A single-layer binder comprising a synthetic resin binder and a phthalocyanine pigment dispersed therein is modified so as to reduce the content of the phthalocyanine pigment while maintaining or further improving the sensitivity of the binder. A coating material comprising a synthetic resin binder comprising as a constituent component a polyester resin containing halogen atoms, e.g., chlorine or bromine, and a phthalocyanine pigment dispersed in the binder is applied to a conductive base to produce an electrophotographic binder.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electrophotographic photoreceptor. More 
particularly, it relates to a photoreceptor applicable to LED printers, 
digital copiers and digital image output apparatus such as laser printers. 
2. Description of Related Art 
The Carlson-type electrophotography utilized in printers or copiers (hard 
copy imaging systems) is based on a combination of a photoreceptor having 
photoconductivity and a technique of electrostatic charging. A surface of 
the photoconductive photoreceptor is evenly charged in the dark by means 
of a corona charging or similar device and is then irradiated with optical 
information, whereby the static charge is removed from the parts 
corresponding to the light image to form an electrostatic latent image 
corresponding to the light image. The latent image is developed with a 
colored charged powder to visualize the latent image. 
The basic properties of a photoreceptor for use in electrophotography 
include the ability to be charged in the dark to an appropriate potential, 
the ability to retain the charged state for a given period, and the 
ability to enable the charges to be rapidly neutralized upon light 
irradiation. 
Inorganic compounds such as amorphous silicon, amorphous selenium, cadmium 
sulfide, and zinc oxide have conventionally been used widely as preferred 
photoconductive materials for such electrophotographic photoreceptors. 
None of these materials has proven to be adequate. Amorphous selenium and 
cadmium sulfide are harmful to the human body, have been designated as 
pollutants and have been prohibited from use in the future. Amorphous 
silicon has a high production cost because it is produced by a special 
vapor deposition technique. 
Recently, investigations on organic photoreceptors as substitutes for 
inorganic photoreceptors have been made. Various compounds for use as 
organic photoreceptors have been proposed. Among these are phthalocyanine 
pigments, which have come to be used in photoreceptors for laser printers 
and related devices because of their sensitivity to light having long 
wavelengths. 
However, the organic photoreceptors currently used in the market have 
various drawbacks. They are fabricated as two-layer photoreceptors 
composed of a charge-generating layer and a charge-transporting layer. 
Since these multilayered photoreceptors are negatively charged in use, the 
corona charging device for negative electrification generates about ten 
times the ozone than that generated by a corona charging device for 
positive electrification. The excess ozone must be removed to meet the 
requirements of the Industrial Safety and Health Law. In addition, ozone 
attacks the photoreceptor surface to, causing a decrease in photoreceptor 
life. 
In contrast to photoreceptors of the negative electrification type, 
single-layer photoreceptors are usable in a positively charged state. 
Because of this, many proposals have been made for the improvement of a 
photoreceptor containing a phthalocyanine pigment. 
For the purpose of improving the printing durability of a photoreceptor, a 
mixture of an acrylic resin and a melamine resin was used in Japanese 
Patent Laid-Open No. 15250/1984 and 219752/1984. In Japanese Patent 
Laid-Open No. 207145/1985, a mixture of a polyester resin, a polycarbonate 
resin, and an acrylic resin was used as a binder in order to improve 
moisture resistance. 
As a means for improving sensitivity, use of a mixture of 
poly-N-vinylcarbazole and a polyester resin as a binder was disclosed in 
Japanese Patent Laid-Open No. 185044/1982. 
In Japanese Patent Laid-Open No.105550/1984, a specific phenolic resin was 
used for sensitivity improvement. Another technique for sensitivity 
improvement incorporates an electron-accepting substance into a 
photosensitive layer, as disclosed in Kitamura and Komon, "Denshi Shashin 
Gakkai-shi (Journal of The Soc. of Electrophotography)," 20 (2) 10 (1982) 
and Kitamura and Komon, "Denshi Shashin Gakkai-shi," 20 (3) 2 (1982). In 
Japanese Patent Laid-Open No. 187248/1988, a polyester resin and a 
melamine resin were used as a binder together with a modified 
phthalocyanine in order to improve durability. 
The functions required of a photoreceptor for printers include high 
durability and high sensitivity. This durability is expressed in terms of 
the total number of prints that the photoreceptor can yield while 
retaining its printing function and an acceptable image quality. A 
photoreceptor is chemically deteriorated during image printing by the 
ozone generated by corona, and it suffers surface wear due to mechanical 
friction during development, cleaning and paper transfer. This causes 
scratch marks, resulting in reduced image quality. In addition, the 
thickness of the photosensitive layer decreases, causing a decrease in 
electrification potential and an increase in fogging. None of the 
conventional multilayered photoreceptors has a durability of 100,000 
sheets or higher under these conditions. 
On the other hand, wear resistance in conventional single-layer 
photoreceptors has been obtained by binder improvement. However, their 
sensitivity has still been insufficient in practical use. Although an 
essential requirement for a high-speed printer is that the photoreceptor 
combines high sensitivity with high durability, the attainment of the two 
properties with any conventional technique for photoreceptor production 
has not been achieved. 
Prior art organic photoreceptors have been regarded as unsuitable for 
high-speed printers because of their insufficient durability. The inventor 
has investigated improvements of single-layer photoreceptors for many 
years. Attempts were made to improve the durability of a photoreceptor by 
heightening its surface hardness. As a result, the inventor succeeded in 
improving the surface hardness of a photoreceptor to 3H pencil hardness. 
With respect to durability, it was found that the photoreceptor surface 
had not developed scratches leading to a decrease in image quality even 
after a 300,000-sheet printing, 
However, a high-speed printer is required to exhibit not only durability, 
but also high sensitivity and attenuation of surface potential upon 
exposure to a small amount of light. A conventional technique for 
imparting high sensitivity to a single-layer photoreceptor has been to 
increase the proportion of its photoconductive pigment. For example, in a 
single-layer photoreceptor containing a phthalocyanine pigment, increasing 
the proportion of the phthalocyanine pigment improves photosensitivity but 
accelerates dark decay to the point that the surface becomes potentially 
unstable. In addition, the pigment dispersion has abnormal viscosity 
characteristics, causing coating troubles. 
Therefore, with respect to achieving high sensitivity in a single-layer 
photoreceptor, it is necessary to find a method for maintaining 
sensitivity even when a phthalocyanine pigment is incorporated in a small 
proportion to obtain a high-sensitivity photoreceptor with a desirable 
balance of all properties. The inventor felt a need to develop a new 
synthetic resin for use as a binder for a phthalocyanine pigment, and 
investigated this subject. 
SUMMARY OF THE INVENTION 
This invention relates to a composition of a single-layer photoreceptor 
obtained by dispersing a phthalocyanine pigment into a synthetic resin 
binder and applying the dispersion to a conductive base. In particular, 
this invention is intended to optimize the composition of a polyester 
resin for use as a binder to improve the photosensitivity of the 
photoreceptor. 
The sensitivity of a photoreceptor varies with the proportion of a 
phthalocyanine pigment dispersed in a synthetic resin binder. Some 
proportions may accelerate the dark decay of the photoreceptor to impair 
electrification stability. The proportion of a phthalocyanine pigment is 
an important determinant of photoreceptor properties. As the proportion of 
a phthalocyanine pigment increases, the sensitivity becomes higher but 
dark decay is accelerated simultaneously, causing trouble in practical 
use. In addition, the coating material displays highly thixotropic or 
other abnormal viscosity properties, and is apt to cause unevenness of the 
coating when applied to form a photosensitive layer. 
A photosensitive layer in which a surface part has a composition containing 
a smaller proportion of a phthalocyanine pigment has higher mechanical 
surface strength. According to the inventor's experiences, when the 
content of a phthalocyanine pigment is higher than 20% by weight, troubles 
are apt to arise during photoreceptor production, and a weakened 
photoreceptor surface results. Where a photosensitive layer is formed to 
compensate for the above problems by using a binder content exceeding 80% 
by weight, the photoreceptor has reduced sensitivity and this must be 
compensated for. 
An effective means for improving the sensitivity of a photoreceptor 
comprising a photosensitive layer containing a phthalocyanine pigment 
dispersed in a synthetic resin binder is to use a polyester resin 
containing, as a constituent component, a halogenated organic acid 
containing at least one halogen atom in the synthetic resin binder. 
The synthetic resin binders usable in the photosensitive layer includes 
acrylic resin, polyester resin, styrene/butadiene copolymer resin, 
polycarbonate resin, vinyl chloride/vinyl acetate copolymer resin, 
polyurethane resin, epoxy resin, and polyvinyl butyral resin, though 
polyester resins are preferable. By mixing an amino resin as a 
crosslinking agent with a polyester resin, a photoreceptor can be obtained 
that has a higher mechanical strength and a higher surface hardness. 
Improving the dispersibility of a pigment during photoreceptor production 
is an important factor in ensuring evenness of film thickness, evenness of 
electrification potential, and evenness of photosensitivity. The inventor 
attained optimization by using starting materials for a binder resin in 
combination with a starting material effective in dispersing pigments. 
Phthalocyanine pigments suitable for this invention include copper 
phthalocyanines in .alpha., .beta., .gamma., .delta., .epsilon., and .chi. 
forms and metal-free phthalocyanines in .alpha., .beta., .gamma., .delta., 
.epsilon., and .chi. forms. Titanyl phthalocyanines in various crystal 
forms are also usable and effective. 
Examples of dibasic saturated acids usable as starting materials for the 
polyester resin include phthalic anhydride, isophthalic acid, terephthalic 
acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, and 
endomethylenetetrahydrophthalic anhydride. Examples of dibasic unsaturated 
acids usable as the starting materials include maleic anhydride and 
fumaric acid. 
Usable halogenated saturated acids are limited in kind. Examples thereof 
include tetrachlorophthalic anhydride, tetrabromophthalic anhydride, 
chlorendic anhydride, and an adduct of hexachlorocyclopentadiene with 
tetrahydrophthalic anhydride. Of these, chlorendic anhydride (also called 
HET acid) has a high chlorine content and high reactivity and is hence 
effective as a material for a polyester resin for use in this invention. 
A usable glycol ingredient comprises a combination of neopentyl glycol as 
the main component with ethylene glycol, propylene glycol, diethylene 
glycol, dipropylene glycol, etc. Since use of neopentyl glycol in excess 
results in impaired solubility in organic solvents, a combination of 
neopentyl glycol with other glycol(s) is preferred. 
For producing a polyester resin suitable for photoreceptor production, a 
mixture of a halogenated organic acid and one or more other dibasic 
organic acids is used, in which the proportion of the halogenated organic 
acid is preferably from 0.5 to 10% by mole. The halogen content of all the 
binders in the photosensitive layer is preferably from 1.0% to 10% by 
weight. 
The appropriate range of the content of halogen atoms in the binder is 
narrow. If the content is too low, it is difficult to achieve sensitivity 
improvement. Although sensitivity heightens gradually with increasing 
halogen content, a halogen content exceeding the limits stated above 
results in accelerated dark decay in photoreceptor surface potential. If 
the halogen content is increased further, the potential decreases 
immediately after charging so that a given surface potential cannot be 
maintained. Since this photoreceptor has no practical use, it is necessary 
to incorporate halogen atoms in an amount within the appropriate range. 
When a polyester resin synthesized from starting materials containing a 
halogenated organic acid was used as a binder for a phthalocyanine pigment 
in combination with an amino resin as a crosslinking agent in producing a 
photoreceptor, a photosensitive layer reduced in coating unevenness and 
having a low phthalocyanine pigment content was obtained from a coating 
material having appropriate viscosity during application. The 
photoreceptor produced had evenness of surface potential and improved 
photosensitivity suitable for practical use. 
The photoreceptor having a low phthalocyanine pigment content and an 
acceptable sensitivity was mounted in a printer and subjected to an 
imaging test. The photoreceptor was found to have excellent half-tone 
reproducibility and to yield prints having high evenness of image density, 
a factor that is indispensable for the design of a printer capable of 
printing high-precision images. The organic photoreceptor discussed above 
can be useful in the future printer market. Moreover, since the 
photosensitive layer has an increased binder content, it has an increased 
surface hardness and is resistant to surface wear. As a result, the 
photoreceptor can have excellent durability. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
While the invention is susceptible to various modifications and alternative 
forms, a specific embodiment thereof has been shown by way of example 
described in detail herein below. It should be understood, however, that 
it is not intended to limit the invention to the particular form 
disclosed, but on the contrary, the invention is to cover all 
modifications, equivalents, and alternatives falling within the spirit and 
scope of the invention as defined by the appended claims.

This invention is explained below with reference to examples of production 
processes. 
EXAMPLE 1 
A method of synthesizing a binder is explained first. A 1-liter flask was 
charged with 114 parts (1.5 mol) of propylene glycol, 104 parts (1.0 mol) 
of neopentyl glycol, 355.2 parts (2.4 mol) of isophthalic acid, and 38.9 
parts (0.1 mol) of HET acid. The contents were heated to 80 to 90.degree. 
C. while introducing a small amount of nitrogen gas into the flask. Gentle 
stirring was initiated, and the temperature of the mixture was elevated to 
150 to 160.degree. C. over 1 to 1.5 hours. The mixture was further heated 
to 190.degree. C. over 3 to 4 hours. After the mixture was maintained at 
190.degree. C. for 1 hour, it was kept from being further heated until the 
acid value of the resin ingredient had decreased to 50 or below, while 
removing unreacted starting materials at a reduced pressure. 
When the acid value had reached a given value, the temperature was lowered 
and the resin was taken out of the flask. The resin was an amber fragile 
solid. It was powdered to an appropriate size to be used as a binder. 
Ten parts of the synthesized polyester resin, 16.7 parts of a butylated 
melamine resin (U-Van 20SE60, manufactured by Mitsui Toatsu Chemicals, 
Inc.), 5 parts of a metal-free phthalocyanine (manufactured by Dainichi 
Seika Colour & Chemicals Mfg. Ltd.), and 0.2 part of an antioxidant 
(Irganox 565, manufactured by Ciba-Geigy Japan Ltd.) were introduced into 
a sand mill together with 100 parts of cyclohexanone. The mixture was 
kneaded for 2 hours. The resultant binder had a chlorine content of 1.7%. 
The resulting solution had a viscosity of 400 mPa.s (20.degree. C.), which 
was suitable for dip coating. The surface of an aluminum pipe was 
dip-coated with this solution and heated at 120.degree. C. for 1 hour to 
dry the coating. The resultant photosensitive layer had a thickness of 18 
m. The surface of the photosensitive layer was smooth, even and 
semiglossy. The layer had a surface hardness of 2H pencil hardness. 
This photoreceptor was examined for electrophotographic properties. The 
photoreceptor surface was charged at a voltage of +600 V and fluctuations 
of its potential were measured while rotating the photoreceptor. The 
fluctuations were within 20 V, showing that the charged state was 
extremely stable. The photoreceptor had a sensitivity of 0.5 to 0.6 
J/cm.sup.2 in terms of half-decay exposure to 780-nm light. From these 
found values, the photoreceptor was judged to be capable of practical use. 
EXAMPLE 2 
A sand mill was charged with 27 parts of the polyester resin synthesized in 
Example 1, 10.7 parts of a butylated melamine resin (U-Van 20SE60, 
manufactured by Mitsui Toatsu Chemicals, Inc.), 5 parts of a metal-free 
phthalocyanine (manufactured by Dainichi Seika Colour & Chemicals Mfg. 
Ltd.), and 0.1 part of an antioxidant (Irganox 565, manufactured by 
Ciba-Geigy Japan Ltd.) together with 130 parts of cyclohexanone. The 
mixture was kneaded for 2 hours. The resultant binder had a chlorine 
content of 2.8%. 
The resulting solution had a viscosity of 350 mpa.s (20.degree. C.), which 
was suitable for dip coating. The surface of an aluminum pipe was 
dip-coated with this solution and heated at 120.degree. C. for 1 hour to 
dry the coating. The resultant photosensitive layer had a thickness of 18 
.mu.m. The surface of the photosensitive layer was smooth, even, and 
glossier than that obtained in Example 1. The layer had a surface hardness 
of 3H pencil hardness. 
This photoreceptor was examined for electrophotographic properties. The 
photoreceptor surface was charged at a voltage of +600 V and fluctuations 
of its potential to were measured while rotating the photoreceptor. The 
fluctuations were within 20 V, showing that the charged state was 
extremely stable. The photoreceptor had a sensitivity of 0.6 to 0.7 
.mu.J/cm.sup.2 in terms of half-decay exposure to 780-nm light. From these 
found values, the photoreceptor was judged to be capable of practical use. 
EXAMPLE 3 
A 1-liter flask was charged with 159 parts (1.5 mol) of diethylene glycol, 
104 parts (1.0 mol) of neopentyl glycol, 325.6 parts (2.2 mol) of 
isophthalic acid, and 139.1 parts (0.3 mol) of tetrabromophthalic 
anhydride. The contents were heated to 80 to 90.degree. C. while 
introducing a small amount of nitrogen gas into the flask. Gentle stirring 
was initiated, and the temperature of the mixture was elevated to 150 to 
160.degree. C. over 1 to 1.5 hours. The mixture was further heated to 
190.degree. C. over 3 to 4 hours. After the mixture was maintained at 
190.degree. C. for 1 hour, it was further heated until the acid value of 
the resin ingredient decreased to 50 or below, while removing unreacted 
starting materials at a reduced pressure. 
When the acid value had reached a given value, the temperature was lowered 
and the resin was taken out of the flask. The resin was a light-brown 
fragile solid. It was powdered to an appropriate size to be used as a 
binder. 
Ten parts of the synthesized polyester resin, 16.7 parts of a butylated 
melamine resin (U-Van 20SE60, manufactured by Mitsui Toatsu Chemicals, 
Inc.), 4.8 parts of a metal-free phthalocyanine (manufactured by Dainichi 
Seika Colour & Chemicals Mfg. Ltd.), and 0.2 part of an antioxidant 
(Irganox 565, manufactured by Ciba-Geigy Japan Ltd.) were introduced into 
a sand mill together with 100 parts of cyclohexanone. The mixture was 
kneaded for 2 hours. The resultant binder had a bromine content of 1.6%. 
The resulting solution had a viscosity of 380 mpa.s (20.degree. C.), which 
was suitable for dip coating. The surface of an aluminum pipe was 
dip-coated with this solution and heated at 120.degree. C. for 1 hour to 
dry the coating. The resultant photosensitive layer had a thickness of 18 
.mu.m. The surface of the photosensitive layer was smooth and even, and 
had a dull gloss. 
This photoreceptor was examined for electrophotographic properties. The 
photoreceptor surface was charged at a voltage of +600 V and fluctuations 
of its potential were measured while rotating the photoreceptor. The 
fluctuations were within .+-.20 V, showing that the charged state was 
extremely stable. The photoreceptor had a sensitivity of 0.5 to 0.6 
.mu.J/cm.sup.2 in terms of half-decay exposure to 780-nm light. From these 
found is values, the photoreceptor was judged to be capable of practical 
use. 
Comparative Example 
A photoreceptor was produced from the same composition as in Example 1, 
except that a polyester resin containing no halogen molecules was used. A 
sand mill was charged with 16.7 parts of the polyester resin (Almatex 
P645, manufactured by Mitsui Toatsu Chemicals, Inc.), 16.7 parts of a 
butylated melamine resin (U-Van 20SE60, manufactured by Mitsui Toatsu 
Chemicals, Inc.), 5 parts of a metal-free phthalocyanine (manufactured by 
Dainichi Seika Colour & Chemicals Mfg. Ltd.), and 0.2 part of an 
antioxidant (Irganox 565, manufactured by Ciba-Geigy Japan Ltd.) together 
with 90 parts of cyclohexanone. The mixture was kneaded for 2 hours. 
The obtained solution had a viscosity of 450 mpa.s (20.degree. C.), which 
was suitable for dip coating. The surface of an aluminum pipe was 
dip-coated with this solution and heated at 120.degree. C. for 1 hour to 
dry the coating. The resultant photosensitive layer had a thickness of 18 
.mu.m. The surface of the photosensitive layer was smooth, even, and 
semiglossy. The layer had a surface hardness of 2H pencil hardness. 
This photoreceptor was examined for electrophotographic properties. The 
photoreceptor surface was charged at a voltage of +600 V and fluctuations 
of its potential were measured while rotating the photoreceptor. The 
fluctuations were within .+-.20 V, showing that the charged state was 
extremely stable. The photoreceptor had a sensitivity of 3.50 to 4.0 
.mu.J/cm.sup.2 in terms of half-decay exposure to 780-nm light. From this 
sensitivity, it was judged that this photoreceptor had a sensitivity about 
one seventh the sensitivity of the photoreceptor prepared in Example 1. 
The photoreceptor of this Comparative Example was difficult to use in 
practical printers. 
As described above, this invention is based on the finding that, in a 
single-layer photoreceptor containing a phthalocyanine pigment dispersed 
in a synthetic resin binder, the photoreceptor sensitivity is 
significantly improved by using a synthetic resin binder comprising a 
polyester resin containing halogen atoms, such as chlorine or bromine, as 
a constituent component. Due to this finding, the poorly dispersible 
phthalocyanine pigment can be used in a reduced amount and easily 
dispersed to achieve a stable dispersed state. Thus, coating unevenness 
and thickness unevenness, which are apt to occur during coating for 
photoreceptor production, can be diminished. 
The photoreceptors produced in Examples 1, 2, and 3 were each mounted in a 
printer to conduct image evaluation as described above. As a result, it 
was found that the photoreceptors gave prints having excellent evenness of 
half-tone image density. They were superior in density gradation to 
commercial photoreceptors. The photoreceptors produced in the Examples 
were further evaluated for durability in a printing test. As a result, it 
was found that the photoreceptors had a life about 2 times that of 
commercial photoreceptors. 
While the invention has been illustrated and described in detail in the 
foregoing description, such illustration and description is to be 
considered as exemplary and not restrictive in character, it being 
understood that only the preferred embodiment and minor variants thereof 
have been shown and described and that all changes and modifications that 
come within the spirit of the invention are desired to be protected.