Note: Descriptions are shown in the official language in which they were submitted.
1~)71~:?0~
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
--2--
: ,
~'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
--10--
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
1071004
;.
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
-12-
