductivity of 10- mho/cm g 50 2% aqueous solution of carboxymethyl cellulose ccs 500 Water ccs 100 Ethanol ccs 400 A photoconductive layer as described in Example 1 is applied thereon.

After charging the layer as in Example 1, an image is episcopically projected thereon for 30 sec. For this purpose two Sylvania Green (trademark) exposure lamps of 14 watt and a Schneider Kreuznach Xenar (trademark) 4.4 lens with a focal distance of 21 cm. are used, placed at a distance of 84 cm. and set at a diaphragma aperture of 5.6.

The positive latent electrostatic image is developed in the same way as in Example 1 but with a developing liquid of the following composition:

Ccs. 15% aqueous dispersion of colloidal carbon having a specific conductivity of 82x10" mho/crn. 40

Water 100 For increasing the density of the developed image, the rotation speed of the ink roller is increased to 20 m./min. and the travelling speed of the paper to 3 m./min. The distance between the photoconductive layer and the roller is 1 .mm. A vigorous positive image is obtained.

EXAMPLE 3 The following composition is ground for 24 hours in a ball-mill:

Zinc oxide (from Example 1) (neige extra pure, type A, marketed by Vielle Montagne S.A., Liege, Belgium) k-g 4.5 4% solution of Flexbond D-13 (trademark) in ethyl alcohol litres 9 15 After grinding the mixture is diluted with the following mixture:

4% solution of Flexbond D-l3 (trademark) in ethyl alcohol litres 21 10% solution of monobutyl phosphate in ethyl alcohol -Q ccs 300 10% solution of succinic acid in dimethyl formamide ccs 300 1% solution of fiuorescein in ethyl alcohol ccs 300 This dispersion is coated onto a baryta-coated paper of 90 g./sq. m. according to the knife-coating system and in such a way that with 1 litre 10 sq. m. are covered.

After drying, a uniform charge to a tension of 400 v. is applied on these layers by means of a corona discharging of 6000 v. Then the charged layer is image-wise exposed for 0.7 second, through a negative transparency with a lamp of 75 w. placed at a distance of 10 cm. The formed latent image is developed in the following dyestuff solution:

G. Methylene blue 2 Water 100 having a specific conductivity of 1.2 10- mho/cm., by means of the apparatus as described with reference to FIG. 3 while an alternating tension of 100 v. and 50 cycles is applied between the rollers and 11 during development. The contact time of liquid and photoconductive layer was 0.1 second. A positive legible image is obtained.

At 50 cycles a vigorous image is obtained but a little ink is also deposited upon the non exposed areas. This undesired ink-deposit can be removed by increasing the frequency of the applied tension during development, so that already at 1000 cycles a positive image with a pure background is obtained.

Only in the areas wherein wetting really occurs, so wherein the air adsorbed to the charge carrier is replaced by the polarizable conductive liquid discharge of the surface charge pattern can take place.

The charge built up by corona discharge in fact does not consist of free electrons but rather of adsorbed negative oxygen ions created by the ionization of the air. The desorption and discharge of said ions takes time in the order of tenths of seconds so that a suflicient time interval for development e.g. with rollers exist to build up an electrical double layer between the polarizable liquid and the corona-charged layer.

Of course in the case a P.I.P. charge pattern has to be developed according to the present liquid contact development method no problem in respect of a critical developing time arises.

We claim:

1. A process of producing liquid images in which a normally insulating electrostatic charge carrier having a hydrophobic surface and bearing an electrostatic charge pattern detectable at such surface is developed by wetting with a developing liquid according to such charge pattern thereby to record such pattern by means of surface Wetting, which includes the step of substantially uniformly contacting the hydrophobic surface of said carrier, progressively or simultaneously, with an aqueous conductive developing liquid in the presence of a superimposed direct current electric field extending generally at right angles to said surface, said surface being normally unwettable by said aqueous developing liquid but selectively wettable thereby under the influence of attractive electrostatic field forces, the period of said contact being less than the discharge time of the charge pattern on contact of the charge carrier with the developing liquid.

2. A process of producing liquid images according to claim 1, wherein the total electrostatic field forces created by said charge pattern and said external field is at least about 100 volts.

3. A process according to claim 1 wherein the said superimposed direct current field has a polarity opposite to that of the field created by said electrostatic charge pattern and the liquid wets the surface of said charge carrier in the area bearing said electrostatic charge pattern.

4. A process according to claim 1 wherein the electrostatic charge carrier is a normally insulating photoconductive layer that after uniform electrostatic charging and imagewise exposure to electromagnetic radiation to which said layer is sensitive, contains a charge pattern, said layer having a hydrophobic surface, and wherein said superimposed direct current field has the same polarity as the field created by said electrostatic charge pattern and causes the liquid to wet said surface in the exposed and uncharged areas thereof and to be repelled in the unexposed, charged areas thereof.

5. The process of claim 1 wherein said hydrophobic charge carrier surface has a total surface roughness of about 2l5,u..

6. A process of developing a normally insulating electrostatic charge carrier having a hydrophobic surface and which exhibits rectifying properties when placed in an alternating current field, said carrier bearing an electrostatic charge pattern, which comprises the step of contacting said surface substantially uniformly and within a superimposed alternating current electric field directed generally at right angles to said surface, with an aqueous conductive polarizable liquid by which said surface is normally unwettable but by which such surface is selectively wettable under the influence of attractive electrostatic field forces, whereby said carrier is wetted with liquid according to such charge pattern thereby to record such pattern by means of surface wetting.

7. A process according to claim 6 wherein said alternating current field has a frequency substantially above 50 cycles per second, and the total electrostatic field forces created by said charge pattern and said superimposed field is at least about volts.

8. A process according to claim 7 wherein the alternating current field has a frequency of more than 500 cycles.

9. A process according to claim 6 wherein said charge carrier includes a normally insulating photoconductive layer comprising photoconductive zinc oxide grains dispersed in a hydrophobic binder, forming a hydrophobic surface on said layer, and said electrostatic charge pattern is created by means of corona discharge.

10. A process according to claim 9 wherein the photoconductive layer comprises the reaction product of photoconductive zinc oxide grains and a monoor di-basic aliphatic carboxylic acid or organic phosphorous compound.

11. A process according to claim 6 wherein said developing liquid is conductive and said electric field is maintained between said conductive liquid and a relatively conductive backing for said charge carrier, said conductive liquid and said backing being connected to the opposite poles of an external alternating voltage source, said backing being insulated against electrical contact with said conductive liquid by means of the normally insulating charge carrier intervening therebetween.

12. The process of claim 6 wherein said hydrophobic charge carrier surface has a total surface roughness of about 2-15 References Cited UNITED STATES PATENTS 3,076,722 2/1963 Greig 11737 3,080,251 3/1963 Claus 961 3,084,043 4/1963 Gundlach l17-37 3,096,198 7/1963 Schaffert 1l737 X 3,102,045 8/1963 Metcalfe et a1. l17-37 3,106,157 10/1963 Reithel 117-37 X (Other references on following page) UNITED STATES PATENTS Cassiers et a1. 961 Middleton et a1. 96-1 Blake et a1. 961 Castle 96-1 Cassiers et a1. 96-18 Crumley et a1 96-1 Kurz 96-1 Metcalfe et al. 117-37 Olson 117-37 Richer 117-37 Mayer 117-175 X Schaflfert 118-637 Greig 961 Reithel 117-37 X Greig 117-37 Gundlach 117-37 Murray 96-1 X Walkup 961 X Staicopoulos 260-41 Mayer et a1. 117-17.5 X

Mayer 117-175 X FOREIGN PATENTS Great Britain. Great Britain.

OTHER REFERENCES Bikerman: Surface Chemistry, pp. 422-423, 1948. Schwartz-Perry: Surface Active Agents, pp. 473-76,

WILLIAM D. MARTIN, Primary Examiner E. J. CABIC, Assistant Examiner US. Cl. X.R.

Images