Note: Descriptions are shown in the official language in which they were submitted.
- 2125431
-
PATENT APPLICATION
Attorney's Docket No. D/91642
LAYERED PHOTORECEPTOR STRUCTURES WITH OVERCOATINGS
CONTAINING A TRIPHENYL METHANE
BACKGROUND OF THE INVENTION
This invention relates in general to electrophotographic imaging
members and more specifically, to layered photoreceptor structures with
overcoatings containing a triphenyl methane and process for making and
using the photoreceptors.
Electrophotographic imaging members, i.e. photoreceptors,
typically include a photoconductive layer formed on an electrically
conductive substrate. The photoconductive layer is a good insulator in the
dark so that electric charges are retained on its surface. Upon exposure to
light, the charge is dissipated.
A latent image is formed on the photoreceptor by first uniformly
depositing an electric charge over the surface of the photoconductive layer
by one of any suitable means well known in the art. The photoconductive
layer functions as a charge storage capacitor with charge on its free surface
and an equal charge of opposite polarity (the counter charge) on the
conductive substrate. A light image is then projected onto the
photoconductive layer. On those portions of the photoconductive layer
that are exposed to light, the electric charge is conducted through the layer
reducing the surface charge. The portions of the surface of the
photoconductive not exposed to light retain their surface charge. The
quantity of electric charge at any particular area of the photoconductive
surface is inversely related to the illumination incident thereon, thus
forming an electrostatic latent image.
The photodischarge of the photoconductive layer requires that
the layer photogenerate conductive charge and transport this charge
through the layer thereby neutralizing the charge on the surface. Two
types of photoreceptor structures have been employed: multilayer
- 2125431
structures wherein separate layers perform the functions of charge
generation and charge transport, respectively, and single layer
photoconductors which perform both functions. These layers are formed
on an electrically conductive substrate and may include an optional charge
blocking and an adhesive layer between the conductive layer and the
photoconducting layer or layers. Additionally, the substrate may comprise
a non-conducting mechanical support with a conductive surface. Other
layers for providing special functions such as incoherent reflection of laser
light, dot patterns for pictorial imaging or subbing layers to provide
chemical sealing and/or a smooth coating surface may be optionally be
employed.
One common type of photoreceptor is a multilayered device that
comprises a conductive layer, a blocking layer, an adhesive layer, a charge
generating layer, and a charge transport layer. The charge transport layer
can contain an active aromatic diamine molecule, which enables charge
transport, dissolved or molecularly dispersed in a film forming binder. This
type of charge transport layer is described, for example in US-A 4,265,990.
Other charge transport molecules disclosed in the prior art include a variety
of ele~l-on donor, aromatic amines, oxadiazoles, oxazoles, hydrazones and
stilbenes for hole transport and electron acceptor molecules for electron
transport. Another type of charge transport layer has been developed
which utilizes a charge transporting polymer wherein the charge
transporting moiety is incorporated in the polymer as a group pendant
from the backbone of the polymer backbone or as a moiety in the
backbone of the polymer. These types of charge transport polymers
include materials such as poly(N-vinylcarbazole), polysilylenes, and others
including those described, for example, in US-A 4,618,551, 4,806,443,
4,806,4~4, 4,818,650, 4,935,487, and 4,956,440. The disclosures of these
patents are incorporated herein in their entirety.
Charge generator layers comprise amorphous films of selenium
and alloys of selenium and arsenic, tellurium, germanium and the like,
hydrogenated amorphous silicon and compounds of silicon and
germanium, carbon, oxygen, nitrogen and the like fabricated by vacuum
-- 2125431
evaporation or deposition. The charge generator layers may also comprise
inorganic pigments of crystalline selenium and its alloys; Group Il-VI
compounds; and organic pigments such as quinacridones, polycyclic
pigments such as dibromo anthanthrone pigments, perylene and perinone
diamines, polynuclear aromatic quinones, azo pigments including bis-, tris-
and tetrakis-azos; and the like dispersed in a film forming polymeric binder
and fabricated by solvent coating techniques.
Phthalocyanines have been employed as photogenerating
materials for use in laser printers utilizing infrared exposure systems.
Infrared sensitivity is required for photoreceptors exposed to low cost
semiconductor laser diode light exposure devices. The absorption spectrum
and photosensitivity of the phthalocyanines depend on the central metal
atom of the compound. Many metal phthalocyanines have been reported
and include, oxyvanadium phthalocyanine, chloroaluminum
phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine,
chlorogallium phthalocyanine, magnesium phthalocyanine and metal-free
phthalocyanine. The phthalocyanines exist in many crystal forms which
have a strong influence on photogeneration.
One of the design criteria for the selection of the photosensitive
pigment for a charge generator layer and the charge transporting molecule
for a transport layer is that, when light photons photogenerate holes in the
pigment, the holes be efficiently injected into the charge transporting
molecule in the transport layer. More specifically, the injection efficiency
from the pigment to the transport layer should be high. A second design
criterion is that the injected holes be transported across the charge
transport layer in a short time; shorter than the time duration between the
exposure and development stations in an imaging device. The transit time
across the transport layer is determined by the charge carrier mobility in the
transport layer. The charge carrier mobility is the velocity per unit field and
has dimensions of cm2/volt sec. The charge carrier mobility is a function of
the structure of the charge transporting molecule, the concentration of the
charge transporting molecule in the transport layer and the electrically
"inactive" binder polymer in which the charge transport molecule is
- 2125431
dispersed. It is believed that the injection efficiency can be maximized by
choosing a transport molecule whose ionization potential is lower than
that of the pigment. However, low ionization potential molecules may
have other deficiencies, one of which is their instability in an atmosphere of
corona effluents. A copy quality defect resulting from the chemical
interaction of the surface of the transport layer with corona effluents is
referred to as "parking deletionH and is described in detail below.
Reprographic machines often utilize multilayered organic
photoconductors and also employ corotrons or scorotrons to charge the
photoconductors prior to imagewise exposure. During the operating
lifetime of these photoconductors they are subjected to corona effluents
which include ozone, various oxides of nitrogen etc. It is believed that
some of these oxides of nitrogen are converted to nitric acid in the presence
of water molecules present in the ambient operating atmosphere. The top
surface of the photoconductor is exposed to the nitric acid during
operation of the machine and photoconductor molecules at the very top
surface of the transport layer are converted to what is believed to be the
nitrated species of the molecules and these could form an electrically
conductive film. However, during operation of the machine, the cleaning
subsystem continuously removes (by wear) a region of the top surface
thereby preventing accumulation of the conductive species. However, such
is not the case when the machine is not operating (i.e. in idle mode)
betvveen two large copy runs. During the idle mode between long copy
runs, a specific segment of the photoreceptor comes to rest (is parked)
beneath a corotron that had been in operation during the long copy run.
Although the high voltage to the corotron is turned off during the time
period when the photoreceptor is parked, some effluents (e.g. nitric acid,
etc.) continue to be emitted from the corotron shield, corotron housing,
etc. This effluent emission is concentrated in the region of the stationary
photoreceptor parked directly underneath the corotron. The effluents
render that surface region electrically conductive. When machine
operation is resumed for the next copy run, a loss of resolution, and even
deletion, is observed in the affected region. Thus, the corona induced
212S431
changes primarily occur at the surface region of the charge transport layer.
These changes are manifested in the form of increased conductivity which
results in loss of resolution of the final toner images. In the case of severe
increases in conductivity, there can be regions of severe deletions in the
images. This problem is particularly severe in devices employing the charge
transport molecule N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-
4,4'-diamine represented by the following structural formula:
H3C )~\N~N)~CH3
Thus, although the charge transport molecule meets most other
electrophotographic criteria such as being devoid of traps, having high
injection efficiency from many pigments, ease in synthesizing, and
inexpensive, it encounters serious parking and other deletion problems
when an idle mode is interposed bet~veen extended cycling runs.
INFORMATION DISCLOSURE STATEMENT
US-A 4,297,425 to Pai et al., issued October 27, 1981 - A layered
photosensitive member is disclosed comprising a generator layer and a
transport layer containing a combination of diamine and triphenyl
methane moleculff dispersed in a polymeric binder.
US-A 4,050,935 to Limburg et al., issued September 27, 1977 - A
layered photosensitive member is disclosed comprising a generator layer of
trigonal selenium and a transport layer of bis(4-diethylamino-2-
methylphenyl)phenylmethane molecularly dispersed in a polymeric binder.
US-A 4,457,994 to Pai et al. et al, issued July 3 1984 - A layered
photosensitive member is disclosed comprising a generator layer and a
- 2125431
transport layer containing a diamine type molecule dispersed in a polymeric
binder and an overcoat containing triphenyl methane molecules dispersed
in a polymeric binder.
US-A 4,515,882 to Mammino et al, issued May 7, 1985 - An
electrophotographic imaging system is disclosed which utilizes a member
comprising at least one photoconductive layer and an overcoating layer
comprising a film forming continuous phase comprising charge transport
molecules and finely divided charge injection enabling particles dispersed
in the continuous phase, the insulating overcoating layer being
substantially transparent to activating radiation to which the
photoconductive layer is sensitive and substantially electrically insulating at
low electrical fields.
US-A 4,599,286 to Limburg et al., issued July 8, 1982 - An
electrophotographic imaging member is disclosed comprising a charge
generation layer and a charge transport layer, the transport layer
comprising an aromatic amine charge transport molecule in a continuous
polymeric t ~der phase and a chemical stabilizer selected from the group
consisting of certain nitrone, isobenzofuran, hydroxyaromatic compounds
and mixtures thereof. An electrophotographic imaging process using this
member is also described.
In copending application entitled UOVERCOATlNG FOR
MULTILAYERED ORGANIC PHOTORECEPTORS CONTAINING A STABILIZER
AND CHARGE TRANSPORT MOLECULESn Docket Number D/91641, filed on
the same day as the instant application, an electrophotographic imaging
member is disclosed comprising a substrate, a charge generating layer, a
charge transport layer comprising electrically active charge transporting
molecules dissolved or molecularly dispersed in a first electrically inactive
polymer binder, and an overcoat layer comprising charge transporting
molecules and a chemical stabilizer additive selected from the group
consisting of a nitrone, isobenzofuran, fused hydroxyaromatic compound,
phenolic compound and mixtures thereof molecularly dispersed in an
electrically inactive second polymer binder, the second polymer binder
being soluble in a solvent in which the first polymer binder is insoluble. The
2125431
device may also include any suitable optional charge blocking, adhesive
and other sub layers. This electrophotographic imaging member is
fabricated by forming on a charge generating layer a first coating
comprising charge transporting molecules dispersed in a solution of a first
polymer binder dissolved in a first solvent, drying the coating to remove the
solvent to form a substantially dry charge transport layer, forming on the
charge transport layer a second coating comprising charge transporting
molecules and a chemical stabilizer additive selected from the group
consisting of a nitrone, isobenzofuran, hydroxyaromatic compound and
mixtures thereof molecule molecularly dispersed in an electrically inactive
second polymer binder in a solution of a second polymer binder dissolved in
a second solvent, the first polymer binder being insoluble in the second
solvent, and drying the second coating to remove the second solvent to
form a substantially dry overcoat layer. This electrophotographic imaging
member may be utilized in an electrophotographic imaging process. The
entire disclosure of this copending application is incorporated herein by
reference.
Although acceptable images may be obtained when chemical
triphenyl methanes are incorporated within the bulk of the charge
transport layers, the photoreceptor can exhibit at least two defficiencies
when subjected to extensive cycling. One is that the presence of the
triphenyl methane in the bulk of the charge transport layer results in
trapping of photoinjected holes from the generator layer into the transport
layer giving rise to higher residual potentials. This can cause a condition
known as cy~le-up in which the residual potential continues to increase
with multi-cycle operation. This can give rise to increased densities in the
background areas of the final images. A second undesirable effect due to
the addition of the triphenyl methane in the bulk of the transport layer is
that some of these molecules migrate into the generator layer during the
process of the fabrication of the transport layer. The presence of these
molecules on the surface of the pigment in the generator layer could result
in cyclic instabilities, particularly in long image cycling runs. These two
212S431
-
defficiencies limitsthe concentration of the triphenylmethanes that can be
added in the transport layer.
Where photoreceptors containing triphenylmethanes in the
charge transport layer are overcoated, intermixing of the overcoat and the
transport layers occur which can render the overcoat very ineffective. This
intermixing leads to the incorporation of triphenyl methanes in the bulk of
the transport layer causing the aforementioned cycle-up. Also, the
intermixing causes a reduction of the concentration of triphenyl methanes
on the outer surface of the photoreceptor. The concentration of triphenyl
methanes in the outer surface region of the photoreceptor prevents the
aforementioned deletion.
Thus, there is a continuing need for photoreceptors having
improved resistance to increased densities in the background areas of the
final images, migration of additives into the generator layer during
fabrication of the transport layer, and cyclic instabilities.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an
improved electrophotographic imaging member which overcomes the
above-noted deficiencies.
It is yet another object of the present invention to provide an
improved ele~l,ophotographic imaging member capable of reproducing
extremely high resolution images.
It is still another object of the present invention to provide an
improved electrophotographic imaging member having a surface region
stable against loss of resolution.
It is another object of the present invention to provide an
improved electrophotographic imaging member having a surface region
stable against copy defects such as print deletion.
It is yet another object of the present invention to provide an
improved electrophotographic imaging member having greater stability
against corona effluents without an attendant reduction in transport
efficiency of transport layers
- 2125431
-
It is still another object of the present invention to provide an
improved electrophotographic imaging member having greater stability
against corona effluents without an attendant redunion of the sensitivity
of the imaging member.
The foregoing objects and others are accomplished in
accordance with this invention by providing an electrophotographic
imaging member comprising a substrate, a charge generating layer, a
charge transport layer comprising charge transporting molecules dispersed
in a first polymer binder, and an overcoat layer comprising a triphenyl
methane molecule dispersed in a second polymer binder, said second
polymer binder being soluble in a solvent in which said first polymer binder
is insoluble. The device may also include any suitable optional charge
blocking, adhesive and other sub layers. This electrophotographic imaging
member is fabricated by forming on a charge generating layer a first
coating comprising charge transporting molecules dispersed in a solution of
a first polymer binder dissolved in a first solvent, drying the coating to
remove the solvent to form a substantially dry charge transport layer,
forming on the charge transport layer a second coating comprising
triphenyl methane molecules and charge transporting molecules dispersed
in a solution of a second polymer binder dissolved in a second solvent, the
first polymer binder being insoluble in the second solvent, and drying the
second coating to remove the second solvent to form a substantially dry
overcoat layer. This electrophotographic imaging member may be utilized
in an electrophotographic imaging process.
Electrophotographic imaging members are well known in the
art. Electrophotographic imaging members may be prepared by any
suitable technique. Typically, a flexible or rigid substrate is provided with
an electrically conductive surface. A charge generating layer is then
applied to the electrically conductive surface. A charge blocking layer may
optionally be applied to the electrically conductive surface prior to the
application of a charge generating layer. If desired, an adhesive layer may
be utilized between the charge blocking layer and the charge generating
layer. Usually the charge generation layer is applied onto the blocking
2125431
-
layer and a charge transport layer is formed on the charge generation layer.
This structure may have the charge generation layer on top of or below the
charge transport layer.
The substrate may be opaque or substantially transparent and
may comprise any suitable material having the required mechanical
properties. Accordingly, the substrate may comprise a layer of an
electrically non-conductive or conductive material such as an inorganic or
an organic composition. As electrically non-conducting materials there may
be employed various resins known for this purpose including polyesters,
polycarbonates, polyamides, polyurethanes, and the like which are flexible
as thin webs. An electrically conducting substrate may be any metal, for
example, aluminum, nickel, steel, copper, and the like or a polymeric
material, as described above, filled with an electrically conducting
substance, such as carbon, metallic powder, and the like or an organic
electrically conducting material. The electrically insulating or conductive
substrate may be in the form of an endless flexible belt, a web, a rigid
cylinder, a sheet and the like.
The thickness of the substrate layer depends on numerous
factors, including strength desired and economical considerations. Thus,
for a drum, this layer may be of substantial thickness of, for example, up to
many centimeters or of a minimum thickness of less than a millimeter.
Similarly, a flexible belt may be of substantial thickness, for example, about
250 micrometers, or of minimum thickness less than 50 micrometers,
provided there are no adverse effects on the final electrophotographic
device.
In embodiments where the substrate layer is not conductive, the
surface thereof may be rendered electrically conductive by an electrically
conductive coating. The conductive coating may vary in thickness over
substantially wide ranges depending upon the optical transparency, degree
of flexibility desired, and economic factors. Accordingly, for a flexible
photoresponsive imaging device, the thickness of the conductive coating
may be between about 20 angstroms to about 750 angstroms, and more
preferably from about 100 angstroms to about 200 angstroms for an
-10-
2125431
-
optimum combination of electrical conductivity, flexibility and light
transmission. The flexible conductive coating may be an electrically
conductive metal layer formed, for example, on the substrate by any
suitable coating technique, such as a vacuum depositing technique or
cle~l,odeposition. Typical metals include aluminum, zirconium, niobium,
tantalum, vanadium and hafnium, titanium, nickel, stainless steel,
chromium, tungsten, molybdenum, and the like.
An optional hole blocking layer may be applied to the substrate.
Any suitable and conventional blocking layer capable of forming an
ele~lronic barrier to holes between the adjacent photoconductive layer and
the underlying conductive surface of a substrate may be utilized.
An optional adhesive layer may applied to the hole blocking
layer. Any suitable adhesive layer well known in the art may be utilized.
Typical adhesive layer materials include, for example, polyesters,
polyurethanes, and the like. Satisfactory results may be achieved with
adhesive layer thickness between about O.OS micrometer (500 angstroms)
and about 0.3 micrometer (3,000 angstroms). Conventional techniques for
applying an adhesive layer coating mixture to the charge blocking layer
include spraying, dip coating, roll coating, wire wound rod coating, gravure
coating, Bird applicator coating, and the like. Drying of the deposited
coating may be effected by any suitable conventional technique such as
oven drying, infra red radiation drying, air drying and the like.
Any suitable polymeric film forming binder material may be
employed as the matrix in the charge generating (photogenerating) binder
layer. Typical polymeric film forming materials include those described, for
example, in U.S. Patent 3,121,006, the entire disclosure of which is
incorporated herein by reference. Thus, typical organic polymeric film
forming binders include thermoplastic and thermosetting resins such as
polycarbonates, polyesters, polyamides, polyurethanes, polystyrenes,
polyarylethers, polyarylsulfones, polybutadienes, polysulfones,
polyethersulfones, polyethylenes, polypropylenes, polyimides,
polymethylpentenes, polyphenylene sulfides, polyvinyl acetate,
polysiloxanes, polyacrylates, polyvinyl acetals, polyamides, polyimides,
212~431
-
amino resins, phenylene oxide resins, terephthalic acid resins, phenoxy
resins, epoxy resins, phenolic resins, polystyrene and acrylonitrile
copolymers, polyvinylchloride, vinylchloride and vinyl acetate copolymers,
acrylate copolymers, alkyd resins, cellulosic film formers, poly(amideimide),
styrene-butadiene copolymers, vinylidenechloride-vinylchloride
copolymers, vinylacetate-vinylidenechloride copolymers, styrene-alkyd
resins, polyvinylcarbazole, and the like. These polymers may be block,
random or alternating copolymers.
The photogenerating composition or pigment is present in the
resinous binder composition in various amounts. Generally, however, from
about 5 percent by volume to about 90 percent by volume of the
photogenerating pigment is dispersed in about 10 percent by volume to
about 95 percent by volume of the resinous binder, and preferably from
about 20 percent by volume to about 30 percent by volume of the
photogenerating pigment is dispersed in about 70 percent by volume to
about 80 percent by volume of the resinous binder composition. In one
embodiment about 8 percent by volume of the photogenerating pigment is
dispersed in about 92 percent by volume of the resinous binder
composition. The photogenerator layers can also fabricated by vacuum
sublimation in which case there is no binder.
Any suitable and conventional technique may be utilized to mix
and thereafter apply the photogenerating layer coating mixture. Typical
application techniques include spraying, dip coating, roll coating, wire
wound rod coating, vacuum sublimation and the like. For some
applications, the generator layer may be fabricated in a dot or line pattern.
Removing of the solvent of a solvent coated layer may be effected by any
suitable conventional technique such as oven drying, infrared radiation
drying, air drying and the like.
The charge transport layer may comprise a charge transporting
small molecule dissolved or molecularly dispersed in a film forming
electrically inert polymer such as a polycarbonate. The term ndissolvedn as
employed herein is defined herein as forming a solution in which the small
molecule is dissolved in the polymer to form a homogeneous phase. The
- 2125~1
-
expression nmolecularly dispersed~ is used herein is defined as a charge
transporting small molecule dispersed in the polymer, the small molecules
being dispersed in the polymer on a molecular scale. Any suitable charge
transporting or electrically active small molecule may be employed in the
charge transport layer of this invention. The expression charge
transporting "small moleculen is defined herein as a monomer that allows
the free charge photogenerated in the transport layer to be transported
across the transport layer. Typical charge transporting small molecules
include, for example, pyrazolines such as 1-phenyl-3-(4'-diethylamino
styryl)-5-(4"- diethylamino phenyl)pyrazoline, diamines such as N,N'-
diphenyl-N,N'-bis(3-methylphenyl)-(1 ,1 '-biphenyl)-4,4'-diamine,
hydrazones such as N-phenyl-N-methyl-3-(9-ethyl)carbazyl hydrazone and
4-diethyl amino benzaldehyde-1,2-diphenyl hydrazone, and oxadiazoles
such as 2,5-bis (4-N,N'-diethylaminophenyl)-1,2,4-oxadiazole, stilbenes and
the like. However, to avoid cycle-up, the charge transport layer should be
substantially free of triphenyl methane. As indicated above, suitable
electrically active small molecule charge transporting compounds are
dissolved or molecularly dispersed in electrically inactive polymeric film
forming materials. A small molecule charge transporting compound that
permits injection of holes from the pigment into the charge generating
layer with high efficiency and transports them across the charge transport
layer with very short transit times is N,N'-diphenyl-N,N'-bis(3-
methylphenyl)-(1, 1 '-biphenyl)-4,4'-di-amine represented by the formula:
H3C )~3\N~N)~\CH3
- Z125~31
The electrically inert polymeric binder generally used to disperse
the electrically active molecule in the charge transport layer is a poly(4,4'-
isopropylidene-diphenylene)carbonate (also referred to as bisphenol-A-
polycarbonate) represented by the formula:
CH3 O
~ o~- c~o-ll
CH3 n
Any suitable electrically inactive resin binder insoluble in the
solvent used to apply the overcoat layer may be employed in the charge
transport layer of this invention. Typical inactive resin binders include
polycarbonate resin, polyester, polyarylate, polyacrylate, polyether,
polysulfone, and the like. Molecular weights can vary, for example, from
about 20,000 to about 150,000.
Any suitable and conventional technique may be utilized to mix
and thereafter apply the charge transport layer coating mixture to the
charge generating layer. Typical application techniques include spraying,
dip coating, roll coating, wire wound rod coating, and the like. Drying of
the deposited coating may be effected by any suitable conventional
technique such as oven drying, infra red radiation drying, air drying and the
like.
Generally, the thickness of the charge transport layer is between
about 10 and about 50 micrometers, but thicknesses outside this range can
also be used. The hole transport layer should be an insulator to the extent
that the electrostatic charge placed on the hole transport layer is not
conducted in the absence of illumination at a rate sufficient to prevent
formation and retention of an electrostatic latent image thereon. In
general, the ratio of the thickness of the hole transport layer to the charge
generator layers is preferably maintained from about 2:1 to 200:1 and in
2125431
some instances as great as 400:1. In other words, the charge transport
layer, is substantially non-absorbing to visible light or radiation in the
region of intended use but is electrically "active" in that it allows the
injection of photogenerated holes from the photoconductive layer, i.e.,
charge generation layer, and allows these holes to be transported through
itself to selectively discharge a surface charge on the surface of the active
layer.
The overcoat layer of this invention comprises three principal
components: (1) a polymeric film forming binder which is soluble in and
coated from a solvent in which the charge transport layer film forming
binder is insoluble, (2) an optional charge transporting monomer, and (3) a
triphenyl methane monomer which functions as both a stabilizer and as a
charge transporting monomer. In US-A 4,457,994 the same film forming
binder is used in both the charge transport layer and in the overcoat (e.g.
see Examples ll through IV of US-A 4,457,994) and, therefore, undesirable
intermixing of the materials from each layer can occur. This is significantly
unlike the different film forming binders employed in the overcoating and
charge transport layer combination of this invention. The choice of the film
forming binder for the overcoat depends on the choice of the film forming
binder for the transport layer. In a specific preferred embodiment, the
charge transport layer binder is poly(4,4'-isopropylidene-diphenylene)
carbonate (i.e. bisphenol-A-polycarbonate), the overcoat film forming
binder can be, for example, a poly(4,4'-cyclohexylidine-diphenylene)
carbonate (also referred to as bisphenol-Z-polycarbonate) having a
structure represented by the following formula:
O ~ ~ ~ O--C--
212~431
Bisphenol-Z-polycarbonate is soluble in and coated from toluene. The
expression nsoluble" as employed herein is defined as capable of forming a
solution with which a film can be applied to a surface and dried to form a
continuous coating. Bisphenol-A-polycarbonate is insoluble in toluene.
The expression ninsolublen as employed herein is defined as not capable of
forming a solution so that the solvent and the solid remain in two separate
phases and a continuous coating cannot be formed. Molecular weights can
vary, for example, from about 20,000 to about 150,000.
The charge transporting monomer (small molecule) in the
overcoat layer can be any one of the aforementioned monomers employed
to fabricate the transport layer. In one embodiment the molecule in both
the charge transport and overcoat layers is N,N'-diphenyl-N,N'-bis(3-
methylphenyl)-(1 ,1 '-biphenyl)-4,4'-di-amine.
The triphenyl methane molecule in the overcoat layer can be any
one of several compounds represented by the following generic formula:
~N l ~ N \
Where R1 is H, CH3, Cl and R2 and R3 are are alkyl groups containing from 1
to 5 carbon atoms. Typical triphenyl methane compounds represented by
this structural formula include, for example, bis (4-diethylamino-2-
chlorophenyl)phenylmethane, bis- (4-N,N'-dibenzyl -2-chlorophenyl)phenyl
methane, and the like.
The concentration of the charge transporting molecules in the
overcoat can range up to about 50 percent by weight based on the total
weight of the dried overcoat. The concentration of the triphenyl methane
molecule in the overcoat layer is between about 0.5 percent by weight and
' _ Z125431
about 50 percent by weight based on the total weight of the dried
overcoat. When less than about 0.5 percent by weight of triphenyl
methane molecule is present in the overcoat, the photoreceptor still
exhibits considerable deletion. If the amount of triphenyl methane
molecule in the overcoat exceeds about 50 percent by weight,
crystallization may occur resulting in residual cycle-up. The total combined
concentration of the charge transporting molecule and the triphenyl
methane molecule should be between about 5 percent by weight and
about 50 percent by weight based on the total weight of the dried
overcoat.
The thickness of the overcoat layer selected depends upon the
abrasiveness of the cleaning system employed and can range from about
0.5 micrometer to about 10 micrometers in thickness. Any suitable and
conventional technique may be utilized to mix and thereafter apply the
overcoat layer coating mixture to the charge generating layer. Typical
application techniques include spraying, dip coating, roll coating, wire
wound rod coating, and the like. Drying of the deposited coating may be
effected by any suitable conventional technique such as oven drying,
infrared radiation drying, air drying and the like.
The composition and materials employed in the overcoat layer
must meet several requirements: (1) it should be charge transporting to
prevent a residual build up across the overcoat, and (2) it should not
intermix into the charge transport layer during the process of fabricating
the overcoat. The second requirement can be met by the judicious selection
of binders for the charge transport layer and the overcoat layers whereby
the polymer binder for the overcoat is soluble in a solvent in which the
poly...er binder for the charge transport layer is insoluble.
Other suitable layers may also be used such as a conventional
electrically conductive ground strip along one edge of the belt or drum in
contact with the conductive surface of the substrate to facilitate connection
of the electrically conductive layer of the photoreceptor to ground or to an
electrical bias. Ground strips are well known and usually comprise
conductive particlesdispersed in a film forming binder.
212~i431
In some cases an anti-curl back coating may be applied to the
side opposite the photoreceptor to provide flatness and/or abrasion
resistance for belt or web type photoreceptors. These anti-curl back
coating layers are well known in the art and may comprise thermoplastic
organic polymers or inorganic polymers that are electrically insulating or
slightly semiconducting.
The photoreceptor of this invention may be used in any
conventional electrophotographic imaging system. As described above,
electrophotographic imaging usually involves depositing a uniform
electrostatic charge on the photoreceptor, exposing the photoreceptor to a
light image pattern to form an electrostatic latent image on the
photoreceptor, developing the electrostatic latent image with
electrostatically attractable marking particles to form a visible toner image,
transferring the toner image to a receiving member and repeating the
depositing, exposing, developing and transferring steps at least once. The
serious parking and other deletion problems when an idle mode is
interposed between extended cycling runs become especially pronounced
when the depositing, exposing, developing and transferring steps are
repeated at least 1,000 times in a single run, followed by resting of the
photoreceptor between about S minutes and about 30 minutes, and
repeating said depositing, exposing, developing and transferring steps at
least 10 additional times in another single run.
A number of examples are set forth hereinbelow and are
illustrative of different compositions and conditions that can be utilized in
practicing the invention. All proportions are by weight unless otherwise
indicated. It will be apparent, howevcr, that the invention can be practiced
with many types of compositions and can have many different uses in
a~ordance with the disclosure above and as pointed out hereinafter.
-18-
~12~431
EXAMPLE I
Scanner Characterization
Each photoconductor device to be evaluated is mounted on a
cylindrical aluminum drum substrate which is rotated on a shaft. The device
is charged by a corotron mounted along the periphery of the drum. The
surface potential is measured as a function of time by capacitively coupled
voltage probes placed at different locations around the shaft. The probes
are calibrated by applying known potentials to the drum substrate. The
devices on the drums are exposed by a light source located at a position
near the drum downstream from the corotron. As the drum is rotated, the
initial (pre exposure) charging potential is measured by voltage probe 1.
Further rotation leads to the exposure station, where the photoconductor
device is exposed to monochromatic radiation of known intensity. The
device is erased by light source located at a position upstream of charging.
The measurements made include charging of the photoconductor device in
a constant current or voltage mode. The device is charged to a negative
polarity corona. As the drum is rotated, the initial charging potential is
measured by voltage probe 1. Further rotation leads to the exposure
station, where the photoconductor device is exposed to monochromatic
radiation of known intensity. The surface potential after exposure is
measured by voltage probes 2 and 3. The device is finally exposed to an
erase lamp of appropriate intensity and any residual potential is measured
by voltage probe 4. The process is repeated with the magnitude of the
exposure automatically changed during the next cycle. The photodischarge
characteristics is obtained by plotting the potentials at voltage probes 2
and 3 as a function of light exposure. The charge acceptance and dark
decay can also be measured in the scanner.
Parking Deletion Test
A negative corotron is operated (with high voltage connected to
the corotron wire) opposite a grounded electrode for several hours. The
high voltage is turned off, and the corotron is placed (or parked) for thirty
minutes on a segment of the photoconductor device being tested. Only a
_19_
- 2i254~1
-
short middle segment of the device is thus exposed to the corotron
effluents. Unexposed regions on either side of the exposed regions are
used as controls. The photoconductor device is then tested in a scanner for
positive charging properties for systems employing donor type molecule~
These systems are operated with negative polarity corotron in the latent
image formation step. An electrically conductive surface region (excess
hole concentration) appears as a loss of positi - charge acceptance or
increased dark decay in the exposed regions (compared to the unexposed
control areas on either side of the short middle segment) Since the
electrically conductive region is located on the surface of the device, a
negative charge acceptance scan is not affected by the corotron effluent
exposure (negative charges do not move through a charge transport layer
made up of donor molecules). However, the excess carriers on the surface
cause surface conductivity resulting in loss of image resolution and, in
severe cases, causes deletion.
EXAMPLE 11
A photoreceptor is prepared by forming coatings using
conventional techniques on a substrate comprising a vacuum deposited
titanium layer on a polyethylene terephthalate film. The first deposited
coating is a siloxane barrier layer formed from hydrolyzed gamma
aminopropyl triethoxy silane having a thickness of 100 angstroms. The
second coating is an adhesive layer of polyester resin (PE 49,0000, available
from E. I. duPont de Nemours & Co.) having a thickness of S0 angstroms.
The next coating is a charge generator layer containing 35 percent by
weight vanadyl phthalocyanine particles obtained by the process as
disclosed in US-A 4,771,133, dispersed in a polyester resin (Vitel PE100,
available from Goodyear Tire and Rubber Co.) having a thickness of 1
micrometer. The next layer was a transport layer and was coated with a
solution containing one gram of N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-
(1,1'biphenyl)-4,4'~iamine and one gram of polycarbonate resin [poly(4,4'-
isopropylidene-diphenylene carbonate, available as MakrolonR from
Farbenfabricken Bayer A. G.l, dissolved in 1 1.5 grams of methylene chloride
-20-
- 212~431
solvent using a Bird coating applicator. The N,N'-diphenyl-N,N'-bis(3-
methyl-phenyl)-(1,1'biphenyl)-4,4'-diamine is an electrically active aromatic
diamine charge transport small molecule whereas the polycarbonate resin
is an electrically inactive film forming binder. The coated device was dried
at 80~C for half an hour in a forced air oven to form a 25 micrometer thick
charge transport layer.
EXAMPLE lll
A second photoreceptor device was coated as in Example ll. It
was thereafter coated with a 2 micrometer thick overcoat from a solution
containing 0.9 gram of N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-
(1,1'biphenyl)-4,4'-diamine, 0.1 gram of bis(4-diethylamino-2-
methylphenyl)phenylmethane and one gram of polycarbonate resin
[poly(4,4'-cyclohexylidine-diphenylene carbonatel dissolved in 23 grams of
toluene solvent using a Bird coating applicator. The device was dried in
forced air oven at 80~C for 30 minutes.
EXAMPLE IV
A third device was coated as in Example ll. It was thereafter
coated with a 2 micrometer thick overcoat from a solution containing 0.8
gram of N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-(1,1'biphenyl)-4,4'-
diamine, 0.2 gram of bis(4-diethylamino-2-methylphenyl)phenylmethane
and one gram of polycarbonate resin ~poly(4,4'-cyclohexylidine-
diphenylene carbonatel, dissolved in 23 grams of toluene solvent using a
Bird coating applicator. The device was dried in forced air oven at 80~C for
30 minutff.
EXAMPLE V
A fourth device was coated as in Example ll. It was thereafter
coated with a 2 micrometer thick overcoat from a solution containing 0 5
gram of N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-(1,1'biphenyl)-4,--
diamine, 0.5 gram of bis(4-diethylamino-2-methylphenyl)phenylmethane
and one gram of polycarbonate resin [poly(4,4'-cyclohexylidine-
212~431
diphenylene carbonate], dissolved in 23 grams of toluene solvent using aBird coating applicator. The device was dried in forced air oven at 80~C for
30 minutes.
EXAMPLE Vl
the four devices of Examples ll, Ill, IV and V were evaluated for
their sensitivity and cyclic stability properties in the scanner described in
Example 1. A slight increase in sensitivity was observed in the overcoated
devices. This increase corresponded to the increase in thickness by two
microns. The residual potential was equivalent (15 volts) for all four devices
and no cycle-up was observed when cycled for 10,000 cycles in a continuous
mode. The overcoat clearly did not introduce any defficiencies.
EXAMPLE Vll
The four devices of Examples ll, Ill, IV and V were evaluated for
their deletion properties by the parking deletion test described in Example
1. The corotron exposed region of the device in Example ll (without the
overcoat) was found to be very conductive (a loss of positive charge
acceptance of 600 volts). The loss of positive charge acceptance of the
overcoated devices in Examples lll, IV and V was very slight, indicating that
the surface region had been stabilized against corona induced conductivity
increases.
Although the invention has been described with reference to
specific pref~rred embodiments, it is not intended to be limited thereto,
rather those skilled in the art will recognize that variations and
modifications may be made therein which are within the spirit of the
invention and within the scope of the claims.
