US3468660A - Electrophotographic material comprising a multilayer support having a barrier layer over a duplex paper base - Google Patents

Description

p 3. 1969 R. M. DAVENPORT, JR.. ETAL 3,468,660

ELECTROPHOTOGRAPHIC MATERIAL COMPRISING A MULTILAYER SUPPORT HAVING A BARRIER LAYER OVER A DUPLEX PAPER BASE Filed Feb. 15, 1963 INVENTORS RALPH M. DAVENPORT, JR. ALFRED L. KRATZER ROBERT w. SAPP BY WILLIAM mrnonus HEINZ H- WEICHARDT United States Patent U.S. C]. 96-15 2 Claims This invention relates to electrophotographic material useful in electrophotographic processes, i.e., processes in which an electrostatic latent image is produced by utilizing the property of photoconductivity, which is a variable conductivity dependent upon the intensity of illumination to which the photoconductive material is exposed. The electrostatic latent image may be produced in a conventional exposure operation, for example by means of a lens-projected image or by contact printing techniques, whereby an invisible electrostatic charge pattern, the electrostatic latent image, is created on a surface, in which pattern the charge density at any point is related to the intensity of illumination obtaining at that point during exposure to light. The latent image may be developed, i.e., rendered visible, by means of an electroscopic powder, such as a colored synthetic resin powder, and the resulting visible image may be fixed by rendering the powder permanently adherent to a support on which the image is desired, for example, by heating to soften or melt the powder particles and/or the surface of the image support, or by treating the non-fixed powder image with volatile solvents.

In electrophotographic processes, the electrostatic latent image is formed on the surface of a photoconductive insulating layer carried on a support. For example, material comprising such support and photoconductive layer may be sensitized by applying an electrostatic charge to the free surface of the photoconductive layer. Such surface charge may be applied, for example, by means of a corona discharge, the charge being retained due to the substantial insulating character i.e., the low conductivity of the layer, when not exposed to light. Upon exposure, as described above, the photoconductive property of the layer causes the conductivity to increase in the illuminated areas to an extent dependent upon the intensity of illumination, whereby the surface charge in the illuminated areas leaks away,

leaving the charge located in the unilluminated areas. This remaining charge constitutes the charge pattern or electrostatic latent image.

Latent electrostatic images which are developed by treatment with a suitable electroscopic powder and which are then fixed by heating, for example, can be used in the preparation of printing plates by hydrophilizing the nonimage areas of the photoconductive layer in known manner.

In accordance with the present invention, a novel electrophotographic material is provided which comprises a photoconductive insulating layer on a support, the latter comprising a conductive paper web as a base, a conductive barrier film on the top surface of the base, intermediate the base and the photoconductive layer, and an effectively integral layer on the bottom surface of the conductive web.

The barrier film is so constituted as to be unaffected by contact with the solvents used in applying the photoconductive layer thereto and, also, the barrier film prevents the penetration of aqueous media such as fountain solution, for example, into the paper base web when the electrophotographic material is being run as a printing plate on an oltset press, for example. Further, the barrier film is highly conductive so that it will conduct away the electrostatic charge from the photoconductive layer in those areas exposed to illumination.

The barrier film, having the aforementioned characteristics, is an important feature of the invention since it enables the support to withstand long runs on an offset press due to the prevention of loss of integrity or dimensional stability by penetration of fountain solution into the conductive paper web utilized as a base.

The electrophotographic material of the invention incorporates, as a base web, a duplex paper web comprising a highly conductive upper layer which may be carbon-filled, for example, and an effectively integral, carbon-free lower layer. The carbon-free lower layer can have such character and thickness as to accommodate the passage of electrostatic charges from the carbon-filled upper stratum to an electrode plate or element contacting the surface of the carbon-free lower layer. The provision of an integral carbon-free lower layer of a duplex base web enables the finished electrophotographic material to be either rolled or stacked with the photoconductive layer in contact with the lower surface of the base web without causing the photoconductive surface to become smudged or grayed by carbon rub-off from the overlying base web. Otherwise, it is necessary, as a practical matter, to employ interleaving sheets between adjacent layers of the stacked or rolled electrophotographic material.

The carbon-free layer of the duplex base web is as light, i.e., thin, as practicable, consistent with the primary function thereof of preventing carbon rub-off between adjacent layers of the finished material. This provides for minimum electrical resistance through the relatively poorly conducting carbon-free layer as a carbon-free layer of optimum weight will both prevent carbon rub-off and facilitate the passage of electrostatic charges in the desired manner. Further, it may be desirable or appropriate, in certain cases, to impregnate the carbon-free layer with a suitable electrolyte, such as an electroconductive resin, for example, in which case greater thicknesses of the carbonfree layer may be utilized, if desired.

The invention will be further illustrated by reference to the accompanying drawings in which:

FIGURE 1 is an enlarged, fragmentary, cross-sectional view of an electrophotographic material according to the invention and FIGURE 2 is a schematic representation of the procedures involved in the manufacture of the material shown in FIGURE 1.

Referring to the drawings, there is shown in FIGURE 1 a section of an electrophotographic material, designated generally by the reference numeral 10, which includes a photoconductive layer 11 and a supporting base 12, the latter in the form of a dimensionally stable sheet. The base sheet 12 is electrically conductive, as that term is understood in the electrophotographic art, so that the photoconductive layer 11, when appropriately electrostatically charged, can be exposed to light under a pattern or master whereupon it is discharged in the illuminated areas.

In addition to the basic requirements of electrophotographic materials, significant additional advantages are achieved by the electrophotographic material of the invention. Thus, it is desired to use as a base sheet a suitable paper web which may be inexpensively manufactured and has the desired physical characteristics. The paper base sheet must possess a desired level of conductivity and special provisions must be made to enable the photoconductive layer to be properly coated on one surface of the base. Further provisions must be made for rendering the material dimensionally stable and resistant to fountain solutions. Further, the material must be capable of being rolled or stacked without marring or otherwise damaging the photoconductive layer thereof.

According to one specific embodiment of the invention, the base material comprises a conductive duplex paper web having a carbon-filled upper layer 13 and a substantially carbon-free bottom layer 14. Other materials such as aluminum or copper powders may be used instead of carbon as a conductive material for loading the paper layer but greater weights of these materials are required to impart equivalent conductivity to the paper. The duplex web is formed in accordance with known procedures so that the top and bottom layers are merged while still Wet from the web formation, the two layers being effectively integrally joined along the contacting interface in the finished web.

From the standpoint of conductivity alone, a simple, carbon-filled sheet might be a preferable base. However, since the lower surface of a carbon-filled sheet would be exposed to the photoconductive layer of another sheet upon rolling or stacking of the sheets, it is advantageous to form a protective layer over the bottom of the carbonfilled layer. This is of particular importance where the photoconductive layer is white in color such as a zinc oxide layer, for example. In such cases, there is a tendency for the zinc oxide in the photoconductive layer to become grayed or darkened if contacted directly by an unprotected carbon-filled sheet.

Thus, the duplex sheet includes the carbon-free bottom layer 14 which has a thickness or weight sufiicient to serve as a protective layer under the carbon-filled layer 13 while at the same time being sufiiciently thin to afford an adequately high level of electrical conductivity, top to bottom, through the duplex sheet.

The duplex base sheet may be of about 70 pounds total basis weight (70 pounds per ream of 3,000 square feet), the carbon-filled layer 13 having a basis weight of about 50 pounds and the substantially carbon-free bottom layer 14 having a basis weight of about pounds. If desired, the carbon-free bottom layer 14 of the duplex sheet may be impregnated or otherwise treated with suitable conductive resins in order to increase the conductivity thereof but a 20 pound carbon-free layer does not unduly impair the conductivity of the duplex sheet.

Adequate conductivity of the described 70 pound basis weight duplex sheet is achieved by incorporating about 10 to 20 percent, preferably 11 to 12 percent, of carbon black in the layer 13. The carbon is suitably incorporated into the pulp furnish prior to web formation, utilizing known procedures.

The duplex sheet 13-14 is rendered capable of receiving the photoconductive layer 11 and, at the same time, is protected against dimensional change and disintegration in the presence of fountain solutions by forming a special barrier film or subcoat 16 on the upper surface of the carbon-filled layer 13 of the duplex sheet.

The special film coating 16 serves as a barrier layer first to prevent penetration of solvents, in which the photoconductive layer 11 is applied to the duplex sheet, and also prevents penetration of aqueous fountain solution.

The special film coating 16 may be a nitrocellulose lacquer which incorporates sufiicient quantities of high purity carbon black to render the solidified film conductive, i.e., it is significantly more conductive than the carbon-filled layer of the duplex base sheet. The carbonfilled nitrocellulose lacquer film is initially applied in a solvent solution, advantageously by a gravure applicator or other known methods. The initial solution may, for example, comprise about 23-25 parts by weight of Hercules nitrocellulose lacquer, /2 second grade AS or RS; about 4-5 parts by weight of dibutyl phthalate plasticizer, or other known nitrocellulose plasticizers, CP grade; about 6-8 parts by weight of high purity grade carbon black, such as Conductex SC manufactured by Columbia Carbon Company and about 165 parts by weight of a solvent vehicle comprising a 50-50 mixture of ethyl acetate and toluol. This solution is applied to the carbonfilled layer 13 of the dry duplex web in amounts sufiicient to form a pinhole-free film after the solvent is removed from the coating composition. Instead of nitrocellulose, other resins possessing resistance to both organic solvents and water, such as oil-modified alkyds, epoxy resins, and polyvinyl chloride may be employed and the film may be loaded with aluminum or copper powders, for example, instead of carbon black. If desired, the film 16 may be applied in two light applications rather than one heavy application, as a smoother, more uniform layer results.

As a minimum, the film thickness of the barrier layer 16 must be such as to provide a pinhole-free barrier film, whereas, the maximum film thickness may be determined by economic factors, drying capacity, and the like. In the specifically described duplex base sheet 13-14, the maximum film thickness is limited by a tendency of the base sheet to curl undesirably with excessive film thicknesses. It has been established that optimum film thickness is achieved when the film-forming material is applied in weights of l to 2 pounds, residual (after removal of the solvent), per 3,000 square feet of surface area. When less than this amount is employed, the film may not be reliably pinhole-free and when greater amounts are used, the sheet begins to exhibit a tendency to curl which becomes undesirable when the film weight reaches 3.5 pounds and above, residual, per 3,000 square feet.

The carbon-filled nitrocellulose film, being highly conductive, has no adverse effect on the conductivity of the composite base material and, in fact, enhances the surface conductivity thereof. Simultaneously, the film provides an effective barrier against solvent vehicles and, thus, the film weight of l to 2 pounds per 3,000 square feet provides a toluol holdout upwardly of seconds, which is entirely adequate for proper application of the photoconductive material in a toluol vehicle and subsequent removal of the toluol. Furthermore, the pinhole-free nitrocellulose film is an effective barrier against fountain solutions.

The photoconductive layer 11 may include either an inorganic or organic photoconductor. Where an inorganic photoconductor, such as zinc oxide, for example, is employed, the photoconductive layer may be applied to the base in accordance with the teachings of U.S. Patents Nos. 3,052,539 and 3,052,540.

Where an organic photoconductor is employed, it may be applied to the base material in accordance with the teachings of U.S. Patent No. 3,041,165.

Where, for example, zinc oxide is employed as a photoconductor in the photoconductive layer 11, it is typically applied in a toluol vehicle together with a silicone resin by a wire or similar coater enabling the coating composition to be applied at a relatively high solids content, for example, 70 percent, in a high density, for example, 14 pounds per gallon, dispersion. In the described electrophotographic material, a zinc oxide photoconductive composition may be applied at weights of about 30 pounds per 3,000 square feet of surface area, which is sufiiciently heavy to provide adequate coverage of the black base sheet combination 12 and to present a desired level of surface whiteness.

For best results, the photoconductive layer 11 should have the highest practicable surface smoothness as well as uniform thickness. This is accomplished by the use of the nitrocellulose film barrier 16 which presents a substantially smooth surface to which the photoconductor is applied. Moreover, further improvements may be realized, where desirable or appropriate, by subjecting the duplex base web 13-14 to a supercalendering operation, the supercalendering also having the beneficial effect of increasing the conductivity of the duplex sheet.

Where zinc oxide is used as the photoconductor in the photoconductive layer 11, the layer may be hydrophilized in the non-image areas, after exposure, development, and fixing, in accordance with the procedure of US. Patents Nos. 2,952,536, 2,988,988, and 3,001,872. Where an organic photoconductor is employed, the photoconductive layer may be treated, after exposure, development, and fixing, in accordance with the procedure of US. Patent No. 3,041,165.

In some cases, it has been found desirable to apply a carbonfilled clay coating on top of the nitrocellulose barrier layer 16 and then apply the photoconductive layer 11 over it. Such carbon-filled clay coatings may comprise any of the standard clays used in paper making and may be a carbon-filled conventional starch-clay or casein-clay coating including, if desired, a butadiene-styrene latex to increase water resistance, the latter being resent in a range of about to 20 percent by weight.

Manufacture of the electrophotographic material of FIGURE 1 may be performed in a continuous process but generally is performed in separate stages, as shown in the schematic diagram of FIGURE 2. Thus, the first manufacturing stage is the formation of the duplex base sheet 13-14 which may be performed by known techniques using a pair of related cylinder machines 17, 18. The first cylinder machine 17 is supplied with a carbonbearing pulp furnish of predominantly cellulosic fibers and forms the carbon-bearing web 13. The second cylinder machine is supplied with a carbon-free pulp furnish of predominantly cellulosic fibers and forms the carbon-free web 14. The two separately formed webs are brought together while still wet and are effectively merged along their contacting interface to form a single integrated duplex web, which is thereafter passed through a suitable drier 19. If desired, the duplex web may be machine calendered and/or supercalendered to improve the conductivity and surface characteristics thereof. Alternatively, a combination of a single cylinder machine and a Fourdrinier machine may be employed.

The dried duplex web 13-14 then has a barrier film solution applied thereto, advantageously by a conventional gravure applicator 20. Normally, the gravure application of the described film-forming solution will result in an extremely smooth outer film surface. However, if even greater smoothness is desired, a suitable smoothing bar 21 may be provided in trailing relation to the gravure applicator 20. In either case, the coated web is passed through a curing oven 22 to volatilize the solvent and leave a thin, waterand solvent-resistant, smooth, con ductive barrier film 16.

Then, the composite base sheet, including the conductive duplex web 13-14 and the conductive barrier film 16, is passed through a blade or wire coater 23 or similar apparatus, which applies a high solids zinc oxide-toluol composition to the barrier film 16. A blade type coater is particularly desirable in that it enables a high solids composi- [1011 to be applied in relatively heavy coating weights with excellent surface smoothness.

After coating with the zinc oxide composition, the sheet is passed directly into a curing oven 24 which volatilizes the toluol vehicle, the total elapsed time between coating and curing generally being significantly less than the 120 second toluol holdout time provided by the barrier film.

The electrophotographic material of the invention, while affording exceptionally good toluol holdout and water resistance characteristics, and while effectively avoiding carbon rub-01f from one sheet to the photoconductive layer of an adjacent sheet, also provides exceptionally good surface and through conductivity. This is achieved by incorporating desired quantities of carbon black or other conductors in the nitrocellulose lacquer and in the upper layer 13 of the duplex web as well as by properly proportioning the effective thickness of the carbon-free bottom layer 14 of the duplex web. Thus, whereas it is believed that though resistance of the composite base 12 (duplex sheet 13-14 and barrier film 16) should be less than 1X10 ohms-ems. and that the surface resistance should be less than 1x10 ohms/ cm. (both measurements made using clean liquid mercury electrodes), the composite base of the electrophotographic material has actual resistance characteristics well below the theoretical maximums. For example, a typical test sheet had a meas ured through resistance of 1.4 10 ohms-ems. and a surface resistance of 8X10 ohms/cm.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. An electrophotographic material comprising a photoconductive insulating layer on a support, the latter comprising an electrically conductive liquid-impervious barrier film on a through-conductive duplex paper base, which latter comprises an upper carbon-filled layer and a lower carbon-free layer.

2. An electrophotographic material according to claim 1 in which the duplex base web has a basis weight of about pounds per 3,000 square feet, the carbon-filled upper layer having a basis weight of about 50 pounds per 3,000 square feet, and the lower carbon-free layer having a basis weight of about 20 pounds per 3,000 square feet.

References Cited UNITED STATES PATENTS Bornath et al 96-1.8

NORMAN G. TORCHIN, Primary Examiner I. C. COOPER, Assistant Examiner US. Cl. X.R.

Claims (1)

1. AN ELECTROPHOTOGRAPHIC MATERIAL COMPRISING A PHOTOCONDUCTIVE INSULATING LAYER ON A SUPPORT, THE LATTER COMPRISING AN ELECTRICALLY CONDUCTIVE LIQUID-IMPERVIOUS BARRIER FILM ON A THROUGH-CONDUCTIVE DUPLEX PAPER BASE, WHICH LATTER COMPRISES AN UPPER CARBON-FILLED LAYER AND A LOWER CARBON-FREE LAYER.

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