CA1071004A - Xeroradiographic plate with coating of charge conductive metal on margin edge - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Xerographic plates, particularly xeroradiographic plates, suitable for soft or hard x-ray exposure require margins or edges suitable for handling and mounting. In order to fabricate such plates, it is convenient to apply a peripheral mask over the substrate during application of at least the photo-conductive layer. Use of masks, however, tend to promote peripheral plate irregularities, usually because of scratches or other uneveness at or under the margin of the applied mask.
Such imperfections spawn corresponding electrical field irregularities which can now be minimized or avoided altogether by grounding the margins with metal overcoats.

Description

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sAcKGRouND OF THE INVENTION
This invention relates to xeroradiographic plates and to method for preparing such plates while maintaining the high degree of diagnostic accuracy, sensitivity, speed and convenience S necessarily associated with modern medical and industrial usage.
In xerography as originally disclosed by Carlson, U.S.
Patent 2,297,691, there is employed a member sensitive to activating radiation such as light or photon-type radiation and generally comprising a photoconductive insulating layer disposed on a conductive backing member. Electrostatic latent images are formed on the latent member by selective conduction or dissipation of an electrostatic charge by the action of activating radiation such as a light or optical image either of the visible or invisible spectra on the photoconductive layer. Usually this is accomplished by placing a uniform electrostatic charge on the layer and exposing the charged layer to an optical image, whereby the layer becomes selectively conductive in the activated area.
Because of a relatively more efficient utilization of x-rays by photoconductive layers as compared with corresponding silver halide-based elements (absent fluorescent screens) and the inherent speed and convenience of dry development, it is much to be desired that xerographic principles be utilized for medical diagnostic purposes. Unfortunately, however, this highly specialized use does not permit avoidance of a number of funde-metal problems faced in general xerography and raises a few additional ones as well. By way of example, general radiation-sensitive members must support an electrostatic charge for a much longer time than required by high speed copiers. A small dark discharge, however, must be coupled with a comparatively high conductivity upon exposure so that the plate charge can be rapidly : , ~'7~0~

dissipated; so great is the need for both micro and macro differentiation that the charge in a radiation-struck area should approximate a zero potential. A still further problem relates to the necessity for maintaining very careful control over the continuity, integrity and thickness of the various plate elements such as the substrate, charge blocking and photoconductive layers, so that highly desired characteristics such as a diagnostic level of resolution will be consistently maintainable. In short, it is desirable that the plate variables be limited primarily to mathematical functions of the amount of x-ray tissue penetration.
In preparing such plates it is necessary to apply a number of very thin, precisely applied coats while maintaining a very high degree of purity in a carefully controlled environment.
In addition to the usual fabrication problems, however, it is found that the masks or patterns needed to precisely lay down photoconductive material and to maintain suitable margins around the edges of the finished xeroradiographic plate are respon-sible for some defects. In particular, it has been found that particles of dust and particularly scratches on the inside face of a mask or pattern permits the formation of very small irregularities such as pimples, ridges or other projections in the photoconductive layer bordering or within the plate margins. Normally, the presence of such small defects and the resulting irregular field currents are not ascertainable until the finished plate has been test charged and developed.
It is an object of the present invention to minimize or a~oid the electronic effect of such marginal imperfections in a finished xeroradiographic plate so as to render the plate suitable for its intended purpose.

, 1~1004 . .
THE INVENTION
This invention encompasses margin metallized xero-radiographic plates and the process for minimizing undesired anomalous electrical field patterns in the plates due primarily ' 5 to peripheral imperfections in the photoconductor layer and/or overlaying coats.
'~ Plates of the type here contemplated best comprise a metal or metal clad substrate, a charge blocking layer, and at least one photoconductive layer applied thereto. The photocon-ductive layer being locally grounded along the plate periphery by a metal charge conductive layer applied essentially over the margin and edge area of the plate.
The present invention also relates to a method for minimizing electrical field patterns leading to undesirable printout aæsociated with edge and margin plate surface irregularities by applying to the margins and edges of the plate a coating of charge-conductive metal.
For such purpose, it is found that almost any charge-conductive metal can be used if applied evenly and is reasonably adherent to the margin area of the finished plate as a film or pattern. It is preferred, however, if the charge-conductive metal is at least one of an aluminum, aluminum alloy, nickel, silver, silver alloy, copper, steel or brass. Of particular use are high purity metals such as aluminum alloys which can be obtained commercially and which can be easily vacuum coated onto the margin area of the finished or partly finished plate. If desired, however, the charge-conductive metal can also be one which is amenable to painting, silk screening or ink printing, etc., and still fall within the scope of the present invention.
Suitable xeroradiographic plates within the present ~07~CIV4 ~IL p~ftla, I
invention are schematically shown in~cross-section in Figures 1 and 2, Fi~ure 2 being essentially a blown-up, partial cross-qs ,~
section ~x~ Figure 1.
In the Figures, element 11 represents a suitable xeroradiographic substrate, inclusive of aluminum, aluminum-clad materials such as a plastic or less pure metals, nickel, brass, stainless steel, tantalum, magnesium, molybdenum or combinations thereof.
Element 12 represents a suitable xerographic charge-blocking layer such as an organic dielectric material or a metal oxide layer.
Where the substrate metal is a spontaneous oxide former such as aluminum, it is found useful to degrease and to remove existing porous surface oxides by the use of solvent baths and/or commerical caustic cleaners prior to application of a thin uniform dielectric blocking layer. These coats can usefully include an oxide coat such as aluminum oxide (25 - 200 Angstrom) or one or more polymeric dielectric coats (about .1 - 2~). Both types are described in U.S. Patent 2,901,348. Suitable organic charge-blocking coats can include, for instance, a polybenzimidazole, a polyester, a polyurethane, a polycarbonate or an epoxy resin.
Organic dielectric charge blocking material as described above, can be applied onto a base or substrate by solution casting or other art-recognized techniques and the corresponding metal oxide layers are conveniently applied, for instance, by thermal oxidation, anodic oxidation or by glow discharge under a partial atmospheric pressure.
This latter step is best carried out, for instance, by evacuating a suitably modified vacuum coater down to a pressure of about 5 x 10-5 Torr and then backfilling with up to about 30 . .

~0~004 microns of air. A pressure of about 5 - 20 microns is generally preferred for this purpose, depending upon the gases utilized.
While air under reduced pressure is acceptable, it is also found convenient, on occasion, to utilize various alternative mixtures of inert ion producing and oxidizing gases at comparable pressures. Such include, for instance, argon-oxygen, argon-air, argon-C02, or a mixture of nitrogen and oxygen, etc. In each case, however, the amount of available oxygen for initial oxidation of the substrate should not be less than about 1% by volume of the available gases, and a glow discharge must be maintainable.
Maintenance of a satisfactory glow discharge for purposes of effecting an initial ion bombardment and oxidation of the substrate can be satisfactorily effected for purposes of the present invention under a DC field at a potential ranging from about 1500 to about 3500 volts and a cathode current density of about .05 - .5 ma/cm2, depending upon the type and pressure of gas used to form the ions. Alternatively, a low frequency AC
glow discharge of about 60 - 400 cycles, a potential of about 500 to about 1400 volts and a substantially reduced current density of about .01 - .15 ma/cm2 is also found to be sufficient.
Element 13 of Figures 1 and 2 represent one or more photoconductive layers, preferably although not exclusively, inorganic in nature. Of particular interest are ambipolar ionizable photoconductive material. Such as exemplified, for instance, by selenium and corresponding alloys thereof with arsenic, tellurium, germanium, antimony, bismuth, and/or one or more halogens such as chlorine, bromine, or iodine. Such photoconductive materials are obtainable by subjecting selenium, plus small amounts of one or more of the above alloy elements and/or a halogen to ~071004 heat in a sealed container. Such a layer can conveniently, although not exclusively, range from about 100 to 350 microns in thickness, depending upon the relative "hardness" of the radiation to be used in exposing the plate. This, in turn, depends primarily upon the depth and nature of the area being diagnosed and the relative sensitivity of the organs to be exposed to radiation generally.
The photoconductive layer indicated supra, is conveniently applied in several different ways, the preferred method being by vapor deposition in a coater at about 5 x 10 5 Torr.
Prior to completion of a period of time sufficient to form an oxide barrier layer of about 10 - 200 Angstrom thickness on the substrate, and assuming that the substrate has been brought A up to a suitable temperature (55 - 60C ) by ion bombardment or otherwise~the margin-masked oxide-bearing substrate can also be simultaneously exposed to a vapor cloud of charged and uncharged photoconductor particles evolved from a heated photoconductive source by introducing the vapor into and adjacent to an area of a DC glow discharge or into a low frequency AC glow discharge.
Under the latter conditions, it has been found that both negative and posi~ive high energy ions of the ambipolar photoconductive material are formed in good yield under conditions favoring efficient deposition onto the substrate electrode.
As a practical matter, the described photoconductive layer deposition is conveniently accomplished, when desired, by increasing the amount of vacuum to 5 x 10-5 Torr and then back-filling the coating chamber with up to about 5 - 30 microns of argon, nitrogen, xenon or similar glow discharge maintaining inert gas. This technique effectively reduces the relative concen-tration of oxygen and assures adequate displacement of the more . .

lQ'~004 loosely adhering photoconductive material.
Since the chief advantage of depositing ionized photo-conductor material on the metallic substrate relates to improved durability and adhesion under flex, a relatively thin deposition S is very adequate for plates. Generally speaking, about 1 - 10%
of the total thickness is sufficient but not limiting, with the balance of the deposition completed by conventional vacuum deposition techniques at about 5 x 10-5 Torr. As previously indicated, either all or none of the photoconductive layer or the blocking layer need be deposited by means of a glow discharge process.
In Figures 1 and 2, a marginally located imperfection in the photoconductive layer of a plate is demonstrated as a small projection or tip (#16) which is most generally attributed to a scratch, warpage or dent in the applied mask (not shown) during the photoconductor coating operation. Such warpage, etc.
~?ermits vaporized photoconductive material to migrate or infiltrate beneath and along the edge of the mask.
Element 14 represents an optionally applied protective overcoating which is shown to be affixed to the xeroradiographic plate prior to initial testing and metallization (#15). This material can be applied by spraying or other conventional means and is usefully present in a thickness of about 1000 - 5000 Angstrom, and must be capable of permitting charge and/or hole migration depending upon the nature of the intended initial sur-face charge to be imaged on the plate. Such material is optionally a conductive, moisture resistant organic polymeric material.
Suitable material of this type is desired, for instance, by Gerace et al in Canadian Patent 938,143, filed January 22, 1971, and including coating compositions containing (a) about 1% to 71%

10710~4 by weight of a moisture insensitive, film-forming organic solvent soluble resin, (b) from about 16% to 71% polyester and (c) from about 5% to 35% polyurethane having a volume resistivity of from 1011 to 1013 ohm-cm.
The moisture insensitive, organic solvent soluble film forming resins desired can include, for instance, most thermo-plastic resins. Typically effective resins include polyvinyl chloride, polyvinyl fluoride, polyvinylidene chloride, poly-isobutylene and copolymers thereof. Preferred components include vinyl copolymers and copolymers of vinylidene chloride and acrylonitrile.
Film forming polyesters which can be used in the three , component overcoat c~mposition include a film forming polyester such as prepared in the conventional manner by reaction of an anhydride or diacidchloride with a diol in the presence of a catalyst. Typical of this group of resins are the polyterephtha-lates of either ethylene glycol or 1,4-bis-(hydroxymethyl) cyclohexane; polycarbonates such as polybisphenol-A-carbonate;
and polyadipates of ethylene, propylene and butylene glycols.
The third or polyurethane resin component to be used in the overcoat includes highly cross-linked polyurethanes having a volume resistivity of from about 1011 to 1013 ohm-cm and prepared by the basic reaction of an isocyanate with an alcohol or an ester.
Typical of such polyurethanes are those prepared by the reaction of toluene diisocyanate or diphenylmethane-4,4'-diisocyanate with any alcohol.
~A Element 15 schematically represents a grounding charge-conductive metal layer applied onto the margin and edge surfaces of the plate.

_g_ -. . : .. . .~ . . . ..

~a~Q~4 As a practical matter, the metallized layer (15) over-laps or at least contacts the free margin and edge of the plate along the locale of the imperfection(s). This is done whether or not a polymeric overcoat (14) has been previously applied and whether or not the charge blocking layer in the vicinity of the imperfection has been spontaneously formed or otherwise applied as described above.
In order to metal coat the margin areas of defective plates of the type described, it is found most convenient to affix a stainless steel or similar mask over the active (image receiving) part of the plate and thereafter paint, spray or vacuum coat the metal layer on the exposed margin area(s). The metal coating can vary substantially depending upon relative conductivity, flexibility and adhesion to the underlying layers. Generally speaking, however, a coating of about 2000 - 5000 Angstrom is found to be sufficient.
As previously noted, the charge-conductive metal used can vary in type provided it is compatible with basic xerographic functions, is sufficiently durable, and will adhere to the top and edge of the plate.
The following example specifically demonstrates preferred embodiments of the present invention with limiting it thereby.
EXAMPLE I
Six 9" x 12" 9 mil sheets of #1175 aluminum alloy are degreased in a trichloroethylene bath, water rinsed, immersed for 10 minutes at 60C. in a dilute caustic solution, water washed, immersed for about 1/2 minute in concentrated nitric acid, and then rinsed and washed for about 30 minutes in deionized water.
The resulting sheets, identified as S-l through S-6, are mounted in a vacuum coater on a grounded six-sided rotatable mandrel about 71~4 15" above a floating ~-heating stainless steel crucible containing a photoconductiue- selenium alloy consisting essentially of about 99.5% selenium, .5% arsenic, and about 10 ppm of chlorine. A
high voltage glow bar (5000 v) is stationary mounted about 10"
from the axis of the mandrel in the 10 o'clock position relative to the mandrel axis and directed towards the mandrel and mounted sheets, a hollow stainless steel rectangular-shaped mask being bolted over each sheet to form the plate margins.
After evacuating the coater to 5 x 10-5 Torr and back-filling the coater with 20 micron (Hg) air~pressure, negative 3000 volts is applied to the glow bar for about 10 minutes to heat and uniformly oxidize the three test sheets. The glow bar is then turned off, coater pressure once more lowered to about 5 x 10 5 Torr and the crucible heated up to 280C. and retained at this temperature for about 80 minutes to obtain photoconductor coatings .i~out 300 microns in thickness. During both steps, the mandrel is constantly rotated at 10 revolutions per minute to obtain uniform exposure of the three test sheets. The resulting plates are then cooled and removed from the coater. The rectangular-shaped marginal masks are then removed from the three plates, S-l through S-3, then dip-coated into a three component polymeric resin com-position prepared in accordance with Example 1 of Canadian Patent 938,143 and dried. All six plates are then placed in a xerographic Model D Machine, charged to 800 volts, and exposed to a standard pattern. The developed copies are then checked for characteristic printout irregularities along the margins. Plates S-l, S-4 and S-6 produced four or more irregularities. Plates S-l and S-4 are then remounted inside a vacuum coater, the margin areas being covered with stainless steel masks, and then exposed for 1 minute in a vacuum coater at 5 x 10 5 Torr on a water cooled plate above ;.
a shuttered resistance heated insulated crucible charged with pure ,' aluminum and operating at about 1800C. The metallizing step is repeated with sample S-6, except that pure silver is used as a crucible charge, and the crucible is operated at about 1000C.
Plates S-l, S-4 and S-6 are then once more charged, exposed and developed on the Model D Machine, and the resulting positive prints evaluated and repeated in Table I below:

TABLE_I
SamPlePolYmeric Overaoat Metal Coat Plate Quality*
; S-l yes aluminum Ex S-4 no aluminum VG
.~-6 no silver VG

*EX - none of original marginal irregularities noted in positive print VG - one original marginal irregularity noted but substantially reduced in size G - all original marginal irregularities noted but sub-stantially reduced in size

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for minimizing electrical field patterns leading to undesirable xerographic printout associated with edge and margin surface irregularities in xerographic plates coincident with the charging, exposure and development of a xeroradiographic plate having a charge conductive substrate, a charge blocking layer and at least one photoconductor layer exhibiting such marginal irregularities, comprising applying to the margin and corresponding edge of said plate a coating of charge-conductive metal.

2. A method of Claim 1 wherein the charge conductive metal is at least one of aluminum, aluminum alloy, silver, silver alloy, copper, steel or brass coating.

3. A method of Claim 1 wherein the applied charge-conductive metal is aluminum or an aluminum alloy and is applied to at least one plate margin.

4. A method of Claim 1 wherein the applied charge-conductive metal is silver or a silver alloy and is applied to at least one plate margin.

5. The method of Claim 1 wherein the charge-conductive metal is applied by vacuum coating.

6. The method of Claim 1 wherein the charge-conductive metal is applied over a thin charge- or hole-conductive organic overcoat.

7. The method of Claim 6 wherein the overcoat com-prises polyurethane.

8. A xeroradiographic plate comprising in combination, a metal or metal clad charge-conducting substrate, a charge blocking layer and at least one photoconductive layer applied thereto, said photoconductive layer being locally grounded along the plate periphery by a metal charge-conductive layer applied essentially over the upper margin and edge areas of the photo-conductive layer side of the plate.

9. A xeroradiographic plate of Claim 8 wherein the peripheral metal charge-conductive layer comprises aluminum.

10. A xeroradiographic plate of Claim 8 wherein the peripheral metal charge-conductive layer comprises silver.

11. A xeroradiographic plate of Claim 8 wherein a polymeric charge-conductive overcoat is interposed over the photoconductive layer and beneath at least part of the peripheral metal charge-conductive layer.

12. A xeroradiographic plate of Claim 8 wherein the photoconductive layer has a thickness of about 100µ- 350µ and comprises a selenium alloy.

13. A xeroradiographic plate of Claim 9 wherein the photoconductive layer comprises a selenium/arsenic alloy doped with a halogen.