US3447957A - Method of making a smooth surfaced adhesive binder xerographic plate - Google Patents

Description

United States Patent 3,447,957 METHOD OF MAKING A SMOOTH SURFACED ADHESIVE BINDER XEROGRAPHIC PLATE Arthur J. Behringer, Webster, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York No Drawing. Filed Aug. 19, 1964, Ser. No. 390,738 Int. 'Cl. B44c 1/22; (30811 13/24 US. Cl. 117-201 11 Claims ABSTRACT OF THE DISCLOSURE obtained.

This invention relates in general to xerography and more specifically to a xerographic binder plate and a method for its production.

In the art of xerography as originally disclosed by Carlson in US. Patent 2,297,691 and as further amplified by many related patents in the field, an electrostatic latent image is formed on a photoconductive insulating layer and is developed through the deposition thereof on finely divided electroscopic materials. The developed image may then be fixed in place or transferred to a copy sheet Where it is permanently fixed. In most applications, the photoconductive insulating layer, which is referred to in the art as a plate regardless of its shape or flexibility, is first charged to sensitize it and is then exposed to a light image or other pattern of activating electromagnetic radiation which serves to dissipate the charge in radiation-struck areas, thus forming a charge pattern which conforms to the electromagnetic radiation pattern which impinges upon the plate. This charge pattern is then developed or made visible by the charge-wise deposition on the plate of an electroscopic or electrostatically, attractable, finely divided, colored material which is referred to in the art as toner.

Although reusable, homogeneously structured xerographic plates, such as those made with amorphous selenium, have enjoyed extremely wide commercial success because they are capable of producing many thousands of extremely high quality copies, attention has recently been drawn to heterogeneous xerographic plates known in the art as binder plates and composed of finely divided, photoconductive materials dispersed in an insulating, adhesive, film-forming binder. Xerographic plates of this latter type are described, for example in US. Patents 2,663,636 to Middleton and 3,121,006 to Middleton and Reynolds.

Owing to the fact that xerographic plates are generally used in photographic imaging systems, it is very important that they have smooth surfaces and when they are being used in high resolution imaging systems, the surfaces must be extremely smooth in order to correctly reproduce the image. Extreme smoothness of the plate surface is also preferred to produce good-quality images in electroded charge transfer systems such as the one described in FIGURE 6 of my copending application, Serial No. 309,665, filed Sept. 18, 1963, now US. Patent No. 3,253,65 3 since this smoothness not only provides a good focal plane for light impingement, but also allows for "ice very uniform spacing between it and the surface to which its charge is later transferred in image configuration.

Although extremely smooth xerographic plates of the homogeneous variety have been produced by vacuum evaporating material such as amorphous selenium, it is much more difficult to produce smooth-surfaced bindertype xerographic plates since because of their heterogeneous nature, these plate materials cannot be vacuum evaporated and further because the particulate photoconductive material included therein tends to make for a rough plate surface.

Accordingly, it is an objective of this invention to describe a method for the production of extremely smooth binder-type xerographic plates.

Yet another object of this invention is to define a method of making xerographic plates under pressure with highly adhesive materials.

The above and still further objects may be accomplished in accordance with the present invention, generally speaking, by coating a composite mixture of the photoconductive particles and an insulating, adhesive, film-forming resin on a substrate having a coeflicient of heat expansion roughly the same as that of the composite binder plate material, pressing an extremely smooth surface cover over the free top surface of the binder coating and holding the sandwich formed by these elements under pressure with the resin in liquid form and then in solid form. Since the cover material is selected so as to have a distinctly different coefficient of thermal expansion from that of the composite binder layer, separation of the adhering cover from the binder is achieved by a large temperature change of the sandwich structure as by dipping in liquid nitrogen or the like.

The xerographic plate of this invention is constructed by first blending the desired photoconductive insulating particles with an insulating film-forming adhesive resin and coating this blend on a supporting substrate. Although particulate zinc oxide, particulate selenium and particulate cadmium sulfide are the most well-known and widely-used materials for inclusion in binder-type xerographic plates, any suitable particulate photoconductive insulating material may be employed. Typical photoconductive insulating materials include zinc sulfide, cadmium selenide, red lead (Pb O arsenic disulfide, arsenic trisulfide, titanium dioxide, zinc titanate, zinc silicate, zinc magnesium oxide, mercuric iodide, mercuric oxide, mercuric sulfide, indium trisulfide and calcium strontium sulfide.

The particular resin employed as the adhesive binder material may be either a natural or synthetic resin having either thermoplastic or thermosetting characteristics. Here again, as with the photoconductive materials, although silicones such as phenyl-methylpolysiloxane resins and vinyl chloride-acetate copolymer resins are, perhaps, the most widely-known binder resins for this type of xerographic plate, any suitable insulating, adhesive, filmforming resin may be employed. Typical insulating adhesive film-forming resins including both natural and synthetic resins of both thermoplastic and thermosetting nature are as follows: polystyrene, polybutylmethacrylate, chlorinated rubber, cellulose esters and ethers such as ethyl cellulose and nitrocellulose; alkyd resins such as linseed oil-glycerol alkyds as well as other polyesters, polyurethanes, polyamides, polycarbonates, styrene-butadiene copolymers, soya-alkyds, shellac, paraffin wax, carnauba Wax, beeswax, cumarone resins, indene resins, balsam resins modified with colophony, polyvinyl acetals, polyacrylic and polymethacrylic esters such as polybutylmethacrylate, polymethylmethacrylate, copolymers of methylacrylate and ethylmethacrylate, copolymers of nbutyl and iso-butylmethacrylate, polyvinylidene chloride, polyvinyl butyral, epoxides and amine-formaldehyde con;

densation products such as melamine formaldehyde. It should be noted, however, that epoxies, constitute a preferred form of the invention because they make very hard smooth layers with excellent electrical properties. The adhesive resin may be blended with the particulate photoconductor in the powdered form or in any suitable liquid form such as from a hot melt of the resin, from a solution of the resin in an organic solvent or from a water emulsion of the resin. Many suitable hardenable resins are, of course, liquid at room temperature and may be used as such. If the resin is of the thermosetting or hardenable variety, a catalyst or accelerator is included in the resin-photoconductor blend. The ratio between the binder resin and the photoconductive particles is the same as that generally employed with binder-type xerographic plates of conventional fabrication. These plates usually contain a ratio between resin and photoconductive particles of from about 1 part resin and parts photoconductive particles to about 2 parts resin and 1 part photoconductive particles (all parts by weight). The actual proportions will, of course, depend upon the specific materials employed as well as on the properties and characteristics desired in the final plate produced.

' The substrate upon which the resin-photoconductive particle blend is coated is employed to provide physical support for the layer and in some cases to act as a ground, thereby permitting the photoconductive insulating layer to receive an electrostatic charge in the dark and permitting charges to migrate when exposed to light. Obviously, where the composite layer of resin binder and photoconductive particles have sufiicient strength to form a self-supporting layer the support may be eliminated from the final plate construction. In this case (or if a non-conductive substrate is used) the self-supporting layer may be brought in contact with a grounded conductive plate during charging or if positive electrostatic charges are placed on one side of the plate, as by corona charging, as described in US. Patent 2,777,957 to Walkup, the simultaneous deposition of negative charges on the other side of the plate, also by corona charging, will create an induced or virtual ground plane within the body of the plate just as if the charges of opposite polarity had been supplied to the interface by being induced up from a grounded substrate. When it is desired to include a substrate in the final plate structure, this type of substrate may be produced by coating a thin layer of tin oxide, or indium oxide on glass or a heat resistant plastic such as polyethylene terephthalate or tetrafluoroethylene or other material having a coefficient of thermal expansion about the same as that of the binder. If, on the other hand, the substrate is only to be used temporarily in the fabrication of the plate, and it is desired that the substance be removed at the completion of fabrication, it is selected to have a coefiicient of thermal expansion significantly diflferent from that of the photoconductive binder film and may consist, for example, of brass, copper, steel, zinc or the like.

Once the photoconductor-resin blend (hereafter referred to as the binder film) is coated on the supporting substrate and any water or solvent is completely removed from the binder film, a very smooth cover plate is clamped thereon. This plate is also selected to have a coefiicient of thermal expansion significantly different from that of the binder film and may also consist of brass, stainless steel, copper, nickel, zinc or the like. The thus formed clamped sandwich is then subjected to heat if the resin is initially in solid form or is a liquid resin to be heat cured. This tends to soften and reduce the viscosity of the resin so that it will more readily wet and come into intimate contact with the pressure plate clamped to its surface. In the case where a thermosetting resin is employed, the application of this heat treatment serves the additional function of curing the resin. The amount and duration of heat application will, of course,

vary depending upon the particular resin selected for use in the plate, its melting point, or the time required for a cure of the resin if it is to be heat cured. Obviously if the resin is initially liquid and it is desired to cure it at room temperature, it is merely held in the clamped condition until the cure is complete resulting in resin solidification. In this case the heating step is eliminated. After completion of this heating step, the sandwich is allowed to cool to about room temperature and is then unclamped and cooled to a low enough temperature so that the difierences in thermal expansion between the cover plate and the resin film cause the two layers to separate. Upon removal of the cover plate, an extremely smooth, uniform, resin binder film surface is found to exist. As will be clear, if the substrate is also selected so as to have a significant difference in coeflicient of thermal expansion from that of the resin binder film it will also separate from this film upon cooling. If, on the other hand, the coefficient of the thermal expansion of the substrate and the binder film are roughly the same, separation of these two layers will not take place, and the resulting xerographic binder plate will include the supporting substrate firmly bonded beneath it.

This technique for the production of smooth surfaced binder-type xerographic plates is particularly effective with some photoconductive materials which cannot be made in sizes smaller than about 25 micronsin diameter without losing their photoconductivity, whereas in the past, it has been virtually impossible to make smooth surfaced binder plates with these materials by any of the known techniques.

The general nature of the invention having been at forth, the following examples are now presented as illustrations of the method of carrying out the invention.

Example I 100 parts by weight of powdered photoconductive cadmium sulfide having an average size of about 25 microns in diameter are blended with 61 parts by weight of a powdered bisphenol A-epichlorohydrin-type epoxy resin and 2.56 parts by weight of a diethylamine triamine curing agent and made into a slurry with deionized water. The slurry is then coated on a NESA coated Pyrex glass plate (a thin coating of tin oxide on borosilicate glass) with a stainless steel spatuala. A polished molybdenum plate is clamped against the top surface of the belnd, resulting from the slurry after air drying, and the sandwich structure is then cured at 250 F. for 10 minutes. Since this temperature is well above the melting point of the epoxy resin, a smooth, continuous binder film is formed which also takes on a good hard cure under these conditions. After removal from the oven and cooling to room temperature to facilitate handling, the sandwich structure is dipped in liquid nitrogen and upon removal from the nitrogen bath, the molybdenum plate is easily separated from the binder film leaving behind an extremely, smooth surfaced photoconductive binder film.

Example [Ill The method of Example I is repeated except for the fact that the powdered epoxy resin is replaced with the same amount of epoxy in an acetone solution and curing conditions are 122 F. for 45 minutes. These conditions produce equal good results with those of Example 'I. Curing is carried out only after the acetone is fully evaporated 01f.

Example III A polystyrene emulsion, made with a polymer having a melting point of (3., is blended with powdered zinc sulfide (C.P. grade) in sufiicient quantity to produce a l to 1 ratio by volume between the resin solids and the zinc sulfide photoconductor. The water emulsion is then agitated to promote thorough mixing and coated on a NESA glass substrate. After thorough air drying, a polished aluminum plate is clamped face down on the surface of the binder film formed by the dried emulsion and this sandwich structure is baked for 15 minutes at 150 (3. followed by cooling to room temperature. The sandwich is then dipped in liquid nitrogen and after removal of the clamp, the aluminum plate is easily separated from the underlying binder film, thus producing an extremely smooth surface.

Example IV The procedure of Example H1 is repeated except that the zinc sulfide photoconductor is replaced with phosphor No. 1200 (zinc cadmium sulfide, available from E. I. du Pont de Nemours and Company) with equally good results.

Example V One part by weight of 'French process zinc oxide, available from the 'New Jersey Zinc Company under the trade name, Florence Green Seal No. 8, is added to a solution of 1 part by weight of polybutylmethacrylate in 4 parts by weight of toluene and ball milled for 3 hours. This blend is then coated on a NESA glass substrate and air dried. Following drying, a polished aluminum plate is placed with its polished side facing the coating and clamped thereon under pressure. This sandwich is then baked for minutes at 115 C. and after cooling, it is dipped in liquid nitrogen. Upon removal of the clamp, the polished aluminum plate falls away from the sandwich structure leaving behind an extremely smooth surfaced binder film.

Example VI 0.45 gram of an epoxy resin of the type described in connection with Example I are mixed with a few drops of a cross linking agent and 1.00 gram of photoconductive cadmium sulfide. This mixture is placed on a NESA glass plate and a polished molybdenum sheet is clamped over the mixture, the clamped plate is then cured for about 5 hours at 50 C., at which time the clamps are removed and liquid nitrogen is then poured on the molybdenum sheet which is then simply stripped free of the cadmium sulfide-epoxy layer. The separation is very clean resulting in an extremely smooth and glossy cadminum sulfide-epoxy surface. The surface is very hard and well cured and is well bonded to the NESA glass substrate.

Example VII A slurry is first prepared consisting of 11% by weight of poly-methyl-methacrylate and 89% by Weight of cadmium sulfide photoconductor with the resin incorporated as a xylene solution and viscosity adjusted with the solvent for easy application. This mix is then applied to a NESA glass substrate through a stainless steel screen after which the screen is separated from the mix and the solvent is thoroughly removed by heating in a drying oven set at 65 C. The layer is then cooled in a dessicator. A flexible metal sheet is then placed on the binder layer surface and the surface is smoothed by rolling a heated roller over the [cover plate. The 5 mil thick aluminum cover plate has a size such that it extends well beyond the area of the plate so that after the heated roller has passed over the cover plate and it has cooled to room temperature, the cover plate is removed by immersing only its portion which extends beyond the plate into liquid nitrogen. Because of the high heat conductivity of the aluminum, even the portion of the aluminum over the plate is cooled very rapidly and contracts sufficiently to break the bond between the cover plate and the binder film beneath it result ing in a very smooth surfaced binder film.

What is claimed is:

1. The method of making a xerographic plate comprising blending a particulate photoconductor with an adhesive, insulating film-forming resin to form a uniform mix, coating said mix on a supporting substrate to form a binder film thereon, pressing a smooth-surfaced cover plate against said binder film to form a fabricating laminate while holding said resin in liquid form, to adhere said binder film to said cover plate, said cover plate having a significantly different coefficient of thermal expansion than said binder film, solidifying said resin, cooling said laminate sufficiently to break the bond between said binder film and said cover plate due to the differences in contraction of said binder film and said cover plate and removing said cover plate whereby an extremely smooth surfaced xerographic binder plate is formed.

2. A method according to claim 1 including employing a thermosetting resin to form said binder film and heating said fabricating laminate at a high enough temperature for a long enough time to cure said resin.

3. A method according to claim 2 in which said thermosetting resin includes an epoxy polymer and a curing agent.

4. A method according to claim 1 in which said photoconductor is cadmium sulfide.

5. A method according to claim 1 in Which said photoconductor is zinc oxide.

6. A method according to claim 1 including using a substrate having a coefficient of thermal expansion closer to that of said binder film than to the coefficient of thermal expansion of said cover plate and cooling said fabricating laminate sufficiently to break the bond between said binder film and said cover plate but not sufliciently to break the bond between said binder film and said substrate.

7. A method according to claim 1 including using a substrate having a coefficient of thermal expansion significantly different from that of said binder film, and cooling said fabricating laminate sufficiently to break the bonds between said binder film and said cover plate and between said binder film and said substrate.

8. A method according to claim 1 wherein said laminate is cooled by immersing in a liquified gas.

9. A method according to claim 1 wherein said laminate is cooled by immersing in liquid nitrogen.

10. The method of making a xerographic plate comprising blending a particulate photoconductor with an adhesive, insulating film-forming resin to form a uniform mix, coating said mix on a supporting substrate to form a binder film thereon, pressing a smooth-surfaced cover plate against said binder film to form a fabricating laminate while holding said resin in viscous form, to adhere said binder film to said cover plate, said cover plate having a significantly dilferent coefiicient of thermal expansion than said binder film, solidifying said resin, cooling said laminate sufiiciently to break the bond between said binder film and said cover plate due to the differences in contraction of said binder film and said cover plate and removing said cover plate whereby an extremely smooth surfaced xerographic binder plate is formed.

11. The method of making a xerographic plate comprising blending a particulate photoconductor with an adhesive, insulating film-forming resin to form a uniform mix, coating said mix on a smooth-surfaced cover plate to form a binder film thereon, pressing a supporting substrate against said binder film to form a fabricating laminate while holding said resin in liquid form to adhere said binder film to said cover plate, said cover plate having a significantly different coefficient of thermal expansion than said binder film, solidifying said resin, cooling said laminate sufficiently to break the' bond between said binder film and said cover plate due to the differences in contraction of said binder film and said cover plate, and removing said cover plate whereby an extremely smooth surfaced xerographic binder plate is formed.

References Cited UNITED STATES PATENTS 2,320,536 2/1943 Pollack et al. 11765.2 2,370,562 2/ 1945 Meunier 11764 (Other references on following page) 1 7 Middleton 96-1.5 Curler et a1 117--64 Jones 117-64 Middleton et a1. 961.5 Strahl 11764 V 8 MURRAY KATZ, Primary Examiner.

US. Cl. X.R.