US3865587A - Electrophographic bichargeable materials and process - Google Patents

Abstract

An electrophotographic recording material comprising a photoconductive binder type layer incorporating a mixture of photoconductive zinc oxide particles and colourless or white positively chargeable pigment particles in a ratio by weight smaller than 50:50 but not smaller than 10:90. Preferred positively chargeable pigment particles are zinc sulphide, barium sulphate, silica or polyethylene particles.

Description

United States Patent 1 1 No'e" 1 Feb. 11, 1975 ELECTROPHOGRAPHIC BICHARGEABLE 3,703,371 11/1972 E [A AND PROCESS 31703372 11/1972 MAT R LS 3,706,560 12/1972 [75] Inventor: Robert Joseph Noe, Mortsel, 3,3 1,316 4 1974 Belg'um FOREIGN PATENTS OR APPLICATIONS 1 1 Asslgneei AgfajGevael't 6 Mortsel, 908,779 10/1962 Great Britain 96/].8 Belglum 941,702 11/1963 Great Britain Jo/1.8

[22] Filed: July 19, 1972 OTHER PUBLICATIONS 21 A 273 05 Hauffe et aL, Electrophotographic Charging and Disl pp 3 chargmg Experiments with Zinc Oxide Single Crystals," Phys. Stat. Sal. (at) 3, No. 1, pp. l73-183 F0re1gn Appllcatlon Prlorlty Data (1970) July 20, l97l Great Britain 34046/71 Hoeffgen et 3],, on the Electrophotographic Behaviour of Pink Zinc Oxide," Phys. Stat. Sal.'(a) 3. No. I, [52] US. Cl 96/1 PC, 96/1.7, 96/l.8 pp, 185-192, (1970). [51] Int. Cl. 603g 5/08, 603g 13/22 [58] Field Of Search 96/l.8, 1 PC; 252/] Primary Examiner-Norman G, Torchin Assistant Examiner.lohn R. Miller [56] References Cite Attorney, Agent, or Firm-Wil1iam J. Daniel UNITED STATES PATENTS 2,287,161 6/1942 Ball 96/l.8 x ABSTRACT 2,555,321 6/1951 Dalton et al. 96/l.8 X An electrophotographic recording material comprising KUI'Z r X a photoconductive binder layer incorporating 3 2983-220 5/1961 Dalton a] 96/l'8 X mixture of photoconductive zinc oxide particles and Elder Q colourless or white positively chargeable pigment par- 3 607 9/1971 8:51: 5? 96/118 X ticles in a ratio by weight smaller than 50:50 but not 3:634:65 1/1972 Akiyama 96/l.8 x Smaller 3.642.430 2/1972 r e X Preferred positively chargeable pigment particles are garmdafth zinc sulphide, barium sulphate, silica or polyethylene ran on 1 partlcles. 3,674,477 7/1972 Carlson 96/].8 3,674,483 7/1972 Vrancken et a1 96/].8 X 2 Claims, N0 Drawings ELECTROPHOGRAPHIC BICHARGEABLE MATERIALS AND PROCESS This invention relates to an electrophotographic recording material and its use in the production of photographic images.

Photoconductive binder type coatings prepared by incorporating a finely divided photoconductor in a binder are well known.

Commonly used are electrophotographic materials containing a photoconductive layer comprising photoconductive zinc oxide dispersed in an insulating binder. Said layer is coated on a sufficiently conductive support, e.g. a clay-coated or glassine type paper base, from a paint-like coating mixture by conventional paper coating techniques.

Since the zinc oxide has a brilliant white colour it forms an excellent base for contrasting with the developer materials applied to it. Moreover, zinc oxide is relatively less expensive and can be spectrally sensitized by a great variety of dyes. However, as already explained in the United Kingdom patent specification No. 1,256,888 filed June 6, 1968 by Gevaert-Agfa N.V. the positive corona charging of photoconductive zinc oxide binder layers is only of an acceptable level if photoconductive zinc oxide is used that has been treated with a special chemical compound improving its positive chargeability.

The present invention is based on the discovery that electrophotographic recording materials containing photoconductive zinc oxide can be used for electrophotographic recording purposes including a positive charging by the use in the photoconductive binder type layer of a mixture of photoconductive particles and colourless or white positively chargeable pigment particles in a ratio by weight range being smaller than 50:50 but not smaller than :90.

Preferred mixtures of dispersed photoconductive zinc oxide and positively chargeable particles are in a ratio by weight range from 30:70 to 40:60.

Remarkable is that the, positively chargeable particles need not to .be photoconductive and that inorganic as well as organic finely divided positively chargeable particles may be used.

For selecting the positively chargeable particles a test has been developed according to the particles involved are dispersed in a ratio of 50 to 95% by weight in an insulating binder having a resistivity of at least 10 ohm.cm and coated on a conductive base forming a layer of 10-30 microns.

The coated layer is subjected in the dark to a positive corona charging of 6000 V till a maximum voltage level is reached.

Thus prepared and treated layers that after terminating the charging step retain for at least 5 seconds in the dark a positive charge with a potential difference of at least 200 V with respect to a contacting conductive carrier or support contain pigments suited for the purpose of the present invention.

The dark decay is registered e.g. by means of an electrometer with vibrating electrode such as that described by C. J. Young and H. G. Greig, RCA Review (1954) XV, 469, switched to a Bruel & Kjaer Voltage Level Recorder.

Substances that are within the criteria of the above test and that for the purpose of the present invention are used advantageously in combination with photoconductive zinc oxide are e.g. the following:

barium sulphate zinc sulphide silica (SiO polyethylene The grain size of the added substances is advantageously about the grain size of the photoconductive zinc oxide particles but may vary within a broad range e.g. from 10' micron to 20-50 micron.

The grain size of the photoconductive zinc oxide is, e.g. of 0.1 to 15 micron.

in admixture with a suitable insulating binder or binder mixture the recording layer has preferably a resistivity which is approximately between 10 and l0 ohm.crn.

The recording layer may contain from 50 to 97% by weight of the dispersed particles with respect to the binder material. A preferred ratio by weight of binder to dispersed particles is in the range of 1:4 to 1:10 parts.

Any polymeric or resinous binding agent or mixture of polymeric binding agents already known for use in electrophotographic recording layers containing photoconductive zinc oxide may be used.

Suitable binding agents have a resistivity that is higher than the inherent resistivity of the photoconductive zinc oxide particles. In that respect are used e.g. electrically insulating organic resinous binders of the type of polyurethanes, polyesters, polycarbonates, polystyrene, chlorinated rubber, acrylic resin, alkyd resins or vinyl acetate copolymers e.g. vinyl chloride-acetate resin.

According to the coating composition the binder/pig ment layers are coated from an organic solvent or solvent mixture, an emulsion of organic solvents and water or from an aqueous medium.

Particularly suitable binding agent combinations for application from organic solvents are described in the United Kingdom patent specification No. 1,199,061 filed Apr. 26, 1967 by Gevaert-Agfa N.V.

Photoconductive coating compositions applied from an aqueous emulsion are described, e.g., in the US. Pat. No. 3,152,895 of Georges H. Tinker, Shu Huai, David Mai and Roger E. Burke issued Oct. 13, 1964.

Photoconductive coating compositions applied from an alkaline aqueous binder solution are described, e.g. in the United Kingdom patent specification Nos. 1,125,579 and 1,125,580 both filed Oct. 25, 1966 by Gevaert-Agfa NV. and the US. Pat. No. 3,160,503 of Joseph H. Cady issued Dec. 8, 1964.

According to a special embodiment the binding agent or binding agent composition has a resistivity that is lower than that of photoconductive zinc oxide grains treated with a compound as described in United Kingdom patent specification Nos. 1,020,504 and 1,020,506 both filed Dec. 29, 1961 by Gevaert Photo- Producten N.V.

According to the inventions described in said Patent Specifications it has been found that a sufficient darkresistivity can be conferred on the zinc oxide to make it unnecessary when forming a photoconductive zinc oxide layer to use a binder with a'high dark-resistivity. Preferred compounds for treating the photoconductive zinc oxide grains and increasing its dark-resistivity are acid ester derivatives of oxyacids of phosphorus e.g. monoand dibutyl orthophosphate.

Other compounds improving the dark-resistivity of photoconductive zinc oxide grains are urazole, urazole derivatives and analogous compounds described in the United Kingdom patent specification No. 1,256,888 filed June 6, 1968 by Gevaert-Agfa N.V. and the U.S. Pat. No. 3,634,080 of Robert Joseph No'e', Jozef Frans Willems, Albert Lucien Foot and' Karel Eugeen Verhille issued Jan. 11, 1972. These compounds correspond with the following general formula:

wherein:

each of R, and R represents hydrogen, an alkyl group including a substituted alkyl group, and aryl group including a substituted aryl group, an acyl group including a substituted acyl group, e.g. an aliphatic acyl group (alcoyl) or substituted aliphatic acyl group, e.g. an acetyl group, or an aromatic acyl group (aroyl), e.g. a benzoyl group, including a substituted aromatic acyl group, or a heterocyclic nucleus including a substituted heterocyclic nucleus, e.g. a pyridine nucleus, or a thiazole nucleus, or R, and R together represent the necessary atoms to close a fused ring or ring system,

Z the atoms necessary to close a 5- or 6-membered heterocyclic ring including such ring in substituted form and including such ring making part 'of an either or not substituted fused ring system, and

X represents oxygen or imino, and at least one of the nitrogen atoms making part of the ring closed by Z carries a hydrogen atom, that optionally may be replaced by a cation when using the compound in salt form Said compoundsincreasing the dark-resistivity are used in combination with the photoconductive zinc oxide in preferred amounts of 0.01 to 10% by weight.

In the manufacture of the electrophotographic material according to the present invention in practice any commercially available white zinc oxide prepared e.g. by oxidation of zinc vapour (French process) may be used.

Especially suited zinc oxide types are:

the zinc oxide marketed under the trade name BLANC DE ZINC Neige extra pure, type A, B and C, by Vieille Montagne S.A. Liege, Belgium, and the zinc oxide marketed by Durham Chemical Ltd. Birtley England under the trade names MlCROX and ELECTROX.

In addition to the binding agent(s) the positive by chargeable particles, the photoconductive zinc oxide grains and the compounds increasing the darkresistivity the recording layers of the present invention may contain all other additives known in the art, e.g., compounds influencing the mechanical strength, gloss, viscosity, thermal stability and compounds that increase the spectral sensitivity and/or general sensitivity of the recording layer.

Examples of suitable spectral sensitizing agents that increase the spectral sensitivity of photoconductive zinc oxide are described in the United Kingdom patent specification Nos. 1,020,504 filed Dec. 29, 1961 by 4 Gevaert Photo-Producten N.V. 1,199,062 1,200,901 both filed Apr. 26, 1967, 1,183,044 filed Apr. 5, 1966 and 1,198,994 filed Dec. 20, 1966 all by Gevaert-Agfa N.V.

For masking the colour of the applied spectral sensitizing agent, dyes complementary in colour to the sensitizing agent giving the recording layer an off-white appearance may be applied as described, e.g. in the United Kingdom patent specification No. 1,142,509 filed May 20, 1966 by Gevaert-Agfa N.V.

According to a preferred embodiment for preparing photoconductive recording materials of the invention both the photoconductive zinc oxide particles and the positively chargeable particles are dispersed homogeneously in a solution containing the dissolved insulating binding agent(s) and the obtained dispersion is applied to an electro-conductive support.

In the coating of the photoconductive layer several techniques may be used, e.g. centrifuging, spraying, brushing, roller coating, dip-coating, knife-coating or extrusion.

The formed layer is dried, e.g., by a hot air current or .by infrared radiation.

The support for the recording layer preferably has a resistivity that is times as small as that of the dried recording layer in the dark. In general its resistivity is smaller than 10 ohm.cm, but preferably smaller than 10 ohm.cm.

The supports may be made of metal, resin sheets coated with conductive layers or paper supports that contain conductive substances and/or are coated with conductive interlayers. Particularly suitable paper supports are those that have a high impermeability to organic solvents such as glassine paper described in the United Kingdom patent specification No. 995,491 filed Mar. 16, 1962 by Gevaert Photo-Producten N.V.

Paper sheets that have an insufficient electric conductivity are coated or impregnated with substances enhancing their conductivity, e.g. by means of a conductive polymer overcoat or by means of a metal sheet e.g. aluminium sheet laminated to the paper.

Substances suited for enhancing the conductivity'of a paper sheet and which can be applied'in the paper mass are hygroscopic compounds and antistatic agents are described, e.g. in the United Kingdom patent specification No. 964,877 filed May 2, 1960 by Gevaert Photo-Producten N.V. and antistatic agents of the polyionic type, e.g. CALGON CONDUCTIVE POLY- MER 261 (registered trade-mark of Calgon Corporation, Inc., Pittsburgh, Pa., USA.) for a solution containing 39.1% by weight of active conductive solids, and which contain a conductive polymer having recurring units of the following type:

Electrophotographic materials according to the present invention can be used in any of the different techniques known in recording with the aid of photoconductors.

Recording materials according to the present invention can be used in a recording technique comprising a negative corona charging as well as in a recording technique comprising a positive corona charging.

Thanks to this property it is possible, with one and the same developer, e.g. and electrophoretic developer with toner particles carrying, e.g., a positive charge, to reproduce one and the same original at will as a positive or a negative copy depending on the sign of the electrostatic charge applied to the recording layer of said recording material.

Thus, it is possible, e.g. by starting from a microfilm image as an original, to prepare positive as well as negative enlarge copies of that original by means of the recording materials of the present invention.

The following examples illustrate the present inven' tion.

EXAMPLE 1 binder composition The average grain size of the photoconductive zinc oxide and the zinc sulphide were 0.l7 micron and 1.8

micron respectively.

The obtained dispersion was dip-coated onto a glassine type paper support having a resistivity of 5.10 ohm.cm at relative humidity and a weight of g per sq.m and dried in a laminar air current drier by conveying air at 40C over the coating at a speed of 2 m per sec. The coated layer contained 35 g of pigment per sq.m.

Column A of the following Table 1 contains the amount of zinc oxide of each coating composition expressed in by weight with respect to the total weight of zinc oxide and zinc sulphide.

Column B contains analogously to column A the by weight of zinc sulphide.

In column C the maximum positive charge expressed by the obtained voltage level (V) built up under the same positive charge conditions is listed.

In column D the maximum negative charge expressed by the obtained voltage level (V) built up under the same negative charge conditions is listed.

In columns E and F is listed the percentage inherent decay of charge in the dark 3 see after the end of the positive and negative charging respectively.

In columns G and H the percentage decay of charge under same light (incandescent lamp light) exposure conditions (750 lux.sec) is listed for the positively and negatively charged recording layers respectively. The charge decay under exposure is measured after a constant charge decay in the dark has been reached.

a) ALKYDAL V '15 (trade name for a styrolated alkyd resin of Bayer AG w.-Germany) by 16 g weight solution in toluene [,hcopolymeriof ethylene, vinylsulphonyl chloride and vinyl chloride (26.l/6.9/67 by. weight) g urazole 052 g spectral sensitizing agents lmmophenol blue 28 mg and o o \-CH=CH-CH=] '--l i+ N ((iH ea Br 8 i0 2 NH-COCH l\ lH COCH silicone oil 0.2 ml

solvent mixture dichloroethane 500 in] ethanol 50 m1 toluene 500 in] methyl ethyl ketone 'l7O ml The total pigment content (ZnO 2118) was 400 The ingredients were mixed in a bail 'l-mi l'l In column I the relative speed (expressed in steps) of the described materials obtained by a negative corona charging, exposure through a step-wedge of constant 0.1 and development with positive electrophoretic toner is listed.

In columns 1 and K are listed the contrast and image acutance appreciation of the obtained results on development of the produced positive charge images. For obtaining these results after the positive charging under the same conditions, the recording materials were exposed through a step wedge with constant 0.3 superposed on a transparency representing a copy of a printed text. Then the acutance of the produced letters in the areas of different contrast was examined.

EXAMPLE 2 Example 1 was repeated with the difference, however, that the zinc sulphide was replaced by barium sulphate (average grain size 1.7 micron) The obtained results are listed and discussed in Table 2 hereinafter analogously to those in Table 1.

EXAMPLE 3 Example I was repeated with the difference, however, that the zinc sulphide was replaced by silica (average grain size 5 micron).

The obtained results are listed and discussed in Table 3 herinafter analogously to those in Table I.

Table l A B C D E F G H l J K (ZnO) (ZnS) 70 Decay ("/1 Decay (7: Decay (7t Decay (steps) (contrast) (acutance) (Volts+) (Volts) in Dark) in Dark) in Dark) in Dark) I 0 210 800 86 4 87 l i no image 90 i0 160 840 90 2 76 13 very weak bad 80 20 I90 830 92 7 72 l 1 weak bad 70 30 170 840 94 9 3] l4 weak bad 60 40 190 830 90 4 20 l 1 weak bad 50 50 360 840 89 2 l4 1 1 good bad 40 60 700 900 21 2 59 l l 10 good fairly good 30 70 I000 840 8.2 2.5 H l3 l0 good good 20 80 I030 840 6 2.4 6 8 5 fairly good good 90 I040 840 5 2.1 5 6 l fairly weak good 0 l00 lOlO 840 6 4 2 2 none no image Table 2 A B C D E F G H l J K (ZnO) (B1150 (steps) (contrast) (acutance) I00 210 800 86 4 87 II no image 90 10 760 850 22 7 94 68 16 weak bad i 80 820 840 I8 9 92 42 i4 weak bad 70 800 840 l9 12 23 I4 weak fairly good 60 810 840 l8 I4 66 43 i6 fairly good good 50 840 840 I7 i6 41 35 I5 good good 40 800 850 22 I4 33 3 l 14 good good 30 760 860 2i I7 27 26 l 1 good good 20 700 840 25 22 l9 l8 6 weak good 10 700 840 24 24 l0 10 l weak good 0 I00 660 800 33 29 7 7 none no image no image Table 3 A B D E F G H l J K (ZnO) (sio, (steps) (contrast) (acutance) I00 2l0 800 86 4 87 l 1 no image no image 90 I0 340 830 78 5 90 13 rather weak bad 80 20 750 840 27 8 97 90 14 rather weak bad 70 30 790 840 27 8 97 75 14 weak bad 60 40 850 850 18 9 96 77 15 weak bad 50 50 900 s40 12 l 1 63 I4 good bad 40 60 970 850, 12 ll 53 95 14 good good 30 70 1020 830 l0 16 25 40 14 good good 20 80 950 840 l3 I9 21 29 6 fairly good good If) 90 1030 850 l0 l3 l7 l9 1 weak good I00 l 960 4 5 4 2 none no image no image EXAMPLE 4 Example I was repeated with the difference, how ever, that the zinc sulphide was replaced by polyethylene particles having an average grain size of 15 micron.

The obtained results analogously to those discussed in Table l for columns 1, J and K are listed in Table 4 hereinafter.

A homogeneous mixture containing the following ingredients was prepared in a sand mill:

dichloroethane 7.4 1

ethanol 0.4 l

cyclohexanone l l 50 by weight solution in toluene of the terpolymer of vinyl acetate,

styrene, aliphatic acrylic acid ester and acrylic acid (25/30/44/1 by weight) 1.2 l

ALKYDAL V 15 (trade name) as a 75 by weight solution in toluene 240 ml zinc oxide, type ELECT ROX (trade name) 1,440 kg colloidal silica (average particle size 5 1) 2.160 kg by weight solution of urazole in dimethylformamide 36 ml 1 by weight solution of bromophenol blue in methanol 19 ml 0.5 by weight solution in dimethylformamide of the cyanine sensitizing dye of Example I 22 ml The dispersion was roller-coated in a ratio of 40 g of solids per sq.m. The coated layer was dried in a laminar air current drier. As supported for the coated layer a glassine type paper of 72 g per sq.m being made more conductive with sodium sulphate and calcium chloride was used.

After charging with a positive corona the recording layer was exposed through a negative transparency of a printed text and developed in an electrophoretic developer having the following composition:

500 g of ALKYDAL L 67 (trade-name of Farbenfabriken Bayer AG, Leverkusen, W.-Germany, for a linseed oil (67 by weight) modified alkyd resin) and 500 ccs of white spirit containing 1 l by weight of aromatic compounds are heated at 60C till a clear solution A is obtained. After cooling the solution A to room temperature, 150 g thereof are used as fixing agent for the following dispersion.

carbon black (average particle size 20 mp.)

zinc monotridecyl phosphate SHELLSOL T (trade name for a mixture of aliphatic hydrocarbons having the following characteristics: density at 15C: 0.764 boiling range: 180-200C viscosity at 25C: 1.62 c? Kauri'Butanol number (KB): 31)

Mixing of the solution A with the said composition occurs as follows:

150 g of said solution are first ground in a ball-mill with 1.5 g of the zinc monotridecyl phosphate. whereupon 30 g of carbon black are added and at least 750 ccs of Shellsol T (trade name).

After grinding the whole composition for 15 h the ob tained concentrated developer is diluted with Shellsol T (trade name) in a ratio of 15/1000.

The developer thus obtained is very stable and yields quite erasure-proof images after evaporation of the solvent from the recording layer.

A contrasty image having reversed image values with respect to the original and with excellent actuance was obtained.

EXAMPLE 6 A homogeneous mixture of the following ingredients was prepared in a sand mill:

dichloroethane 2.300 1 ethanol 0.300 I toluene 2.300 1 n-butanone 1.700 1 ALKYDAL V 15 (trade name) as a 7: by weight solution in toluene) 210 ml ZnO type MICROX (trade name) 1.600 kg silica (average grain size 5 1.) 2,400 kg HYPALON 30 (trade name for a copoly (ethylene/vinylsulphonyl chloride/vinyl chloride) (26.1/69/67 in by weight) marketed by E. I. du Pont de Nemours & Co (1nd.) Wilmington, Del., U.S.A.) as a 25 by weight solution in dichloroethane 4,600 kg at 3 by weight solution of silicone oil in dichloroethane 66 ml The obtained dispersion was coated and processed as described in Example 5.

A contrasty image having reversed image values with respect to the original was obtained.

We claim:

I. An electrophotographic process for producing selectively positive or negative copies of a given original comprising the steps of: uniformly corona charging a bi-chargeable photoconductive recording material comprising a photoconductive recording layer com prising an electrically insulating binder and dispersed therein a mixture of photoconductive zinc oxide particles and positively chargeable particles of the group consisting of barium sulphate, silica, and polyethylene, the ratio by weight in said layer of the photoconductive zinc oxide particles with respect to said positively chargeable particles ranging from 10:90 to less than 50:50, the recording layer containing the dispersed mixture of particles in a proportion with respect to the binder material of from 50 to 97% by weight, the zinc leaving the charges retained in the non-image aroxide particles having a grain size in the range of 0.1 to eas, and 7 15 micron and the positively chargeable particles 21 developing the exposed sheets by contacting the grain size in the range of to 50 microns, same with toner particles charged with the same said uniform charge having one polarity for positive 5 polarity for both positive and negative copies.

copy and the opposite polarity for a negative copy, 2. An electrophotographic process according to exposing said layer to a light image of said original to claim 1 wherein said original is a microfilm image.

dissipate said charges in the exposed areas while

Claims (2)

1. AN ELECTROPHOTOGRAPHIC PROCESS FOR PRODUCING SELECTIVELY POSITIVE OR NEGATIVE COPIES OF A GIVEN ORIGINAL COMPRISING THE STEPS OF: UNIFORMLY CORONA CHARGING A BI-CHARGEABLE PHOTOCONDUCTIVE RECORDING MATERIAL COMPRISING A PHOTOCONDUCTIVE RECORDING LAYER COMPRISING AN ELECTRICALLY INSULATING BINDER AND DISPERSED THEREIN A MIXTURE OF PHOTOCONDUCTIVE ZINC OXIDE PARTICLES AND POSITIVELY CHARGEABLE PARTICLES OF THE GROUP CONSISTING OF BARIUM SULPHATE, SILICA, AND POLYETHYLENE, THE RATIO BY WEIGHT IN SAID LAYER OF THE PHOTOCONDUCTIVE ZINC OXIDE PARTICLES WITH RESPECT TO SAID POSITIVELY CHARGEABLE PARTICLES RANGING FROM 10:90 TO LESS THAN 50:50, THE RECORDING LAYER CONTAINING THE DISPERSED MIXTURE OF PARTICLES IN A PROPORTION WITH RESPECT TO THE BINDER MATERIAL OF FROM 50 TO 97% BY WEIGHT, THE ZINC OXIDE PARTICLES HAVING A GRAIN SIZE IN THE RANGE OF 0.1 TO 15 MICRON AND THE POSITIVELY CHARGEABLE PARTICLES A GRAIN SIZE IN THE RANGE OF 10**-2 TO 50 MICRONS, SAID UNIFORM CHARGE HAVING ONE POLARITY FOR POSITIVE COPY AND THE OPPOSITE POLARITY FOR A NEGATIVE COPY, EXPOSING SAID LAYER TO A LIGHT IMAGE OF SAID ORIGINAL TO DISSIPATE SAID CHARGES IN THE EXPOSED AREAS WHILE LEAVING THE CHARGES RETAINED IN THE NON-IMAGE AREAS, AND DEVELOPING THE EXPOSED SHEETS BY CONTACTING THE SAME WITH TONER PARTICLES CHARGED WITH THE SAME POLARITY FOR BOTH POSITIVE AND NEGATIVE COPIES.

2. An electrophotographic process according to claim 1 wherein said original is a microfilm image.