US3687658A - Electrostatic recording processes and materials for chargeless electrophotography - Google Patents

Abstract

A METHOD OF RECORDING WITHOUT EXTERNAL ELECTROSTATIC CHARGING WHEREIN AN UNCHARGED RECORDING MATERIAL HAVING RECORDING LAYER CONTAINING PARTICLES OF AN N-TYPE PHOTOCONDUCTOR DISTRIBUTED IN A HYDROPHILIC POLYMERIC BINDING AGENT CONSTITUTING ABOUT 50-97% BY WEIGHT OF THE RECORDING LAYER IS IMAGEWISE EXPOSED TO RADIATION AND IS THEREAFTER ELECTROPHORETICALLY DEVELOPED WITH A DISPERSION OF ELECTROSTATICALLY ATTRACTABLE SUBSTANCES IN AN ELECTRICALLY INSULATING LIQUID MEDIUM.

Description

United States Patent 3,687,658 ELECTROSTATIC RECORDING PROCESSES AND MATERIALS FOR CHARGELESS ELECTROPHOTOGRAPHY Karel Verhille, Mortsel-Antwerpen, Andr Jan Conix,

Antwerpen, and Robert Joseph Noe, Mortsel-Antwerpen, Belgium, assignors to Agfa-Gevaert N.V., Mortsel, Belgium No Drawing. Original application June 27, 1966, Ser. No. 560,900. Divided and this application Oct. 27, 1969, Ser. No. 869,867 Claims priority, application Great Britain, June 25, 1965, 27,129/65, 27,130/65; Apr. 14, 1966, 16,459/ 66 Int. Cl. G03g 13/22 US. Cl. 96-1 R 11 Claims ABSTRACT OF THE DISCLOSURE A method of recording without external electrostatic charging wherein an uncharged recording material having recording layer containing particles of an n-type photoconductor distributed in a hydrophilic polymeric binding agent constituting about 50-97% by weight of the recording layer is imagewise exposed to radiation and is thereafter electrophoretically developed with a dispersion of electrostatically attractable substances in an electrically insulating liquid medium.

This application is a division of US. application Ser. No. 560,900, filed June 27, 1966, now abandoned.

The present invention relates to an electrostatic recording process. It relates especially to the production of images by means of exposed but non-pre-charged photoconductive materials suited for that purpose.

The production of visible images by electrophotography is well known. In this system of photography, a photoconductive layer is provided with a surface electrostatic charge. On exposure to a radiation pattern, the electric charges are dissipated in proportion to the intensity of the radiation.

The so formed charge pattern can be made visible by a special development or toning technique based on the electrostatic attraction of finely divided particles. Well-established methods of developing the electrostatic latent image include cascade, powder cloud, powder suspension, magnetic brush, and fur-brush development. These are all based on the presentation of charged toner to the surface bearing the electrostatic image, Where coulomb forces attract the toner from carrier particles or a liquid dispersing medium. The use of an electrophoretic developer wherein an insulating liquid medium contains dispersed electrostatically attractable particles is particularly interesting for the development of electrostatic charge images which possess no very high charge density and can be used, if the dispersed charged particles have colloidal dimensions, for very-high-resolution work.

Besides the above described technique of forming an electrostatic latent image, an electrostatic latent image may also be formed starting from a so-called latent electron image also known as a latent conductivity image which is produced e.g. by normal exposure of a darkadapted photoconductive zinc oxide-insulating binder containing layer. The latent electron image is made up of excited electrons and bound holes. At the termination of the exposure an equilibrium exists between the electrons in the conduction band and the trapped charge carriers. By applying an appropriate external field, those few electrons present in the conduction band may be extracted from the layer. As they leave, the electrons in the shallow traps are thermally excited into the conduction band from which,

Patented Aug. 29, 1972 "Ice in turn, they may also be extracted from the material to a contacting positive electrode.

So, in the latter system no precharging of the photoconductive material but the application of an electric field during or after the exposure is necessary to form a detectable latent electrostatic image.

A method of recording and reproducing information has now been found, which method comprises record-wise or image-wise irradiating a non-precharged recording material, which in addition to an n-type photoconductor incorporates a substance e.g. a polymeric substance acting as a binder preferably a hydrophilic polymer, which substance contains one or more groups selected from the class consisting of nitro groups, nitroso groups, aldehyde groups, acyl groups, e.g. acetyl groups, carboxylic acid anhydride groups, carboxylic acid groups in acid or salt form, amido groups, e.g. carbonamido groups, ureido groups, hydroxyl groups and cyano groups, which groups accept electrons set free from said photoconductor, whereby a positive charge pattern in correspondence with the exposed areas is formed by that irradiating alone, and developing the exposed material by electrostatically attractable substances, preferably finely dispersed particles in an insulating liquid or finely divided particles applied from an aerosol.

The content of such polymeric substance may vary from 50% to 3% by weight of the recording layer of the recording material.

By record-wise or image-wise exposing to electromagnetic radiation is meant that the exposure may be progressive (in the sense that the recording of spoken information on a recording tape is progressive), or simultaneous, e.g. as is the case of reflectographic or transmission exposure respectively to or through an original, e.g., a printed text or silver image transparency.

The recording process according to the present invention can be successfully applied for the reproduction in enlarged form of microfilm transparencies and X-ray photography.

As n-type photoconductor e.g. titanium dioxide and zinc oxide but preferably photoconductive zinc oxide is used.

Other n-type photoconductive substances are described Zeitschr. fiir Physik (1936) 5079Tabelle 2, p. 61-62.

n-Type photoconductors such as zinc oxide and titanium dioxide are especially sensitive to UV. and X-ray electromagnetic radiation and their sensitivity to visible light can be increased or extended by sensitization.

Polymeric substances which have proved to be particularly suited for use in combination with photoconductive zinc oxide and titanium dioxide in order to obtain, by simple exposure and without precharging a positive charge image of improved intensity are those containing one or more of the cited groups in at least a part of their structural units and which have a hydrophilic character. Polymers which are soluble in pure water or in an alkaline aqueous liquid are preferably used. Some of them can be cured by heating which can be done after coating.

The following is an illustrative list of such preferred polymers for use as binders:

Resins or condensates known as formaldehyde resins, e.g. urea, phenol, and melamine resins, containing methylol groups,

Resins containing carboxylic acid groups, in free, or salt form e.g. in ammonium salt form, e.g. crotonic acid polymers more particularly copoly(vinyl acetate/crotonic acid) and copoly styrene/crotonic acid,

Polymers containing hydroxyl groups e.g. vinyl alcohol polymers, methylol melamine resins and polysaccharides e.g. starch, alginic acid, cellulose and derivatives of said products wherein still hydroxy groups are present,

Polymers containing amido and/or ureido groups, e.g. protein colloids such as gelatin.

As resins suitable for use according to the present invention may be more particularly mentioned formaldehyde condensation products e.g. those listed in Table 1.

TABLE 1 (l) Parez Resin 613 (a dimethyltrimethylolmelamine marketed by American Cyanamid Company).

(2) Parez Resin 607 (a melamine-formaldehyde resin marketed by American Cyanamid Company).

(3) Epok-W-980l (a 72 to 75% aqueous solution of a melamine-formaldehyde resin marketed by British Resin Products Ltd., London).

(4) Cymel 405 (a melamine-formaldehyde resin marketed by American Cyanamid Company).

(5) Resloom HP (a methylolmelamine marketed by Monsanto Chemical Company).

(6) Cassurit-MLP (partially etherified melamineformaldehyde resin marketed by Cassella 'Farbwerke Mainkur A. G. Frankfurt (Main)--Fechenheim, Germany).

(7) Urecoll P (urea formaldehyde resin marketed by Badische Anilin & Sodafabrik Ludwigshafen/Rh.--Germany).

Two basic types of melamine resins are commercially available for use according to the present invention: unmodified and modified (etherified) melamine-formaldehyde polymers. Both types are marketed as spray-dried powders and as solutions of about 60% of solid resin in water. The solutions may contain some alcohol for improvement of the storage stability. Spray-dried resin powders are produced under the trade name Cymel by American Cyanamid Company, New York, N.Y., U.S.A. and under the trade name Resimene by Monsanto Chemical Company, St. Louis, Mo., U.S.A. Melamine resin solutions are marketed under the trade name Catalin Resin by Reichhold Chemicals Inc., White Plains, N.Y., U.S.A.

The water-soluble melamine-formaldehyde resins contain at least 2 moles of formaldehyde per mole of melamine.

As suitable melamine-formaldehyde resins containing several N-methylol groups may further be mentioned: dimethylolmelamine (e.g. Resloom HP of Monsanto Chemical Company); dimethyltrimethylolmelamine (Aerotex.

M3 or Parez 613 of American Cyanamid Company); trimethylolmelamine (Aerotex 605 or Parez 607 of American Cyanamid Company); tetramethylolmelamine (Resloom HP Special, Monsanto Chemical Company); tetramethylpentamethylolmelamine (Resloom LC-48, Monsanto Chemical Company); tetramethylhexamethylolmelarnine and pentamethylhexamethylolmelamine.

Further may be mentioned polymers containing carboxylic anhydride and/or carboxylic acid groups in acid or salt form e.g.:

(1) copoly(vinyl acetate/crotonic acid) (94.4/ 5.6)

(2) copoly(vinyl acetate/methacrylic acid) (95/5) (3) alkyd resins containing carboxylic acid groups in acid or salt form (4) copolymers of maleic anhydride.

Polymers containing hydroxyl groups:

( 1) dextrine (2) potato starch (3) Solvitose H 4 (a hydroxyethyl starch marketed by Scholten N.V. FoxholNetherlands; Molar substitution degree of hydroxyethyl groups (MS) =0.27).

(4) Ceron--A (a hydroxyprqpylstarch marketed by Hercules Powder Company Inc., Wilmington, Del. U.S.A., viscosity of a 5% aqueous solution at C. =500 c.p.).

(5) Sumstar J (a dialdehydylstarch marketed by Miles Interchemical -U.S.A.; the hydroxyl groups being for 94% substituted by aldehyde).

'(6) Carboxymethyl cellulose (7) CollateX-EH (amine-aliginate marketed by Alginate Ind. Ltd. London Eng.)

(8) Olgitex-768 (hydroxypropyl alginic acid marketed by Henkel--Germany).

(9) Arabic gum (10) Polyviol W 28/20 [copoly(vinyl alcohol/vinyl acetate) (98/2) marketed by Wacker Chemie G.m.b.H. Miinchen, Germany].

The order of mixing steps for the constituents to form the coating is not important. The kind of photoconductor does not play a part; any known photoconductor, inorganic or organic, may be used. Since known photoconductive substances are not water-soluble they have to be dispersed preferably with a dispersing agent that does not markedly lower the dark-resistivity of the photoconductive element.

The quantitative ratios of the photoconductive substances to the binding agents may vary within wide limits. It is preferred to apply the photoconductive substance in a ratio of 1 part by weight of photoconductive substance to 0.1 to 0.6 part by weight of total content of binder.

The coating mixture may contain a dispersed n-type photoconductor in a weight ratio of 97% to 50% in respect of the total solids content of the coated and dried layer.

The thickness of the photoconductive layer may be chosen between wide limits according to the requirements of each case. Good recording and reproduction results are attained with electrophotographic layers of a thickness between 1 and 20 and preferably between 3 and 10 The photoconductive recording layers prepared according to the present invention may contain, in additon to the photoconductive substance(s) and binder optical sensitizers, e.g., those mentioned in Belgian patent specification 612,102 :filed Dec. 29, 1961 by Gevaert Photo- Producten N.V. and additives known in coating techniques, e.g., pigments (see e.g. Belgian patent specification 609,056 filed Oct. 12, 1961 by Gevaert Photo-Pro dncten N.V., compounds influencing the gloss and/or the viscosity, and compounds which counteract ageing and/ or oxidation of the layers, or which influence the thermal stability of the layers.

A very substantial increase in image density, which is probably due to an increase in sensitivity, can be obtained by the use in the photoconductive recording element, preferably in combination with the above mentioned resins and polymers of substances increasing the darkresistivity as e.g. described and claimed in Belgian patent specification 612,102 filed Dec. 29, 1961 by Gevaert Photo- Producten N.V.

Said substances are preferably added in an amount of 10 mg. to 10 g. in respect of 1000 g. of the photoconductor, although larger amounts may be present.

Whereas those compounds do effectively lower the electrical conductivity of the zinc oxide, it is clear that they also lower the concentration of free electrons in the recording element. 'It is believed that as a consequence hereof a greater amount of electron accepting levels in the system composed of the binder and the photoconductor are not occupied; the absorption of light by the photoconductor and the inherent creation of hole-electron pairs thus leads to an increased electron capture and increased sensitivity of the recording element.

Compounds which are especially useful to yield improved image recording results in combination with the electron-accepting binder are monomeric carboxylic acids preferably containing at least one hydroxyl group, and/ or keto group and/ or carbonamido group. When the carboxylic acid contains a hydroxyl group, said group preferably stands in the ot-position with respect to the carboxylic acid group.

In the following Table 2 compounds are listed which yield improved image recording results (an increase in image density) when incorporated in the photoconductive recording element.

TABLE 1 1. coon CHz-OH HOOc-( -ooon HC on C II H3C(|JC0OH OH 4 H2C-(|3H2 o-c=o HOOC- c0orr COOH 8. OHz-CHg-CHg-CO o o. coon c0011 coon OHQOHQOH -0Hi CH2 CH2- L J 10. HO(CH2)10C OOH 11. crr-cn ll ii on COOH I coon N/ 1'1 13. (FE:

no CH-CHz-CHz-COOH The photoconductive zinc oxide need not necessarily be prepared by the French process. Any type of photoconductive zinc oxide may be used. So, it has been stated that direct process zinc oxide also known as zinc oxide prepared according to the American process is suited and more particularly that photoconductive zinc oxide having a high water adsorption power (0.2 by weight) otfers very good results. Further it is not necessary to use a very pure zinc oxide since zinc oxide e.g., doped with cadmium or gallium trioxide has proved to be suited as well.

The photoconductive layers for use according to the present invention preferably have a hydrophilic character as can be seen from the type of some of the binding agents mentioned above and the binder need not necessarily be electrically insulating or more insulating than the photoconductive zinc oxide. Owing to their hydrophilic character and the ions which are incorporated in the coating composition e.g. for dispersing purposes said layers are as compared with the classical electrophotoconductive recording layers fairly electrically conductive also in the dark. However, for obtaining a good contrast and high density the recording materials for use according to the invention are preferably kept in the dark for a few days before image wise exposure.

Dispersing agents which can be used in the preparation of photoconductive recording layers containing zinc oxide or titanium dioxide for use according to the present invention are e.g. sodium hexametaphosphate, propionic acid, and monobutylphosphate.

The photoconductive layer may be coated dependent on the binder composition from an organophilic or an aqueous medium. When it is coated from an aqueous medium the binding agent is preferably dissolved in an aqueous alkaline medium e.g. in aqueous ammonia, so that ammonium ions are left to some extent in the recording layer after drying.

The photoconductive layer is preferably applied to a support in such an amount that between 8 and g. is present per sq. m. of dried coating composition.

The electric properties of the support for the photo conductive layer do not play a fundamental role in the process of image recording according to the present invention. The recording process of the present invention can be performed with a recording layer coated on an electrically insulating support as well as with a recording layer coated on a conductive support. Thus, contrasty and dense images can be obtained e.g. by electrophoretic development of a charge pattern present in a recording layer applied to an insulating resin support (specific resistivity 10 ohm cm.) as well as with a charge pattern formed by simple exposure and without precharging in a recording layer applied to a conductive metal or paper support.

Suitable electrophoretic developers for detecting the charge pattern obtained by exposure alone without precharging the recording layer are of the negative type. Such developers are e.g. described in the published Belgian patent specification 650,423 filed July 10, 1964 by Commonwealth of Australia, the UK. patent specification 897,903 filed Dec. 30, 1958 by Commonwealth of Australia, Belgian patent specification 594,907 filed Sept. 6, 1960 by Laboratories of Australia, UK. patent specifications 902,928 filed Sept. 18, 1958 by Commonwealth of Australia and 1,016,072 filed Nov. 8, 1961 by General Aniline & Film Corporation. The electrophoretic development may be carried out in the presence of an electric field e.g. as described in the Australian patent specification 227,951 filed Apr. 5, 1957 by Commonwealth of Australia.

Another development technique which has proved to be suited for developing rather weak charge images is known under the name aerosol-developmen which includes the development with a smoke or mist. For such type of development reference is made e.g. to the US. patent specifications 2,297,691 and 2,551,582 issued Oct. 6, 1942 and May 8, 1951 both by Chester F. Carlson.

Although the working principles of the production of the positive charge pattern cannot be explained with complete certainty it is assumed without limiting the invention hereby that at the exposure the n-type photoconductor e. g. photoconductive zinc oxide effects the following photo-reaction:

Before exposure the electron'accepting groups in the recording layer stand in equilibrium with the free electrons contained in the recording element. On exposure this equilibrium is disturbed since the concentration in free electrons altered. Whereas the temperature and trap density are not changed it is to be expected that a certain fraction of the quantity of electrons set free by the lightenergy absorbed in the photoconductor has to be trapped in the various levels present in the recording material.

It is quite evident that upon capturing a portion of the created photo-electrons, a positive charge must remain on the surface of the recording element. Since in a n-type photoconductor the mobility of the electrons usually is of a higher magnitude than the hole mobility, the electrons diffuse further into the layer than the holes. Whereas the electrons are captured in the recording element and are only slowly excited out of their capturing levels, a positive charge must be acquired by the surface of the recording element.

It is surprising however, that the trapping of these electrons, is a rather rapidly occurring phenomenon whereas the inverse phenomenon requires, as can be observed from the slow decay of the positive charge, a much longer period of time.

A method which has proved itself to be quite suited to detect this small surface charge to which the developing toner is attracted, consists in measuring the contact potential difference of the layer before and after irradiation with respect to some standard material e.g. stainless steel (18/ 8).

The following examples illustrate the present invention without however limiting it thereto.

EXAMPLE 1 g. of Resloom HP (trade name for a di-methylol melamine resin marketed by Monsanto Chemical Company, Springfield, Mass, U.S.A.) were dissolved in 2.4 liter of water whereupon 600 ccs. of ethanol and 12 ccs. of a 10% aqueous solution of sodium hexametaphosphate were added. Then 750 g. of Blane de Zinc, Neige extra pure, Type A (trade name for zinc oxide prepared by the French process marketed by Vieille Montague S.A., Liege, Belgium) were added to the solution obtained. The mixture was ground in a ball-mill for 24 hours.

The dispersion obtained was coated on a subbed cellulose triacetate support pro rate of 10 g. of zinc oxide per sq. m. The material formed was exposed for 10 seconds through a negative, which was in contact with said material, by means of incandescent bulbs (together 450 watt) placed at a distance of 25 cm.

Immediately after exposure the material was developed for 40 sec. in a negatively charged dispersion of carbon black in a hydrocarbon mixture. Such a developer is the developer No. 3 described in the Belgian patent specification 650,423 filed July 10, 1964 by Commonwealth of Australia. A very contrasty image was obtained.

When development takes place 15 minutes after the exposure, still a good image is obtained. When keeping the exposed material 1 hour in the dark before developing, the contrast is markedly decreased and when the storage in the dark between development and exposure is 1 day no image is obtained anymore.

EXAMPLE 2 Example 1 was repeated with the difference, however, that for forming the binding agent 150 g. of Resloom HP (trade name) and 150 ccs. of a 5% aqueous solution of Polyviol W 28/20 (trade name for a poly/(vinyl alco- 1101) containing 97.5 to 99.5 of vinyl alcohol groups marketed by Wacker-Chemie G.m.b.H., Miinchen, Germany) are used. The results obtained were as good as those of Example 1.

EXAMPLE 3 Example 1 was repeated with the difierence, however, that the cellulose triacetate support was replaced by glazed paper of 90 g./sq. m., coated with a gelatin interlayer.

The results obtained were as good as those of Example 1.

EXAMPLE 4 Example 1 was repeated with the difference, however, that the cellulose triacetate support was replaced by a glassine-type paper support of 65 g./sq. m.

The results obtained were as good as those of Example 1.

EXAMPLE 5 Example 1 was repeated with the difference, however, that to the coating composition 0.5 cc. of a solution in water of cerium(1V) sulphate in respect of 25 g. zinc oxide was added. The obtained image showed an improved contrast.

EXAMPLE 6 Example 1 was repeated with the difierence, however, that to the coating composition 4.5 ccs. of a 5% solution in ethanol of furanwa-carboxylic acid calculated on 22.5 g. of zinc oxide was added. For obtaining a same image density as obtained with the material of Example 1 the exposure time could be reduced to 2.5 seconds.

EXAMPLE 7 Example 1 was repeated with the difference, however, that to the coating composition 0.4 cc. of a 5% solution in ethanol of ot-N-Cil'l'lC acid monododecylamide was added. For obtaining a same image density as obtained with the material of Example 1 the exposure time could be reduced to 2.5 seconds.

EXAMPLE 8 Example 1 was repeated with the difference, however, that the photoconductive layer was composed of the following mixture:

hydroxyethylstarch (MS of hydroxyethyl groups: 0.27):

photoconductive zinc oxide: 10 g.

a mixture of water and ethanol (80/20): 50 ccs.

which was ground in a ball-mill before coating. The exposure was carried out as described in Example 1 but the exposure time was doubled. On electrophoretic development a contrasty image was obtained.

EXAMPLE 9 5 g. of a novolac (phenolformaldehyde resin with melting point 160-170' F.) was diluted with 20 ccs. of cyclohexanon and 80 ccs. of toluol and mixed in a ball-mill with 30 g. of photoconductive zinc-oxide.

The ground composition wascoated on a baryta-coated paper pro rata of 20 g. per sq. m. of solid substance. The processing is the same as described in Example 1 with the difference, however, that an exposure time of 1 min. was applied.

EXAMPLE 10 150 g. of Vinac ASB (trade name for a copoly(vinyl acetate/crotonic acid) (94.4/5.6) marketed by Colton Chem. Co. a division of Air Reduction Co., Inc., Cleveland, Ohio, U.S.A.) were dissolved in a mixture of 3.2 litres of water, 800 ccs. of ethanol and 4 g. of concentrated aqueous ammonium hydroxide. Then 3 g. of sodium hexamethaphosphate as a dispersing agent dissolved in 30 ccs. of water, were added whereupon 750 g. of photoconductive zinc oxide prepared according to the American Process were admixed while strongly stirring. The mixture was ground for 20 hours in a ball-mill.

10 The mixture was then coated on a cellulose triacetate support as described in Example 1. Exposure and development of the material obtained occurred as described in Example 1 with the diiference, however, that the exposure time was 20 seconds.

EXAMPLE 11 A mixture was made of the following ingredients:

75 g. of Resloom HP (trade name) in 500 ccs. of water g. of Urecoll P (trade name for a urea-formaldehyde resin marketed by Badische Anilin & Soda-Fabrik Ludwigshafen/Rh.Germany) in 600 ccs. of Water,

1100 ccs. of water 800 ccs. of ethanol 24 ccs. of a 10% aqueous sodium hexametaphosphate solution.

While strongly stirring, 1200 g. of photoconductive zinc oxide were added and the whole was ground for 18 hours in a ball-mill. Then cos. of water were added.

The dispersion obtained was coated by dip-coating pro rata of 19 g. of dry substance per sq. m. on a glazed paper support of 90 g. per sq. 111. provided with a gelatin interla er.

A silver image transparency was placed in contact with the photoconductive layer and direct exposure for a period of 0.005 sec. carried out with an 80 watt high pressure ultra-violet radiation bulb (marketed under the name U.V. lamp HP 80 by Philips Gloeilampen fabriek N.V., Eindhoven, Netherlands) placed at a distance of 25 cm. A very contrasty image was obtained with the developer number 3 described in the published Belgian patent specification 650,423 filed July 10, 1964 by Commonwealth of Australia.

EXAMPLE 12 300 g. of photoconductive zinc oxide prepared by the French process were ground in a ball-mill for 16 hours in the presence of 1 litre of a 1% aqueous solution of gelatin. The ground composition is coated onto a glassine type paper support of 60 g. per sq. m. at such a rate that 15 g. of solid substances were present after drying.

The image-wise exposure is carried out through a negative transparency with an ultra-violet lamp of 80 watt placed at distance of 15 cm. and irradiating the photo sensitive n aterial for 15 seconds. Development is carried out with the developer described in Example 1.

By adding to the above composition 60 ccs. of a 5% solution in ethanol of one of the compounds mentioned with their structural formula in the Table 4 a higher density was obtained.

EXAMPLE 13 300 g. of titanium dioxide (TiO were ground in a ballmill for 15 h. in the presence of 1 litre of a 1% aqueous solution of gelatin. The ground composition is coated onto a glassine type paper support of 60 g. per sq. m. at such a rate that 15 g. of solid substances were present after drymg.

The image-wise exposure is carried out through a negative transparency with an ultra-violet lamp of 80 watt placed at a distance of 15 cm. and irradiating the photosensitive material for 5 seconds. Development is carried out with the developer described in Example 1.

A cllegible sufiiciently dense and contrasty image is obtaine EXAMPLE 14 g. of Resloom HP (trade name for a di-methylol melamine resin marketed by Monsanto Chemical Company, Springfield, Mass., USA.) were dissolved in 2.4 litres of water whereupon 600 ccs. of ethanol and 12 cos. of a 10% aqueous solution of sodium hexametaphosphate were added. Then 750 g. of Blane de Zinc, Neige extra pure, Type A (trade name for zinc oxide prepared by the French process marketed by Vieille Montague S.A., Lige,

Belgium) were added to the solution obtained. The mixture was ground in a ball-mill for 24 hours.

The dispersion obtained was coated on a subbed cellulose triacetate support pro rata of 10 g. of zinc oxide per The material obtained was directly X-ray irradiated for 1 min. through an aluminium wedge, or the exposure carried out for the same period by means of an intensifying screen (fluorescence maximum at 500 mg). The exposure characteristics of the X-ray radiation bulb are 120 RV. and 5 A. A visible print corresponding to the Wedgedensities can be obtained by a liquid development using an insulating liquid as described in the Australian patent specification 227,951 filed Apr. 5, 1957 by Commonwealth of Australia.

The electrophoretic developing composition for use according to the present invention contains electrostatically attractable substances dispersed in an insulating (preferably at least ohm cm.) liquid medium. For other developing compositions reference is made to the UK. patent specification 897,903 filed Dec. 30, 1958 by Commonwealth of Australia, Belgian patent specification 594,- 907 filed Sept. 6, 1960 by Laboratories of Australia and UK. patent specification 902,928 filed Sept. 18, 1958 'by Commonwealth of Australia.

We claim:

1. A method of recording and reproducing information, which method comprises the steps of (1) image-wise irradiating an uncharged recording material having a recording layer containing an admixture of an n-type photoconductor with a hydrophilic polymer which accepts electrons set free from said photoconductor on irradiation to active electromagnetic radiation and is selected from the class consisting of polymers containing hydroxyl groups, ureido groups, amido groups, tarboxylic acid groups in acid or salt form, acyl groups and aldehyde groups, the amount of said photoconductor being about 50-97% by weight of the total dry solids content of said recording layer, to produce in such layer a positive charge pattern corresponding to the irradiated areas thereof and (2) electrophoretically developing the positive charge pattern in correspondence with the exposed areas by contacting said layers with an electrophoretic developer consisting essentially of negatively charged toner particles dispersed is an electrically insulating liquid medium.

2. A method according to claim 1, wherein the record ing material is produced by coating onto a support a dispersion of said photoconductor in an aqueous medium containing the polymer in dissolved state.

3. A method according to claim 2, wherein the aqueous medium contains ammonia.

4. A method according to claim 1, wherein said polymer is substantially water-soluble.

5. A method according to claim 4, wherein said polymer is a melamine-formaldehyde resin, urea-formaldehyde condensate or phenolformaldehyde condensate capable to be dissolved in water or an aqueous alkaline medium.

6. A method according to claim 1, wherein the n-type photoconductor is photoconductive zinc oxide.

7. A method according to claim 1 wherein the recording material in admixture with the n-type photoconductor contains positive metal ions having strong electron capturing character in their highest oxidation state.

8. A method according to claim 7, wherein the said ions are cerium(IV) ions.

9. A method according to claim 1, wherein said polymer is polyvinyl alcohol.

10. A method according to claim 1, wherein said polymer contains suflicient methylol groups as to be water soluble.

11. A method according to claim 1, wherein the recording material contains in addition to the n-type photoconductor a monomeric carboxylic acid compound containing a group selected from the class consisting of hydroxyl, keto and carbonamido.

References Cited UNITED STATES PATENTS 2,956,874 10/1960 Giaimo 96-1 2,959,481 11/1960 Kacera 96-1 2,990,280 6/1961 Giaimo 96-1 2,997,.3 87 8/1961 Tanenbaum 96-1 3,121,006 2/ 1964 Middleton et a1. 96-1 3,159,483 12/ 1964 Behanenberg 96-1 3,160,503 12/ 1964 Cady 96-1 3,408,183 10/1968 'Mammino 96-15 3,447,957 6/1969 Behringer 117-201 JOHN C. COOPER III, Primary Examiner US. Cl. X.R.