U.S. patent application number 16/414126 was filed with the patent office on 2019-09-05 for photoconductor having an overcoat with tetrafunctional radical polyermizable charge transport molecule and non-radical polymeriz.
The applicant listed for this patent is LEXMARK INTERNATIONAL, INC.. Invention is credited to MARK THOMAS BELLINO, DAVID GLENN BLACK, WEIMEI LUO.
Application Number | 20190271924 16/414126 |
Document ID | / |
Family ID | 67767379 |
Filed Date | 2019-09-05 |
View All Diagrams
United States Patent
Application |
20190271924 |
Kind Code |
A1 |
LUO; WEIMEI ; et
al. |
September 5, 2019 |
PHOTOCONDUCTOR HAVING AN OVERCOAT WITH TETRAFUNCTIONAL RADICAL
POLYERMIZABLE CHARGE TRANSPORT MOLECULE AND NON-RADICAL
POLYMERIZABLE ADDITIVE
Abstract
An organic photoconductor drum includes a support element, a
charge generation layer over the support element, a charge
transport layer over the charge generation layer, and an overcoat
layer. The overcoat layer includes a curable composition having
both a non-radical polymerizable additive and a charge transport
molecule containing four radical polymerizable functional groups
exemplified as: ##STR00001## where R.sub.1 and R.sub.2 contain a
spacer group and a radical polymerizable functional group, R.sub.3
and R.sub.4 are hydrogen atoms, and R.sub.5 and R.sub.6 contain a
spacer group and a radical polymerizable functional group wherein
the spacer group of R.sub.1, R.sub.2, R.sub.5, and R.sub.6, is an
ethyl group and the radical polymerizable functional group of
R.sub.1, R.sub.2, R.sub.5 and R.sub.6 is an acrylate group. The
non-radical polymerizable additive may be a surfactant. The curable
composition may also include a curing agent.
Inventors: |
LUO; WEIMEI; (LOUISVILLE,
CO) ; BLACK; DAVID GLENN; (BROOMFIELD, CO) ;
BELLINO; MARK THOMAS; (LOVELAND, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
LEXMARK INTERNATIONAL, INC. |
LEXINGTON |
KY |
US |
|
|
Family ID: |
67767379 |
Appl. No.: |
16/414126 |
Filed: |
May 16, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15898687 |
Feb 19, 2018 |
10331051 |
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16414126 |
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14983928 |
Dec 30, 2015 |
9927727 |
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15898687 |
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14145309 |
Dec 31, 2013 |
9256143 |
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14983928 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03G 5/14734 20130101;
G03G 7/0006 20130101; C07C 57/48 20130101; G03G 5/14791 20130101;
G03G 5/0525 20130101; C07C 57/50 20130101; G03G 5/14717 20130101;
G03G 5/0614 20130101; G03G 5/1473 20130101; G03G 5/14795 20130101;
C07C 13/567 20130101; G03G 5/071 20130101 |
International
Class: |
G03G 5/05 20060101
G03G005/05; G03G 5/147 20060101 G03G005/147; G03G 5/07 20060101
G03G005/07; C07C 13/567 20060101 C07C013/567; G03G 5/06 20060101
G03G005/06; G03G 7/00 20060101 G03G007/00; C07C 57/50 20060101
C07C057/50 |
Claims
1. An organic photoconductor drum, comprising: a support element; a
charge generation layer disposed over the support element; a charge
transport layer disposed over the charge generation layer; and an
overcoat layer including a curable composition including a
non-radical polymerizable additive and a charge transport molecule
containing four radical polymerizable functional groups exemplified
below: ##STR00012## where R.sub.1 and R.sub.2 contain a spacer
group and a radical polymerizable functional group, R.sub.3 and
R.sub.4 are hydrogen atoms, and R.sub.5 and R.sub.6 contain a
spacer group and a radical polymerizable functional group wherein
the spacer group of R.sub.1, R.sub.2, R.sub.5, and R.sub.6, is an
ethyl group and the radical polymerizable functional group of
R.sub.1, R.sub.2, R.sub.5 and R.sub.6 is an acrylate group.
2. The organic photoconductor drum of claim 1, wherein the
non-radical polymerizable additive is a surfactant.
3. The organic photoconductor drum of claim 2, wherein the
surfactant exists in an amount equal to or less than about 10% by
weight of the curable composition.
4. The organic photoconductor drum of claim 3, wherein the amount
is about 0.1% to about 5% by weight of the curable composition.
5. The organic photoconductor drum of claim 1, wherein the curable
composition further includes a curing agent.
6. The organic photoconductor drum of claim 5, wherein the curing
agent is a monomer or oligomer having at most five radical
polymerizable functional groups.
7. The organic photoconductor drum of claim 6, wherein the radical
polymerizable functional groups is selected from the group
consisting of acrylate group, methacrylate group, styrenic group,
allylic group, vinylic group, glycidyl ether group, epoxy group,
and combinations thereof.
8. The organic photoconductor of claim 1, wherein the charge
transport module exists in an amount of about 20% to about 80% by
weight of the curable composition.
9. The organic photoconductor of claim 1, wherein the spacer group
of R.sub.1 and R.sub.2 is an unbranched alkyl group-containing
between 1 and about 6 carbon atoms.
Description
CROSS REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
patent application Ser. No. 15/898,687, filed Feb. 19, 2018,
entitled "METHOD TO MAKE A PHOTOCONDUCTOR HAVING AN OVERCOAT WITH
TETRAFUNCTIONAL RADICAL POLYMERIZABLE CHARGE TRANSPORT MOLECULE",
which is a continuation application of U.S. patent application Ser.
No. 14/983,928, filed on Dec. 30, 2015, now U.S. Pat. No. 9,927,727
and issued Mar. 27, 2018, entitled "METHOD TO MAKE A PHOTOCONDUCTOR
HAVING AN OVERCOAT WITH TETRAFUNCTIONAL RADICAL POLYMERIZABLE
CHARGE TRANSPORT MOLECULES", which is a continuation application of
U.S. patent application Ser. No. 14/145,309, now U.S. Pat. No.
9,256,143, filed Dec. 31, 2013 and issued Feb. 9, 2016, entitled
"PHOTOCONDUCTOR OVERCOAT HAVING TETRAFUNCTIONAL RADICAL
POLYMERIZABLE CHARGE TRANSPORT MOLECULE", all of which are assigned
to the assignee of the present application.
BACKGROUND
1. Field of the Disclosure
[0002] The present disclosure relates generally to
electrophotographic image forming devices, and more particularly to
a method to make an organic photoconductor drum having and overcoat
with a tetrafunctional radical polymerizable charge transport
molecule.
2. Description of the Related Art
[0003] Organic photoconductor drums have generally replaced
inorganic photoconductor drums in electrophotographic image forming
device including copiers, facsimiles and laser printers due to
their performance and advantages. These advantages include improved
optical properties such as having a wide range of light absorbing
wavelengths, improved electrical properties such as having high
sensitivity and stable chargeability, availability of materials,
good manufacturability, low cost, and low toxicity.
[0004] While the performance and advantages offered by organic
photoconductor drums are significant, inorganic photoconductor
drums offer much higher durability. Inorganic photoconductor drums
(e.g., amorphous silicon photoconductor drums) are ceramic-based,
thus being extremely hard and abrasion resistant. The surface of
organic photoconductor drums is typically comprised of a low
molecular weight charge transport material, and an inert polymeric
binder. Therefore, the failure mechanism for organic photoconductor
drums typically arises from mechanical abrasion of the surface
layer due to repeated use. Abrasion of photoconductor drum surface
may arise from its interaction with print media (e.g. paper), paper
dust, or other components of the electrophotographic image forming
device.
[0005] The abrasion of the photoconductor drum surface degrades its
electrical properties, such as sensitivity and charging properties.
Electrical degradation results in poor image quality, such as lower
optical density, and background fouling. When a photoconductor drum
is locally abraded, images often have black toner bands due to the
inability to hold charge in the thinner regions. This black banding
often marks the end of the life of the photoconductor drum.
[0006] Increasing the life of the photoconductor drum will allow
the photoconductor drum to become a permanent part of the
electrophotographic image forming device. In other words, the
photoconductor drum will no longer be a replaceable unit nor be
viewed as a consumable. Photoconductor drums with a
life-of-the-printer will allow the printer to operate with lower
cost-per-page, more stable image quality, and less waste.
[0007] To achieve a long life photoconductor drum, especially with
an organic photoconductor drum, a protective overcoat layer may be
coated onto the surface of the photoconductor drum. An overcoat
layer formed from a crosslinkable silicon material has been known
to improve life of the photoconductor drums used for color
printers. However, such overcoat layer does not have the robustness
for edge wear of photoconductor drums used in mono printers. A
photoreceptor overcoat comprising a UV crosslinked layer of
hexacoordinate urethane acrylate and UV crosslinkable charge
transport molecule is disclosed in U.S. patent application Ser. No.
13/731,594 entitled "PHOTOCONDUCTOR OVERCOATS COMPRISING RADICAL
POLYMERIZABLE CHARGE TRANSPORT MOLECULES AND HEXA FUNCTIONAL
URETHANE ACRYLATES", which is assigned to the assign ee of the
present application and is incorporated by reference herein in its
entirety. While this overcoat system improves paper edge wear
versus crosslinkable silicon materials, a still greater improvement
is needed and desired.
[0008] While a robust overcoat layer improves the life of
photoconductor drums, a suitable overcoat layer is required that
does not significantly alter the electrophotographic properties of
the photoconductor drum. If the overcoat layer is too electrically
insulating, the photoconductor drum will not discharge and will
result in a poor latent image. On the other hand, if the overcoat
layer is too electrically conducting, then the electrostatic latent
image will spread resulting in a blurred image. Thus, a protective
overcoat layer that improves life of the photoconductor drum must
also allow charge migration to the photoconductor surface for
development of the latent image with toner.
SUMMARY
[0009] The present disclosure provides for an organic
photoconductor drum having a support element, a charge generation
layer over the support element, a charge transport layer over the
charge generation layer, and an overcoat layer. The overcoat layer
includes a curable composition having both a non-radical
polymerizable additive and a charge transport molecule containing
four radical polymerizable functional groups exemplified as:
##STR00002##
where R.sub.1 and R.sub.2 contain a spacer group and a radical
polymerizable functional group, R.sub.3 and R.sub.4 are hydrogen
atoms, and R.sub.5 and R.sub.6 contain a spacer group and a radical
polymerizable functional group wherein the spacer group of R.sub.1,
R.sub.2, R.sub.5, and R.sub.6, is an ethyl group and the radical
polymerizable functional group of R.sub.1, R.sub.2, R.sub.5 and
R.sub.6 is an acrylate group. The non-radical polymerizable
additive may be a surfactant. The curable composition may also
include a curing agent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying drawings incorporated in and forming a part
of the specification, illustrate several aspects of the present
disclosure, and together with the description serve to explain the
principles of the present disclosure.
[0011] FIG. 1 is a schematic view of an electrophotographic image
forming device.
[0012] FIG. 2 is a cross-sectional view of a photoconductor drum of
the electrophotographic image forming device.
DETAILED DESCRIPTION
[0013] It is to be understood that the disclosure is not limited in
its application to the details of construction and the arrangement
of components set forth in the following description or illustrated
in the drawings. The disclosure is capable of other embodiments and
of being practiced or of being carried out in various ways. Also,
it is to be understood that the phraseology and terminology used
herein is for the purpose of description and should not be regarded
as limiting. The use of "including," "comprising," or "having" and
variations thereof herein is meant to encompass the items listed
thereafter and equivalents thereof as well as additional items.
Further, the terms "a" and "an" herein do not denote a limitation
of quantity, but rather denote the presence of at least one of the
referenced item.
[0014] FIG. 1 illustrates a schematic representation of an example
electrophotographic image forming device 100. Image forming device
100 includes a photoconductor drum 101, a charge roll 110, a
developer unit 120, and a cleaner unit 130. The electrophotographic
printing process is well known in the art and, therefore, is
described briefly herein. During a print operation, charge roll 110
charges the surface of photoconductor drum 101. The charged surface
of photoconductor drum 101 is then selectively exposed to a laser
light source 140 to form an electrostatic latent image on
photoconductor drum 101 corresponding to the image being printed.
Charged toner from developer unit 120 is picked up by the latent
image on photoconductor drum 101 creating a toned image.
[0015] Developer unit 120 includes a toner sump 122 having toner
particles stored therein and a developer roll 124 that supplies
toner from toner sump 122 to photoconductor drum 101. Developer
roll 124 is electrically charged and electrostatically attracts the
toner particles from toner sump 122. A doctor blade 126 disposed
along developer roll 124 provides a substantially uniform layer of
toner on developer roll 124 for subsequent transfer to
photoconductor drum 101. As developer roll 124 and photoconductor
drum 101 rotate, toner particles are electrostatically transferred
from developer roll 124 to the latent image on photoconductor drum
101 forming a toned image on the surface of photoconductor drum
101. In one embodiment, developer roll 124 and photoconductor drum
101 rotate in the same rotational direction such that their
adjacent surfaces move in opposite directions to facilitate the
transfer of toner from developer roll 124 to photoconductor drum
101. A toner adder roll (not shown) may also be provided to supply
toner from toner sump 122 to developer roll 124. Further, one or
more agitators (not shown) may be provided in toner sump 122 to
distribute the toner therein and to break up any clumped toner.
[0016] The toned image is then transferred from photoconductor drum
101 to print media 150 (e.g., paper) either directly by
photoconductor drum 101 or indirectly by an intermediate transfer
member. A fusing unit (not shown) fuses the toner to print media
150. A cleaning blade 132 (or cleaning roll) of cleaner unit 130
removes any residual toner adhering to photoconductor drum 101
after the toner is transferred to print media 150. Waste toner from
cleaning blade 132 is held in a waste toner sump 134 in cleaning
unit 130. The cleaned surface of photoconductor drum 101 is then
ready to be charged again and exposed to laser light source 140 to
continue the printing cycle.
[0017] The components of image forming device 100 are replaceable
as desired. For example, in one embodiment, developer unit 120 is
housed in a replaceable unit with photoconductor drum 101, cleaner
unit 130 and the main toner supply of image forming device 100. In
another embodiment, developer unit 120 is provided with
photoconductor drum 101 and cleaner unit 130 in a first replaceable
unit while the main toner supply of image forming device 100 is
housed in a second replaceable unit. In another embodiment,
developer unit 120 is provided with the main toner supply of image
forming device 100 in a first replaceable unit and photoconductor
drum 101 and cleaner unit 130 are provided in a second replaceable
unit. Further, any other combination of replaceable units may be
used as desired. In some example embodiment, the photoconductor
drum 101 may not be replaced and is a permanent component of the
image forming device 100.
[0018] FIG. 2 illustrates an example photoconductor drum 101 in
more detail. In this example embodiment, the photoconductor drum
101 is an organic photoconductor drum and includes a support
element 210, a charge generation layer 220 disposed over the
support element 210, a charge transport layer 230 disposed over the
charge generation layer 220, and a protective overcoat layer 240
formed as an outermost layer of the photoconductor drum 101.
Additional layers may be included between the support element 210,
the charge generation layer 220 and the charge transport layer 230,
including adhesive and/or coating layers.
[0019] The support element 210 as illustrated in FIG. 2 is
generally cylindrical. However, the support element 210 may assume
other shapes or may be formed into a belt. In one example
embodiment, the support element 210 may be formed from a conductive
material, such as aluminum, iron, copper, gold, silver, etc. as
well as alloys thereof. The surfaces of the support element 210 may
be treated, such as by anodizing and/or sealing. In some example
embodiment, the support element 210 may be formed from a polymeric
material and coated with a conductive coating.
[0020] The charge generation layer 220 is designed for the
photogeneration of charge carriers. The charge generation layer 220
may include a binder and a charge generation compound. The charge
generation compound may be understood as any compound that may
generate a charge carrier in response to light. In one example
embodiment, the charge generation compound may comprise a pigment
being dispersed evenly in one or more types of binders.
[0021] The charge transport layer 230 is designed to transport the
generated charges. The charge transport layer 230 may include a
binder and a charge transport compound. The charge transport
compound may be understood as any compound that may contribute to
surface charge retention in the dark and to charge transport under
light exposure. In one example embodiment, the charge transport
compounds may include organic materials capable of accepting and
transporting charges.
[0022] In an example embodiment, the charge generation layer 220
and the charge transport layer 230 are configured to combine in a
single layer. In such configuration, the charge generation compound
and charge transport compound are mixed in a single layer.
[0023] The overcoat layer 240 is designed to protect the
photoconductor drum 101 from wear and abrasion without altering the
electrophotographic properties, thus extending the service life of
the photoconductor drum 101. The overcoat layer 240 has a thickness
of about 0.1 .mu.m to about 10 .mu.m. Specifically, the overcoat
layer 240 has a thickness of about 1 .mu.m to about 6 .mu.m, and
more specifically a thickness of about 3 .mu.m to about 5 .mu.m.
The thickness of the overcoat layer 240 is kept at a range that
will not provide adverse effect to the electrophotographic
properties of the photoconductor drum 101.
[0024] The overcoat layer comprises the cured, or substantially
crosslinked, product of a crosslinkable hole transporting material
containing four radical polymerizable functional groups and at
least one curing agent. The overcoat layer may further comprise an
optional non-crosslinkable additive.
[0025] The terms "crosslinkable" and "radical polymerizable," and
derivatives thereof, may be used interchangeably. "Cured" herein
refers to, for example, a state in which the crosslinkable charge
transport molecule containing four radical polymerizable groups and
the at least one curing agent in the coating solution form a
crosslinked or substantially crosslinked product. "Substantially
crosslinked" in embodiments refers to, for example, a state in
which about 60% to 100% of the charge transport compounds in the
overcoat composition, for example about 70% to 100% or about 80% to
100%, are covalently bound in the composition. Curing in the
present invention occurs by exposing the curable composition to
ionizing electromagnetic radiation of suitable wavelength or by
exposure to an electron beam. Crosslinking of the reactive
components occurs following application of the overcoat coating
composition to the photoconductor.
[0026] The following discussion seeks to define the crosslinkable
charge transport molecule containing four radical polymerizable
groups by describing the fluorene and diphenylamine fragments
separately. The crosslinkable charge transport compound of the
present invention comprises a nitrogen atom bonded directly to one
fluorene group and two phenyl groups. Fluorene has the following
molecular structure:
##STR00003##
[0027] The fluorene moiety may be fluorene or a fluorene
derivative. Fluorene derivatives have the above core structure,
with different groups bonded to the core structure. By convention,
the carbon atoms are numbered 1-9. The crosslinkable compound of
the present invention is substituted in the 9-position with two
functional groups comprising a radical polymerizable group. The
substituted fluorene group thus has the molecular structure show
below:
##STR00004##
[0028] The substituent's R.sub.1 and R.sub.2 are functional groups
containing a spacer group and a radical polymerizable group. The
spacer group separates the 9-position of the fluorene ring from the
radical polymerizable functional group. The bonds are represented
with R.sub.1 coming out of the page, and R.sub.2 going behind the
page. In other words, R.sub.1 and R.sub.2 are oriented
perpendicular to the plane of the fluorene group. In principle,
R.sub.1 and R.sub.2 may be the same or different functional group
containing a spacer group and a radical polymerizable group. In a
practical sense, R.sub.1 and R.sub.2 are the same functional group
containing a spacer group and a radical polymerizable group. The
radical polymerizable group is separated from the 9-position of the
fluorene ring by a spacer group of at least one carbon. In
principle, this spacer group may be any branched or unbranched
saturated hydrocarbon group having the general formula
C.sub.nH.sub.2n, wherein n is, for example, a number from 1 to
about 100 or more, such as methyl, ethyl, n-propyl, isopropyl,
n-butyl, isobutyl, tert-butyl, octyl, decyl, tetradecyl, hexadecyl,
eicosyl, tetracosyl, and the like. As used herein, the term spacer
group is defined as a C.sub.nH.sub.2n group. In a practical sense,
the spacer group of R.sub.1 and R.sub.2 is an unbranched alkyl
group containing between 1 and about 12 carbon atoms. More
specifically, the alkyl group is an unbranched alkyl group
containing between 1 and 6 carbon atoms. In an example embodiment,
the spacer group of R.sub.1 and R.sub.2 is an ethyl group.
[0029] The radical polymerizable group of R.sub.1 and R.sub.2 is
any functional group that is capable of undergoing radical
polymerization (crosslinking) upon absorption of electromagnetic
radiation, or by exposure to an electron beam. The radical
polymerizable group may be selected from the group consisting of
acrylate group, methacrylate group, styrenic group, allylic group,
vinylic group, glycidyl ether group and epoxy group. In an example
embodiment, the radical polymerizable group is an acrylate group.
The substituted fluorene group thus has the molecular structure
shown below:
##STR00005##
[0030] R.sub.3 and R.sub.4 may be the same or different from
R.sub.1 and R.sub.2, R.sub.3 and R.sub.4 may both contain a radical
polymerizabie functional group, both contain a non-radical
polymerizable group, or contain one each of a radical polymerizable
group and a non-radical polymerizable group.
[0031] The radical polymerizable groups maw be selected from the
group consisting of acrylate group, methacrylate group, styrenic
group, allylic group, vinylic group, glycidyl ether group and epoxy
group. The non-radical polymerizable groups may be any group
without limitation, including H, an alkyl group, an alkoxyl group,
an aryl group, an aryl alkyl groups, an alcohol group, an alkyl
alcohol group and the like. R.sub.3 and R.sub.4 may be covalently
bonded to any of the 1-8 positions of the fluorene group. In an
example embodiment, R.sub.3 and R.sub.4 are hydrogen atoms. The
substituted fluorene group thus has the molecular structure shown
below:
##STR00006##
[0032] The crosslinkable compound of the present invention also
comprises two phenyl groups. The phenyl groups further contain
substituents R.sub.5 and R.sub.6. The substituted phenyl groups are
exemplified for clarity as the diphenylamine fragment shown
below:
##STR00007##
[0033] The substituents R.sub.5 and R.sub.6 comprise a spacing
group and a radical polymerizable group. The spacer group separates
the phenyl group from the radical polymerizable functional group.
The bonds are represented with the bonds for R.sub.5 and R.sub.6 in
the same plane as the phenyl group. In principle, R.sub.5 and
R.sub.6 may be the same or different functional group containing a
spacer group and a radical polymerizable group. In a practical
sense, R.sub.5 and R.sub.6 are the same functional group containing
a spacer group and a radical polymerizable group. The radical
polymerizable group is separated from the phenyl group by a spacer
group of at least one carbon. In principle, this spacer group may
be any branched or unbranched saturated hydrocarbon group having
the general formula C.sub.nH.sub.2n, wherein n is, for example, a
number from 1 to about 100 or more, such as methyl, ethyl,
n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, octyl, decyl,
tetradecyl, hexadecyl, eicosyl, tetracosyl, and the like. In a
practical sense, the spacer group of R.sub.5 and R.sub.6 is an
unbranched alkyl group containing between 1 and about 12 carbon
atoms. More specifically, the alkyl group is an unbranched alkyl
group containing between 1 and 6 carbon atoms. In an example
embodiment, the spacer group of R.sub.5 and R.sub.6 is an ethyl
group.
[0034] The radical polymerizable group of R.sub.5 and R.sub.6 are
both located in the para position relative to the nitrogen. The
inventors have found that this regiochemistry gives a crosslinkable
charge transport compound with electrical properties suitable for a
photoconductor of the present invention. The radical polymerizable
group of R.sub.5 and R.sub.6 is any functional group that is
capable of undergoing radical polymerization (crosslinking) upon
absorption of electromagnetic radiation, or by exposure to an
electron beam. The radical polymerizable group may be selected from
the group consisting of acrylate group, methacrylate group,
styrenic group, allylic group, vinylic group, glycidyl ether group
and epoxy group. In an example embodiment, the radical
polymerizable group is an acrylate group. The radical polymerizable
group of R.sub.5 and R.sub.6 may be the same or different from the
radical polymerizable group of R.sub.1 and R.sub.2. In an example
embodiment, the radical polymerizable group is an acrylate
group.
[0035] Combining the fluorene and diphenylamine fragments gives the
crosslinkable charge transport molecule of the present invention
with the molecular structure shown below:
##STR00008##
[0036] The curable composition of the present invention also
includes a curing agent. The general purpose of the curing agent is
to further improve the abrasion resistance of the overcoat. The
curing agent may also improve the adhesion of the cured overcoat to
the underlying layer. In one embodiment, the curing agent is a
urethane resin containing six radical polymerizable functional
groups. The radical polymerizable groups may be selected from the
group consisting of acrylate group, methacrylate group, styrenic
group, allylic group, vinylic group, glycidyl ether group and epoxy
group. In another embodiment, the radical polymerizable group is an
acrylate. In an example embodiment, the curing agent is a urethane
acrylate containing six acrylate groups of the following
structure:
##STR00009##
and is commercially available from Cytec Industries under the trade
name EBECRYL 8301.
[0037] The synthesis of urethane acrylates involves reaction of a
diisocyanate with pentaerythritol triacrylate in the presence of a
catalyst. In a general sense, the choice of isocyanate and/or
hydroxy acrylate plays a large role in determining the mechanical
and thermal properties of the radically cured material. Curing of
urethane acrylates, such as the material described above, creates a
3-dimensionally crosslinked structure. Increasing the crosslink
density of the radically cured material is one way to improve the
mechanical and thermal properties of the materials. Urethane resins
containing six acrylate groups are preferred curing agents for the
present invention since crosslink density increases with the number
of radical polymerizable functional groups. High crosslink density
is known to improve properties such as abrasion and chemical
resistance. The crosslinked 3-dimensional network should be
homogeneous throughout the cured material, since this improves
mechanical and thermal properties. Homogeneous crosslinking is also
important for applications requiring a high degree of optical
transparency.
[0038] The curable composition comprising a charge transport
molecule containing four radical polymerizable functional groups
and optional urethane resin containing six radical polymerizable
functional groups exemplifies the overcoat layer 240 with excellent
abrasion resistance. In an electrophotographic printer, such as a
laser printer, an electrostatic image is created by illuminating a
portion of the photoconductor surface in an image-wise manner. The
wavelength of light used for this illumination is most typically
matched to the absorption max of a charge generation material, such
as titanylphthalocyanine. Absorption of light results in creation
of an electron-hole pair. Under the influence of a strong
electrical field, the electron and hole (radical cation) dissociate
and migrate in a field-directed manner. Photoconductors operating
in a negative charging manner moves holes to the surface and
electrons to ground. The holes discharge the photoconductor
surface, thus leading to creation of the latent image. Cured
overcoats comprising a charge transport molecule containing four
radical polymerizable functional groups provides electrical
properties that approach those of the underlying charge transport
layer 230. Combining a charge transport molecule containing four
radical polymerizable functional groups with a urethane resin
containing six radical polymerizable groups provides an overcoat
240 with improved abrasion resistance, along with excellent
electrical properties for the photoconductor drum 101.
[0039] Ultimately this overcoat formulation of the present
invention leads to a photoconductor drum having an `ultra long
life`, thereby allowing a consumer to successfully print at least
100,000 pages on their printer before they have to go purchase a
replacement photoconductor drum.
[0040] In one embodiment, the curable composition comprises a
combination of a charge transport molecule containing four radical
polymerizable functional groups and a curing agent comprising a
urethane resin containing six radical polymerizable functional
groups. The curable composition includes about 20 to about 80
percent by weight of the charge transport molecule containing four
radical polymerizable functional groups, and about 20 to about 80
percent by weight of urethane resin containing six radical
polymerizable functional groups. In more particular, the curable
composition includes about 40 to about 60 percent by weight of
charge transport molecule containing four radical polymerizable
functional groups and about 40 to about 60 percent by weight of the
urethane resin containing six radical polymerizable functional
groups. This combination provides both the charge transporting
properties of the charge transport molecule containing four radical
crosslinkable functional groups with the abrasion resistance of the
urethane resin containing six radical polymerizable functional
groups. Additionally, the overcoats of the present invention have
(1) excellent adhesion to the photoconductor surface, (2) optical
transparency and (3) crack free. Overcoat delamination (poor
adhesion) from the photoconductor surface has been noted as a
problem in the prior art.
[0041] Overcoat layers are typically coated in solvent systems
designed to solubilize components of the overcoat formulation,
while minimizing dissolution of the underlying photoconductor
structure. Dissolution of components comprising the underlying
photoconductor results in materials with no radical polymerizable
functionality entering the overcoat layer. The result is
dramatically lower crosslinking density and lower abrasion
resistance since the properties of the overcoat layer are optimized
by an uninterrupted 3-dimensional network. Ideally, the overcoat
layer is distinct from the underlying photoconductor surface.
However, the interface between the overcoat and the photoconductor
surface often lacks the chemical interactions required for strong
adhesion. The overcoats of the present invention have excellent
adhesion to the photoconductor surface throughout the print life of
the photoconductor. The overcoat must also be optically
transparent. Illumination of the photoconductor in an image-wise
manner requires that layers not involved in the charge generation
process be transparent to the incident light. Additionally, optical
transparency is an indicator of material and crosslink homogeneity
within the overcoat structure. The overcoats of the present
invention have a high degree of optical transparency throughout the
print life of the photoconductor. The overcoat must also be crack
free. UV or Electron beam cured films often exhibit cracks as a
result of unrelieved internal stress. These cracks will manifest
immediately in print and will dramatically decrease the functional
life of the overcoat. The overcoats of the present invention are
crack free throughout the print life of the photoconductor.
[0042] The curable composition may further include a curing agent
comprising a monomer or oligomer having at most five radical
polymerizable functional groups. The radical polymerizable
functional groups of the monomer or oligomer may be selected from
the group consisting of acrylate group, methacrylate group,
styrenic group, allylic group, vinylic group, glycidyl ether group,
epoxy group, or combinations thereof. Suitable examples of
mono-functional monomer or oligomer include, but are not limited
to, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl
methacrylate, propyl acrylate, propyl methacrylate, isobornyl
acrylate, isobornyl methacrylate, 2-phenoxyethyl acrylate, and
lauryl methacrylate.
[0043] Suitable examples of di-functional monomer or oligomer
include, but are not limited to, diacrylates and dimethacrylates,
comprising 1,4-butanediol diacrylate, 1,4-butanediol
dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol
diacrylate, diethylene glycol dimethacrylate, triethylene glycol
diacrylate, triethylene glycol dimethacrylate, 1,3-butylene glycol
diacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol
dimethacrylate, 1,12-dodecanediol methacrylate, tripropylene glycol
diacrylate, 1,3-butylene glycol dimethacrylate, 1,3-butylene glycol
dimethacrylate, cyclohexane dimethanol diacrylate esters, or
cyclohexane dimethanol dimethacrylate esters.
[0044] Suitable examples of tri-functional monomer or oligomer
include, but are not limited to, trimethylolpropane triacrylate,
trimethylolpropane trimethacrylate, hydroxypropyl acrylate-modified
trimethylolpropane triacrylate, ethylene oxide-modified
trimethylolpropane triacrylate, propylene oxide-modified
trimethylolpropane triacrylate, and caprolactone-modified
trimethylolpropane triacrylate. More specifically, the
tri-functional monomer or oligomer includes propoxylated (3)
trimethylolpropane triacrylate, ethoxylated (6) trimethylolpropane
triacrylate, propoxylated (6) trimethylolpropane triacrylate, and
ethoxylated (9) trimethylolpropane triacrylate.
[0045] Suitable examples of monomers or oligomers having four
radical polymerizable functional groups include, but are not
limited to, pentaerythritol tetraacrylate, di-trimethylolpropane
tetraacrylate, and ethoxylated pentaerythritol tetraacrylate.
[0046] Suitable examples of monomers or oligomers having five
radical polymerizable functional groups include, but are not
limited to, pentaacrylate esters and dipentaerythritol
pentaacrylate esters.
[0047] The curable composition may further include a non-radical
polymerizable additive such as a surfactant at an amount equal to
or less than about 10 percent by weight of the curable composition.
More specifically, the amount of non-radical polymerizable additive
is about 0.1 to about 5 percent by weight of the curable
composition. The non-radical polymerizable additive may improve
coating uniformity of the curable composition.
[0048] The curable composition is prepared by mixing the charge
transport molecule containing four radical polymerizable groups and
optional curing agent in a solvent. The solvent may include organic
solvents such as tetrahydrofuran (THF), toluene, alkanes such as
hexane, butanone, cyclohexanone and alcohols. The solvent may
include a mixture of two or more organic solvents to solubilize the
crosslinkable charge transport compound containing four radical
polymerizable functional groups and the urethane resin having six
radical polymerizable functional groups.
[0049] The curable composition may be coated on the outermost
surface of the photoconductor drum 101 through dipping or spraying.
If the curable composition is applied through dip coating, an
alcohol is used as the solvent to minimize dissolution of the
components of the charge transport layer 230. The alcohol solvent
includes isopropanol, methanol, ethanol, butanol, or combinations
thereof.
[0050] The coated curable composition is then exposed a radiation
source of sufficient energy to induce formation of free radicals to
initiate the crosslinking reaction. The exposed composition is then
post-baked to anneal and relieve stresses in the coating. The
radiation source of sufficient energy to induce formation of free
radicals is either a UV source, or an electron beam source. If a UV
source is used to generate free radicals, the curable composition
may contain a photoinitiator.
[0051] Specific examples of photo initiators for use under UV cure
conditions include acetone or ketal photo polymerization initiators
such as diethoxyacetophenone,
2,2-dimethoxy-1,2-diphenylethane-1-one,
1-hydroxy-cyclohexyl-phenyl-ketone,
4-(2-hydroxyethoxy)phenyl-(2-hydroxy-2-propyl)ketone,
2-benzyl-2-dimethylamino-1-(4-molpholinophenyl)butanone-1,2-hydroxy-2-met-
hyl-1-phenylpropane-1-one and
1-phenyl-1,2-propanedion-2-(o-ethoxycarbonyl)oxime; benzoinether
photo polymerization initiators such as benzoin,
benzoinmethylether, benzoinethylether, benzoinisobutylether and
benzoinisopropylether; benzophenone photo polymerization initiators
such as benzophenone, 4-hydroxybenzophenone,
o-benzoylmethylbenzoate, 2-benzoylnaphthalene, 4-benzoylviphenyl,
4-benzoylphenylether, acrylated benzophenone and
1,4-benzoylbenzene; thioxanthone photo polymerization initiators
such as 2-isopropylthioxanthone, 2-chlorothioxanthone,
2,4-dimethylthioxanthone, 2,4-diethylthioxanthone and
2,4-dichlorothioxanthone; phenylglyoxylate photoinitiators such as
methylbenzoylformate and other photo polymerization initiators such
as ethylanthraquinone,
2,4,6-trimethylbenzoyldiphenylphosphineoxide,
2,4,6-trimethylbenzoyldiphenylethoxyphosphineoxide,
bis(2,4,6-trimethylbenzoyl)phenylphosphineoxide,
bis(2,4-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphineoxide,
methylphenylglyoxyester, 9,10-phenanthrene, acridine compounds,
triazine compounds and imidazole compounds. Further, a material
having a photo polymerizing effect can be used alone or in
combination with the above-mentioned photo polymerization
initiators. Specific examples of the materials include
triethanolamine, methyldiethanol amine,
4-dimethylaminoethylbenzoate, 4-dimethylaminoisoamylbenzoate,
ethyl(2-dimethylamino)benzoate and 4,4-dimethylaminobenzophenone.
These polymerization initiators can be used alone or in
combination. The loading of photoinitiator is between about 0.5 to
about 20 parts by weight and more specifically from about 2 to
about 10 parts by weight per 100 parts by weight of the curable
composition.
[0052] Curing the composition by electron beam does not require the
presence of a photoinitiator and thus may result in greater
crosslink density. In an example embodiment, the radiation source
of sufficient energy to induce formation of free radicals is
electron beam.
Preparation of Example Photoconductor Drum
[0053] An Example Photoconductor Drum was formed using an aluminum
substrate, a charge generation layer coated onto the aluminum
substrate, and a charge transport layer coated on top of the charge
generation layer.
[0054] The charge generation layer was prepared from a dispersion
including type IV titanyl phthalocyanine, polyvinylbutyral,
poly(methyl-phenyl)siloxane and polyhydroxystyrene at a weight
ratio of 45:27.5:24.75:2.75 in a mixture of 2-butanone and
cyclohexanone solvents. The polyvinylbutyral is available under the
trade name BX-1 by Sekisui Chemical Co., Ltd. The charge generation
dispersion was coated onto the aluminum substrate through dip
coating and dried at 100.degree. C. for 15 minutes to form the
charge generation layer having a thickness of less than 1 .mu.m,
specifically a thickness of about 0.2 .mu.m to about 0.3 m.
[0055] The charge transport layer was prepared from a formulation
including terphenyl diamine derivatives and polycarbonate at a
weight ratio of 50:50 in a mixed solvent of THF and 1,4-dioxane.
The charge transport formulation was coated on top of the charge
generation layer and cured at 120.degree. C. for 1 hour to form the
charge transport layer having a thickness of about 17 .mu.m to
about 19 .mu.m as measured by an eddy current tester.
Example 1
[0056] The inventive overcoat layer of the present invention was
prepared from a formulation including EBECRYL 8301 (20 g) and
ethanol (80 g) and the crosslinkable charge transport molecule
containing four radical polymerizable functional groups (20 g)
shown below:
##STR00010##
[0057] The formulation was coated through dip coating on the outer
surface of the Example Photoconductor Drum formed as outlined
above. The coated layer was then exposed to an electron beam source
at an accelerating voltage of 90 kV, a current of 3 mA, and an
exposure time of 1.2 seconds. The electron beam cured photoreceptor
was then thermally cured at 120.degree. C. for 60 minutes. The
cured layer forms the overcoat layer having a thickness of about
4.3 m as measured by an eddy current tester.
Comparative Example 1
[0058] An overcoat layer was prepared from a formulation including
EBECRYL 8301 (20 g) and ethanol (80 g) and a crosslinkable charge
transport molecule containing two radical polymerizable functional
groups (20 g) shown below:
##STR00011##
[0059] The formulation was coated through dip coating on the outer
surface of the Example Photoconductor Drum formed as outlined
above. The coated layer was then exposed to an electron beam source
at an accelerating voltage of 90 kV, a current of 3 mA, and an
exposure time of 1.2 seconds. The electron beam cured photoreceptor
was then thermally cured at 120.degree. C. for 60 minutes. The
cured layer forms the overcoat layer having a thickness of about
4.5 m as measured by an eddy current tester.
[0060] Photoconductor drums Prepared in Example 1 and Comparative
Example 1 were installed in an electrophotographic image forming
device. The electrophotographic image forming device was then
operated at 50 ppm in a four-page and pause run mode. After 40 k
prints, the test was stopped and the overcoat thickness loss was
determined using an eddy current tester. The initial overcoat
thickness (.mu.m), image quality throughout the test, and maximum
overcoat thickness loss (.mu.m) after 40 k prints are summarized in
Table 1.
TABLE-US-00001 TABLE 1 Max. Overcoat Overcoat Thickness Loss
(.mu.m) Example Thickness (.mu.m) Image Quality (After 40k Prints)
Example 1 4.3 Excellent 0.3 Comparative 4.5 Excellent 2.2 Example
1
[0061] The overcoat thickness loss from the overcoat of Example 1
is dramatically lower than that of Comparative Example 1. Without
wishing to be bound by theory, the inventors believe that there are
at least three reasons for the improved abrasion resistance
imparted by the crosslinkable charge transport molecule containing
four functional groups of the present invention. (1) The presence
of four acrylate groups, or more generally, four radical
polymerizable functional groups, increases the crosslink density
relative to a charge transport molecule with only two radical
polymerizable functional groups. (2) The ethyl acrylate groups in
the crosslinkable are arrayed such that the groups bonded to the
9-position of the fluorene are perpendicular to the groups bonded
to the para position of the two phenyl rings. The inventors believe
that this orientation further improves the 3-dimensional
crosslinking imparted by the four radical polymerizable functional
groups. (3) The spacer group separating the acrylates from the
fluorene group and the phenyl groups is an ethyl group. The spacer
group separating the acrylate from the phenyl group in Comparative
Example 1 is a propyl group. The ethyl group allows for still
greater crosslink density versus the propyl group by simply
removing a methylene fragment.
[0062] The foregoing description illustrates various aspects of the
present disclosure. It is not intended to be exhaustive. Rather, it
is chosen to illustrate the principles of the present disclosure
and its practical application to enable one of ordinary skill in
the art to utilize the present disclosure, including its various
modifications that naturally follow. All modifications and
variations are contemplated within the scope of the present
disclosure as determined by the appended claims. Relatively
apparent modifications include combining one or more features of
various embodiments with features of other embodiments.
* * * * *