US3501295A - Electrophotographic reproduction system utilizing lightweight copy papers - Google Patents

Description

March 17, 1970 D. E. HANSON 3,501,295

ELECTROPHOTOGRAPHIC REPRODUCTION SYSTEM UTILIZING LIGHTWEIGHT COPY PAPERS Filed June 17 1966 INVENTOR DONALD E. HANSON BYW a ATTOFNEY 9 war United States Patent Ofice 3,501,295 Patented Mar. 17, 1970 3,501,295 ELECTROPHOTOGRAPHIC REPRODUCTION SYS- TEM UTILIZING LIGHTWEIGHT COPY PAPERS Donald E. Hanson, Flemington, N.J., assignor to Riegel Paper Corporation, New York, N.Y., a corporation of Delaware Filed June 17, 1966, Ser. No. 558,503 Int. Cl. G03g /08, 13/22 US. Cl. 961.8 4 Claims ABSTRACT OF THE DISCLOSURE A light-Weight zinc oxide coated paper is used in a reverse contact electrophotographic process. The paper has an at least partially transparent base sheet with a zinc oxide-binder coating of from 5l5 pounds per ream.

The present invention relates to electrophotographic reproduction, commonly referred to as xerography, and is directed more particularly to specific improvements in electrophotographic reproduction systems of the general type utilizing zinc oxide coated copy papers, to the particular end that the copy papers may be of extremely low weight, relative to conventional, while retaining the quality characteristics of more conventional reproduction systems using copy papers of conventional weight.

Xerographic reproduction systems are broadly divided into two types: In the first type, a reusable photoconductive transfer element (e.g., a selenium coated drum) is electrophotographically imaged and is arranged to transfer the image, as by a suitable printing technique, to a copy sheet, which may be of ordinary paper. In the second broadly classified type of xerographic reproduction, the copy sheet itself is provided with a surface coating of photoconductive material, and the imaged copy sheet forms the finished product of the system. Most typically, for routine office copy work, the sensitized copy sheet is adapted for one-time use. In other words, the xerographic image produced thereon is rendered permanent and the copy sheet then forms a permanent document.

In reproduction systems in which the copy sheet itself is provided with an imageable photoconductive surface coating, extensive practical commercial experience has led to the almost universal adoption of photoconductive zinc oxide compositions as the surface coating material because of the many advantages of this material over known alternatives. One of the Widely recognized and important disadvantages, however, of the zinc oxide coating material, which has tended to impose significant limitations on the utilization of the reproduction systems utilizing the coated copy sheets, resides in the significantly high weight of the zinc oxide material which must be applied to the surface of the copy sheet, in order to achieve satisfactory copy quality. In other words, with previous technology, the copy quality demanded by the consumer for commercial acceptance of the product has necessitated the use of zinc oxide coatings above 15 pounds per ream (3,000 square feet) for minimum acceptable quality, and typically on the order of 20 to 25 pounds of coating per ream for good quality reproduction. For a coat weight of this order it has been the practice heretofore to utilize a base sheet having a basis weight on the order of 45 pounds, such that the overall weight of a typical, good quality copy sheet has been on the order of about 65 or 70 pounds per ream, which is significantly and detectably greater than the basis weight of an ordinary bond paper, for example. The substantial weight of conventional zinc oxide coated copy papers prevents the papers from having the desirable look and feel of ordinary bond paper and, per- 'haps more importantly, a collation of a relatively large number of conventional copy sheets is noticeably and objectionably heavy, as compared to ordinary papers.

The present invention is directed and limited to electrophotographic reproduction systems utilizing zinc oxide coated copy sheets and has, as its principal objective, the improvement and optimization of one of the heretofore known (in the broad sense) xerographic reproduction processes, which will enable commercally acceptable copy quality to be achieved with copy papers having a desirable look and feel, favorably comparable to ordinary papers, and being drastically reduced in overall weight, as compared to conventional zinc oxide coated copy papers. Specifically, the present invention enables the use of zinc oxide coated copy papers having an overall weight typically less than 55 pounds per ream, as compared to conventional papers of usually almost at least one third greater weight for comparable copy quality. Quite apart from the obvious and significant economies thus made available to the paper consumer, the improved system has special attractiveness in that the weight, feel, and appearance of the copy sheet more closely approach the corresponding characteristics of a good quality bond paper, for example.

More specifically, the present invention is directed to improvements, focused particularly in the area of the copy sheet itself, in the system of xerographic reproduction descriptively referred to herein as the reverse contact exposure system. In the reverse contact exposure system, an original document, bearing on one surface thereof an image to be reproduced, is placed in direct contact with the back or uncoated surface of a zinc oxide coated copy sheet. The photoconductive zinc oxide coating is electrostatically charged, either before or after contact is established with the original, or master, sheet, and the charged photoconductive layer is exposed by the projection of light through the master sheet and through the back or base sheet component of the coated copy sheet. The light-exposed areas of the copy sheet are electrophotographically discharged by the exposure, to form a latent image on the coated surface of the copy sheet. This latent image thereafter is developed by bringing the copy sheet into proximity With electroscopic developer or toner particles of suitable electrostatic polarity, causing the particles to adhere selectively to the copy sheet and develop the latent image thereon. Most advantageously, the developer or toner consists of a liquid suspension of the electroscopic toner particles. Broad aspects of this overall process are described in, for example, British Patent No. 902,558.

The present invention is based, at least in significant part, upon the discovery that proper (in the sense of commercial acceptability) imaging and development of the copy sheet is a function of surface charge voltage differential between light and dark areas of the latent image, more than a function of the absolute values of surface charge voltages. While it is important, on the one hand, for the surface charge voltage of the copy sheet to be reduced to or below the toner threshold voltage, in nonimage or background areas, a wholly acceptable, sharp, contrasty copy is obtainable if the image areas retain a charge of as much as to 200 volts above the background charge voltage. In the overall process of the invention, by controllably assuring sharp exposure contrast between image and background areas, it is made possible to achieve the necessary surface charge voltage differential with a significantly reduced initial charge one that does not grossly exceed the voltage differential sought for on the exposed copy sheet. This being so, it is also made possible to reduce the weight of the photoconductive zinc oxide surface coating to a level at which the maximum charge acceptance of the coating equals the reduced initial charge required for acceptable copy re sults. Additional coat weight, while not harmful to the image quality, would be unnecessary and wasteful and would add undesirably to the weight and cost of the sheet. With a photoconductive zinc oxide coating of typical charge acceptance characteristics, a coat weight in the range of 5 to 15 pounds gives a commercially good copy under commercial copy conditions suitable for office application whereas, heretofore, coat weights of at least 20 to 25 pounds of equivalent zinc oxide photoconductive composition have been considered necessary for commercially acceptable results according to conventional processes.

In part, the invention is based upon the realization that significantly improved imaging or exposure of the photoconductive surface coating of the copy sheet could be achieved in the reverse contact exposure system by, in the first instance, properly controlling the light transmission characteristics of the base sheet component of the copy sheet. By enabling the base sheet to transmit the imaging light without undue distortion, diffusion, and blocking, desirably high contrast is maintained between exposed areas of the zinc oxide coating corresponding to light and dark (typically black and white) areas of the master sheet. By thus achieving significantly high contrast in the electrophotographic exposure, it is possible to effect significant reductions in thea-bsolute values of the electrostatic surface charge voltages required to be applied to the zinc oxide coating in the first instance without loss of image quality.

In accordance with the specific teachings and practices of the present invention, realization of the significance of surface charge differential to the copy process, coupled with proper control of exposure light transmission through the base sheet component of the copy sheet, enables the surface charge voltage to be reduced from the typical 600-450 volt range conventionally required down to the 350-150 volt range, without compromising the quality of the copy image ultimately produced. Since the maximum charge acceptance of a photoconductive zinc oxide coating is a function of the coat weight, for a given composition, the process of the invention has thus enabled the coat weight of the zinc oxide to be reduced from 15 pounds and up per ream (and typically 20 to 25 pounds per ream for commercially acceptable good quality) to from 15 pounds down to about 5 pounds per ream (and typically around pounds per ream).

A key aspect of the present invention resides in the use, in a reverse contact exposure system of xerographic reproduction, of a novel, indeed revolutionary in the light of prior knowledge and commercial experience, form of copy sheet which has an appearance and feel more closely approximating that of ordinary bond paper and has an overall weight not significantly exceeding 55 pounds per ream, and which, at the same time, accommodates the production of xerographic copies of commercially acceptable quality and at speeds and under conditions suitable for and acceptable to the making of ofiice copies, for example. The new copy sheet most advantageously includes a special base sheet component which, while being a cellulosic, felted, fibrous paper and having the look and feel of paper, is controllably translucent to the imaging light without excessive diffusion or blocking, forms a suitable substrate for the solvent coating of the zinc oxide photoconductive surface layer, and is suitably conditioned to be passed through a toner solution including a carrier such as odorless mineral spirits or kerosene. Most particularly suitable for this purpose is a high density, semi-glassine paper sheet having abasis weight of, say, 3550 pounds per ream and which affords not only an advantageously low caliper and degree of translucency but provides superior hold-out characteristics with respect to both the toner vehicle and the solvent carrier for the zinc oxide surface composition. The semiglassine sheet minimizes the need for special hold-out coatings, which could adversely affect the sharpness of the image, and in some cases altogether avoids the need for such special coatings.

The improved system of the invention takes advantage of the fact that commercially acceptable electrophotographic copy depends to a large extent upon achieving a satisfactory level of contrast between the black and white areas of a printed or a typewritten master sheet. In order to achieve adequate contrast after toner development, a surface charge voltage differential between the black and white" areas of an exposed copy sheet carrying an electrostatic latent image should be on the order of 200 volts. Where exposure contrast is particularly sharp and controllable, as is made possible by the optimized system of the present invention, the maximum initial surface charge voltage required on the unexposed copy sheet is not substantially in excess of the overall voltage differential required for adequate contrast in the developed copy, whereas conventionally two to three times this amount of voltage is required in the initial charge. Byenabling commercially satisfactory copy quality to be achieved using an initial surface charge voltage on the order of, say, 200 volts, the invention makes it possible and desirable to reduce the weight of the zinc oxide photoconductive surface coating to as low as 7 to 10 pounds per ream, perhaps even as low as 5 pounds per ream, the coat weight requirement being that minimum weight having a charge acceptance of about 200 volts. And, while it is sometimes possible as a laboratory experiment to produce xerographic copy using zinc oxide photoconductive coatings of low coat weight, it has not been possible, heretofore, to do so on a basis which would be commercially feasible, taking into consideration such factors as acceptable copy quality and copy-making speed, feel and appearance of the copy sheet, physical and electrical requirements of the necessary equipment, etc. With the applicants optimized process, on the other hand, commercially good quality copy is obtainable on a copy sheet favorably comparable in weight and to ordinary bond paper, using an extremely low-cost copy-making machine suitable for desk-top utilization and easily capable of copy-making speeds comparable to or exceeding those of conventional commercially acceptable systems.

For a better understanding of the invention, reference should be made to the following detailed description and to the accompanying drawing, in which:

FIG. 1 is a simplified schematic representation of a typical reverse contact exposure system of electrophotographic reproduction suitable for carrying-out the principles of the invention; and

FIG. 2 is a highly enlarged, fragmentary, cross-sectional view of an electrophotographic copy sheet according to the invention shown in reverse contact relationship with a master sheet.

Referring now to the drawings, and initially to FIG. 1, the reference numerals 10, 11 designate, respectively, a master sheet and a copy sheet, separated by a divider plate 12 but otherwise in position to be brought into surface contacting relation and conveyed through a sequence of electrophotographic copy-making operations. The master sheet 10 is an original document, such as a printed or typewritten page, which is oriented with its printed surface facing downward. The copy sheet 11, which forms a key aspect of the invention and will be described in greater detail, consists of a base sheet component 13 and a photoconductive zinc oxide surface. coating 14 (see FIG. 2), and the copy sheet is arranged, when in its feed-in position as shown in FIG. 1, with the photoconductive zinc oxide coating facing in a downward direction.

In the illustrated arrangement, the photoconductive zinc oxide coating 14 of the copy sheet is conveyed over a corona discharge or like facility 15, which serves to impart to the photoconductive coating 14 a predetermined initial surface chargevoltage. Assuming, as is intended in the present case, that the capacity of the corona discharge facility is adequate in relation to the speed of conveyance of the copy sheet, the photoconductive zinc oxide coating 14 will assume a maximum surface charge voltage, as a function of the overall thickness of the coating, as will be discussed in greater detail. In the arrangement specifically illustrated, the electrostatic charge is imparted to the photoconductive Zinc oxide coating 14 while the master sheet 10 and copy sheet 11 remain separated by the plate 12. However, it may be desired in certain cases to effect the electrostatic charging of the photoconductive surface 14 subsequent to bringing the sheets 10, 11 into surface contacting relation.

In the reverse contact exposure system, to which the present invention is specifically directed, the imaged (i.e.,

downwardly facing) surface 16 of the master sheet is brought into direct and intimate contact with the back or uncoated surface 17 of the copy sheet. In this connection, reference to the uncoated surface 17 of the copy sheet is intended to mean only that the surface does not have a photoconductive zinc oxide coating thereon, but it does not preclude the possibility of its having some other kind of non-opaque coating, such as a hold-out coating, if such a coating is desired or necessary. The contacting sheets are then conveyed through an exposure or imaging step, in which an imaging light beam is directed through the back of the master sheet 10 and, where permitted by the imaged surface 16 of the master sheet, through the base sheet component 13 of the copy sheet. In the areas of the copy sheet opposite non-imaged areas of the master, the exposure light travels through to the photoconductive zinc oxide surface coating 14, dissipating the surface charge thereon and causing an electrostatic latent image to be formed on the photoconductive coating 14.

After complete exposure of the contacting master and copy sheets, the sheets may be separated, with the master sheet 10 being returned and the copy sheet 11 being developed, so that the electrostatic latent image thereon becomes visible. Typically, and most advantageously, the development is accomplished by conveying the electrophotographically exposed copy sheet through a toner bath 18 containing suitable, colored (usually black) electroscopic toner particles suspended in a suitable vehicle such as mineral spirits, kerosene, or the like. The electroscopic particles, being polarized oppositely to the undissipated charges of the exposed copy sheet, are attracted specifically to the latently imaged areas of the copy sheet to form the visible image. Thereafter, the copy sheet is conveyed out of the toner bath, through means such as squeegee rollers 19 to remove excess toner liquid, and thereafter to a discharge point. If necessary, heat and/or air flow is provided following the squeegee rollers 19, in order to help dry the developed copy sheet.

In accordance with one of the important concepts of the invention, provision is made for optimizing the transmission imaging light from the imaged surface of the master sheet 10 to the photoconductive zinc oxide coating 14 of the copy sheet 11, to assure that the electrostatic latent image produced at the exposed surface of the copy sheet is, to the greatest practicable degree, faithfully correspondent to the imaged areas 20 of the master sheet 10. To this end, the base sheet component 13 of the copy sheet is desirably formed to afford maximum translucency and minimum caliper, consistent with other important requirements, such as adequate solvent and vehicle hold-out, adequate strength and body for processing conditions, appearance and feel corresponding to ordinary paper, etc. According to the invention, the base sheet component 13 of the copy sheet has a Bausch & Lonrb opacity rating not in excess of about 85 and a caliper of not more than about 3.0 mils, and most advantageously in the range of 2.2 mils to 3.0 mils. Most desirably, the base sheet component has an apparent density of at least about 16 and perferably about 18, which is relatively high and assures a minimum caliper for a given basis weight. The most preferred caliper range of the sheet is about 2.2 to 2.5 mils, which corresponds to a basis weight range of about 35 to 50 pounds per ream, at an apparent density of about 18. The caliper of the sheet, for a given basis weight deemed necessary or desirable for adequate strength and other characteristics, is a significant consideration, in that opportunity for diffusion of the image projected upon the photoconductive zinc oxide coating is a direct function of the caliper or thickness of the sheet.

A most advantageous specific base sheet for use in the practice of the invention is a 45 pound basis weight conductive base sheet paper as manufactured by the assignee of this application. The papermaking furnish for this base sheet advantageously consists of 40% bleached softwood kraft pulp, such as that commercialy available under the trade designation Hi-Brite, and 60% bleached hardwood kraft pulp, such as that made available under the trade designations Jessup or Natchez. To this pulp furnish the additives titanium dioxide, clay, and alum may be incorporated in respective amounts of 3%, 6%, and 3.6% of the fiber weight. The furnish is highly refined, to a machine slowness of at least about 250-300 cc. or higher Schopper-Riegler. After web formation, but before significant drying, the web is advantageously size press impregnated .with a 4% solution of carboxymethyl cellulose (Hercules Grade 7A), with a small amount of Kymene 557 insolu'bilizer. This wet or plastic press impregnation of CMC is made on the wire side only of the web and serves to improve the hold-out of the web to liquid toner vehicles, such as mineral spirits, kerosene, etc.

After substantial but still incomplete drying of the web, a second size press impregnation (this time on both sides) is made of a 10% solution of Dow QX2611.7 polyelectrolyte resin. The pick-up of the polyelectrolyte resin is approximately 3% of the fiber weight of the sheet, on a dry basis.

After drying, the web is supercalendered, advantageously to an apparent density of about 18. A 45 pound basis weight conductive base sheet paper according to the foregoing specifications has a particularly advantageously low caliper of about 2.4 mils.

The specific resistivity of the above-described base sheet, initially dried to about 6% moisture content, is l 10 ohms/sq. at 50% relative humidity and 5 l0 ohms/sq. at 20% relative humidity. The opacity of the sheet (Bausch & Lomb) is less than 80.

Because of the high density and high translucence of the base sheet, transmission of exposure light through the base sheet will be realized with optimum faithfulness and with a minimum opportunity of dispersion of light into areas intended to be shaded or for blocking of light from areas intended to be fully exposed. And this, of course, enables the highest degree of contrast to be realized in the exposure process. In this connection, although the' translucence and density characteristics of the master sheet 10 may be substantially out of the operators control, the effect of these characteristics of the master sheet are largely unimportant (as long as the sheet passes adequate amounts of light) inasmuch as the image areas 20 thereof directly contact the base sheet 13, and the light is not required to travel through the body of the master sheet after passing the image areas. The character of the source of exposure illumination does not appear to be critical, as long as it is adequate to complete the necessary exposure in a commercially acceptable period of time, typically a few seconds. In this connection, however, the illuminating source, designated by the reference numeral 21 in FIG. 1, may be a fluorescent bulb of about 8 watts output. A commercially available fluorescent General Electric Green F12T5 bulb is suitable for the purpose.

By reason of the extremely high image contrast ratios between black and white image areas obtainable through the reverse contact exposure system utilizing a copy paper having a base sheet component on the order of that above described (e.g., easily 10:1 and up to as high as 30:1, depending somewhat on the characteristics of the master), the maximum surface charge voltage required to obtain a black-to-White charge dilferential of on the order of 150 to 200 volts is not significantly in excess of the amount of the desired ditferentia1150 to 200 volts. In conventional systems, an initial surface charge of 450 to 600 volts normally is required in order to get an image contrast voltage differential high enough for adequate contrast.

As a most significant aspect of the invention, advantage is taken of the fact that copy image quality is a function of latent image voltage differential, as distinguished from any absolute values of surface charge voltage, and of the fact that the initial surface charge voltage is a function of the coat weight of the photoconductive zinc oxide. To achieve an initial surface charge voltage of 150 to 200 volts, a suitable formulated photoconductive zinc oxide coating of 5 to 7 pounds has been found to be adequate. Somewhat higher coat weights, say around pounds as an optimum and pounds as about a maximum upper limit, may be utilized in appropriate cases to provide a margin of charge acceptance capacity adequate to accommodate a wide variety of master sheets, some of which may, by reason of poor original images, make high image contrast ratios more difficult to obtain.

In a particularly advantageous copy sheet for use in the practice of the invention, the photoconductive zinc oxide is applied at a weight of about 10 pounds per ream and is formulated to have a wide spectrum response at an electrophotographic exposure speed somewhat corresponding to an ASA paper speed (tungsten) of from 0.1 to 5 and most preferably about 1. A particularly advantageous photoconductive zinc oxide formulation for this purpose comprises a mixture of about 1500 pounds New Jersey Zinc Photox 80 zinc oxide, about 1500 pounds New Jersey Zinc Photox 801 zinc oxide, about 607.5 pounds Monsanto GMS264 binder resin (55% solids), about 1270 pounds of toluene, and a sensitizing dye system comprising about 40.8 grams of Bromphenol Blue, about 45.5 grams of Dibromofluorescein, about 50.0 grams of Acridine Yellow, in about gallons of methanol. It will be understood, of course, that the speed rating and charge acceptance ability of the photoconductive zinc oxide will vary from formulation to formulation, and the various coat weights specified herein are related to photoconductive compositions having properties not significantly dissimilar from those of the described formulation. For differing photoconductive zinc oxide formulations, compensating adjustments in coat Weight may be necessary or desirable to achieve corresponding results.

The copy sheet and exposure system described, while being theoretically operative with both liquid and dry toner systems, are most advantageously utilized in connection with liquid toner systems comprising electroscopic toner particles suspended in a suitable liquid vehicle, such as mineral spirits. An advantageous form of liquid toner system for this purpose is available from Electrocopy Corporation, 10401 Decatur Road, Philadelphia, Pa., under the trade designation Electrocopy Toner.

According to the invention, the new copy paper, consisting of a base sheet having a basis weight of less than about pounds per ream, a caliper of less than about 3.0 mils, a Bausch & -Lomb opacity of not higher than about 85 and a photoconductive zinc oxide surface coating of not in excess of about 15 pounds, when electrostatically charged and exposed by reverse contact exposure under an 8 watt fluorescent bulb for a period of 10 seconds or less, will form an electrostatic latent *Total time for exposure of entire sheet conveyed past exposure window; actual exposure period for any area of sheet, approximately one-half second.

image having a charged-to-discharged area surface charge voltage differential of at least about 150 volts. When developedby a suitable tonersystem, the thus imaged low coat weight copy paper can and will provide a .finished copy having commercially satisfactory image characteristics, as well as a desirable weight, feel, and appearance.

According to available objective standards of commercially acceptable copy images, image (black) density must be greater than about 0.5 (corresponding to a reflection of 32% or less) with the preferred commercially good copy having an image density of at least 0.75 corresponding to a reflection of 18% or less. Background density, for commercial acceptability, must be less than about 0.15, corresponding to a reflection of at least 71%, with the preferredbackground density being less than about 0.10, corresponding to a reflection of at least 79%. In the optimized system of the invention, the described copy paper, having a zinc oxide photoconductive surface coating of between 15 and 5 pounds per ream, exposed by reverse contact exposure processes, falls well within these commercial acceptance ranges of image and background density, and is readily brought within the preferred region of the acceptance range.

The invention is based in part upon the realization that commercially acceptable electrophotographic copy is as much a function of the voltage differential between light and dark image areas of an electrostatic latent image as it is a function of the absolute electrostatic charge voltages of these areas. By providing a sharp, high contrast exposure in the first instance, achievable with the contact exposure technique, the necessary charge differential is obtainable using an exceptionally low initial charge voltage. In turn, this enables the coat weight of the zinc oxide photoconductive coating to bereduced to the point where its maximum charge acceptance equals the desired, low initial charge voltage. The overall process enables an extraordinary reduction in zinc oxide coating while retainingthe ability to makecomrnercially good copy at commercially acceptable speeds, with low cost, commercially feasible equipment. In conjunction with the foregoing, the process of the invention enables the reverse contact copy procedure to be performed on an optimized basis with a copy paper having the look, feel, weight, and appearance of ordinary bond paper, which is a particularly important factor affecting suitability for oflice copy Work.

The present invention significantly obviates what has been perhaps the major disadvantage of xerographic copy systems utilizing zinc oxide coated copy sheetsthat being the excessive weight of the sheet, as compared to ordinary bondpapers and the like which oflice personnel are accustomed to handling. Conventional zine oxide coated copy sheets, capable of producing a commercially satisfactory copy on a commercially feasible basis, commonly have 25 to 30 pounds per ream of zinc oxide coating, in addition to the basis Weight of the paper of, typically, 4050 pounds per ream. The resulting sheet may have an overall weight of 75 pounds per ream, which compares very unfavorably with a typical grade of bond paper for oflice use, having a basis weight on the order of 50 pounds per ream. By contrast, in accordance with the present invention it is readily possible and practical to employ a 45 pound base sheet with a 10 pound zinc oxide coating, for a total copy sheet weight of about 55 pounds.

One of the economically significant benefits of the invention is, of course, the extraordinary savings in the zinc oxide coating composition, which represents .a costly component of the copysheet. Savings of from about 50 to 80% are possible in-this area. Y

The overall, optimized system is adapted most particularly for low-cost, desk-top copy service, in which the typical master to be copied is a typewritten sheet, for example. Since the reverse contact exposure system may be carried out with extremely simple and inexpensive equipment, the system is basically suitable for decentralized use. The advances of the invention, by optimizing the contact exposure system and making it possible to utilize therein copy paper of very low zinc oxide coat weight and favorable overall sheet weight without compromising image quality, tend to widen substantially the available market area for zinc oxide based electrophotographic copy reproduction systems.

It should be understood that the specific form of the invention herein illustrated and described is intended to be representative only, as certain changes may be made therein without departing from the clear teachings of the invention. Accordingly, reference should be made to the following appended claims in determining the full scope of the invention.

I claim:

1. The process of producing an electrophotographic copy, which comprises (a) placing a master sheet, carrying an original image,

in direct contact with a copy sheet,

(b) said copy sheet having a base sheet component and a surface coating component comprising photoconductive zinc oxides dispersed in a resin binder,

(c) said base sheet component having a Bausch & Lomb opacity not greater than about 85 and a caliper not greater than about 3.0 mils,

(d) said photoconductive zinc oxide surface coating being present at a substantially uniformly thick layer of not substantially less than pounds per ream nor substantially more than pounds per ream,

(e) said master sheet and copy sheet being placed with the imaged side of the master sheet in contact with the base sheet component of the copy sheet,

(f) before or after contacting said master and copy sheets, imparting to the photoconductive surface coating an initial surface charge not substantially in excess of 350 volts and not substantially less than 150 volts,

(g) electrophotograhically exposing the charged surface coating by projecting light through the master sheet and base sheet to the photoconductive surface coating, and

(h) imaging the exposed copy sheet by causing toner particles to be selectively attracted to the copy sheet surface in accordance with the electrophotographically exposed latent image pattern of said surface coating.

2. The process of claim 1, further characterized by (a) the base sheet component of said copy sheet comprising a sheet having a basis weight on the order of -50 pounds per ream and having a fibrous cellulosic structure in which the fibers have been highly hydrated,

(b) said base sheet having an apparent density not substantially less than about 16.

3. The process of producing an electrophotographic copy, which comprises (a) supplying a master sheet carrying an original image,

(b) supplying a copy sheet comprising base sheet and surface coating components,

(c) said base sheet component having a density of about 16 and being controllably translucent to accommodate the controlled passage of light without excessive diffusion to obtain an image contrast ratio of at least about 10:1 during imaging,

(d) said surface coating component being on one side only and comprising a substantially uniformly dis tributed coating of photoconductive zinc oxide dispersed in a resin binder having a coat weight not substantially outside the range of five to fifteen pounds per ream,

(e) applying an initial surface charge of from about volts to about 350 volts to said photoconductive coating,

(f) bringing the master and copy sheets into contact, with the imaged surface of the master sheet in direct contact with the surface of the copy sheet opposite the photoconductive coating,

(g) selectively discharging the surface charge on said photoconductive coating by directing image exposure light through the master sheet and through the base sheet component of the copy sheet to the coating, whereby to impart to image areas of the coating corresponding to relatively dark and light areas of the master sheet image a surface charge voltage differential of at least about 150 volts, and

(h) imaging the exposed photoconductive surface coating by selectively attracting electroscopic toner materials thereto in accordance with said voltage differentials.

4. An electrophotographic copy sheet which comprises (a) a paper like base sheet having a basis weight of from about 30 to 50 pounds per ream,

(b) said base sheet having a Bausch and Lomb opacity not greater than about 85, and

(c) a coating on a surface of said base sheet comprising photoconductive zinc oxide dispersed in a resin binder,

(d) said zinc oxide surface coating having a maximum surface charge acceptance in the range of not less than about 150 volts and not more than about 350 volts,

(c) said zinc oxide surface coating having a coating weight in the range of not less than about 5 pounds per ream to not more than 15 pounds per ream,

(f) whereby an image contrast ratio of at least about 10:1 is obtained during imaging by a reverse contact exposure method.

References Cited UNITED STATES PATENTS 2,663,636 12/1953 Middleton 96-1.5 2,993,787 7/1961 Sugarman 961 2,999,750 9/1961 Miller 96-1 3,121,006 2/1964 Middleton et al. 96-1.8 3,160,503 12/1964 Cady 961 3,165,405 l/l965 Hoesterey 96l 3,245,786 4/1966 Cassiers et al. 96--1 3,335,003 8/1967 Snelling 96l GEORGE F. LESMES, Primary Examiner J. C. COOPER III, Assistant Examiner US. Cl. X.R. 961

Patent No. 3,5 95 Dated 7, 97

Inventor(e) Donal E. Hansen It is certified that error and that said Letters Patent are Column 6, line 45, '1 x 10 should read ----1 x 10 Column 7, line 1 "suitable" should read --suitably--. Column 9, line 2 "at" should read --es--.

appears in the above-identified patent hereby corrected as shown below:

0 QIL-Luw SEALED $720M SEA- M M mm x. uownm. :8- MO oomissioner of Pat-8n

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