Note: Descriptions are shown in the official language in which they were submitted.
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IMAGING PROCESS
This invention relates to a xerographic imaging process and
apparatus and, more particularly, to a process and apparatus for uniformly
charging a photosensitive member which is overcoated with an electrically
5insulating layer.
For many years during the development of the xerographic process,
the need for protecting the photosensitive layer has been appreciated. ~qany
proposals have been made to protect the photosensitive layer by coating the
layer with various materials. By coatin~ the photosensitive layer, the wear
10occasioned by repeated use is absorbed by a tough, polymeric surface rather
than the more delicate, expensive photosensitive material itself. One example
of such an attempt to protect the photosensitive layer in the xerographic
process is described by Blakney et al. in U.S. Patent No. 3,041,167 wherein the
problem of charge build-up in the photoreceptor was observed. An electrically
15insulating, protective overcoating on a photosensitive layer was utilized to
support the electrostatic latent image in a process disclosed in U.S. Patent No.3,234,019 to Hall. In this process, as in the Blakney et al. process, an initialcharge of one polarity is provided on the photoreceptor followed by a second
charge of opposite polarity in order to establish an electrical field across the20photoreceptor. While one may calculate voltages to be utilized in order to
arrive at a condilion in the photoreceptor wherein the electrical field is
entirely across the photoreceptor and none is across the bonded, electrically
insulating protective coating, such condition is difficult to achieve in practice.
Actually, there is always a small imbalance of charges resulting in a small
25charge residing on the surface of the electrically insulating overcoating which
charge may build up with repeated use of the photoreceptor and ultimately
interfere with the quality of images provided by the process.
Other examples of photoreceptors having protective, electrically
insulating coatings over the photosensitive layers include U.S. 3,895,943 and
30U.S. 3,904,409 to lIanada wherein the electrically insulating overcoating is
utilized in conjunction with persistent internal polarization and contains the
problem of charge build-up on the surface of the electrically insulating layer.
As a practical matter, it ls very difficult to operate a process wherein said
layer is returned, at the beginning of each imaging cycle, to zero voltage.
35Small variations in the operation of the DC corotrons particularly utilized inthe prior art to provide the sequential charging steps make the condition of
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zero voltage on the electrically insulating layer
nearly impossible with ordinary equipment.
It is an object of an aspect of this invention to
provide an imaging process utilizing a photoreceptor
containing an electrically insulating protective layer
over the photosensitive layer wherein charge build-up
on the electrically insulating layer is prevented.
An object of an aspect of this invention is to
simplify the operation of xerographic processes
utilizing a photoconductive element containing an
electrically insulating protective layer over the
photosensitive layer.
An ob~ect of an aspect of this invention is to
provide an apparatus having a reduced number of
charging units required to establish an electric field
across the photosensitive layer having bonded thereto
an electrically insulating, protective coating.
In accordance with an aspect of this invention,
there is provided an apparatus and process wherein an
electric field is provided in a single charging step
across the photosensitive layer of a photoconductive
member having an electrically insulating protective
layer over the photosensitive layer, which step
eliminates or prevents the build-up of electrical
charge on the surface of the electrically insulating
layer. Briefly, the process of an aspect of this
invention -comprises charging such a photoreceptor
containlng a photoconductive layer on an electrically
conductive substrate, which photosensitive layer is
protected by an electrically insulating layer of
sufficient thickness to support the electrostatic
charges utilized in the imaging process, by means of an
AC corotron having a bias voltage applied to the
corotron shield. By adjusting the voltage applied to
the corotron shield, a condition of zero voltage on the
electrically insulating overcoating is achieved and an
electrical field established across the photosensitive
layer in a single charging step. As in any xerographic
process, the charged photoreceptor is exposed to an
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imagewise pattern of electromagnetic radiation to which
the photosensitive layer is sensitive to provide a
latent electrostatic image entirely within the
photosensitive layer. Typically, the latent image is
developed by electroscopic materials applied to the
electrically insulating overcoating, which image is
then transferred to an image receiving sheet thereby
allowing erasure of the electrostatic latent image by
flood exposure of the photosensitive layer and reuse of
the photoreceptor.
Other aspects of this invention are as follows:
An electrophotographic imaging process comprising
providing a photoreceptor comprising a conductive
substrate, a photosensitive layer, and an electrically
insulating layer over said photosensitive layer;
charging said photoreceptor by means of an AC
corotron, said corotron having a shield bias voltage
adjusted so as to provide substantially no voltage
across said electrically insulating layer; and
exposing said photoreceptor to an imagewise
pattern of electromagnetic radiation to which said
photosensitive layer is sensitive whereby an
electrostatic latent image is formed in said
photosensitive layer.
In an electrophotographic imaging process
comprising providing a photoreceptor comprising a
conductive- substrate, photosensitive layer and an
electrically insulating layer over said photosensitive
layer and
a) charging said photoreceptor by means of an AC
corotron, said corotron having an adjustable shield
bias voltage;
b) exposing said photoreceptor to an imagewise
pattern of electromagnetic radiation to which said
photoreceptor is sensitive, whereby an electrostatic
latent image is formed in said photosensitive layer;
c) visibly developing said latent image on said
photoreceptor;
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2b
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d) transferring said developed image from said
photoreceptor to an image receiving substrate;
e) erasing said latent image by means of flood
exposi.ng said photosensitive layer to electromagnetic
radiation to which it is sensitive;
f) measuring the voltage remaining on said
photoreceptor subsequent to said flood exposure;
g) utilizing said measured voltage to adjust said
shield bias voltage so as to maintain zero voltage on
the surface of said electrically insulating layer; and
h) repeating steps (a) - (f) at least once.
An electrophotographic imaging apparatus
comprising a photoreceptor comprising an electrically
conductive substrate, a photosensitive layer and an
electrically insulating layer over said photosensitive
layer, an AC corotron charging device to provide an
electrical field across the photosensitive layer of
said photoreceptor, said corotron having means to apply
a bias voltage to the shield thereof, means to expose
said photoreceptor to an imagewise pattern of
electromagnetic radiation to which said photosensitive
layer is sensitive, means to develop said image on said
electrically insulating layer, means to transfer said
image from said electrically insulating layer, means to
erase said latent image subsequent to said transfer and
to clean untransferred imaging material from said
;~ electrically insulating layer.
- Of fundamental significance in the present
invention is the presence of a rectifying layer at the
interface of.the photosensitive layer and the
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electrically conductive substrate. In the case of an amorphous ~e alloy, one
typically chooses a rectifying contaet which injects positive charges into the
photoconductive layer and blocks negative charges. If negative charges are
deposited on the top of the photoreceptor, positive charges are injected from
5 the interface into the photoconducting layer. They travel through the
photoconducting layer to the interface between the photoconducting layer and
the insulating layer where they are trapped. If the surface charge on top of
the photoreceptor is positive, the negative counter charge remains at the
conductive substrate because of the blocking nature of the interface. If one
10 wants to operate the photoreceptor with negative surface charges, the
interface would have to be injecting for electronics and blocking for positive
charges. The preferred operating mode depends on the photoreceptor mate-
rials used. For example, in the case of the utilization of a selenium alloy
photosensitive layer, positive charges are injected into the photosensitive
15 layer during periods of negative charge on the surface of the electrically
insulating layer. Because of the rectifying properties of the photosensitive
layer, no negative charge is injected during those periods when the charge on
the surface of the electrically insulating layer is positive. In such instance, a
positive voltage is established across the photosensitive layer. With the proper20 adjustment of bias voltage on the shield of the AC corotron, the total current
influx integrated over the time of exposure to charge can be made zero on the
surface of the electrically insulating layer. Such condition also eliminates
charge build-up due to the polarization of the overcoating since the net total
charge deposited by the AC corotron is of such polarity to counteract the
25 overcoating polarization.
As noted above, there is thus provided, in a single charging step, an
electrical field across the photoconductive layer by a single corotron in place
of the two corotrons required in the prior art. Further, since the polarization
or charge buil~up on the surface of the electrically insulating layer is
30 eliminated, there is no need for a corotron, typically an AC corotron, to level
the charge subsequent to image development and transfer. In accordance with
this invention, a single corotron replaces three corotrons required in the priorart in order to properly provide an electrical field across the photoreceptor
only and to eliminate residual charge on the electrically insulating, protective35 overcoating on the photosensitive layer. Typically, the AC corotron is
operated in the Irequency range of from about 50 Hz to about 1000 ~Iz in the
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process of this invention. Preferably, the frequency is in the range of from
about50Hztoabout400Hz.
The invention will be more fully described with reference to the
attached drawings wherein:
Figure 1 is a diagrammatic representation of a section of a
xerographic photoreceptor utilized in the process of this invention.
Figure 2 is a diagrammatic representation of a section of a
photoreceptor indicating an intermediate charging condition during the process
of this invention.
Figure 3 is a diagrammatic representation of a section of a
photoreceptor indicating the charged condition subsequent to the charging step
in the process of this invention.
Figure 4 is a diagrammatic representation of a section of a
photoreceptor indicating the creation of an electrostatic latent image upon
light exposure in accordance with the process of this invention.
Figure 5 is a schematic representation of a xerographic printing
apparatus incorporating the process of this invention.
In Figure 1, there is shown photoreceptor l comprising a conductive
substrate 3 supporting a photosensitive layer 5. An electrically insulating
layer 7 resides on photosensitive layer 5 to provide protection from wear and
contamination due to the repeated toning, transferring and cleaning which
occurs in each imaging cyele and retards crystallization in the event a Se alloyis used.
In Figure 2, there is shown the intermediate charge condition of
the photoreceptor during the charging step in the process of this invention. In
Figure 2, photoreceptor l is shown receiving, at the surface of the electricallyinsulating layer 7, both positive and negative charges, which are provided by
an AC corotron. The charged designations indicating positive and negative
charges contained within circles indicate a transitory condition or charges in
motion while those charge designations, both positive and negative, without
circles indicate stable charges which remain in the photoreceptor until further
processing occurs. Thus, there is shown both positive and negative charges on
the surface of electrically insulating layer 7 which are alternately supplied byan AC corotron. With a proper voltage bias on the corotron shield, these
charges will equal each other thereby resulting in a net zero charge residing onthe surface of electrically insulating layer 7. However, during periods of
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negative charge deposition on the surface of the electrically insulating layer 7,
positive charges are presented to the photosensitive material at the interface
of electrically conductive layer 3 and photosensitive layer 5. These positive
charges are shown in a circle at the interface and, because of the negative
charge, residing simultaneously on the surface of electrically insulating layer
7, the positive charges are drawn to the interface of photosensitive layer 5 andelectrically insulating layer 7 where they are trapped. During periods in which
the AC corotron is depositing positive charges on the surface of electrically
insulating layer 7, negative charges are presented a~ the interface between
conductive layer 3 and photosensitive layer 5. These charges remain trapped
at said interface because of the rectifying nature cf the photosensitive
material in layer 5.
While Figure 2 indicates the segment of a photoreceptor, during
the charging step, said segment is considered to be extremely small at any
particular point in time during the charging step in the process of this
invention. Figure 2 illustrates the condition for purposes of illustration only.In Figure 3, the photoreceptor is illustrated in its charged condition
wherein there are stable negative charges residing at the interface of
electrically conductive layer 3 and photosensitive layer 5 while equal charges
of opposite polarity reside at the interface of photosensitive layer 5 and
electrically insulating layer 7. These charges provide an electrical field across
the photosensitive layer with no charge residing on the surface of electrically
insulating layer 7.
The thus charged photosensitive layer is ready for imagewise light
exposure to establish a latent image therein as is illustrated in Figure 4. Light
rays 9 are shown impinging on the surface of electrically insulating layer 7
which is transparent to said electromagnetic radiation allowing charge carriers
to be created in the photosensitive material thereby eliminating the equal
amounts of charge residing at the interfaces of said layer. There are thus
provided areas of charged and uncharged photoreceptor which can be detected
at the surface of electrically insulating layer 7 in any suitable manner.
Obviously, subsequent to image formation, development and transfer, the
remaining electrical field within the photoreceptor 1 is eliminated by flood
exposure of the photoreceptor.
The photosensitive layer 5 may be exposed from either side as is
known in the prior art when providing a transparent conductive substrate 3 or,
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more commonly, a transparent electrically insulating layer 7. Apparatus
convenient for the purpose of the user is constructed utilizing the principals of
the process of this invention in either case. In addition, intermediate layers
may be placed between the conductive layer 3 and photosensitive layer 5 to
5 enhance the charge injecting nature of the interface. Such materials are well
known in the prior art and are chosen with regard for the type of photo-
sensitive material utilized in layer 5. In addition, adhesive layers may also beapplied to the surfaces of photosensitive layer 5 in order to adhere the
electrically insulating protective layer thereto, as well as providing adhesion
10 of the photosensitive material to the conductive substrate as injection layer.
Typical electrically insulating layers include organic, as well as
inorganic, materials. ~ particularly preferred m,aterial is polyethylene tere-
phthalate available commercially under the~ f~omn~he E.I. du
Pont de Nemours & Company, Inc.. Such material is preferred because of its
15 availability and ease of handling, as well as its electrical properties. Other
materials which can be typically utilized as protective layers include
polyester, polyvinylchloride, polypropylene, polyvinylidenechloride, polycar-
bonate, polystyrene, polyamide, polyfluoroethylene, polyethylene, polyimide,
polyvinylfluoride, polyvinylidene fluoride, polyvinylidenechloride, poly-
20 urethane, etc..
Photosensitive materials utilized in the process of this inventionare typically those which provide a rectifying boundary at the conductive
substrate. Typical photosensitive materials include selenium, selenium alloys
such as selenium-tellurium alloys, selenium-arsenic alloys containing various
25 dopants, such as cadmium sulfide, cadmium selenide, cadmium sulfoselenide,
zinc oxide, zinc sulfide and zinc selenide. Of course, said photosensitive
materials may be dispersed in suitable binder materials as is well known in the
art. Any suitable photosensitive material is included within the scope of this
invention, such as a composite layer leaving fine photoconductive material in
30 contact with the electrically insulating layer and relatively coarse photocon-
ductive particles contacting the base. Each portion of the composite layer is
desirably dispersed in a suitable binder. Such a photoreceptor is more fully
described in U.S. Ratent 3,801,317 to Tanada et al.. If desired, additional
layers may be incorporated into the imaging member to aid in the various
35 desired properties. For example, materials can be utilized at the interface
between the photosensitive layer and the electrically conductive layer which
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promote charge injection Oe one polarity and suppress charge injection of
another. Such materials include trigonal Se, gold, Te-alloys and carbon.
As mentioned above, the AC corotron utilized in the process of this
invention is provided with a voltage bias on the shield thereof. The bias
5 voltage to the shield is adjusted so as to provide the desired zero voltage onthe surface of the electrically insulating layer. This voltage bias is typicallydetermined empirically as it is highly dependent upon numerous operational
factors such as distance between the corotron and the surface being charged,
the amount of voltage desired to be utilized on the corotron wire and the
10 nature of the surface being charged. As a typical example of the operation ofthe process of this invention, there is shown in Fig-ure 5 a schematic of a
xerographic apparatus indicating the major operations of the xerographic
process. In Figure 5, there is shown xerographic apparatus 11, comprising a
photoreceptor 13 having the configuration of the photoreceptor illustrated in
15 Figure 1. In this instance, photoreceptor 13 is in the form of a typical
xerographic rotary drum mounted upon a grounded support 15. In the cyclic
process, corotron 17 is utilized to charge photoreceptor 13 through power
supply 19, either directly coupled to the wire or to the wire via a capacitance.In addition, a variable power supply 21 is utilized to supply a bias voltage to
20 the shield of corotron 17 as indicated in Figure 5. Por testing purposes only, a
probe 23 is inserted subsequent to the charging operation to monitor the
amount of charge on the photoreceptor. The charged photoreceptor is then
rotated past a typical slit scanning optical system 25 whereby the charged
photoreceptor is exposed to a pattern of electromagnetic radiation to which
25 the photosensitive material is sensitive. The exposed photoreceptor is then
rotated past the developing station whereby the electrostatic latent image in
the photosensitive layer is developed. After development, the image is
transferred as shown at transfer station 29 with the aid of transfer corotron
31. After transfer, the photoreceptor 13 is prepared for further use by erase
30 lamp 33 which collapses the remaining field in the photoreceptor followed by
removal of residual toner material at cleaning station 35. Por testing
purposes, a probe 37 is inserted in the cycle after the erase lamp 33 to
determine the amount of charge remaining in the photoreceptor. Since the
erase lamp collapses the field remaining across the photosensitive layer, any
35 voltage detected by probe 37 must represent charge residing on the electri-
cally insulating layer. Power supply 21 is adjusted so as to provide a proper
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bias voltage to the shield of corotron 17 which results in a zero net charge on
the insulating layer as indicated by probe 37. When the voltage indicated by
probe 37 is positive, then the bias voltage to the shield is made more negative.Conversely, when the indicated voltage is negative, the bias voltage to the
5 shield is made more positive.
RXAMPLE I
In an apparatus as illustrated in Figure 5, there is provided a
photoreceptor comprising an electrically conductive substrate having coated
thereon a 3 micron thick trigonal selenium injecting layer over which is coated
10 a 60 micron thick selenium-arsenic alloy doped with chlorine. Over the
photosensitive layer there is applied a 12 micron thick coating of an electri-
cally insulating polyurethane layer. At a surface speed of about 51 cm./sec.
the photoreceptor is rotated past a double wire corotron 13.5 cm in length and
operated at 60 Hz. The corotron shield is biased to a negative 30 volts, while
15 the corotron wire has 16,000 volts AC peak to peak applied thereto. A field
condition of +80 volts was measured at probe 23 subsequent to charging. It
was established that this voltage is completely across the photosensitive layer
by the fact that no voltage was detected at probe 37 subsequent to the erase
lamp. Any voltage detected by probe 37 would indicate a voltage across the
20 overcoating since there would be no field left in the photosensitive layer.
EXAMPLE II
The procedure of Example I is repeated with the exception that the
photoreceptor is moved past a double wire corotron which is the same as that
of Example I except that the length was 12 cm. A positive voltage of 350 volts
25 is measured at probe 23 while the shield voltage is held at +400 volts and the
voltage applied to the corotron wires was 16,000 volts peak to peak. Again,
there is no voltage measured at probe 37, indicating that the entire field of
+350 volts existed across the photosensitive layer of the photoreceptor and no
voltage was left residing on the surface of the electrically insulating poly-
30 urethane layer.
EXAMPLE III
A single wire corotron 20 cm in length is utili~ed in the process of
Example I to establish a field of 500 volts which is measured at probe 23 after
exposure to the AC corotron having a shield bias of +520 volts and 16,000 volts
35 peak to peak applied to the wire. Again, no voltage was detected by probe 37
subsequent to exposure to the erase lamp.
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Other modifications and ramifications of the present invention will
occur to those skilled in the art upon a reading of the present disclosure.
These are intended to be included within the scope of this invention. Of
especial note is the fact that the procedures described herein are not limited
5 to structures with dimensions typical of Rxample I. The insulating protective
layer may vary in thickness from a few microns to in excess of 20 m and the
60 m photosensitive layer may vary in thickness from approximately 5 m
to 80 m so that operation with a large range of electroscopic image
development materials may be accommodated.
