US3627526A - Electrophotographic recording elements with half-tone screen coatings thereon - Google Patents

Abstract

AN ELECTROPHOTOGRAPHIC RECORDING ELEMENT COMPRISES A PHOTOCONDUCTIVE LAYER ON A RELATIVELY CONDUCTIVE SUBSTRATE, AND A COATING OF A PARTIALLY LIGHT-TRANSMITTING MATERIAL ON THE PHOTOCONDUCTIVE LAYER. THE COATING OF THE LIGHT-TRANSMITTING MATERIAL IS IN A PATTERN OF A HALF-TONE SCREEN, COVERING ONLY A PORTION OF THE PHOTOCONDUCTIVE LAYER, SO THAT THE RECORDING ELEMENT PROVIDES HALF-TONE TYPE-PRINTS IN AN ELECTROPHOTOGRAPHIC PROCESS.

Description

DBC. 14, 1971 P. J. DONALD 3,627,526

ELECTROPHOTOGRAPHIC RECORDING ELEMENTS WITH HALF-TONE SCREEN COATINGS THEREON Filed DBC. 29, 1959 INVIL'N'IUK FHJLJPJUNAL United States Patent O 3,627,526 ELECTROPHOTOGRAPHIC RECORDING ELE- MENTS WITH HALF-TONE SCREEN COAT- INGS THEREON Philip `Ioseph Donald, Woodbury, NJ., assignor to RCA Corporation Filed Dec. 29, 1969, Ser. No. 888,248 Int. Cl. G03g 5/04 U.S. Cl. 96-1.5 1 Claim ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to electrophotographic recording elements, and more particularly to electrophotographic recording elements with half-tone screen coatings thereon. The novel electrophotographic recording elements are particularly useful for copying continuous-tone (continuously variable shades of gray) images by electrophotographic methods.

Most electrophotographic recording elements have relatively short gray scales and by themselves reproduce continuous-tone images poorly. It has been proposed to reproduce a continuous-tone image on an electrophotographic recording element by employing an external halftone screen during the exposure operation in an electrophotographic process. While such a procedure is satisfactory for some applications, it leaves something to be desired in others. For example, in projecting a half-tone screened light image, the individually projected elements of the half-tone screen should be clearly resolvable. Hence, the optical system employed for projecting the image imposes a limitation on `both the resolution and the number of resolvable elements to be copied. If direct screening is used, whereby the half-tone screen is placed in contact with a charged electrophotographic element, there is a danger of displacing some of the charge of the recording element in the process, thus spoiling the resulting reproduction.

Each of the novel electrophotographic recording elements provides half-tone type copies of continuous-tone originals and eliminates the need for an external halftone screen, thereby obviating the aforementioned disadvantages in the use of external half-tone screens.

SUMMARY OF THE INVENTION The novel electrophotographic recording element comprises a relatively conductive substrate, a photoconductiye layer thereon, and a coating of a partially light-transmltting material in the pattern of a half-tone screen on a portion of the photoconductive layer.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a fragmentary plan view of one embodiment of the novel electrophotographic element;

FIG. 2 is a fragmentary cross-sectional View of the recording element illustrated in FIG. 1, taken along the line 2-2 and viewed in the direction indicated by the arrows;

FIG. 3 is a fragmentary plan View of another embodiment of the novel electrophotographic recording element;

FIG. 4 is a fragmentary cross-sectional view of the recording element illustrated in FIG. 3, taken along the line 4-4 and viewed in the direction indicated by the arrows; and

FIG. 5 is a fragmentary cross-sectional View of the novel recording element illustrated in FIG. 3, taken along the line 5 5 and viewed in the direction indicated by the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing, there is shown an electrophotographic recording element 10 of the type commonly employed in the electrophotographic art. The recording element 10 comprises a relatively electrically conductive substrate 12, such as paper or metal foil, and a relatively electrically insulating (in darkness) photoconductive layer 14 adhered to the substrate 12. The photoconductive layer 14 comprises a photoconductor, such as zinc oxide in a suitable resin binder. The photoconductive layer 14 may also be dye sensitized to render the photoconductive layer 14 responsive to light in a desired spectral range.

Recording elements of the type thus far described have a relatively short gray scale in comparison to photographic films and do not, by themselves, reproduce continuous-tone images well in a typical electrophotographic printing system. For example, if the recording element thus far described were uniformly charged with an electrostatic charge and exposed through a photographic, stepwedge gradient, light filter (a transparency of successive steps of light transmission ranging from black, through various shades of gray, to clear) only a few steps would appear on the developed recording element. It is well recogniZed, that a recording element should have the characteristic of a long gray scale to reproduce continuoustone images faithfully.

Means are provided to increase the gray scale of the recording element 10. To this end, the recording element 10 is provided with a coating of a partially light-transmitting material on portions of the photoconductive layer 14 in the pattern of a half-tone screen. In FIGS. 1 and 2, the coating of partially light-transmitting material, in the pattern of one kind of half-tone screen, is shown in a pattern of dots 16. In addition to being partially light-transmitting, the dots 16 possess the characteristic of changing the time of an electrostatic discharge across both it and the underlying photoconductive layer 14, in response to light of a given intensity directed onto the recording element, in comparison to the electrostatic discharge across the photoconductive layer 14 alone. Preferably, the change in the time of electrostatic discharge across both a dot 16 and the underlying photoconductive layer 14, in response to light of a given intensity, should be longer than the time of electrostatic discharge across the photoconductive layer alone.

Examples of materials that are suitable for coating the surface of the photoconductive layer 14 of the recording element 10 in patterns of half-tone screens include milled lithographie inks, as follows:

'Example I (white ink) Anatase titanium dioxide, dry 61 lb. 0 oz.. Alumina hydrate, dry 5 lb. 12 OZ. Long oil alkyd resin, 100% 32 lb. 8 oz. Cobalt linoleate drier, 3% metal 0 lb. l2 oz.

100 lb. 0 oz.

Example II (medium yellow ink) #l Transparent varnish 20 lb. 0 oz. #2 Transparent varnish 4 lb. l2 oz. Transparent Varnish 2 lb. 0 oz. #3 Transparent varnish 56 lb. 8 oz. Gloss white, dry 13 lb. 8 oz. Alumina hydrate, dry l lb. 12 oz.

On last pass over mill add:

#7 Lithographie varnish 1 lb. 8 oz.

100 lb. 0 oz.

Example III (reddish blue ink) #l Lithographie varnish 29 lb. 8 oz. #0 Lithographie varnish 5 lb. 0 oz. #3 Lithographie varnish 1 0 lb. 8 oz. Phosphotungstic lake of methyl violet, dry 16 lb. 8 oz. Bronze blue, dry 25 lb. 8 oz. Alumina hydrate, dry 6 lb. 0 oz. Paste drier 7 lb. 8 oz.

100 lb. 0 oz.

tExarnple lV (white ink) Percent by wt. Titanium dioxide 70 Linseed varnish 30 Cobalt drier Trace `Coatings of these materials in the form of half-tone patterns can be applied to the photoconduct-ive layer 14 of the recording element 10 by lithographic printing techniques. Thus, the aforementioned inks can be printed on the photoeonductive layer 14 from a lithographie plate. The coatings can also be applied to the photoconductive layer 14 by silk screen techniques, well known 1n the art. The thickness of the dried half-tone coating may vary in the range between 0,01 mil and 1 mil, but preferably should be in the order of about 0.1 mil or less. The thickness of the coating should be such that 1t is partially light-transmitting,

While it is desirable that the color of the half-tone coating be substantially similar to that of the photoconduetive layer 14, as for example the white ink of Example I when the photoconduetive layer 14 is white, it has been found that colored lithographie inks in partially lighttransrnitting coatings, such as described in Examples II and II-I, can also be used to produce acceptable halftone images. The aforementioned and other suitable inks are described in the book, Printing and Litho Inks, by Herbert Jay Wolfe, th edition, published by MacNair- Dorland Co., New York, NY`

Regardless of the particular pattern of the half-tone screen coating, it should be a regular or random pattern substantially equivalent to a half-tone screen of between 50 and 250 lines per inch, and the percent of the area of the photoconductive layer 14 covered by the coating may be between and 90%, depending on the images to be copied. The coating of dots 16, for example, may comprise 125 dots per linear inch, regularly or randomly spaced, may cover 50% of the area of the photoconductive layer 14, and may be of any color if the overall appearance of the recording element is not considered objectionable. The particular half-tone screen coating to be used in any particular case may be determined 4 by the criteria considered for the selection of a contact screen in a photographic process.

The novel recording element 10 is adapted for use in a typical electrophotographic process. For example, the surface of the recording element 10, including the photoconductive layer 14 and the coat-ing of dots 16 thereon, is uniformly electrostatically charged negatively in the dark by any electrostatic charging means well known in the art. The uniformly charged surface is exposed to a light image to selectively discharge it and to provide thereon an electrostatic latent image. The latent image is then developed by any suitable electroscopie toner, well known in the art, to provide a visible image of the light image.

In theory, it is believed that since each dot 16 is only partially light-transmitting, only a portion of the light impinging thereon affects the underlying portion of the photoconductive layer 14 on which the dot 16 is coated. Therefore, the portion of the photoconductive layer 14 directly underneath a dot 16 is not rendered as conductive in response to a given intensity of light (impinging on the overlying dot 16) as is a portion of the photoconductive layer 14 not covered by a dot 16. Hence, each dot 16 decreases the response of the photoconductive layer 14 beneath it to light and extends the over-all response of the recording element 10 to a greater range of light intensity from the original image than would be possible without the half-tone pattern of dots 16.

With a coating of partially light-transmitting material in the pattern of a half-tone screen on a recording element, as described herein, it is possible to extend the gray scale of the photoconductive layer 14, and thereby produce a half-tone reproduction with better resolution of gray tones than possible without the use of the coating.

Referring now to FIGS. 3, 4, and 5, there is shown another embodiment of a novel electrophotographic recording element 20 wherein parts similar to those in the recording element 10 are designated by the same reference numerals. The recording element 20 comprises a substrate 12 and a photoconductive layer 14 of the type previously described. A coating of light-transmitting material on the photoconduetive layer 14 comprises a halftone screen in the pattern of two sets of parallel strips intersecting each other at an angle` Thus, for example, one set of strips 22 is disposed at right angles to the other set of strips 24. Each of the strips 22 and 24 is between 0.01 mil and 1 mil in thickness (in a direction normal to the surface of the photoconductive layer 14) and can comprise a light-transmitting material such as described supra in Examples I, II, III, and IV. The strips 22 and 24 provide a pattern of a half-tone screen that may be substantially equivalent to between 50 and 250 lines per inch, and may cover an area of between 10% and 90% of the photoconductive layer 14` In coating the strips on the surface of the photoconductive layer 14, one of the sets of strips, such as the set of strips 24, is coated first and the other set of strips 22 is coated over the strips 24. Thus, the thickness of the cross-over portions, hereinafter called points 26, Where the strips 22 and 24 cross each other is relatively greater than the thickness of any one of the strips (measured in a direction normal to the major surface of the photoconductive layer 14). With this arrangement, the cross-over points 26 of the strips 22 and 24 are less light-transmitting than the other portions of the strips 22 and 24 and exclude more light from an image to their underlying portions of the photoconductive layer 14 than do the single strips 22 and 24 alone. For example, let it be assumed that the surface of the recording element 20 is uniformly charged negatively with an electrostatic charge, -Let it also be assumed that a given intensity of light just completely discharges the unscreened (noncoated) photoconductive layer 14. Since the strips 22 and 24 are only partially light-transmitting, the same quantity of light that just discharges the unscreened portions of the photoconductive layer 14 does not discharge the underlying portions of the photoconductive layer 14 beneath the coated strips 22 and 24. Hence, the coating on these latter portions still retain a portion of the electrostatic charge and can attract electroscopic developer.

Again, let it be assumed that a given intensity of light just completely discharges the. electrostatic charge on a single thickness of either of the strips 22 or 24. This latter quantity of light does not discharge the electrostatic charge at the cross-over points 26 because the light-transmitting characteristic of the cross-over points 26 is less than that of a single strip 22 or 24. Hence, the coating of the cross-over points 26 retains an electrostatic charge when the electrostatic charge on the other portions of the coating of the strips 22 and 24 is just completely discharged. Thus, the half-tone screen coated recording element 20 is responsive to a greater range of light intensities than a non-coated recording element, thereby effectively extending the gray scale of the coated recording element for providing developed prints of improved continuous-tone in an electrophotographic process.

What is claimed is:

1. In an electrophotographic recording element of the type comprising a substrate and a photoconductive layer of a substantially uniform thickness thereon, said recording element being adapted to be rst electrostatically charged with a uniform charge and then to be selectively discharged by exposure to a light image, the improvement comprising:

optical lter means permanently adhered to selected areas of said layer, opposite said substrate, rendering said selected areas less sensitive to light impinging on said recording element, said selected areas being in the pattern of a half-tone screen,

said optical lter means comprising a coating of a partially light-transmitting ink having a thickness of between 0.01 and 1.0 mil and a characteristic such that the time of electrostatic discharge of said um` form charge through both said coating and said layer,

in response to light of a given intensity, is different 40 in comparison to the time of discharge of said uniform charge through said layer only, in response to said light of given intensity,

said coating comprising a tirst plurality of substantially parallel strips disposed in one direction, and a second plurality of substantially parallel strips disposed at an angle of substantially 90 to said one direction and crossing said rst plurality of strips in a regular pattern substantially equivalent to a half-toned screen of between and 250 lines per inch and covering between 10% and 90% of one major surface of said layer, and

the combined thickness of said irst and said second plurality of strips at their cross-over points being greater than the thickness of any one of said strips alone, whereby said cross-over points are less light transmitting than any one of said strips alone, so that said photoconductive layer of substantially uniform thickness may be exposed, through said regular pattern, by light of three different intensities, in response to light of a given intensity impinging on said recording element.

References Cited UNITED STATES PATENTS CHARLES E. VAN HORN, Primary Examiner U.S. Cl. X.R.

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