Note: Descriptions are shown in the official language in which they were submitted.
This inven-tion relates to charge transfer imaging and
more particularly -to charge transfer imaging employing a modified
photorecep-tor assembly to provide reduced image breakup during
-the transfer process.
In charge transfer electrophotography, a photosensitive
material is provided with the electros-tatic counterpart of an
image that is to be reproduced. The elec-trostatic image is then
transferred to a member possessing a dielectric surface.
Unfortunately, in the ordinary usage of the foregoing
charge transfer techni~ue, a disrup-tive image breakup has been often
encountered when the charge photosensitive surface has been brought
to the vicinity of the carrier surface. This effect is described
in detail in Xerography and Relate'd Process, edited by Jo~m H.
Desrauer and Harold E. Clark, the Focal Press, London and New
York 1~65 at Page ~3~.
Accordingly the present invention is advantageous in
attempting to reduce the extent of image degradation that takes
place when a surface with an electrostatic image is brought to the
vicinity of a surface to which the electrostatic image is to be
transferred.
Still another advantage of the present invention is to
modify a conventional photorecep-tor assembly to achieve reduced
image degradation in electrophotography and to achieve a modified
photoreceptor assembly for use in reducing the extent of image
degradation when a charged surface of the photoreceptor assembly
is brought to the vicinity of a receptor dielectric surEace.
'['he pres~nt invention provides an electrophotographicrr.ethod oor~
prising the s-teps of charging a photoconductive surface of a photorec~ptor asser~:ly,
said photoreceptor asserrbly :Eur-ther including a conducting sl1bstrate,
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and a semiconductor with a -thickness of at leas-t 1 mil, in-terposed
between the photoconductive surface and the conduc-ting substrate;
exposing the char~ecl pho-toconductive surface -to a pa-ttern represen-
ting an original to he reproduce~, whereby the surface is selectively
discharged and a latent electrostatic image is produced -thereon;
moving the image bearing portion of said photoconductive surface
into the -transfer region of a dielectric member; and transferring
the latent electrostatic image to said dielectric member by the
ionization of air in a gap between said photoconductive surface
and said dielectxic member.
According to another aspect, the present invention
provides a method of reducing image degradation during the
transfer of an elec-trostatic image -to a receptor which comprises
providing a semiconduc-tor underlying a photoconductive surtace from
which the transfer is to take place~
In accordance with a further aspect, the present
invention also provides an electrophotographic apparatus of the type
including a photoreceptor assembly, means for charging a photo~
conductive surface of the photoreceptor assembly, means for exposing
the.charged photoconductive surface layer to a pattern representing
an original to be reproduced to form a latent electrostatic image
thereon, an image receptor, and means for moving the image bearing
portion of the photoconductive surEace into the transfer region of
t.he image recep-tor, `characterized in that the photoreceptor member
comprises: a photoconductive surface; a conducting substrate; and
a semiconductor in-terposed between khe photoconductive surface and
the conducting substra-te, said semiconductor having a thickness of
at least 1 mil.
other aspects of the invention will become apparent
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af-ter consiclerinc3 several illustrative embodimen-ts taken in
conjunction wi-th the drawinys in which
FIG. 1 is a perspec-tive view of a schematic charge
transfer member employing a photoreceptor
assembly in accordance with the invention; and
FIG. 2 is a perspec-tive view of an alternative photo-
receptor assembly.
Turniny to the drawinys, a schema-tic representation of
a charye transfer imaging assemb]y, in accordance with the invention,
is outlined in FIG. 1.
In the particular embodiment of FIG. 1, the photorecep-
tor assembly 20 is a drum with a photoconductive layer 21 overlying
a semiconductor layer 23 on a conducting substrate 25.
In the conven-tional transfer process, the presence of
the electric field associated with the charges of the electrostatic
image formed on the drum 20 results in image deyradation in the
transfer process. The effect of such image degradation is miti~ated
in accordance with -the invention by the inclusion of the semiconduc-
tor 23 between the conducting substrate 25 and the photoconductor 21.
Other forms of photoreceptor assembly in accordance with
the invention can be provided, for example, by the flexible belt
20' o~ FIG. 2 in which a photoconductive layer 21' overlies a
semiconductive layer 23' which is in turn positioned on a conductive
substrate 25'. In order -to achieve the desired conductive substrate
25' a conductive coatiny may be applied to a plastic ~ilm or the
substrate may be a thin metallic foil, for example nickel.
The conclucting substrate 25 of the drum 20 in FIG. 1 is
illustratively of aluminum, but any combination of materials which
provides the desiréd conductivity may also be employed.
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It has been empirically discovered that the semi-
conducting layers 23 and 23' preferably have a thickness in the
range from 1 mil to 750 mlls. The reslstivity of the semiconductive
layer must be such -that charge will pass through the layer in a
reasonable time. ~ccordingly the resis-tivity is advantageously
less than 1012 ohm centimeters.
On the other hand the resistivity must be sufficiently
high to provide a time constant for smoothing the charge transfer
and thus reduce -the degradation of the transfer image as heretofore
encountered. The lower level of resistivity for the semiconductive
layers 23 and 23' depends on the thickness of the layers, the thick-
ness of the superimposed photoconductive layer, and the operating
speed. It has been discovered generally that a resistivity of
more than 10 ohm centimeters is suitable.
The semiconductive layer may be realized in a variety of
ways. It may be formed by a semiconductive plastic or a semi-
conductive elastorner. A suitable conducting agent is carbon black,
while a suitable matrix for receiving the carbon black is an epoxy
resin. Thus the semiconductor layer may be formed by dispersing
carbon black in a resin matrix to achieve a resistivity within the
range set forth above. Similarly a wide variety of rubbers can be
used with carbon black to obtain the desired resistivity.
The photoconductor~may be of the type generally employed
in electrostatic imaging. Ma-terials which have been found to
function satisfactority with the semiconductive layer 23 or 23
include polyvinylcarbazole complexed with -trini-troEluorenone;
cadmium sulfide disporsed in a variety of binders includiny epoxies,
, . . .
silicones and thermoplastics; selenium and selenium alloys,
including amorphous selenium, and low fatigue zinc oxide.
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In ~eneral, for binder layer pho-toconductors, the semi-
conducting layer may also be formed of the same material as the
photoconductor, but with a higher photosensitive element con-
centration, -thus a photoconductive layer of cadmium sulfide in
epoxy with an 18~ concentration behaves as an insulator in the dark,
while the same layer with a 30% cadmium sulfide concentration behaves
as a semiconductor in the dark.
With any of the foregoing photoconductive layers, disrupti~e
image breakdown of the kind discussed is observed when the la-tent
image receptor consists of a dielectric surace contiguous to a
conducting surface. The presence of this semiconducting layer 23
or 2~' between the photoconductor 21 or 21' and the substrate 25 or
25', however, significantly reduces degradation due to disruptive
breakdown. Although -the phenomenon by which the semiconducting layer
eliminates the disruptive breakdown is not completely understood~ it
is believed that the time constant introduced by this semicondu~ting
layer has the effect of smoothing or reducing the precipitous
behaviour otherwise associated with disruptive breakdown.
The teachings of this invention are useful in situations
where it is desirable to transfer a latent electrostatic charge
image to any dielectric member, for example, an intermediate dielect-
ric member which is subsequently toned and the image produced by
toning is then transferred to a plain paper copy or a dielectric
sheet which is itself toned to produce a copy.
While various aspects of the invention have been set forth
by the drawings and the speciication, it is to be understood that
the foregoing detailed description is for illustration only and that
various changes in parts, as well as -the substitution of equivalent
constituents for those shown and described, may be made without
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departincJ from the spiri-t and scope of the invention as set
forth in the appended claims.
