Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROP~OTOGRAPHIC P~OTOR~CEPTOR
E~IE~ OF THE INVENTION
The invention relates to an electrophotographic
photoreceptor. More particularly, it relates to the electro-
photographic photoreceptor having an excellent durability.
BA~KGOUND OF THE INVENTION
In recent years, the electrophotography has been applied to
copying machines as well as various printers, because they can
give images with high qualities without delay. As a
photoreceptor which plays an important role in the
electrophotography, the photoreceptor comprising an inorganic
photoconductive substance such as selenium, arsenic-selenium
alloy, cadmium sulfide, zinc oxide and the like has been used.
More recently, the photoreceptor comprising an organic
photoconductive substance was proposed. The latter has
advantages, i.e. it is not a pollutant and it has a film-
formability and a shapability.
As one of the organic photoreceptors, the so-called
"laminated-type photoreceptor" in which a charge generation
layer and a charge transport layer are successively laminated
was developed. The laminated-type photoreceptor is increasingly
interested in and is expected to be widely used in the near
future because it has the following advantages:
(1) the photoreceptor having a high sensitivity can be obtained
by suitably selecting and combining a charge generation material
and a charge transport material;
(2) the photoreceptor having a high safety can be obtained
because the charge generation material and the charge transport
material can be selected from a wide range of the materials; and
(3) the photoreceptor can be prepared by a simple coating and
thus, it can be prepared with low costs.
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;owever, tr.e ?rior laminated-type pho.oreceptors nave poor
durabllities. '~ en they are repeatedly used, electric problems
such as a lowerins in charyed potential, an accumulation of a
residual potential and a change in sensitivity are caused. The
problem as to the accumulation of the residual potential is
especially serious because if the residual potential 1s
accumulated, much copies could not be obtained. Such an
accumulation of the residual potential is considered to arise
from some causes, among which impurities present in the charge
transport layer are important. The impurities include
impurities originally present in a composition used for forming
the charge transport layer, impurities produced after the charge
transport layer is subjected to a corona discharge and
impuritles produced by decomposing after the charge transport
layer is exposed repeatedly during an exposure step and an
erasing step and after the charge transport layer is subjected
to an outside light during a maintenance operation. These
impurities trap carriers, thereby unmovable space charges which
remain as the residual charges in the charge transport layer are
produced.
As an other cause of lowering the durability of the
laminated-type photoreceptor, the reduction in thickness of the
charge transport layer due to mechanical stresses, for example
an abrasion such as a blade cleaning, leading to the lowering in
electric properties is mentioned.
An increase in thickness of the charge transport layer is
effective for preventing the reduction in thickness of the
charge transport layer and increasing the sensitivity of the
photoreceptor, but it is accompanied with the increase of
amounts of the impurities, thereby the accumulation of the
residual potential makes more remarkable.
For preventing the accumulation of the residual potential
caused by the impurities present in the charge transport layer,
an addition or a specific com?ound in the charge transport layer
is attempted. However, the prior known compounds are not
satisfactory because they prevent the accumulation of the
residual potential insufficiently and they affect the electric
properties including the charge-ability and the sensitivity.
The present inventors have been investigated the specific
compound which can prevent the accumulation of the residual
potential sufficiently without affecting the electric properties
and now they found that specific organic acids satisfy the above
requirements.
SUMMARY OF THE INVENTION
According to the invention, an electrophotographic
photoreceptor has on a conductive base a photoconductive layer
containing at least a charge generation material and a charge
transport material, the photoconductive layer further containing
an organic acid, whose acid dissociation constant pKa in its
aqueous solution is in a range of 2.1 to 4.3, in an amount of
0.001 to 10 parts by weight per 100 parts by weight of the
charge transport material.
pE~ EXPLANATION OF THE INVENTION
The photoreceptor according to the invention has the
conductive base, on which the photoconductive layer is provided.
The photoconductive layer is a laminated-type photoconductive
layer in which the charge transport layer and the charge
generation layer are successively laminated, or a dispersion-
type photoconductive layer in which the particulate charge
generation material is dispersed in a medium containing the
charge transport material.
As the conductive base, any of the known conductive base
can be used. Examples of the conductive base include a base
comprising a metallic material such as aluminium, stainless
steel, copper and nickel as well as a base comprising an
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insulating mate~ial such as polyester film or paper on which a
conductive laver such as a layer of aluminium, copper,
palladium, tin oxide or indium oxide is provided.
The organic acid contained in the photoconductive layer in
the photoreceptor of the invention should have the acid
dissociation constant pKa in its aqueous solution in the range
of 2.1 to 4.3. As the usable organic acid, aromatic carboxylic
acids, saturated or unsaturated aliphatic carboxylic acids,
sulfur-containing organic acids and phosphorus-containing
organic acids, which may have any substituents such as halogen
atom, and hydroxy, thiol, alkyl, aryl, nitro, amino,
dialkylamino, alkoxy and aryloxy groups, are exemplified.
Especially, the aromatic carboxylic acid which may be
substituted as well as the saturated or unsaturated aliphatic
carboxylic acid which may be substituted are preferable.
Typical examples of the organic acids include acrylic acid,
adipic acid, adenosine-2'-phosphate, valeric acid, 3-
chloropropionic acid, acetic acid, cyanoacetic acid,
phenylacetic acid, phenoxyacetic acid, bromoacetic acid, butyric
acid, cyclohexanecarboxylic acid, o-anisic acid, 1-
anilinesulfonic acid, p-aminobenzoic acid, benzoic acid, p-
chlorobenzoic acid, m-hydroxybenzoic acid, o-nitrobenzoic acid,
salicylic acid, 1-naphthoic acid, oxalacetic acid, succinic
acid, tartaric acid, lactic acid, malic acid, phthalic acid,
terephthalic acid, mandelic acid and crotonic acid.
In the laminated-type photoconductive layer, the organic
acid can be contained in the charge generation layer and/or the
charge transport layer. When it is contained in the charge
generation layer, its amount is generally 0.001 to 10 parts by
weight, preferably 0.01 to 2 parts by weight per 100 parts by
weight of the charge transport material in the charge transport
; layer. On the other hand, when it is contained in the charge
transport layer, its amount is generally 0.001 to 10 parts by
weight, preferably 0.01 to 2 parts by weight per 100 parts by
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weight or ~he charge transport material in the charge tr~nsport
layer. ~h- inc1usion of the organic acid in the charge
transport layer is more preferable.
.
The charge generation layer in the laminated-type
photoconducti.ve layer contains the charge generation material.
As the charge generation material used in the charge generation
layer, various inorganic photoconductive substances such as
selen um or its alloys, arsenic-selenium alloy, cadmium sulfide
and zinc oxide as well as various organic photoconductive
substances such as phthalocyanine, azo pigment, quinacridone,
polycyclic quinone, pyrylium salt, thiapyrylium salt, indigo,
thioindigo, anthoanthrone, pyranthrone and cyanine can be used.
Among them, the phthalocyanine without a metal; the
phthalocyanine coordinated with the metal or its compound such
as tin, zinc, vanadium, oxytitanium, tin chloride, copper indium
chloride and gallium chloride; and the azo pigment such as
monoazo, bisazo, trisazo and polyazo are preferable.
The charge generation material can be used in the charge
generation layer together with at least one of binder resins
such as polyester resin, polyvinyl acetate, polyacrylate,
polymethacrylate, polycarbonate, polyvinyl acetacetal, polyvinyl
propional, polyvinyl butyral, phenoxy resin, epoxy resin,
urethane resin, cellulose ester and cellulose ether.
The charge generation material is preferably used in an
amount of 30 to 500 parts by weight per 100 parts by weight of
the binder resin.
If necessary, the charge generation layer may contain
various additives such as a leveling agent, an antioxidant and a
sensitizer.
The charge generation layer is usually formed on the
conductive base according to any one of the known methods,
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preferably a coating method wherein a coating solution
containing the charge generation material and the binder resin
together with any optional additives in a suitable solvent is
coated. Alternatively, the charge generation layer may be
formed by directly depositing the charge generation material on
the conductive base.
The thickness of the charge generation layer is generally
0.1 to 2 ~Im~ preferably 0.15 to 0.8 ~m.
The charge transport layer contains the charge transport
material and the binder resin. The charge transport material
used in the charge transport layer is an electron donative
material, the examples of which include heterocyclic compounds
such as carbazole, indole, imidazole, oxazole, pyrazole,
oxadiazole, pyrazoline and thiadiazole; aniline derivatives;
hydrazone compounds; arornatic amine derivatives; stilbene
derivatives and polymers having the above compound in their main
chain or their side chain.
As the binder resin used together with the charge transport
material in the charge transport layer, a vinyl polymer such as
polymethyl methacrylate, polystyrene and polyvinyl chloride and
its copolymer, polycarbonate, polyester, polyester carbonate,
polysulfone, polyimide, phenoxy resin, epoxy resin and silicone
resin can be used. Their partially crosslinked products may be
used.
The charge transport material is generally used in an
amount of 30 to 200 parts by weight, preferably ~0 to 150 parts
by weight per 100 parts by weight of the binder resin.
If necessary, the charge transport layer may contain
various additives such as the antioxidant and the sensitizer.
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The charge transport layer is usually formed on the charge
generation layer according to any of the known methods,
preferably the coating method wherein the coating solution
containing the charge transport material and the binder res~in
together with any optional additives in a suitable solvent is
coated.
.The thickness of the charge transport layer is generally 10
to 60 ~m, preferably 10 to 45 ~m.
The laminated-type photoconductive layer described in the
above has the charge generation layer on which the charge
transport layer is provided, but the order of laminating the
charge generation layer and the charge transport layer may be
changed, if necessary.
As an outer layer, a known overcoat layer, for example. a
layer mainly composed of a thermoplastic or thermosetting
polymer can be provided.
As mentioned in the above, the dispersion-type
photoconductive layer comprises the particulate charge
generation material dispersed in the medium containing the
charge transport material and the binder resin. The ratio of
the charge transport material to the binder resin is the same as
that in the laminated-type photoconductive layer.
In the dispersion-type photoconductive layer, the charge
generation material should have a small particle size. Its
particle size is preferably 1 ~m or less, more preferably 0~5 ~m
or less.
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The charge generation material dispersed in the medium ispreferably in an amount of 0.5 to 50 % by weight, more
preferably in an amount of 1 to 20 % by weight based on the
medium.
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When the organic acid is contained in the dispersion-type
phot:oconductive layer, its amount is generally 0.001 to 10 parts
by weight, preferably 0.01 to 2 parts by weight per 100 parts by
weight of the charge transport material in the photoconductive
layer.
If necessary, the dispersion-type photoconductive layer may
contain various additives such as a plasticizer, a dispersion
assistant, a leveling agent and a surfactant.
The thickness of the dispersion-type photoconductive layer
is generally 5 to 50 um, preferably 10 to 45 ~m.
EFFECT OF THE I~y~TION
The electrophotographic photoreceptor containing the
specific organic acid in the photoconductive layer according to
the invention shows very high sensitivity and low residual
potential. It hardly shows the accumulation of the residual
potential and the change in charge-ability and sensitivity, even
if its repeated use. Therefore, it is excellent in the
durability.
FXa~PT~
The invention will be better understood by reference to
certain examples, which are included herein for the purpose of
illustration only and are not intended to limit the invention.
Example 1
10 Parts by weight of a bisazo compound having the
following formula:
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o ~ o , o
~ O
were added to 150 parts by weight of 4-methoxy-4-methyl-
pentanone-2 and they were subjected to the grinding and
dispersion treatment using a sand grind mill. The thus obtained
dispersion was added to 100 parts by weight of a S % solution of
polyvinyl butyral (~6000-C ~trade name), ex DENKI KAGAKU KOGYO
K.K.) in 1,2-dimethoxyethane so as to prepare a dispersion with
a solid concentration of 4.0 %.
The above dispersion was coated using a wire bar on a
surface of a polyethylene terephthalate film having an aluminium
deposited layer followed by drying, thereby the charge
generation layer with a thickness of 0,4 g/m2 was formed.
On the charge generation layer, a solution of 95 parts by
weight of a hydrazone compound having the following formula:
CH=~y_N
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1.5 parts by weight of 4~(2,2'-dicyanovinyl)phenyl 2,4,5-
trichlorobenzene sulfonate, 0.2 part by weight of p- -
hydroxybenzoic acid ~pKa 4.58) and lOO parts by weight of a
polycarbonate resin having the following formula: ~
C~3 C~3 C~30
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in a mixed solvent of 1,4-dioxane and tetrahydrofuran (volume
ratio of 50:50) was coated using an applicator followed by
drying at room temperature for 30 minutes and 125C for 30
minutes, thereby the charge transport layer with a thickness of
35 ~m was formed.
In this way, a laminated-type electrophotographic
photoreceptor (sample A) was prepared.
~xa~ples 2 to 6
The procedures in Example 1 were repeated, except that p-
hydroxybenzoic acid was replaced with other organic acids, o-
nitrobenzoic acid of pKa 2.17 (Example 2, sample B), o-
chlorobenzoic acid of pKa 2.92 (Example 3, sample C), 2-
bromovaleric acid of pKa 2.7 (Example 4, sample D), 3-.
phenylsalicylic acid of pKa 2.8 (Example 5, sample E) and p-
tert-butylbenzoic acid of pKa ~.2 (Example 6, sample F).
Comparative Example 1
The procedure in Example 1 was repeated, except that p-
hydroxybenzoic acid was omitted so as to prepare the
photoreceptor (sample G).
Comparative Example 2
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The procedure in Example 1 was repeated, except that p-
hydroxybenzoic acid was replaced with trichloroacetic acid (pKa
0.66) so as to prepare the photoreceptor (sample H).
am~le 7
10 Parts by weight of a bisazo compound having the
following formula:
O ~ ~, O O
~0 C~H
~ ~N=N ,~0O~
and 0.2 part by weight of 2-bromovaleric acid (pKa 2.7) were
added to 150 parts by weight of 9-methoxy-4-methyl-pentanone-2
and they were subjected to the grinding and dispersion treatment
using a sand grind mill. The thus obtained dispersion was added
to 100 parts by weight of a 5 % solution of polyvinyl butyral
(~6000-C (trade name), ex DENKI KAGAKU KOGYO K.K.) in 1,2-
dimethoxyethane so as to prepare a dispersion with a solid
concentration of 4.0 %.
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The above dispersion was coated on a surface of a
polyethylene terephthalate film having an aluminium deposited
layer using a wire bar followed by drying, thereby the charge
generation layer with a thickness of 0.4 g/m2 was formed.
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On the charge generation layer, a solution of 95 parts by
weight of a hydrazone compound having the following formula:
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1.5 parts by weight of 4-(2,2'-dicyanovinyl)phenyl 2,4,5-
trichlorobenzene sulfonate and 100 parts by weight of a
polycarbonate resin having the following formula:
CH3 CH3
\ CH3 /
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CH3
in a mixed solvent of 1,4-dioxane and tetrahydrofuran (volume
ratio of 50:50) was coated using an applicator followed by
drying at room temperature for 30 minutes and 125C for 30
minutes, thereby the charge transport layer with a thickness of
35 ~m was formed.
In this way, a laminated-type electrophotographic
photoreceptor (sample I) was prepared.
F.~amRle 8
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The procedure in Example 7 was repeated, except that 2-
bromovaleric acid was replaced with o-nitrobenzoic acid of pKa
2.17 so as to prepare the photoreceptor (sample J).
Comparative Example 3
The procedure in Example 7 was repeated, except that 2-
bromovaleric acid was omitted so as to prepare the photoreceptor
(sample K).
Test Example
The characteristics of the photoreceptors A to K prepared
in Examples and Comparative Examples were tested.
Each photoreceptor was placed in an apparatus for
determining the characteristics of the photoreceptor (Model EPA-
8100, ex KAWAGUCHI DENKI K.K.) and charged so that the current
flowing into the aluminium layer was 72 ~A, followed by exposing
and erasing. Then, the charge-ability (Vo), the dark decay (DD,
the rate of decaying of the electrical potential in the dark
after 2 seconds from the charging), the sensitivity of half
decay exposure (E1/2, reference potential 500 V) and the residual
potential (Vr) were determined.
The results are shown in Table 1.
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Table 1 _ .
photo- Vo DD ' Vr E1/2
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A 1299 '~ I 0.65
B 1347 32 E _ 9 , O . 64
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_ C l330 27 ~ 0.63
_ D _ 1352 31 I 7 0.65
E 1369 _ 33 5 0.63
300 ~ 0 64
G 1290 33 E 17 0.67
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H 641 125 I 4 determination
(comparative) ~ conducted
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I 1156 33 ,~ ~0 ~ 0.65
~ 1122 ~ 0.60
K 1151 26 ~Ej 35 0.68
(com~arative) . _
As clear from the results in Table 1, the electro-
photographic photoreceptor according to the invention has the
excellent properties.
Example 9
The procedure in Example 1 was repeated, except that an-
~ aluminium cylinder having a diameter of 80 mm, a length of 340
`~ mm and a thickness of 1 mm was used as the conductive base so as
~ to prepare the cylindrical photoreceptor.
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.~ : This photoreceptor was set in a commercial copying machine.
The images with high qualities could be obtained.
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