US 3092493 A
Description (OCR text may contain errors)
June 4, 1963 c. B. KAISER 3,
PROTECTED XEROGRAPHIC PLATE Filed Feb. 2, 1961 FIG. 1
23 5 I I I v 1 2 20 FIG? INVENTOR. CARL B. KAISER BY 52 ,m
' ATTORNEY United States Patent 3,092,493 PROTECTED XEROGRAPHIC PLATE Carl B. Kaiser, Rochester, N.Y., asslgnor to Xerox Corporation, a corporation of New York Filed Feb. 2, 1961, Ser. No. 86,709 2 Claims. (Cl. 96-1) This invention relates to xerography and in particular to an improved xerographic plate for use therein. More specifically the invention relates to an improved xerographic plate having a vitreous selenium photoconductor with an inorganic protective overcoating.
The art of xerography involves the utilization of a xerographic plate .whose essential elements are a layer of photoconductive insulating material in electrically coupled relation with respect to an electroconductive backing member. The term photoconductive insulating material is applied to those substances which have the special property of being very high in insulating value when not exposed to light or other activating radiation but become conductive when exposed to activating radiation. The photoconductive insulating material being essentially non-conductive in the absence of activating radiation can have an electrostatic charge imposed thereon by means of apparatus known in the art. The imposed voltage is largely retained for a substantial period of time if the plate remains unexposed to activating radiation. Usually a potential is imposed on the exposed surface of the layer of photoconductive insulating material which may be of the magnitude of 300 to 500 volts, or even higher.
Of the various photoconductive insulating materials known to the art, vitreous selenium and alloys of vitreous selenium that include small but significant amounts of additional material including arsenic, arsenic trisulfide and tellurium have been found to be preferred for commercial applications of xerography. Photoconductive insulating materials having a predominance of vitreous selenium have a hard, vitreous appearance and may be used thousands of times in the xerographic process. Nevertheless, under the extrimely exacting conditions of the repetitive developing, cleaning and other operations of the automatic xerographic process as disclosed in Carlson Patent 2,357,809, the useful life of a selenium xerographic plate is limited.
Preservation and protection of the selenium layer has been achieved by coating the selenium with a thin but durable transparent layer of an inorganic insulating material as disclosed in Owens Patent 2,886,434 which disclosure is incorporated herein. Despite the use of such a protectivelayer, however, overcoatediplates have not enjoyed commercial utility for reasons as will become apparent from the discussion below.
In the Owens patent there is illustrated a xerographic plate being comprised of a conductive backing member supporting a photoconductivev insulating layer that is covered with a durable transparent layer as aforesaid. The three layers are illustrated as being coextensive and arranged in sandwich form. As a practical matter, however, commercial xerographic plates, whether they are fiat in the manner illustrated in Owens or cylindrical, such as commonly utilized in commercial xerographic machines, the photoconductive insulating layer is seldom if ever completely coextensive with the conductive sup port member. Instead the photoconductive insulating layer usually occupies an area less than the whole area of the backing member which may be of metal such as brass, aluminum, etc. such that there remains bare uncoated portions of the backing member adjacent the photoconductive insulating layer. These bare portions are designated as margins which are not of themselves 3,092,493 Patented June 4, 1963 utilized in the xerographic process but have practical value for reasons described below. According to teachings of Owens above cited the durable protective layer is applied substantially coextensive with the photoconductive layer, the same mask being utilized to deposit both photoconductor and overcoating. This has been found to adequately preserve the photoconductive insulating layer from mechanical wear and tear but under certain ambient conditions, particularly at high relative humidities, these margin-bearing overcoated plates, when charged, suffer from localized defects such that they are rendered unable to fully retain a developable electrostatic charge in image configuration. Developed images in these defective areas are blurry in contrast to the sharp delineation of light and shadow reproduction as is conventional in xerography. This problem does not occur in the absence of an overcoating, but with the overcoating has been found to manifest itself as poor quality plates that become progressively worse with usage occurring primarily along the edges where the protective layer and photoconductive insulating layer border the bare plate margins of the conductive backing member. Although not fully understood, the phenomenon believed causing this defect is regarded as a form of lateral conductivity within the photoconductive insulating layer owing to the absorption of moisture on the surface at high relative humidities and which is believed to precipitate an electrolytic reaction.
The following examples will more clearly illustrate the problem.
A. A flat xerographic plate was prepared in accordance with the prior art on brass substrate with vitreous selenium as a photoconductor which was half overcoated with zinc sulfide following the process disclosed by Owens. Pools of water were placed in the center of the zinc sulfide overcoating and into three pools strips of copper, aluminum, and brass were dipped separately with the opposite ends being connected to brass plates. The xerographic plate was allowed to stand in this position for two hours, then dried off. Thereafter the xerographic plate was utilized for printing by conventional xerographic techniques. The copy was excellent with the exception of the areas where the metal strips were immersed in the water pools. The area where aluminum was immersed showed only slight blurring, whereas the immersion areas of brass and copper showed sufficient blurring to in effect obliterate the image.
B. A flat xerographic plate in accordance with the prior art was prepared on a brass substrate with vitreous selenium as the photoconductor half overcoated with zinc sulfide. Four droplets of tap water were deposited on the plate at selective locations as follows: (1) On the bordering edge overlapping the coated and non-coated selenium; (2) On the zinc sulfide coating only; (3) On the bordering edge overlapping the zinc sulfide coating and the bare brassmargin; and -(4) Extending over the margin area from the zinc sulfide coating until running over the adjacent edge of the substrate.
After 2% hours the water was siphoned oil and the plate blotted dry. Thereafter the plate was utilized for printing by conventional xerographic techniques. The water at areas (1) and (2) produced no perceptible adverse efiects whereas area (3) developed a blurred image and area (4) developed an image increasingly blurred over the image at area (3 C. A fiat xerographic plate in accordance with the prior art consisting of a brass substrate supporting a vitreous selenium photoconductive layer overcoated with zinc sulfide was utilized. Under controlled humidity conditions in excess of 45% relative humidity (R.H.), and 70 F. dry bulb the plate was repeatedly dustedand brushed with a rabbit for that overlapped the zinc sulfide coating and cent the margin.
D. Three xerographic plates similar to that described in Example C were stored for 19 hours in a humidity controlled atmosphere maintained at approximately 78 F. dry bulb and 80% RJH- During storage one plate was heated well above the dew point temperature of the ambient air to 90 F. Thereafter each plate was brushed approximately a thousand times with a rabbit for under ambient conditions of 30% RH- and 72 F. After brushing the plates were utilized for printing by conventional xerographie techniques. The heated plate produced good prints whereas the others produced blurry prints.
These attending defects are overcome by means of a xerographic plate constructed in accordance with the invention.
It is therefore the principal object of the invention to provide an improved xerographic plate for use in xerography.
It is a further object of the invention to provide an improved xerographic plate having a surface-protective film on the photoconductive layer with margins of non-photoconductive material adjacent thereto.
A still further object of the invention is to provide an improved xerographic plate having margins adjacent the.
protective coated photoconductive layer and which is not susceptible to the attending defects associated with plates of the prior art.
It is still a further object of the invention to provide a surface-protected xerographic plate with photoconduetor edge protection from defects arising during operation under conditions of high relative humidity.
These and other objects of the invention are attained by means of the invention in which the bordering edges of the photoconductive layer on the margins of the xerographic plate are protected with a moisture seal. The seal renders the bordering metals unafiected by moisture such that electrolytic action experienced heretofore is wholly prevented.
Embodiments of xerographic plates constructed in accordance with the invention are illustrated in the drawings in which:
FIG. 1 is a schematic sectional view through a xerographic plate constructed in accordance with one embodiment of the invention; and
FIG. 2 is a schematic sectional view through a xerographic plate construtxed in accordance with a second embodiment of the invention.
Referring now to the drawings there is illustrated in FIG. 1 a xerographic plate designated constructed in accordance with the invention. As shown the xerographic plate comprises a conductive backing member 11 which conveniently may be of brass, aluminum, or the like in rigid or flexible form for supporting a layer 12 of photoconductive insulating material, such as vitreous selenium,
and cover by a thin transparent layer 13 of durable inorganic insulating material of the types disclosed in the Owens patent above-cited. Margin areas 14 are provided on each side of photoconductive layer 12. In accordance with the invention, layer 13 extends over the borders of the photoconductive layer 12 into the margin areas a sulficient distance to establish an ambient vapor barrier where the edges of layer 12 border the margin areas of layer 11 and in the illustrated embodiment extends over the margins and coterminous with the edges of backing member 11. It is to be understood however, that layers 11 and 13 need not necessarily be coterminous and that it is essential only that there be hygroscopic isolation between layers 11, 12 and 13 which, because of the physical relationship between these layers, is required only along the bordering edges of layers 11 and 12. Layer 13 may desirably be zinc sulfide, although other insulating materials as dis- 4 closed in the Owens patent and described below which possess the necessary electrical characteristics in combination with the required durability may be utilized in the alternative.
Referring to FIG. 2 there is illustrated a xerographic plate designated 20 constructed in accordance with the invention. As illustrated herein the plate is comprised of a conductive backing member 21 being partially re cessed along the edges of its uppermost surface to accommodate a layer of inert insulating material 24 which for example may be polyethylene terephthalate, or simi lar material. A photoconductive vitreous selenium layer 22 is supported on member 21 with its edges overlapping the backing member onto the insulating material 24-. The zinc sulfide overcoating 23 is supported totally over the photoconductive insulating layer and extendsdownwardp ly at the edges bordering margins 25 to the insulating material 24 to form a vapor tight seal thereover.
In each of the embodiments illustrated a plate with an inorganic overcoating is constructed in a manner to prevent a simultaneous moisture contact of the three layers thereby preventing the electrolytic action or other phenomenon destroying the utility of these plates for commercial xerography. Plates constructed in the manner illustrated have been found suitable for overcoming the blurring defects occurring at high ambient relative humidities as associated with plates of the prior art. Plates constructed in accordance with the invention are unaffected by moisture and/or brushing under conditions of high ambient humidity.
For purposes of illustration the layers in each of the above embodiments are shown disproportionately large in relation to their true dimensions. As is known to those familiar with the xerographic art, thickness of the photoconductive layer on a commercial xerographic plate usually ranges between 10 and 200 micronsdepending on the particular application for which the plate is to be employed. When utilizing a zinc sulfide layer as the protective coating it has been found preferable to use overcoatings of approximately Ms micron to 2% microns thick.
As stated above the margins have practical value in that the photoconductive layer need not be handled in the manipulation of the plate required to carry out the xerographic process. The margins have practical utility for example, by providing areas for the mounting of insulalating support rails for supporting a light-tight cassette in the manner disclosed in Walkup Patent 2,588,675. By this means a plate bearing a charge may conveniently be transported in a lighted room without dissipation of charge and also to provide a recess for the photoconductive insulating layer to prevent inadvertent damage thereto. In another instance the margins are beneficial in the cleaning operation which may conveniently employ a rotating cylindrical brush in contact with the plate. The brush is usually of greater width than the photoconductive layer but of less width than the width of the substrate. By having margins brush width is not critical and the entire photoconductive layer is assured of complete brushing before reuse without a dribbling of brushed developer over the side of the plate from the unsupported ends of the brush.
Whereas zinc sulfide has been stressed as being a preferred material for overcoating, other premium overcoating materials may be used which are disclosed in the above'cited Owens patent and which include silica, titania, various silicates, alkaline earth fluorides or others having the properties of being hard and resistant to wear, easily cleaned, electrically insulating to retain electrostatic changes, transparent to activating radiation in the thickness used and that may be deposited in intimate contact with a photoconductive insulator comprising predominantly vitreous selenium. In addition to overcoatings disclosed in the Owens patent, an overcoating of zinc sulfide incorporating cadmium sulfide has been found suitable. Any of these overcoatings materials alone or in combination with other insulating and vapor sealing materials when used in accordance with the invention have been found to give xerographic plates commercial utility not heretofore possible.
Although the invention has been described primarily in connection with a protective overcoating on xerographic plates having a single photoconductive insulating layer, it is to be understood that the invention is applicable to xerographic plates having dual photoconductive insulating layers of the type disclosed in Paris Patent US. 2,803,541. Plates constructed in accordance with the Paris patent employ a second photoconductive insulator such as a mixture of selenium and tellurium in vitreous form coated on a first photoconductive insulator predominantly of vitreous selenium. They are characterized by relatively low dark decay and absence of fatigue in combination with increased photoconductive speed and substantially improved sensitivity to red, yellow and green light. However, as stated therein, the top second layer usually ranges in thickness between 0.1 and microns and because of this relative thinness the slightest wearing destroys the eifective advantages of the dual layer plate. Consequently, it is desired to protect the plate with an overcoating protective material of the type described herein and adapted in the manner of the invention. Useful xerographic plates having the same dual layered structure may be constructed with arsenic substituted for the tellurium.
Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter 6 contained in the drawings shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
1. A xerographic plate having a protective overcoating comprising:
(a) a metallic substrate,
(b) a photoconductive insulating layer comprising vitreous selenium overlying a central area of said substrate in between margin areas,
(0) an inert electrically insulating material overlying said margin areas, and
(d) a thin uniform film of insulating material comprising zinc sulfide with a thickness of A to 2% microns overlying said photoconductive layer and overlapping the common boundaries of said photoconductive layer and said inert electrically insulating material without contacting said metallic substrate so that said inert electrically insulating material prevents any reaction between said metallic substrate and said zinc sulfide.
2. A xerographic plate according to claim 1 in which said margin areas are partially recessed with respect to the central area of said substrate.
References Cited in the file of this patent Fieser et al.: Organic Chemistry, 3rd edition,
p Reinhold (1956).