Note: Descriptions are shown in the official language in which they were submitted.
212a429
PATENT APPLICATION
Attorney's Docket No. D/91641
OVERCOATING FOR MULTILAYERED ORGANIC PHOTORECEPTORS
CONTAINING A STABILIZER AND CHARGE TRANSPORT MOLECULES
BACKGROUND OF THE INVENTION
This invention relates in general to electrophotographic imaging
members and more specifically, to layered photoreceptor structures with
overcoatings containing a stabilizer and charge transport molecules 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
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structures wherein separate layers perform the functions of charge
generation and charge transport, respectively, and sing:e 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 optiônally 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 electron 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,
4,806,444, 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
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evaporation or deposition. The charge generator layers may also comprise
inorganic pigments of crystallme selenium and its ~lloy,; ~roup 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
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-
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 deletion" 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 presenc~
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 electricall~
conductive film. Howcvcr, dùring 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)
between t~vo 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
- 2125429
changes primarily occur at the surface region of the charge transport layer.
These changes are manife t~ 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 between extended cycling runs.
INFORMATION DISCLOSURE STATEMENT
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 binder 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.
US-A 4,515,882 to Mammino et al, issued May 7, 1985 - An
electrophotographic imaging system is disclosed which utilizes a member
~125429
comprising at least one photoconductive layer and an overcoating layer
comprising a film forminy 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 4,050,935 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
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 4,297,425 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 molecules dispersed in a polymeric binder.
In copending application entitled "LAYERED PHOTORECEPTOR
STRUCTURES WITH OVERCOATINGS CONTAINING A TRIPHENYL METHANE"
Docket Number D/91642, 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
charge transporting molecules dispersed in a first polymer binder, and an
overcoat layer comprising a triphenyl methane molecule dispersed in a
second polymer binder, the second polymer binder being soluble in a
solvent in which the first polymer binder is insoluble. The overcoat layer
may contain an optional charge transport molecule. 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
2125~29
molecules dispersed in a solution of a first polymer binder dissoived in a
first solvent, drying the c~ating to remove ~he 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. The entire disclosure of this copending application is incorporated
herein by reference.
Although acceptable images may be obtained when a chemical
stabilizer selected from the group consisting of certain nitrone,
isobenzofuran, hydroxyaromatic compounds and mixtures thereof are
incorporated within the bulk of the charge transport layers, the
photoreceptor can exhibit at least two deficiencies when subjected to
extensive cycling. One is that the presence of the nitrone, isobenzofuran,
hydroxyaromatic compounds and mixtures thereof 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 cycle-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 nitrone,
isobenzofuran, hydroxyaromatic compounds and mixtures thereof 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 stabilizers on the surface of the pigment in the
generator layer could result in loss of sensitivity of the device. These two
deficiencies limits the concentration of the nitrone, isobenzofuran,
hydroxyaromatic compounds and mixtures thereof that can be added in the
transport layer.
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Where photoreceptors containing nitrone, isobenzofuran,
hydroxyaromatic compounds and mixtures thereof in the charge transpor~
layer are overcoated, intermixing of the overcoat and the transport layers
occur which render the overcoat very ineffective. This intermixing leads to
the incorporation of nitrone, isobenzofuran, hydroxyaromatic compounds
and mixtures thereof in the bulk of the transport layer causing the
aforementioned cycle-up. Also, the intermixing causes a reduction of the
concentration of nitrone, isobenzofuran, hydroxyaromatic compounds and
mixtures thereof on the outer surface of the photoreceptor. The
concentration of nitrone, isobenzofuran, hydroxyaromatic compounds and
mixtures thereof 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 electrophotographic 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
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against corona effluents without an attendant reduction in transport
efficiency of transport layers
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 reduction 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 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 dissolved or 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 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 dissolved or molecularly dispersed in an
electrically inactiYe 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.
- 212~429
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 l Ir may
optionally be applied to the electrically conductive surface prior ~o 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
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 economicai 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,
-10-
- 212~429
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
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
electrodeposition. Typical metals include aluminum, zirconium, niobium,
tantalum, vanadium and hafnium, titanium, nickel, stainless steel,
chromium, tungsten, molybdenurn, and the like.
An optional hole blocking layer may be applied to the substrate.
Any suitable and conventional blocking layer capable of forming an
electronic 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 0.05 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, airdrying and the like.
2125~29
Any suitable polymeric film forming binder material may be
employed as the matrix in the charge generating (photogenerating) bir.der
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, polyvinyi acetals, polyamides, polyimides,
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
212S429
application techniques include spraying, dip coating, roll coating, wire
wound rod coating, vacuum sublimation and the like. ror 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, airdrying 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 polycarbonate. The term "dissolved" 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
expression "molecularly 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 generator 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. 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:
- 1 3 -
2125429
H3C /~\N~N)~CH3
The electricaily 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_cl
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, polyvinylcarbazole, 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
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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
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
stabilizer selected from the group consisting of certain monomeric nitrone,
isobenzofuran, hydroxyaromatic compounds and mixtures thereof 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:
2125429
O ~ ~ ~ O--C--
Bisphenol-Z-polycarbonate is soluble in and coated from toluene. The
expression "soluble" 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 chemical stabilizer in the overcoat layer can be any one of
several compounds described in US-A 4,599,286, the entire disclosure
thereof being incorporated herein by reference. In one embodiment, the
stabilizer is a nitrone compound represented by the following generic
structural formula:
-16-
212~29
C~
R1 C ~ N-- R2
H
wherein Rl is selected from the group consisting of a substituted and
unsubstituted phenyl group, fused ring aromatic group and heterocyclic
group, and R2 is selected from the group consisting of a linear or branched
alkyl group containing 1 to 20 carbon atoms, a fused ring aromatic group
and a heterocyclic group. Typical nitrone compounds represented by this
structural formula include, for example, t-butylphenylnitrone (also referred
to as N-tert-butyl-alpha-phenylnitrone), i-propylphenylnitrone, 4-
methylphenylphenylnitrone, t-butyl-4-methylphenylnitrone, and the like.
In another embodiment, the stabilizer is a isobenzofuran
compound represented by the following generic structural formula:
R7 R3
R5
R6 iW
R8 R4
wherein R3, R4, Rs, R6, R7, and R8 are independently selected from the
group consisting of substituted and unsubstituted alkyl groups containing 1
to 10 carbon atoms and substituted and unsubstituted phenyl groups.
Typical isobenzofuran compounds represented by this structural formula
include, for example, diphenylisobenzofurans, dimethylisobenzofurans,
2125429
diethylisobenzofurans, dipropylisobenzofurans, diisopropylisobenzofuran,
dibutylisobenzofurans, disQbutylisobenzofurans, diphenylisobenzofurans,
alkyl substituted phenyl isobenzofurans in which the alkyl group contains
from 1 to 4 carbon atoms, di(p-chlorophenyl)isobenzofuran, di(p-
cyanophenyl)isobenzofuran, and the like.
In still another embodiment, the stabilizer is a fused
hydroxyaromatic compounds represented by the following generic
structural formula:
Rg
R10 ~~~ R13
R~ /~R14
R12
wherein Rg, Rto, R11 and Rl2 are independently selected from the group
consisting of hydrogen, a hydroxyl group, an alkoxy group containing 1 to 6
carbon atoms and an alkyl group containing 1 to 6 carbon atoms, wherein
at least one or said Rg, Rlo, R1 1 and R12 is a hydroxyl group, and R13 and R14
are independently selected from hydrogen, an alkenyl group containing 3
to 40 carbon atoms and an alkyl group containing 1 to 40 carbon atoms.
In still another embodiment, the stabilizer is a monomeric or
polymeric phenolic compound represented by the following generic
structural formula:
-18-
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OH
R19 ~R1s
R18 R1 s
R17
wherein Rls, Rl6, Rlg and Rlg are independently selected from hydrogen, a
hydroxyl group, and substituted and unsubstituted groups selected from
the group consisting of a linear alkyl group containing 1 to 20 carbon
atoms, a branched alkyl group containing 1 to 20 carbon atoms, an alkenyl
group containing 1 to 20 carbon atoms, a phenyl group, a napthyl group,
and an alkoxy group containing 1 to 20 carbon atoms. Typical phenolic
compounds include, for example, 2-tert-butyl-4-methoxyphenol, 2,6-di-t-
butyl-4-methoxyphenol, hydroquinones, 2,6-di-tert-butyl-4-ethoxyphenol,
2,6-di-tert-butylphenol, 2,5-di-t-butyl-4-methoxyphenol, 2,6-di-t-butyl-p-
cresol, 2,4,6-triphenol, erythrityl tetrakis [beta-(4-hydroxy-3,5-di-t-
butylphenyl)propionate], and the like and mixtures thereof. Typical
substituted and unsubstituted napthol compounds include 1-hydroxy-4-
methyl-8-tert-butyl napthalene, 1-hydroxy-4-ehtyl-8-tert-butylnapthalene,
1-hydroxy-4-propyl-8-tert-butylnapthalene, 1-hydroxy-4-propoxy-8-tert-
butylnapthalene,1-hydroxy-4-butoxy-8-tert-butylnapthalene, 1-hydroxy-2-
tert-butyl-4-methylnapthalene, 1-hydroxy-2-tert-butyl-4-
ethylnapthalene,1 -hydroxy-2-tert-butyl-4-propylnapthalene, 1 -hydroxy-2-
tert-butyl-4-butylnapthalene, 1-hydroxy-2tert-butyl-4-methoxynapthalene,
1-hydroxy-2-tert-butyl-4-ethoxynapthalene, 1-hydroxy-2-tert-butyl-4-
propoxynapthalene, 1-hydroxy-2-tert-butyl-4-butoxynapthalene, 1-
hydroxy-2,8-di-tert-butyl-4-methylnapthalene, 1 -hydroxy-2,8-ditert-butyl-4-
ethylnapthalene, 1-hydroxy-2,8-di-tert-butyl-4-propylnapthalene, 1-
hydroxy-2,8-di-tert-butyl-4-butylnapthalene, 1-hydroxy-2,8-di-tert-butyl-4-
- 212S~29
methoxynpahthalene,1-hydroxy-2,8-di-tert-butyl-4-ethoxynapthalene, 1-
hydroxy-2,8-di-tert-butyl-~-propoxynapthalene, 1-hydroxy-2,8-di-tert-
butyl-4-butoxynapthalene, and the like and mixturesthereof.
Diphenylisobenzofuran, alpha-tocopherol, tetrakis [beta-(4-hydroxy-
3,5-di-t-butylphenyl) propionate] (Irganox 1010), and tert-
butylphenylnitrone are preferred stabilizers because they are non-toxic,
stable at the temperatures normally employed during photoreceptor
manufacture, soluble in the preferred transparent binders, readily available
and inexpensive.
The concentration of the charge transporting molecules in the
overcoat can be between about S percent by weight and about 50 percent
by weight based on the total weight of the dried overcoat. The
concentration of the chemical stabilizer of this invention in the overcoat
can be between about 0.5 percent by weight and about 30 by weight based
on the total weight of the dried overcoat. When less than about O.S
percent by weight of stabilizer is present in the overcoat, the
photoreceptor still exhibits considerable deletion. If the amount of
stabilizer in the overcoat exceeds about 30 percent by weight,
crystallization may occur resulting in residual cycle-up. The total combined
concentration of the charge transporting molecule and the stabilizer
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
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2125429
prevent a residual build up across the overcoat, and (2) it should not
intermix into the charge transport layer during the process of fabricatir:~
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
polymer 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 particles dispersed in a film forming binder.
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
el~_l,Gslatically 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 5 minutes and about 30 minutes, and
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repeating said depositing, exposing, developing and transferring steps at
least 10 addition~l times in anothersingie 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, however, that the invention can be practiced
with many types of compositions and can have many different uses in
accordance with the disclosure above and as pointed out hereinafter.
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
2125~29
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. However, the excess carried on
the surface can cause surface conductivity resulting in loss of image
resolution and, in severe cases, causes deletion .
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
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 molecules.
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 positive 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).
EXAMPLE ll
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 49,000~, available
from E. I. duPont de Nemours & Co.) having a thickness of 50 angstroms.
2I2~29
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'-diamine and one gram of polycarbonate resin [poly(4,4'-
isopropylidene-diphenylene carbonate, available as MakrolonR from
Farbenfabricken Bayer A. G.] dissolved in 11.5 grams of methylene chloride
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 moiecule 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. However, the excess carriers on the surface cause
surface conductivity resulting in loss of image resolution and, in severe
cases, causes deletion.
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 t-butylphenylnitrone 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 0.9
gram of N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-(1,1'biphenyl)-4,4'-
diamine, 0.1 gram of diphenylisobenzofuran and one gram of
polycarbonate resin lpoly(4,4'-cyclohexylidine-diphenylene carbonate~,
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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 V
A fourth device was coated as in Example ll. It was thereafter
coated with a 2 micrometer thick overcoat from a solution containing 0.9
gram of N,N'-diphenyl-N,N'-bis(3-methyl-phenyl)-(1,1'biphenyl)-4,4'-
diamine, 0.1 gram of alpha-tocopherol and one gram of polycarbonate
resin [poly(4,4'-cyclohexylidine-diphenylene carbonate], 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 Vl
The four devices of Examples ll, lll, 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 deficiencies.
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 preferred embodiments, it is not intended to be limited thereto,
2125~2g
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.
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