Note: Descriptions are shown in the official language in which they were submitted.
BACXGROUND OF THE INVENTION
This invention relates to xerographic copying and
more particularly to a novel method of imaging a particular
type of electrostatographic photoreceptor. The art of
xerography, as originally disclosed in U.S. Patent 2,297,691
by C. F. Carlson, involves the formation of an electrostatic
latent image on the surface of a photosensitive plate normally
- referred to as a photoreceptor. The photoreceptor comprises
a conductive substrate having on its surface a layer of a
photoconductive insulating material. Normally, there is a
- - 25 thin barrier layer between the substrate and the photoconductive
layer to prevent charge injection from the substrate into the
layer upon charging of the plate's surface.
In operation, the plate is charged in the dark,
such as by exposing it to a cloud of corona ions, and imaged
by exposing it to a light shadow image to selectively discharge
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the photoreceptor and leave a latent image corresponding to
the shadow areas. The latent electrostatic image is developed
by contacting the plate's surface with an electroscopic
marking material known as toner which will adhere to the
latent image due to electrostatic attraction. Transfer of
the toner image to a transfer member such as paper with
subsequent fusing of the toner into the paper provides a
permanent copy.
One type of electrostatographic photoreceptor
comprises a conductive substrate having a layer of photo-
conductive material on its surface which is overcoated with
a layer of an insulating organic resin. Various methods of
imaging this type of photoreceptor are disclosed by Mark in
his article appearing in Photogra~hic Science and Engineering,
Vol. 18, No. 3, May/June 1974. The processes referred to
by Mark as the Katsuragawa and Canon processes can basically
be divided into four steps. The first is to charge the
insulating overcoating. This is normally accomplished by
exposing it to d.c. corona of a polarity opposite to that of
the majority charge carrier. When applying a positive charge
to the surface of the insulating layer, as in the case where
an n-type photoconductor is employed, a negative charge is
induced in the conductive substrate, injected into the photo-
! conductor and transported to and trapped at the insulating
layer - photoconductive layer interface resulting in an initial
potential being solely across the insulating layer. The
charged plate is then exposed to a light and shadow pattern
while simultaneously applying to its surface an electronic
field of either alternating current (Canon) or direct current
of polarity opposite that of the initial electrostatic charge
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(Katsuragawa). This step is carried out until the plate's
surface potential is driven to zero (Canon) or to a chosen
potential opposite in sign to that of the original surface
potential (Katsuragawa). The plate is then uniformly exposed
to activating radiation to produce a developable image with
potential across the insulating overcoating and simultaneously
reduce the potential across the photoconductive layer to
zeros.
As is stated in the Mark article "during exposure
to the optical image, the processes here described [Canon,
Xatsuragawa, etc.] require the simultaneous application of
a corona discharge to hold the front surface of the receptor
at a constant potential." The process of the present invention
is carried out in such a manner that the simultaneous application
of corona discharge and exposure to the imagewise pattern of
activating radiation is avoided.
Simultaneous a.c. shunting and imagewise exposure
involves the disadvantage of the corotron wires being placed
in the light path, thereby introducing the possibility of
image distortion. In addition, this technique is not particularly
adaptable to imaging in the full frame flash exposure mode
because corona efficiency is such that the requisite discharge
cannot readily be achieved in the 5 to 50 microsecond duration
of the flash.
It would be desirable, and it is an object of the
present invention, to provide a novel method for imaging an
electrostatographic photoreceptor comprising a conductive
substrate having on its surface a photoconductive insulating
layer which is in turn overcoated with a layer of an insulating
organic resin.
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A further object is to provide such a method which
is capable of much greater imaging speed than are conventional
methods of imaging the type of photoreceptor described.
An additional object is to provide such a method
S in which the increased speed is obtained by the use of
exposure in the full frame flash exposure mode.
A further object is to provide such a method in
which simultaneous exposure and a.c. shunting of the photo-
receptor is eliminated.
O SUMMARY OF THE INVENTION
The present invention is a novel method for forming
an electrostatic image in a photosensitive plate comprising
a grounded conductive substrate having on its surface a
layer of photoconductive material and an insulating resin
layer overlaying the layer of photoconductive material. The !
process comprises the consecutive steps of:
a) applying an initial electrostatic charge of one
polarity to the surface of said photosensitive plate to
provide an initial potential which is solely across the
insulating layer;
b) applying to the surface of said plate an electronic
field of either alternating current or direct current of
polarity opposite that of the polarity of the initial electro-
static charge for a time sufficient to drive the initial
; potential to a potential included in the range extending from
a potential less than the initial potential through zero to
a chosen potential opposite in sign to the polarity of the
initial potential;
c) exposing said plate to imagewise activating
radiation in the full frame flash exposure mode thereby
forming an imagewise potential distribution
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across the layer of photoconductive material;
d) applying to the surface of said plate an
electronic field of either alternating current or direct
current of polarity opposite that of the polarity of the
initial electrostatic charge for a time sufficient to drive
the imagewise potential to a potential included in the range
extending from a potential less than the initial potential
through zero to a chosen potential opposite in sign to the
polarity of the initial potential; and
e) forming an imagewise potential distribution across
the insulating layer by uniformly exposing the plate to
activating radiation or allowing the inherent dark decay
of the photoconductor to establish the imagewise potential
distribution.
DETAILED DESCRIPTION AND PREFERRED EM~30DIMENTS
Figure 1 is a cross-sectional schematic view of a
photoreceptor utilized in the process of this invention
indicating a three layered plate.
Figure 2 is a schematic view of an apparatus
designed to utilize the process of this invention.
Figure 3(a)-(e) is a diagramatic representation of
the location of the various electrical charges in and on the
photoreceptor during the separate process steps of this
invention.
Figure 4 is a schematic view of another apparatus
utilizing the process described in the specification.
Figure 5 is a graphical representation of the
photodischarge rate of a photoreceptor utilized in the process
of this invention.
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The method by which the present invention is carried
out is further illustrated by the drawings. Figure 1 is a
cross-section of one type of photoreceptor which is used
in the method. The photoreceptor, generally designated as
10, comprises typically a conductive base 11, which is for
example, a metal, a metallized plastic film or a tin oxide
coated glass plate. Next, optionally, is a thin blocking
layer 12 such as an insulating metal oxide or dielectric
plastic film. The blocking layer is provided for the purpose
of blocking injection of the minority carrier from the
substrate. In cases where the mobility of the minority carrier
i8 sufficiently low, a blocking layer is not needed. Above
the blocking layer is a photoconducting layer 13, comprising
photoconductive charge carrier generating pigment particles
14, dispersed in a matrix 15 of an electrically insulating
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organic material. The charge generating layer 13 is depicted
in Fi~ure 1 as being highly loaded with pigment particles
14 to insure particle to particle contact. Alternatively,
the geometry controlled concept in which a smaller volume of
pigment particles can be employed by causing them to form
continuous chains through the photoconductive layer may be
used. This concept is more fully disclosed by R. N. Jones
in U.S. Patent 3,787,208. The photoconducting layer 13
is overcoated with a layer of an organic insulating overcoating
16 which may optionally comprise the same material as the
matrix 15.
The full frame flash exposure method of imaging
the photoreceptor, which method is conveniently employed in
carrying out the present invention, is illustrated by Figure
2. In this drawing, the photoreceptor is depicted as being
in the form of an endless, flexible belt 20 mounted on tri-
rollers 22 a, b and c. The belt rotates around the tri-roller
setup in a clockwise direction as indicated by the arrows.
In operation, the photoreceptor is charged to the desired
initial voltage (VO) to provide an initial potential across
the layer of insulating material at the charging corotron
24. Flood illumination may be used at this point to
eliminate the fiald in the photoconductor thereby placing the
field solely across the insulating layer. As the charged
portion of the photoreceptor rotates past the pre-exposure
shunt corotron 26, the initial potential is driven to a
potential included in the range extending from a potential
less than the initial potential through zero to a chosen
potential opposite in sign to the polarity of the initial
potential. The belt continues its movement until the shunted
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portion of the full desired document size is under flash
lamp 28 whereupon a light and shadow image is flash exposed
onto the photoreceptor to partially create an imagewise
potential distribution across the layer of photoconductive
material. The photoreceptor continues to rotate around the
tri-rollers and the imaged portion passes the post shunt
corotron 30 where the creation of the imagewise potential
is completed by driving the photoreceptor surface to a
potential included in the range extending from a potential
less than the initial potential through zero to a chosen
potential opposite in sign to the polarity of the initial
potential. The exposed portion continues to move until
it passes light housing 32 where it is uniformly exposed to
activating radiation to reduce the voltage across the photo-
conductive layer to its residual voltage, and therefore form
an imagewise potential distribution across the insulating layer.
Alternatively, the inherent dark decay of certain photoconductors
can be used in lieu of flood exposure.
At this point, the imagewise potential distribution
(latent image) can be developed in the conventional manner
which is accomplished at developer station 34. After development,
the image is transferred to a receiving member at the transfer
station (now shown) whereupon the imaged portion of the
photoreceptor moves onto the pre-clean corotron 36 where it
is subjected to an a.c. field or a field of polarity opposite
- to that provided by the charging corotron 24. The photoreceptor
then moves to the cleaning station 38 where any residual
toner from the development process is removed by conventional
cleaning means. The last step in the cycle involves erasing
any residual surface potential by exposing the photoreceptor
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uniformly to activating radiation and the output of an a.c.
corotron or other device such as a contact discharging device
at erasure station 40. After this step, the photoreceptor
is ready to begin the next cycle by being charged at charging
corotron 24.
The mechanism by which the imaging method of the
present invention operates is illustrated by Figures 3a - 3~.
In these figures, there is depicted the photosensitive plate
in which the electrostatic latent image is formed. Only the
layer of photoconductive material and the insulating over-
coating layer is depicted in Figure 3. Thus, the plate is
depicted in less detail than in Figure 1, but the schematic
representation of Figures 3a - 3~ is adequate for this
discussion.
In Figure 3a, a positive electrostatic charge has
been applied to the surface of the plate by a corotron device
(not shown) and a negative charge is induced in the conductive
substrate, injected, transported and trapped near the insulating
layer's interface with the layer of photoconductive material
thereby for~ing negative and positive charge pairs resulting
in a surface potential across the layer of insulating material.
In Figure 3b, the plate is depicted as having been
shunted by the pre-exposure corotron (shown in Figure 2 as
26) to zero surface potential.
In Figure 3c, the plate is depicted as having been
exposed to activating radiation on the left side whereas
the right side was not exposed. Upon exposure, which will
normally be on the order of 50 microseconds in duration
when full frame flash exposure is employed, an imagewise
potential distribution is formed across the layer of photo-
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conductive material.
Figure 3d depicts the plate after it has been
exposed to the post exposure a.c. corotron. It should be
kept in mind that the pre-expose and post-expose corotrons
can form a d.c. field of sign opposite to that of the initial
electrostatic charge although a.c. is preferred since a.c.
corotron current is less sensitive to ambient conditions
and therefore more uniform. Additionally, the use of a.c.
corotron permits one to draw carriers of either sign in the
event that the PIDC ranges across the zero potential level.
It has been determined that simultaneous imagewise exposure
and corotron shunting is not necessary because the charge
carriers generated during the process steps have finite
lifetimes which permit one to carry out the process in a
consecutive mode of operation.
The final step of the process may be uniform exposure
of the imaged plate to activating radiation. In this step,
depicted in Figure 3e, the voltage across the layer of photo-
conductive material is reduced to its residual voltage thereby
forming an imagewise potential distribution across the layer
of insulating material. As previously mentioned, the inherent
dark decay of the photoconductor may be used to achieve this
result.
Upon formation of the latent image by the technique
outlined above, it is developed and transferred and the
photoreceptor prepared for another cycle as previously
described.
The conductive substrate upon which the layer of
photoconductive material is deposited can be made up of any
suitable conductive material. It may be rigid as in the case
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where a flat plate or drum configuration is employed, but
must, of course, be flexible for use in the endless belt
configuration depicted in Figure 2. In this regard, a
continuous, flexible, nickel belt or a web or belt of an
aluminized polymer such as mylar can be conveniently used.
If the substrate is not naturally injecting,
a suitable interface should be provided to cause injection
of the majority carrier from the substrate to the layer of
photoconducting material to cause the initial potential
to reside solely across the overcoating. In the case of an
ambipolar photoconductor, a suitable interface should be
provided to block injection of the carrier of the sign of
the initial surface potential.
The layer of photoconductive material may be either
n-type or p-type, organic or inorganic and is selected from
those materials recognized in the art of xerography as being
useful in photoreceptors. Exemplary of useful photoconductive
materials are CdS, CdSe, CdSxSel x' ZnO, TiO2, and selenium
and selenium alloys such as Se/Te and Se/As. Typically,
these materials are dispersed in an insulating resin as
binder such as the configuration disclosed in U.S. Patent
3,121,0Q6 or the previously mentioned geometry controlled
configuration disclosed in U.S. Patent 3,787,208.
The insulating resin which constitutes the top
layer of the photoreceptor can be any material which has high
resistance against wear, high resistivity and the capability
of binding electrostatic charge, and translucency or transparency
to activating radiation. Examples of resins which may be
used are polystyrene, butadiene polymers and copolymers,
acrylic and methacrylic polymers, vinyl resins, alkyd resins,
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polycarbonate resins, polyethylene resins and polyester resins.
The method by which the present invention is carried
out is further illustrated by the following examples.
EXAMPLE I
A photosensitive imaging device of the type
contemplated for use in the instant invention has the following
construction:
To a 9.6 inch diameter aluminum drum is taped a
4 x 6 inch piece of a photoreceptor made up of an aluminum
substrate having a thin blocking layer on its surface to
prevent charge injection in the dark which is uniformly
coated with a 35~u thick layer of 40 volume percent CdS
doped with 103 ppm chlorine dispersed in a polyester copolymer.
This photoconductive layer is, in turn, overcoated with a
25,u thick layer of Myla ~ polyester insulating resin. These
layers are bonded together with a 1 to 2 ~ thick layer of
adhesive.
The device is put through the process steps of the
present invention as illustrated by Figure 4. In Figure 4,
drum 42 is rotated at a rate of 20 RPM corresponding to
a linear velocity of 10 inches per second. The drum's
surface is charged to an initial potential of 2,500 V at
charging corotron 44 which is a positive d.c. corotron. The
ch~arged portion of the drum then rotates past the pre-shunt
corotron 46 which is a negative d.c. corotron adjusted to
provide zero surface potential. The drum next rotates past
exposure slit 48 where it is exposed to a Xenon arc source
of activating radiation having a relatively flat spectral
content from 4,000 A to 7,000 A obtained using the appropriate
cutoff filters. The exposed portion of the drum next rotates
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past post shunt corotron 50 which is an a.c. corotron employing
a frequency of approximately 1 KHz. A positive d.c. bias
of approximately 1,000 volts is provided to again shunt the
surface potential to zero. As the exposed portion of the
drum rotates past fluorescent lamp 52, it is flood illuminated,
and after flood illumination, the post-exposure potential
is measured by use of probe 54. The data generated by use
of probe 54 is used to plot a photoinduced discharge curve
which is reproduced as Figure 5 in which surface potential in
volts is plotted against exposure in ergs/cm2. From this
curve, it can be determined that approximately 9 ergs/cm2
is required for 600 volts of discharge. In the last step
of the cycle, the drum rotates past the charge/flood erase
station 56 where a 60 Hz a.c. corotron is used to remove any
residual surface potential. No illumination is required to
erase the drum in this experiment.
EXAMPLE II
The photosensitive imaging device described in
Example I is charged and exposed as before except that the
exposure is carried out in imagewise configuration, i.e.
the charged device is exposed to a light and shadow image.
A negative contrast potential is formed as the exposed
portion of the drum rotates past post shunt corotron 50
which is intensified and reversed to form a positive contrast
potential by flood illumination. The drum bearing this
latent image is then developed in the ordinary xerographic
mode by the application of toner. Transfer of the toner to
a receiving member with subsequent removal of residual toner
and erasure results in the drum being ready to begin another
cycle.
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EXAMPLE III
A photosensitive imaging device of the type con-
templated for use in the instant invention has the following
construction:
A carbon black particle coating is applied to an
aluminum substrate and a 39~u thick photoconductive layer
comprised of 35 volume percent CdS 35Se 65 pigment which
has been solvent coated in a polyester copolymer to form
a photoconductive binder layer overlays the carbon black
0 layer. A 25 ~ thick layer of Mylar polyester overcoats
the photoconductive layer. The majority charge carriers
for this photoconductive material are electrons; the
carbon black interface is selected because it injects the
majority carrier telectrons).
The device, which is 4 x 6 inches, is taped to
a 30 inch circumference aluminum drum and cycled as described
in Figure 4.
The process steps are as described in Example I
except that the surface speed of the drum is 30 inches/sec.
0 Step 1 provides an initial charging potential of +2700 volts.
After flood, the dark potential obtained is +900 volts
corresponding to a voltage return of 33%. A discharge curve
is prepared from which it is determined that approximately
3.2 ergs/cm2 of exposure are required for 600 volts discharge.
Acceptably stable cycling characteristics are found to exist
over 103 cycles. .
EXAMPLE IV
A photosensitive imaging device of the type
contemplated for use in the instant invention is prepared
0 as in Example III except that the photoconductive layer is a
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35~u thick layer of 20 volume percent trigonal selenium solvent
coated onto the Myla ~ layer in a polyester copolymer. The
devices of Examples III and IV are both prepared by coating
the photoconductive layer directly onto the overcoating layer
thereby eliminating the need for an adhesive layer. The
trigonal selenium photoconducting layer photogenerates holes
as the majority carrier.
The sample is measured as in Example III except
that Step 1 provides an initial charging potential of -1900
volts. After flood, the dark potential obtained is -880
volts corresponding to a voltage return of 46%. From the
photoinduced discharge curve obtained using this sample, it is
determined that approximately 3.3 ergs/cm2 of exposure energy
are required for 600 volts of discharge.
EXAMPLE V
The-photosensitive imaging device described in
Example I is cut into 14 inch strips and these strips are
taped to a 65 inch long endless, nickel belt. The belt
is mounted on a tri-roller set-up similar to that depicted
in Figure 2. Referring to Figure 2, the photosensitive strips
are charged positively to approximately 2800 volts at charging
corotron 24. The surface of the corotron device is driven
to -350 volts at the pre-shunting station 26 which is a
negative d.c. corotron (alternatively a negatively biased
a.c. corotron). The purpose of this pre-exposure shunting
is to place a field in the photoconductor prior to exposure,
thereby increasing the recombination lifetime of light
generated charges created during exposure. This pre-
exposure shunting is necessary to obtain uniform lead to
trail edge discharge.
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.
Next, the photosensitive device is exposed in the
full frame flash exposure mode at exposure station 28. The
exposure source consists of 4 Xenon flash lamps providing
up to 50 ergs/cm2 in exposure energy. The device is then
shunted to 0 volts by post-exposure corotron 30 which is an
a.c. corotron adjusted to deposit no net d.c. current to
ground. The post-exposure shunting not only completes the
discharge of the background areas but also recharges the
image areas, which have been driven to -350 volts, by
pre-shunting to 0 volts.
The photoreceptor is then exposed to uniform activating
radiation provided by a fluorescent lamp thereby eliminating
the field in the photoconductor.
Dark development potentials and discharge levels
are measured and the device is erased to 0 volts with a.c.
corona (and optionally uniform exposure to light) in preparation
for recharging. Measurement of dark development potentials
reveals a uniform lead to trail edge discharge. Measurement
of discharge levels indicates contrast potentials in excess
of 600 volts,
