US 3658523 A
A photoconductive recording material is superadditively sensitized to visible light by a mixture of 95-50 percent of photoconductive zinc oxide and 5-50 percent of photoconductive crystalline mixed cadmium sulphide-cadmium selenide.
Description (OCR text may contain errors)
United States Patent Noe [ 1 Apr. 25, 1972  PHOTOCONDUCTIVE RECORDING MEMBER UTILIZING A MIXTURE OF ZINC OXIDE AND CADMIUM SULPHIDE-CADMIUM SELENIDE  Inventor: Robert Joseph Noe, Mortsel, Belgium  Assignee: Gevaert-Agia N.V., Mortsel, Belgium  Filed: Apr. 25, 1969  App1.No.: 819,453
 Foreign Application Priority Data Apr. 26, 1968 Great Britain ..l9,888/68 s21 u.s.c1 ..96/l.8,101/456, 101/460,
252/501, 96/17 511 1111.01. ..G03g 5/00,G03g 7/00 58 FieldofSearch ..96/1.8;252/501 Primary Examiner-George F. Lesmes Assistant Examiner-John R. Miller Attorney-William J. Daniel [57} ABSTRACT A photoconductive recording material is superadditively sensitized to visible light by a mixture of 95-50 percent of photoconductive zinc oxide and 5-50 percent of photoconductive crystalline mixed cadmium sulphide-cadmium selenide.
16 Claims, No Drawings PHOTOCONDUCTIVE RECORDING MEMBER UTILIZING A MIXTURE OF ZINC OXIDE-AND CADMIUM SULPI-IIDE-CAIIMIUM SELENIDE' The invention relates to photoconductive recordingtmembers, the manufacture thereof, their'use in the recording and reproduction of information, and the production of planographic printing plates;
ln known photoconductive recording materials a relatively conductive backing is coated with a photoconductive insulat ing composition prepared by intimately mixingandgrinding togethera photoconductive insulating material, a binder of high electrical resistivity, and a solvent. A large range of materials has been used for the backing, which has to:possess a higher conductivity than the photoconductivelayer in order to allow an image-wise discharge of said layer upon image-wise exposure to conductivity-increasing electromagnetic radiat1on.
Photoconductive zinc oxide'in a binder has been usedwith success in thev manufacture of photoconductive recording media particularly because of its brilliant-white'colour, its sensitivity, and its spectral sensitizability.
A white opaque recording member is, however, not always required eg there is no need of a neutral image background in cases wherein an image-wise deposited developing material is transferred and fixed on another support with neutral tone,
7 or the developed material itself is used as a planographic printing master.
Since photoconductive zinc oxide has little or no absorption power for light of the visible spectrum, it has become standard praxis to extend the spectral sensitivity and to increase the total sensitivity of this zinc oxide with dyes absorbing light in one or more ranges of the visible spectrum.
According to the present invention, electrophotographic recording materials are use for recording and reproducinginformation and for the production of planographic printing forms, which materials contain a recording layer comprising .a mixture of photoconductive zinc chalkogenide(s) and cadmium chalkogenide(s) that provides to the recording layer the property of a superadditive sensitivity to light of the visible spectrum. The term chalkogenides stands here for compounds containing the elements oxygen, sulphur, seleniumand telluri- It has now been experimentally observed indeed that from the viewpoint of light-sensitivity (conductivity increase on exposure to active electromagnetic radiation) improved results can be obtained with particular mixtures of photoconductive zinc oxide and cadmium chalkogenide(s) including such compounds that are doped for increasing their photosensitivity.
The photoconductive chalkogenides of cadmium preferably used according to the present invention are cadmium sulphide and cadmium sulphide selenides.
By the term superadditive sensitivity is meant that the said mixture of photoconductive substances in a particular wavelength range of the visible spectrum offers a sensitivity higher (stronger increase of conductivity) than that of the separate photoconductive substances, which are used in the recording member in an amount equal to that of the total amount of photoconductive substances contained in the mixture.
This superadditive sensitivity" effect was proved by the following test.
A photoconductive zinc oxide was dispersed in an insulating binder-solvent mixture and spectrally sensitized in a ratio by weight in respect of the zinc oxide of 0.1 percent with the following dyes:
A same dispersion was prepared containing, however, instead of spectrally sensitized photoconductive zinc oxide a same. amountof non-spectrally sensitized cadmium sulphide containing traces. of cadmium selenide and zinc sulphide. Said dispersions are called dispersion, A and B respectively. The dispersions were coatedas such or coated as a mixture in the ratios, by weight indicated. in the Table 1 furtheron. The
coatings were electrostatically charged, exposed and developed in the same way; The charging was a double corona charging .with a potential difference of- 5,000 V to the grond of the corona wires directed to the recording layer and a potential difference of 5,000 V to the ground of the-corona wires directed to the base. The exposure was carried out with tungsten incandescent lamps providing together 2,400 lux. The exposure lasted 15 sec. and was effected through a step wedge having a constant of 0.l. The development was of the electrophoretical type.
The following Table 1 contains the amount of nondeveloped (non-blackened) steps of the prints, which gives an indication of the relative sensitivity of the coatings. An increase of-three steps means a doubling of the sensitivity.
TABLE 1 A B Amount of non-blackened steps 100 25' 97.5 2.5 26 9510 5.0 27 92.5 7.5 28 90.0 10.0 29 85.0 15.0 31 80.0 200 31 75.0 25.0 29 50;0 50.0 20 100 16 Analogously to the said test a comparison was made between non-spectrally sensitized photoconductive zinc oxide (dispersionC) and the same cadmium sulphide dispersion as A substantial superadditive sensitivity can be obtained with a ratio by weight of photoconductive zinc oxide to photoconductive cadmium sulphide and/or cadmium sulphide selenide in the range of to 40 percent.
As can be learned from the Tables 1, 2 and 3, an optimal superadditive sensitivity is obtained with a ratio by weight of 90 to 60 percent of photoconductive zinc oxide to photoconductive cadmium sulphide or cadmium sulphide-selenide. The present invention includes also the use of said photoconductive substances doped with particular chemical elements in such an amount that the superadditive sensitivity effect is not destroyed.
The increase of sensitivity in the visible wavelength spectrum by inorganic compounds offers in respect of the spectrally sensitization with organic dyes the advantage that no loss of sensitivity due to handling of the recording material in daylight or under conditions of extreme temperatures has to be feared since the inorganic compounds are much more resistant to light and heat than the organic sensitizing dyes are.
The recording layers prepared according to the present invention are particularly suited for being used in combination with an exposure apparatus containing a so-called cold light source such as fluorescent tubes. Among these, those containing a green-light-emitting phosphor (maximum of emission near 527 nm) are the most interesting since they yield the highest amount of photons per watt. The technological properties inherent to such type of light sources are so attractive that the application of this type of light sources in electrophotographic recording and reproduction is particularly desirable.
The photoconductive zinc oxide used in the present invention is preferably prepared by the oxidation of zinc vapour i.e. preferably zinc oxide is used prepared according to the French Process. The cadmium sulphide has not to be completely chemically pure but may contain other compounds, e.g. compounds that have been introduced by doping and that enhance its photoconductivity without substantially lowering its darkresistivity. Preferably cadmium sulphide doped with zinc sulphide (zinc being present as the activating element) and mixed crystals of cadmium selenide (selenium being present as the activating element) are used. It has been experimentally found that the molar ratio of cadmium selenide in a mixed crystal of cadmium sulphide selenide is preferably not higher than 50 percent since larger amounts of cadmium selenide, although they still increase the sensitivity, lower the imagedensity too much. Zinc sulphide activated with manganese, calcium, cadmium or copper may be used in admixture. For cadmium sulphide and cadmium sulphide-selenide classical preparation methods known in pigment chemistry can be applied. Commercially available cadmium compounds suited for use according to the present invention are sold by Farbenfabriken Bayer A.G., Leverkusen, W.Germany, under the trade name CADMOPUR see Bayer Pigment Ratgeber (1964) p. 117-1 l8, and under the trade names CADMlUM GELB, CADMIUM ROT and by G. Siegle and Co., G.m.b.l-l.
Stuttgart Feuerbach, W. Germany.
The weight ratio of photoconductive substances to insulating binder may vary within relatively large limits. A ratio of 1 part by weight of photoconductive substances to 0.1 to 0.6 part by weight of total content of binder is preferred. Depending on the use of the recording member the coating mixture normally contains from 95 percent to 60 percent by weight of photoconductive substances in respect of the total solids content of the coated and dried layer. Preferably such a mixture of photoconductive zinc oxide and photoconductive cadmium chalkogenides is used that the sensitivity maximum for visible light is within the wavelength range of 500-550 nm.
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 having a thickness of l to 20 u, and preferably of 3 to 10 11.. The average size of the pigments is preferably lower than 10 micron, e.g. 0.10 to 0.20 ;t.
Binding agents suitable for being used in combination with the mixture of photoconductive substances according to the present invention are described, e.g. in U.S. Pat. No. 3,245,786, Canadian Pat. No. 680,310 or United Kingdom Pat. No. 1,088,834.
In the preparation of planographic printing plates binding agents that are elastomers and/or that are hydrophobic and can be made hydrophilic or less hydrophobic by means of an aqueous treating liquid are preferably used, e.g.
5 hydrophobic polymers containing sulphonyl chloride groups as contained e.g. in copolymers of vinyl chloride, ethylene and vinyl sulphonyl chloride (preferably at least 5 mole percent vinyl sulphonyl chloride). Such polymers are marketed under the name HYPALON 30 and 20 by El. du Pont de Nemours & Co. (Inc.) Wilmington, Del.,
U.S.A., styrolated alkyd resins e.g., ALKYDAL V l0 and AL- KYDAL V (sold by Farbenfabriken Bayer A.G., l 5 Leverkusen, W.-Germany),
- commonly used alkyd resins, and
All these resins can be used in combination with each other.
The photoconductive recording layers containing difi'erent photoconductive substances as described above may contain in addition to these substances and the binder spectral sensitizing agents of any type e.g. spectrally sensitizing agents for photoconductive zinc oxide described in the U.S. Pat. No. 3,245,786, or in the published Dutch Patent application No. 6717400, 6805983, 6704706 and 6704768. Further the photoconductive coating may contain compounds increasing the dark-resistivity, e.g., the phosphorus compounds described in the U.S. Pat. No. 3,245,786 and additives known in coating techniques, e.g., dispersing agents (see, e.g., U.S. Pat. application Ser. No. 665219 corresponding with the published Dutch Patent application No. 6712156), compounds influencing the gloss and/or the viscosity, and compounds that counteract aging and/or oxidation of the layers or that influence the thermal stability of the layers. When selecting any additives, preference is given to those, which least reduce the dark-resistivity of the photoconductive layer.
The photoconductive dispersion can be prepared, e.g., in a sand-mill, pebble-mill, Waring Blendor or homogenizer.
The photoconductive dispersion applied in the present in- 40 vention may be coated on a support according to a known coating technique, e.g., by spraying, whirling, dip-coating, or by a coating technique wherein use is made of a doctor blade. The supports or base materials are chosen in view of the particular charging, exposure, recording, development and/or transfer technique wherein the recording material is used.
In the manufacture of the electrophotographic recording materials according to the present invention a relatively conductive support for the recording layer is used, e.g. an electroconductive sheet or plate, or an insulating sheet or plate covered with an electroconductive interlayer. By electroconductive plate or sheet there is understood a plate or sheet the electrical resistivity of which is smaller than that of the photoconductive layer i.e. in general smaller than 10 ohm.cm. Supports, the electrical resistivity of which is smaller than 10" ohm.cm, are preferred.
Suitable conductive plates are e.g. plates of metals such as aluminum, zinc, copper, tin, iron, or lead.
Suitable electroconductive interlayers for insulating supports are e.g. sufficiently conductive paper sheets, vacuumcoated metal layers such as silver or aluminum layers, transparent conductive polymer layers, e.g. applied from polymers containing quaternized nitrogen atoms such as those described in the U.S. Pat. Nos. 3,174,858 and 3,174,859, or layers containing conductive particles e.g. carbon black and/or metal particles and/or hygroscopic salts dispersed in a binder. The binder used for said particles preferably has 5 resistivity lower than 10 ohm.cm. Suitable binders for that purpose are gelatin and casein.
Substances suited for enhancement of the conductivity of a paper sheet and for the application in a hydrophilic interlayer, e.g., on the basis of gelatin, are of the polyionic type. Good results are obtained with Calgon Conductive polymer 261 manufactured by Calgon Corporation, Pittsburgh, Pa., U.S.A. (Calgon is a registered trademark).
Paper sheets are preferably impermeabilized for organic solvents, e.g., by means of a water-soluble colloid or by strong hydration of the cellulose fibres such as in glassine paper.
Suitable supports are described, e.g., in the Canadian Patent Nos. 687,440, 771,587 and 802,148.
Of course, the present invention is by no means limited to one or the other particular embodiment as regards the use of electrophotographic materials, the exposure technique, the charging method, the developing method, the transfer (if any), and the fixing.
The photoconductive layer of an electrophotographic material, which is prepared starting from a coating composition according to the present invention, can be used for recording purposes, in which prior to exposure an electric charge is non-differentially applied according to known methods. However, the material can also be used in recording techniques, in which the exposure step precedes the charging step. For such a technique reference may be made to, e.g. the U.S. Pat. Nos. 3,383,209 and 3,425,829.
For comparison of the sensitivity of photoconductive recording elements, said elements are charged and exposed in the same manner, e.g. through a step-wedge, and developed in the same conditions. Well established methods of developing electrostatic images include cascade-, powder cloud-, magnetic brush and fur brush-development. These methods are based on the application of charged dry toner to the surface bearing the electrostatic image. Other methods are based on the use of liquids, either insulating or conductive liquids (see e.g. US. Pat. Nos. 2,907,674, 3,383,209 and 3,425,829, and Canadian Patent No. 802.148). Development ofa conductivity image based on electrolysis is described, e.g., by J.A. Amick, RCA Rev., 20, 753 (1959). Development based on electrophoresis is described in the published Dutch Patent application No. 6701696.
Recording materials prepared according to the present invention are especially suited for the preparation of aplanographic printing master since it is not difficult to hydrophilize the non-developed portions (i.e. the portions not covered with developing material) thereof.
Normally, the difference in hydrophobicity between the developed image parts and the non-covered areas of the recording layer is insufficiently high for high-quality planographic printing and has to be increased. Therefore, it has been proposed to chemically treat the recording layer in the undeveloped areas in order to make these areas highly waterreceptive. Thus, it is known from the U.S. Pat. No. 2,952,536 to image-wise hydrophilize a photoconductive insulating recording layer comprising an electrically insulating binder having suspended therein a finely divided photoconductive pigment by means of an aqueous liquid providing hydrophilic anions forming insoluble compounds with said pigment, and which anions are provided by a solid organic acid at least as strong as acetic acid.
Other suitable hydrophilizing compounds for treating photoconductive recording layers in order to obtain a planographic printing master are described in the published Dutch Patent application No. 6803164.
A preferred hydrophilizing liquid for use in combination with recording materials prepared according to the present invention contains ascorbic acid and a water-soluble hexacyanoferratefll) dissolved in a mixture of a lower aliphatic alcohol, e.g. methanol, and water. Another preferred hydrophilizing liquid is composed as follows:
sodium hexacyanoferrate(ll) g potassium dihydrogen phosphate 13.6 g 0.1 N aqueous sodium hydroxide solution 1 1.4 ml water to 1,000 ml (pl-l 5.2)
The hydrophobic nature of the surface areas of the recording layer, which are developed by means of hydrophobic particles e.g. powder particles on the basis of hydrophobic resins deposited from a dry powder mixture (xerography) or electrophoretically as described (e.g. in the U.S. Pat. application Ser. No. 613,759 (corresponding with published Dutch Patent application No. 6701696) can be enhanced, e.g. according to a technique and with hydrophilizing liquids described in U.S. Pat. application Ser. No. 501,666 corresponding with published Dutch Patent application No. 6513710.
The application of the hydrophilizing liquid proceeds advantageously by means of a roller coating technique, e.g. by means of a lick-roller.
By this technique only the surface used for printing is wetted with aminimum of liquid. The said treating step can be carried out in a processing unit making part of the exposure and developing apparatus. The planographic printing form obtained is suited to be used in combination with the classical lithographic or offset printing inks wherein the printing composition has lipophilic properties. Reversed planographic printing, however, is not excluded and can be carried out with printing inks described in U.S. Pat. application Ser. No. 537,600 corresponding with published Dutch Patent application No. 660,2377.
The following examples illustrate the present invention.
EXAMPLEl 101 g of HYPALON 30 (trade-name for a copoly(ethylene/vinylsulphonyl chloride/vinyl chloride) (26.l/6.9/67 in percent by weight) marketed by El. du Pont de Nemours & Co. (lnd.), Wilmington, Del., U.S.A.) were dissolved in a mixture of 575 ml of dichloroethane, 156 m1 of methyl ethyl Ketone, and 31 ml of ethanol. As dispersing agent for the photoconductive pigments 19.5 ml of a percent solution in toluene of CELLOLYN 95 (trade-name for an alkyd resin marketed by The Hercules Powder Company, lnc., Wilmington, Del., U.S.A.) were added to this solution, and whilst stirring 447 g of photoconductive zinc oxide (Type A Neige extra pur Vieille Montagne S.A., Belgium) of an average particle size of 0. 17 micron and 61 g of CADMOPUR GOLDGELB N (a cadmium sulphide pigment of an average particle size of 0.10-0.20 micron, manufactured by Farbenfabriken Bayer, A.G., Leverkusen, W.-Germany; the pigment contains 76 percent of cadmium, 22 percent of sulphur and 2 percent of a mixture of barium sulphate and silica and traces of zinc and selenium). The pigment composition was put once through a homogenizer.
The dispersion was coated into a glassine paper at a rate of 33 g of solids per sq.m. The coating was dried in a laminar current drier at 30-40 C. (air rate 7 m/min.).
A smooth surface was obtained.
The dried material was charged with a double corona, the corona wires in front of the recording layer having a potential difference of 5000 V in respect of the ground, and the corona wires below the support being at 5000 V in respect of the ground.
A step wedge with constant of 0.1 was projected on the recording layer. The light-source consisted of a set of incandescent tungsten filament lamps. The exposed material was electrophoretically developed with an hydrocarbon solventcarbon black dispersion.
The developed image proved that the recording layer was suited for the production of images with a steep gradation so that text originals are reproduced with a very sharp letter on a clear background.
EXAMPLE 2 The same photoconductive dispersion as prepared according to example 1 was coated onto a high wet-strength paper weighing g/sq.m. The support was shielded for solvent penetration, and the electrical conductivity improved by means of an interlayercomposed of gelatin containing 5 percent by weight of Calgon conductive Polymer 26'] (registered trade-mark).
The light-sensitive material was exposed to an opaque text original in a GEVAFAX 50 copying apparatus equipped with green fluorescent light tubes having an emission maximum at 527 nm, and fitted with an electrophoretic developing unit (GEVAFAX is a registered trademark of Gevaert-Agfa N.V., Mortsel, Belgium).
The exposed and electrophoretically developed recording layer was wetted with a lick-roller by using the following hydrophilizing liquid composition:
Water 90ml methanol 10 ml ascorbic acid 2 g sodium hexacyanoferrate (ll) 2 g The hydrophilized printing master was used on a classical offset printing apparatus. As much as 500 sharp points of good quality were obtained by printing with a lipophilic lithographic printing ink.
EXAMPLE 3 101 g of HYPALON 30 (trade-name) were dissolved in a mixture of 575 ml of dichloroethane, 156 ml of methyl ethyl ketone, and 31 ml of ethanol. As dispersing agent for the photoconductive pigments 19.5 ml ofa 80 percent solution in toluene of ALKYDAL V (trade-name for an alkyd resin marketed by Farbenfabriken Bayer A.G., Leverkusen, W. Germany) were added to this solution, and while stirring 447 g of photoconductive zinc oxide (Type A Neige extra pur Vieille Montagne S.A., Belgium) and 61 g of CADMIUM GELB 46 (a cadmium sulphide selenide pigment manufactured by G. Siegle und Co., G.m.b.H., Stuttgart, Feuerbach, W. Germany; the pigment grains consist of a crystalline mixed carbon black (average particle size: 20 nm) 30 g zinc monotridecyl phosphate as dispersing agent 1.5 g lSOPAR H (trade name) 750 ml resin solution prepared as described hereinafter 150 g The resin binder solution was prepared by heating 500 g of ALKYDAL L 67 (trade name of Farbenfabriken Bayer A.G.. Leverkusen, W. Germany, for a linseed oil-modified (67 percent by weight) alkyd resin and 500 ccs of white spirit containing 1 1 percent by weight of aromatic compounds at 60 C. till a clear solution was obtained, and by subsequent cooling.
The sensitometric results obtained are listed in the Table 3 hereinafter.
The total speed and the speed obtained by exposure through the band filters transmitting light of the wavelengths 405, 445, 475, 505, 515, 530, 555, 575 and 605 nm respectively is expressed by means of the steps that were not covered with developing particles, and consequently correspond with the areas discharged by irradiation.
An increase of 3 steps means a doubling of the sensitivity.
TABLE 3 Ratio by weight of ZnO CdS-Se Speed behind band filters of- Total 405 445 475 530 555 speed hm. hm. nni. hm. nm.
cadmium sulphide selenide (97 percent by weight of CdS and 3 percent by weight of CdSe). The pigment composition was put once through a sand-mill (Sandmill-Sussmeyer, Type IA) at a rate of 24 liters/h.
The dispersion was coated onto a glassine paper at a rate of 33 g of solids per sq.m. The coating was dried in a laminar current drier at 30-40 C. (air rate 7 m/min.).
A smooth surface was obtained.
The dried material was charged, exposed and developed as described in example 1.
The developed image proved that the recording layer was suited for the production of images with a steep gradation so that text originals are reproduced with a very sharp letter on a clear background.
EXAMPLE 4 Example 3 was repeated with the following different ratios by weight of photoconductive zinc oxide and photoconductive cadmium sulphide selenide 100/0 (508 g Zno) 95/5 90/10 85/15 80/20 70/30 60/40 /50 40/60 30/70 20/80 10/90 and 0/100 (508 g ofcrystalline mixed cadmium sulphide selenide) (molar ratio 95/5).
The exposure of the different samples was carried out with ,incandescent tungsten filament lamps through a grey step- EXAMPLE 5 Example 3 was repeated (except that the obtained electrostatic charge pattern was developed by means of a triboelectrically charged positive toner prepared on the base of 3 parts by weight of pitch, 4 parts by weight of colophony and 3 parts by weight of carbon black.
The non-fixed image-wise deposited powder was electrostatically transferred to a sheet of common white writing paper and fixed thereon by heating (fusion of the powder).
1. A photoconductive recording material comprising a conductive support and a recording layer comprised of an elastomeric and/or hydrophobic resin binder having dispersed therein finely divided particles of up to about 10p. average size of a mixture of photoconductive zinc oxide and photoconductive crystalline mixed cadmium sulphide-cadmium selenide, said mixture being in a ratio by weight of 95 to 50 percent of said zinc oxide to 5 to 50 percent of said cadmium sulphideselenide, which superadditively sensitizes the recording layer for visible light.
2. A photoconductive recording material according to claim 1, wherein the recording layer contains to 60 percent by weight of said photoconductive zinc oxide and the balance of said cadmium sulphide-cadmium selenide.
3. A photoconductive recording material according to claim 1, wherein the photoconductive recording layer contains from 95 to 60 percent by weight of photoconductive pigments on the total solid content of the recording layer.
4. A photoconductive recording material according to claim 1, wherein the photoconductive layer contains a spectrally sensitizing dye.
5. A photoconductive recording material according to claim 1, wherein the binding agent is hydrophobic, and can be made hydrophilic by means of an aqueous treating liquid.
6. A photoconductive recording material according to claim 5, wherein the binding agent is a copoly(ethylene/vinylsulphonyl chloride/vinyl chloride). 7
7. A photoconductive recording material according to claim 1, wherein the photoconductive zinc oxide has been prepared by oxidation of zinc vapor, and a crystalline mixture of cadmium sulphide selenide is used, in which the molar ratio of cadmium selenide not is higher than 50 percent.
8. A photoconductive recording material according to claim 1, wherein the mixture of photoconductive substances is so selected that the recording layer has a sensitivity maximum for visible light in the wavelength range of 500 to 550 nm.
9. An electrophotographic recording and reproduction process, which comprises the steps of electrostatically charging and information-wise exposing to electromagnetic radiation a recording material having a photoconductive recording layer comprising a mixture of 95 to 40 percent by weight of photoconductive zinc oxide and to 60 percent by weight of photoconductive cadmium sulphide-selenide, and developing the obtained electrostatic charge pattern with electrostatically attractable powder.
10. An electrophotographic process according to claim 9, wherein the electrostatic charge pattern is developed by contacting the same with a dry hydrophobic powder.
11. An electrophotographic recording and reproduction process according to claim 9, wherein after said development with said electrostatically attractable powder, said powder is transfered to a receiving material and fixed thereon.
12. A method for preparing a planographic printing master, wherein on a recording material including a conductive support carrying a photoconductive recording layer comprising an elastomeric and/or hydrophobic binding agent having dispersed therein finely divided particles of up to about 1011. size of a mixture of 95 to 50 percent by weight of photoconductive zinc oxide and 5 to 50 percent by weight of photoconductive crystalline mixed cadmium sulphide-cadmium selenide, an electrostatic charge pattern is formed by a process comprising the steps of electrostatically charging and information-wise exposing the recording layer to information-wise modulated active electromagnetic radiation, developing the residual electrostatic charge pattern by means of an electrostatically attractable finely divided hydrophobic marking material, and after deposition and fixing of the latter treating the recording layer with a liquid hydrophilizing the surface portions of the recording layer not covered by the said marking material.
13. A process according to claim 12, wherein the recording layer contains to 60 percent by weight of photoconductive zinc oxide in said mixture and the exposure is carried out with visible light.
14. A method for preparing a planographic printing master according to claim 12, wherein said liquid hydrophilizing the said non-covered surface portions is an aqueous liquid containing a water-soluble hexacyanoferrate(ll).
15. A process according to claim 12, wherein the photoconductive substances are dispersed in s resin binder containing a copoly(ethylene/vinylsulphonylchloride/vinyl chloride).
16. An electrophotographic process according to claim 9 wherein said electrostatic charge pattern is developed electrophoretically by contacting the same with a developer liquid comprised of an insulating liquid having hydrophobic powder particles dispersed therein.