US 3676210 A
Description (OCR text may contain errors)
United States Patent 3,676,210 Patented July 11, 1972 3,676,210 PROCESS FOR MAKING ELECTROPHOTO- GRAPHIC PLATES Meredith David Shattuck, San Jose, and William I. Weiche, Los Gatos, Calif., assignors to International Business Machines Corporation, Armonk, N.Y. No Drawing. Filed Nov. 9, 1970, Ser. No. 88,149 Int. Cl. B44d 1/08 US. Cl. 117-201 7 Claims ABSTRACT OF THE DISCLOSURE A conductive substrate is simultaneously sprayed with a resin binder suspended in a carrier liquid and 'w1th the precursors of a photoconductive compound.
FIELD OF THE INVENTION The present invention is concerned with electrophotographic plates. More particularly, it is concerned with a novel method for making such plates. The novel method involves the simultaneous spraying of an electro-conductive substrate with a resin binder and with precursors of a photoconductive compound. The photoconductive compound is for'med in situ.
PRIOR ART The prior art is replete with electrophotographic plates containing photoconductive pigments in resin binders. Numerous examples of such plates are given in U.S. Pats. 3,121,006 and 3,121,007. To the best of our knowledge, however, the process used in the present invention to form the plates is different from any disclosed in the prior art.
The use of spray techniques to form a photoconductive film is disclosed in US. Pat. 3,148,084. The disclosure of that patent differs considerably from the present invention. In particular, the patent does not disclose the use of a binder, and the photoconductive films formed by the process of the patent are too electrically conductive to hold a charge in the dark and to be useful in electrophotography. Furthermore, the patent specifically requires that the substrate be a good electric insulator, while the process of the present invention requires that the substrate be electrically conductive.
SUMMARY OF THE INVENTION According to the present invention, an electrically conductive substrate is sprayed simultaneously with a resin binder suspended in a carrier liquid and with the precursors of a photoconductive compound, which is formed in situ on the surface of the substrate.
The preferred photoconductive compounds for use in the present invention include the oxide, sulfide and selenide of zinc, and the sulfide and selenide of cadmium. It is to be understood that mixtures of these compounds, and also materials such as the so-called sulfoselenides are also included within the scope of the invention. To form the photoconductive compounds in situ, precursors are sprayed onto the conductive surface. In general, it is most convenient to use aqueous solutions of precursors. Cadmium acetate, chloride, nitrate and propionate are suitable sources of cadmium ion. Zinc acetate and zinc chloride are suitable sources of the zinc ion. The sulfur moiety is advantageously provided by using thiourea, N,N-dimethylthiourea, ammonium thiocyanate or thioacetamide. In like manner, the selenium portion of the photoconductive compound is conveniently provided by the use of selenourea or N,N-dimethylselenourea. In the case of zinc oxide, it is preferred to form that compound in situ by the use of an aqueous solution of zinc acetate while simultaneously using gaseous oxygen as the carrier gas to propel the spray.
The present invention requires that the substrate be electrically conductive. Many types of electrically conductive substrates suitable for use in electrophotography are known to those skilled in the art. The substrate for example may be aluminum. In the case of aluminum substrates, it is sometimes advantageous to use a matte-finish surface which seems to improve adhesion between the substrate and the photoconductor. Other metallic materials may also be used, as may various polymeric materials. Particularly suitable polymeric materials include aluminum-laminated polyamide film, such as Du Pont Kapton, and aluminum-laminated nylon fiber paper, such as Du Pont Nomex.
A preferred electrically conductive substrate for use in the present invention is paper which has been treated to make it electrically conductive. Such treatment generally involves the use of a quaternary ammonium complex. The preparation of such paper is well known in the prior art.
In instances where it is desired, the conductive substrate may be covered with a so-called barrier layer, for example a thin film of aluminum oxide or the like.
The prior art is rich in disclosures of resin binder materials suitable for use in the preparation of electrophotographic plates. For the purposes of the present invention, it is preferred to use a binder material which may be readily dispersed, for example as a solution or an emulsion in a carrier liquid, which is preferably a liquid compatible with the carrier liquid for the precursors of the photoconductive compound. Particularly outstanding results have been obtained using aqueous emulsions of polyvinyl acetate. Various acrylic binders have also been found useful. Lower alcohols may also be suitable carrier liquids.
According to the process of the present invention, the binder and the precursors for the photoconductor are sprayed onto the substrate simultaneously. It is not necessary that a single spray applicator be used, although it is preferred, since the use of a single spray applicator permits simpler and more uniform control over the concentration of photoconductor in the binder. The use of separate spray guns for the resin binder and the precursors of the photoconductor extends the range of permissible carrier fluids, as does the use of very small spray droplet size.
The process of the present invention is obviously readily adaptable to form an electrophotographic plate containing multiple layers of photoconductors. It is obviously also readily adaptable to doping, i.e. the addition of minor amounts of ingredients to improve desired characteristics, for example sensitivity. Various inorganic or organic dopants and sensitizers may be added for this purpose. In general, it is preferred that the thickness of the photoconductor-resin binder layer be on the order of about 10-15 microns.
In general, the substrate will be heated during the sprayin-g operations. Substrate temperatures of about 300 F. are very advantageously used. It is, however, an unexpected advantage of the present invention that, when the substrate temperature is maintained below about 275 F., the photoconductive compounds formed in situ are not crystalline but are instead amorphous. It is believed that the use of amorphous photoconductors is in many case advantageous, and leads to the possibility of obtaining high resolution in electrophotographic copying, without loss of sensitivity.
An additional unexpected advantage of the present invention is that electrophotographic plates prepared according to the process of the present invention show symmetry or near symmetry in electrophotographic properties to a much greater degree than previously prepared pigment binder plates. The reason for this is not understood.
The following examples are given solely for purposes of illustration, and are not to be construed as limitations of the invention, many variations of which will occur to those skilled in the art, without departing from the spirit or scope thereof.
Example I A 0.04 molar cadmium acetate and 0.04 molar N,N- dimethylselenourea aqueous solution was prepared. To 100 ml. of this solution were added 2.0 grams of a 50% polyvinyl acetate emulsion in water and 0.2 mg. of Crystal Violet. Using a Paasche air brush Type H (three in one) the solution was sprayed under carbon dioxide pressure at 30 pounds per square inch onto an aluminum substrate maintained at 310 F. 5 ml. of the solution per square inch of substrate were used.
Electrophotographic measurements were made on the resulting cadmium selenide plate using a rotating disk electrometer with a quartz iodide light source (75 watts) with a 1.16 neutral density filter and opal glass. In the positive mode the charge acceptance was 660 volts and T /z was 1.0 second. In the negative mode the charge acceptance was 780 volts and TV: was 0.85 second. (T /2 is defined as the time required in the light to reach /2 the voltage acceptance of the dark voltage.)
Example 11 A spray solution was prepared of 100 ml. of water to which was added 2.0 grams of a 50% emulsion of polyvinyl acetate in water. Sufficient zinc acetate to make the solution 0.04 molar and 4 mg. of rose bengal were added. Using oxygen at 30 p.s.i. as the propellant gas, the mixture was sprayed at a level of 5 ml. p.s.i. onto an aluminum substrate maintained at 280 F. Using full tungsten light, the electrophotographic properties of the resulting zinc oxide plate were a charge acceptance of 360 volts in the negative mode and a T /2 of 1.2 seconds.
Example III By procedure analogous to that in Example I'I, zinc oxide coated electrophotographic paper may be prepared, using electrically conductive paper as the substrate instead of the aluminum of Example II. Zinc oxide coated paper is Well known in the art as being useful in electrophotographic processes where an image is developed directly on the coated paper.
Example IV To 100 ml. of water there were added 2 grams of a 50% emulsion of polyvinyl acetate in water and enough cadmium acetate to make the solution 0.02 molar and enough thiourea to make the solution 0.06 molar. With carbon dioxide at a pressure of 30 p.s.i., the solution was sprayed at the level of 5 mL/in. onto an aluminum substrate held at 310 F. Following the spraying operation, the plate was post-heated for 5 minutes at 400 F. Using a rotating disk electrometer, the charge acceptance of this cadmium sulfide plate was 680 volts in the negative mode and T /2 was 0.80 second. Using this plate in an electrophotographic mode, consisting of corona charging, imagewise exposure to light, development and transfer to paper, very good copies were obtained.
Example V To ml. of water there were added 2 grams of a 50% emulsion of polyvinyl acetate in water and sufficient cadmium acetate to make the solution 0.04 molar and sutficient N,N-dimethylsenenourea to make the solution 0.04 molar. Using carbon dioxide propellant at 30 p.s.i., the solution was sprayed onto an aluminum substrate at 290 F. and at level of 5 ml. p.s.i. Using a rotating disk electrometer with 1.47 neutral density filter plus opal glass, the electrophotographic data for this cadmium selenide plate were a charge acceptance of 365 volts in the positive mode with a TV: of 0.9 second and a charge acceptance of 520 volts in the negative mode with a T /z of 0.75 second.
What is claimed is:
1. A process for preparing an electrophotographic plate, said process comprising:
simultaneously spraying an electrically conductive substrate with a binder resin suspended in a carrier liquid and with the precursors of a photoconductive compound, said compound being selected from the group consisting of the oxide, sulfide and selenide of zinc, and the sulfide and selenide of cadmium.
2. A process as claimed in claim 1 wherein both the resin binder and the precursors of the photoconductive compound are sprayed from a single spray applicator.
3. A process as claimed in claim 1 wherein the electrically conductive substrate is electrically conductive paper.
4. A process as claimed in claim 1 wherein a sensitizing dye is also sprayed on the substrate.
5. A process for preparing an electrophotographic plate, said process comprising:
spraying an electrically conductive paper substrate simultaneously with an aqueous solution of zinc acetate and with a resin binder suspended in a carrier liquid.
6. A process as claimed in claim 5 wherein oxygen gas is used to propel the spray.
7. A process as claimed in claim 5 wherein the spray also contains a sensitizing dye.
References Cited UNITED STATES PATENTS 3,451,846 -6/l969 Lane et a1 117-l04 R 3,121,006 2/ 1964 Middleton et al 96--1.5 3,265,523 8/1966 Schulman et al. 117l04 R RALPH S. KENDALL, Primary Examiner J. H. NEWSOME, Assistant Examiner U.S. Cl. X.R.