US 3140174 A
Description (OCR text may contain errors)
United States Patent 3,140,174 PROCESS FOR OVERCgiTING A XEROGRAPHIC P TE Harold E. Clark, Penfield, N.Y., assignor to Xerox Corporation, a corporation of New York No Drawing. Filed June 23, 1959, Ser. No. 822,188 5 Claims. (Cl. 961) The present invention relates generally to xerographic plates and to techniques for producing such plates, and more particularly to xerographic plates of the binder type wherein the binder layer is provided with an overcoating.
The electrophotographic or xerographic process involves the sensitization of a xerographic plate and the exposure of the sensitized plate to an original image to be reproduced. The exposed plate is developed by the use of electrostatically attractable marking particles to produce a visible image which is thereafter transferred from the plate to a final support, the transferred image being fixed thereon to form the final print.
A standard xerographic plate is constituted by a thin layer of selenium on a metal backing plate, the selenium being deposited thereon in a vitreous form. In the dark state, the selenium layer has an extremely high resistivity, but when exposed to light the resistivity is reduced many orders of magnitude, depending on the intensity and wavelength of the light. By reason of its high electrical resistance in darkness the selenium layer can be charged electrostatically, which charge is retained for a prolonged period should no light impinge thereon. However, when light strikes the xerographic plate the resistivity decreases and the charge is discharged to the backing plate.
A more recent development in the field of xerographic plates is the so-called binder plate. It has been found that an efficient electrophotographic plate can be obtained by spraying or otherwise coating (as by a doctor blade, dip roll, air knife, etc.) a base plate with a photoconducting insulating composition prepared by intimately mixing and grinding together a photoconducting insulating material, a binder of a high electrical resistance and a solvent. On evaporation of the solvent, the composition forms a continuous adherent layer on the plate that will hold electrostatic charges in the dark and rapidly lose them on exposure to light. A typical binder plate is constituted by finely divided amorphous selenium particles dispersed in a thermoplastic synthetic resin binder. A binder plate may be charged, exposed and developed in the same manner as a standard xerographic plate and the image fixed by application of heat. The development system for the plate may be that involving a magnetic brush. In this system the toner is carried by iron filings magnetized by a permanent magnet. By merely brushing the surface of the plate with the clump of filings clinging to the end of the magnet, toner is removed by the electrostatic image which is then developed.
One liimtation in'the use of binder plates is that it is not possible to eifect complete transfer of the toner image by reason of the so-called tooth on the surface of the binder plate. The term tooth refers to the surface condition of the photoconductor binder structure. In the binder plate the surface is relatively rough so that the fine toner particles are mechanically trapped therein and cannot be dislodged by normal cleaning methods. While a standard selenium plate can be wiped clean of toner, a binder plate is difiicult to so clean. A selenium plate reflects light specularly, indicating a glassy surface, whereas binder plates of the type heretofore known reflect light diffusely, indicating a rough surface. Since the toner cannot be effectively removed from known binder plates, the plate is not readily reusable.
In view of the foregoing it is the principal object of the present invention to provide an improved xerographic 3,140,174 Patented July 7, 1964 plate of the binder type having a smooth surface which may be wiped clean, whereby the binder plate is reusable.
More particularly, it is an object of the invention to provide a xerographic plate of the binder type having an overcoating adapted to fill in the irregularities on the surface of the binder plate to produce a smooth, shiny surface from which toner can easily be removed.
Yet another object of the invention is to produce an overcoating for a binder plate having a degree of hardness and resistance to abrasion so that repeated cleanings will not result in a scratched overcoated surface.
A principal object of the invention is to produce a binder plate having a degree of hardness and resistance to abrasion as to permit repeated use in a xerographic process coupled with electrical properties so that there is no loss in image quality on such repetitive use.
In general, the invention resides in the use of an overcoating for a binder-type xerographic plate which is applied to the binder plate in a non-solvent for the binder in the xerographic plate.
Previous investigators have found that the application of protective insulating overcoatings to binder plates significantly reduce desirable electrostatic contrast. See, for example, the article by Dr. E. Wainer in Photographic Engineering, vol. 3, No. 1, pages 12-22 (1952). The reason for this adverse effect on the xerographic properties is not known. However, it has been discovered that application of the overcoating in a non-solvent for the binder in the binder plate not only eliminates the loss in electrostatic contrast, but, in at least some cases, actually increases electrostatic contrast on repetitive cycling. Moreover, there has been no apparent loss of adhesion between the overcoating layer and the photosensitive layer.
One method for applying an overcoating in accordance with the instant invention is to cure or cross-link the binder in the binder plate to a completely or partially thermo-setting condition as by adding a suitable catalyst or polyfunctional reactant to a thermoplastic resin binder or using a thermo-setting resin in an intermediate stage of polymerization for the binder and completing the polymerization on the plate. Due to the generally insoluble nature of such resin binders, a wide variety of solvents may be used for applying the overcoating resin using conventional coating processes without the solvent attacking (i.e., softening or partially solubilizing) the binder.
Alternatively the binder resin and overcoating resin must be so selected that a solvent may be chosen for the overcoating which does not attack the binder.
In any case the binder must be electrically insulating to prevent dissipation of electrostatic charge by lateral conductivity and must be translucent to the radiation activating the photoconductive layer.
The overcoating in accordance with the invention is preferably applied to a xerographic plate of the binder type hicluding a photoconductor constituted by a commercial grade of zinc oxide. Supplied to the zinc oxide is a binder in the form of a 60% solution of silicone resin. The zinc oxide and binder are mixed intimately in the proportion of 1.5 grams of Zinc oxide to 1 gram of binder (60% solids). One gram of toluene is added and the mixture is placed in a ball mill and mixed for about three hours in order to attain a thorough dispersion of the photoconductor.
The mixture is then placed in a large tank. The backing plate to be coated is lowered into the tank so that the area thereof to be coated lies below the surface of the mixture. The backing plate may be in the form of an aluminum sheet, of an inch in thickness. It is to be understood, however, that the backing may be constituted by any suitable electrical conductor or a plate having one conductive surface.
The plate is thereupon withdrawn at a constant speed from the photoconductor binder mixture, the speed of withdrawal determining the thickness of the coating. The mixture in the tank is maintained at room temperature. As soon as the plate is completely withdrawn it is placed in a horizontal plane and permitted to dry. The drying operation normally takes a few seconds. The usual coating thickness obtained and found satisfactory in practice is in the order of 15 to 75 microns although thicknesses of from about 10 to about 200 microns have been found operable.
Among the overcoating materials which are suitable are the following: vinyl resins such as polyvinyl alcohol, polyvinyl chloride, polyvinyl acetate, polyvinyl formal, etc.; polyester resins such as both thermoplastic and thermosetting alkyds, etc. ccllulosics such as cellulose acetate, ethyl cellulose, cellulose acetate butyrate, etc.; acrylic resins such as the polwnerized lower alkyl esters of acrylic and methacrylic acid; and thermosetting resins such as epoxides, urethanes, etc. The best solvent for the overcoating material is that which is fast drying, smooth coating, and produces a clear film.
The overcoating may be applied in several ways as by spraying, a dip roll, air knife, doctor blade; etc. One method is to dip the binder plate in the overcoating solution and to withdraw the plate at a constant rate, and allow it to dry. The solvent evaporates rapidly, leaving a smooth, thin insulating film over the photoconductive binder layer. In the event a thermosetting resin is used, the polymerization must be completed to set the overcoating resin as by heating, or by the application of a suitable catalyst at room temperature conditions. The overcoating should be from about 0.1 to microns thick.
The following examples are presented by way of illustration and not by way of limitation of the invention.
Example 1 Two binder plates were prepared as described above comprising a layer of zinc oxide in a silicone resin obtained from General Electric Company under the trade name SR82. The zinc oxide was a pigment grade material obtained from New Jersey Zinc Company under the trade name Florence Green Seal No. 8. The zinc oxide-silicone layers each had a thickness of about 50 microns and were coated on aluminum backings as described. Half of the one plate was over coated with polyvinyl formal as follows: A 3 /2 by weight, solution of polyvinyl formal (obtained from Shawinigan Products Corp. under the trademark Formvar) dissolved in trichloroethylene was poured onto the plate and allowed to evaporate leaving a coating of polyvinyl formal about one micron thick. Half of the other plate was coated with a water-alcohol solution (76.3% ethanol and 21.7% water) of a polyvinyl alcohol. The polyvinyl alcohol used was obtained from E. I. du Pont de Nemours and Company under the trade name Elvanol 20-105. The resulting overcoating was about 1 micron thick. The plates were then tested in the regular xerographic process as described by Carlson in US. 2,297,691 involving eleven successive cycles of charging, exposure, developing, electrostatic transfer of the image and cleaning by swabbing the surface with a cotton swab. From the very first image there was virtually no electrostatic contrast, i.e., no visible image, on the portion of the control plate overcoated with polyvinyl formal. On the portion of the control plate not overcoated with polyvinyl formal and on the portion of the polyvinyl alcohol plate not overcoated therewith, the initial image had excellent electrostatic contrast for the first image which gradually decreased, i.e., produced successively poorer images until the eleventh plate was of very poor quality. In contrast, the portion of the plate overcoated with the polyvinyl alcohol from the first produced sharp images which actually improved in quality on successive use in the xerographic process so that the last plate was even better than the first and significantly better than the equivalent plate on the unovercoated plate. At the end of of eleventh test cycle there was no detectable deterioration in the polyvinyl alcohol overcoated xerographic plate.
Example 2 A binder plate was prepared by mixing together 2.5 parts of zinc oxide (Florence Green Seal No. 8), 1 part of a styrene-alkyd copolymer obtained from American Cyanamid Co. under the trademark Cycopol S 101 and 3 parts of toluene (all parts by weight) and coating the mixture on an aluminum backing. After drying, the resulting coating was about 1 mil thick. The plate was cured at 140 F. for one-half hour and then overcoated with a urethane resin prepared by adding 10 parts of a polyester (obtained from Mobay Chemical Co. under the trade name Multron R-4), parts of an isocyanate (obtained from Mobay Chemical Co. under the trade name Mondur CB-75 and about 500 parts of cellosolve acetate (all parts by weight) The cured Cycopol was not affected by the Cellosolve acetate solution. The resulting overcoating was about 1 micron thick. The overcoated plate was stored at room temperature under high humidity to complete curing of the isocyanate resin. On removal from storage over high quality xerographic images were developed and cleaned from the plate surface without noticeable deterioration or fouling of the plate. Other plates were successfully prepared with zinc oxide: Cycopol ratios of up to 4: 1, both with and without dye sensitizer (Rose Bengal), at thicknesses of 0.5 to 1 mil, and with isocyanate thicknesses from 1 to 5 microns.
While the invention has been described in connection with the overcoating of binder plates wherein the photoconductor is zinc oxide, it is to be understood that other photoconductors may be used. For example, the invention is also applicable to binder plates wherein the photoconductor is formed by finely divided particles of amorphous selenium in a binder of synthetic resin, such as is disclosed in US. 2,663,636, issued December 22, 1953. Other binder plates to which the instant invention is applicable include those described, for example, in co-pending application Ser. No. 668,165, filed June 26, 1957, by Middleton et al.; Ser. No. 697,601, filed November 20, 1957, by W. G. Van Dorn et al.; Ser. No. 697,602, filed November 20, 1957, by W. G. Van Dorn et al.; and Ser. No. 672,569, filed July 18, 1957, by M. A. Insalaco. The types of photoconductive pigments, operable resins, ratios of pigment to resin, dye sensitization, backing materials and methods of applying the light sensitive coatings are fully described therein.
While there has been disclosed preferred overcoatings and methods of applying same, it will be appreciated that many modifications and changes may be made without departing from the essential features of the invention, and it is intended to cover such changes and modifications within the scope of the appended claims. Thus, the overcoatings may be applied from an emulsion or dispersion as well as from solution.
This application is a continuation-in-part of my copending application S.N. 482,896, filed on January 19, 1955, now abandoned.
1. A method of making a smooth surface binder plate which comprises: depositing a light-sensitive photoconductive material on the surface of an electrically conductive backing plate to form thereby a photoconductive layer having an irregular surface, said photoconductive layer comprising a mixture of photoconductive material and a water insoluble resinous binder, depositing over said irregular surface an aqueous solution of a water soluble electrically insulating material, said solution used being chemically unreactive with said photoconductive material, allowing said liquid carrier to evaporate from said solution and the resinous material therein to thereby deposit in and fill the irregularities of said surface with said resinous material, continuing said deposition until a smooth resinous film of from about 0.1 to about 5 microns is formed on said photoconductive material.
2. An electrophotographic process wherein the reusable binder plate made by the method of claim 1 is sequentially electrically charged, exposed to a light image pattern to be reproduced and developed with electrically attractable marking particles.
3. The method of making a smooth surface reusable electrophotographic binder plate which comprises depositing a zinc-oxide silicone resin material on and in direct physical contact with the surface of an electrically conductive backing plate to form thereby a photoconductive layer having an irregular surface, depositing substantially evenly and entirely over said irregular surface an aqueous solution of a water soluble electrically insulating material, said solution used being chemically unreactive with said photoconducting material, allowing said liquid carrier to evaporate from said solution and the resinous material therein to thereby deposit in and fill the irregulan'ties of said surface with said resinous material, continuing said deposition until a smooth resinous film of from about 0.1 to about 5 microns is formed on said photoconductive material.
4. The method of claim 3 wherein said electrically conductive backing plate comprises predominantly aluminum.
5. An electrophotographic process wherein the reusable binder plate made by the method of claim 1 is sequentially electrically charged, exposed to a light image pattern to be reproduced, and developed with electrostatically attractable marking particles.
References Cited in the file of this patent UNITED STATES PATENTS 2,158,111 Doolittle May 16, 1939 2,476,800 Blackburn July 19, 1949 2,494,920 Warrick Jan. 17, 1950 2,623,030 Cordier Dec. 23, 1952 2,663,636 Middleton Dec. 22, 1953 2,664,044 Dalton Dec. 29, 1953 2,693,416 Butterfield Nov. 2, 1954 2,803,542 Ullrich Aug. 20, 1957 2,808,328 Jacobs Oct. 1, 1957 2,829,025 Clemens et al. Apr. 1, 1958 2,860,048 Deubner Nov. 11, 1958 2,862,815 Sugarman et a1 Dec. 2, 1958 2,901,348 Dessauer et al. Aug. 25, 1959 2,979,402 Greig Apr. 11, 1961 OTHER REFERENCES Wainer: Photographic Engineering, vol. 3, No. 1, pp. 12-422 (1952).
UNITED STATES PATENT OFFICE, CERTIFICATE OF CORRECTION July 7, 1964 Patent No, 3,140,174
I Harold E0 Clark It is hereby certified that error appears in the above numbered pattha,t the said Letters Patent should read as.
ent requiring correction and corrected below for the claim reference numeral "1' Column 6, line 1,
read 3 d sealed this 221001 dayof December 1964,,
Signed an EDWARD J. BRENNER, Commissioner of Patents (SEAL) Attest:
ERNEST W. SWIDER Attesting Officer