JP7700486B2 - Electrophotographic photoreceptor, electrophotographic photoreceptor cartridge and image forming apparatus - Google Patents

Description

The present invention relates to an electrophotographic photoreceptor used in copiers, printers, etc., and a cartridge and image forming device using the same.

In printers and copiers, when a charged organic photoconductor (OPC) drum is irradiated with light, the part is discharged and an electrostatic latent image is generated, and an image can be obtained by attaching toner to the electrostatic latent image. In such devices that use electrophotography, the photoconductor is the core material.

Since this type of organic photoreceptor has a large margin for material selection and the properties of the photoreceptor are easy to control, a "function-separated photoreceptor" in which the functions of generating and transporting negative charges are shared by different compounds has become mainstream. For example, a single-layer electrophotographic photoreceptor (hereinafter referred to as a single-layer photoreceptor) having a charge generating material (CGM) and a charge transport material (CTM) in the same layer, and a laminated electrophotographic photoreceptor (hereinafter referred to as a laminated photoreceptor) formed by laminating a charge generating layer containing a charge generating material (CGM) and a charge transport layer containing a charge transport material (CTM) are known. In addition, the charging method of the photoreceptor can be a negative charging method in which the surface of the photoreceptor is negatively charged, or a positive charging method in which the surface of the photoreceptor is positively charged.
Combinations of layer structures and charging methods of photoreceptors currently in practical use include "negatively charged multi-layer photoreceptors" and "positively charged single-layer photoreceptors."

A "negatively charged laminated photoreceptor" generally has a structure in which an undercoat layer (UCL) made of resin or the like is provided on a conductive support such as an aluminum tube, on which a charge generation layer (CGL) made of a charge generation material (CGM) and a resin or the like is provided, on which a charge transport layer (CTL) made of a hole transport material (HTM) and a resin or the like is provided.

On the other hand, a "positively charged single-layer photoreceptor" generally has a structure in which an undercoat layer (UCL) made of resin or the like is provided on a conductive support such as an aluminum tube, and a single-layer photosensitive layer made of a charge generating material (CGM), a hole transport material (HTM), an electron transport material (ETM), and a resin or the like is provided on top of that (see, for example, Patent Document 1).

In either type of photoconductor, the surface of the photoconductor is first charged using a corona discharge method or a contact method, and then the photoconductor is exposed to light to neutralize the surface charge, forming an electrostatic latent image due to the potential difference with the surrounding surface. Toner is then brought into contact with the photoconductor surface to form a toner image that corresponds to the electrostatic latent image, which is then transferred to paper or the like and heated to melt and fix it, completing the print.

As mentioned above, the basic structure of an electrophotographic photoreceptor is a photosensitive layer formed on a conductive support, but a protective layer is provided on the photosensitive layer to improve abrasion resistance, etc.

As a technique for improving the mechanical strength or abrasion resistance of the photoreceptor surface, a photoreceptor has been disclosed in which a layer containing a compound having a chain-polymerizable functional group, i.e., a curable resin compound, is formed as a binder resin in the outermost layer of the photoreceptor, and this is polymerized by applying energy such as heat, light, or radiation to form a cured resin layer (protective layer) (see, for example, Patent Documents 1 and 2).

U.S. Pat. No. 9,417,538 International Publication No. 2010/035683

In recent years, there has been an increasing demand for improved life span, i.e., improved wear resistance, particularly for photoconductors for multifunction machines, from the viewpoint of reducing maintenance costs, etc. In particular, in an image forming apparatus using a contact charging method, how to reduce wear at the end of the photoconductor that comes into contact with the end of the charging roller has become an issue.
One possible solution to this problem would be to increase the thickness of the protective layer (also referred to as "OCL") to improve the wear resistance of the photoreceptor; however, increasing the thickness of the protective layer (OCL) would result in a problem of degraded image characteristics.

The object of the present invention is to provide a new electrophotographic photoreceptor that can enhance abrasion resistance regardless of the thickness of the protective layer, and more preferably, a new electrophotographic photoreceptor that can suppress abrasion of the photoreceptor end when used in a contact charging type image forming apparatus.

In order to solve the above problems, the present invention provides an electrophotographic photoreceptor having a photosensitive layer and a protective layer containing a cured product obtained by curing a curable compound, which are successively provided on a conductive support, the electrophotographic photoreceptor comprising:
We propose an electrophotographic photoreceptor having a current inflow of 15 A/m ² or less when a voltage of −3.4 kV is applied.

The gist of the present invention is as follows: [1] to [13].

[1] An electrophotographic photoreceptor having, on a conductive support, a photosensitive layer and a protective layer containing a cured product obtained by curing a curable compound, in that order, the electrophotographic photoreceptor has a current flow of 15 A/m2 or ^less when a voltage of -3.4 kV is applied.

[2] The electrophotographic photoreceptor according to [1], wherein the photosensitive layer has a thickness of 10 μm or more.
[3] The electrophotographic photoreceptor according to [1] or [2], wherein the protective layer has a thickness of 1 μm or less.
[4] The electrophotographic photoreceptor according to any one of [1] to [3], wherein a ratio A/B of a thickness A of the protective layer to a thickness B of the photosensitive layer is 0.002 or more and 0.3 or less.

[5] The electrophotographic photoreceptor according to any one of [1] to [4], wherein the photosensitive layer contains metal oxide particles having a band gap of 5 eV or more.
[6] The electrophotographic photoreceptor according to [5], wherein the metal oxide particles having a band gap of 5 eV or more are silica particles.

[7] The electrophotographic photoreceptor according to any one of [1] to [6], wherein the protective layer contains metal oxide particles having a band gap of 2 to 4 eV.
[8] The electrophotographic photoreceptor according to [7], wherein the content ratio (mass ratio) of the metal oxide particles having a band gap of 5 eV or more in the photosensitive layer to the content of the metal oxide particles having a band gap of 2 to 4 eV in the protective layer is 0.1 to 0.5.

[9] The electrophotographic photoreceptor according to any one of [1] to [8], wherein the hole transport material contained in the photosensitive layer contains any one of the following formulas (1) or (2), or a combination of two or more of these:

[10] The electrophotographic photoreceptor according to any one of [1] to [9], characterized in that the curable compound is a photocurable compound.

[11] The electrophotographic photoreceptor according to any one of [1] to [10], which is used in an image forming apparatus using a contact charging method.

[12] An electrophotographic photoreceptor cartridge comprising the electrophotographic photoreceptor according to any one of [1] to [11].
[13] An image forming apparatus having the electrophotographic photoreceptor according to any one of [1] to [11].

The electrophotographic photoreceptor of the present invention can enhance abrasion resistance regardless of the thickness of the protective layer, i.e., without making the protective layer thicker than before. In particular, when used in a contact charging type image forming device, abrasion of the photoreceptor end can be suppressed.

1 is a diagram illustrating an example of the configuration of an image forming apparatus that can be configured using an electrophotographic photoreceptor according to an example of the present invention. FIG. 13 is a diagram for explaining a "method of measuring inflow current" in the embodiment.

The following provides a detailed explanation of the form for carrying out the present invention (hereinafter, "embodiment of the invention"). Note that the present invention is not limited to the following embodiment, and can be modified in various ways within the scope of the gist of the invention.

<<Present Electrophotographic Photoreceptor>>
An electrophotographic photoreceptor according to an example of an embodiment of the present invention (also referred to as "the present electrophotographic photoreceptor") is an electrophotographic photoreceptor including, in order on a conductive support, a photosensitive layer and a protective layer containing a cured product obtained by curing a curable compound.

The present electrophotographic photoreceptor may optionally have layers other than the photosensitive layer and the present protective layer.
The electrophotographic photoreceptor may be charged by either a negative charging method in which the surface of the photoreceptor is negatively charged or a positive charging method in which the surface of the photoreceptor is positively charged.

In this electrophotographic photoreceptor, the side opposite the conductive support is the upper side or front side, and the conductive support side is the lower side or back side.

<Characteristics of the present electrophotographic photoreceptor>
It is preferable that the electrophotographic photoreceptor has an inflow current of 15 A/m ² or less when a voltage of −3.4 kV is applied.
The present inventors have investigated the causes of wear on the surface of an electrophotographic photoreceptor, particularly the wear on the surface of the end of the photoreceptor that comes into contact with the end of the charging roller when used in a contact charging type image forming apparatus, and have found that one of the causes is that the charge generated by the charging roller flows as a current inside the photoreceptor (hereinafter, this current is also referred to as "inflow current"). In the contact charging type, the electric field is concentrated particularly at the end of the photoreceptor, so the inflow current to the end of the photoreceptor is also large, and the wear of the end becomes severe.
The reason why a large current inflow causes the surface of the photoconductor to wear more severely is believed to be as follows: It is believed that a large current inflow causes the binder resin of the protective layer and the photosensitive layer to deteriorate. Specifically, it is believed that when a current inflow flows, the polymer chains of the binder resin of the protective layer and the photosensitive layer are destroyed, which reduces the wear resistance of the protective layer and the photosensitive layer, causing the surface of the photoconductor to wear more severely.
Photoconductors, as a necessary characteristic, have the property of allowing electrons and holes to move, but the effect of inflow current on the characteristics of photoconductors has not been examined so far.
In addition, the "photoconductor end" refers to the end in the length direction when the photoconductor is in the shape of a drum (cylindrical), and refers to the end in the width direction when the photoconductor is in the shape of a sheet or belt. Also, the end of the charging roller refers to the end in the length direction of a cylindrical roller.
From this viewpoint, the inflow current of the electrophotographic photoreceptor is preferably 15 A/ ^m2 or less. By setting the inflow current within the above range, the deterioration of the binder resin of the protective layer and the photosensitive layer can be suppressed, and the wear resistance of the photoreceptor can be improved. The inflow current is more preferably 12 A/ ^m2 or less, more preferably 10 A/m2 or ^less , and even more preferably 8 A/ ^m2 or less.
On the other hand, the lower limit of the inflow current is preferably small, but from the viewpoint of maintaining the chargeability and the performance of the photoreceptor, it is usually 1 A/m ² or more.

In the present electrophotographic photoreceptor, the means for making the inflow current 15 A/m2 or ^less can be, for example, a method of increasing the thickness of the photosensitive layer or making the photosensitive layer contain metal oxide particles (also referred to as "insulating metal oxide particles" in the present invention) with a band gap of 5 eV or more. It is surprising and noteworthy that the wear resistance can be increased by improving the photosensitive layer, which is the intermediate layer, rather than improving the protective layer, which is the outermost layer. However, the specific means for making the inflow current 15 A/m2 or ^less are not limited to these methods.

The method for measuring the inflow current into the photoconductor is based on the method described in the examples below.
It is sufficient that the inflow current of the photoconductor is in the above range at least at one point of the photoconductor. However, when it is clear that the inflow current value differs depending on the part of the photoconductor, it is preferable that the average value of any part of the photoconductor is in the above range, and it is more preferable that all parts of the photoconductor are in the above range.
Since the inflow current is measured in the thickness direction of the photoreceptor, the above-mentioned portion of the photoreceptor means any position on the plane of the photoreceptor.

(Thickness of the Photosensitive Layer)
The thickness of the photosensitive layer in the present electrophotographic photoreceptor (also referred to as "the present photosensitive layer") is preferably 10 μm or more, more preferably 15 μm or more, even more preferably 19 μm or more, even more preferably 20 μm or more, even more preferably 22 μm or more, and even more preferably 24 μm or more, from the viewpoint of suppressing the inflow current.
On the other hand, from the viewpoint of electrostatic chargeability, the thickness is preferably 40 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less.
The thickness of the photosensitive layer referred to here means the thickness of a single-layer photosensitive layer, and means the total thickness of the charge generating layer and the charge transport layer in the case of a multi-layer photosensitive layer.

(Thickness of this protective layer)
The thickness of the protective layer is appropriately selected depending on the material used, etc. From the viewpoint of the service life of the photoconductor, the thickness of the protective layer is preferably 0.1 μm or more, more preferably 0.2 μm or more, and particularly preferably 0.5 μm or more. From the viewpoint of electrical properties, the thickness of the protective layer is preferably 10 μm or less, more preferably 5 μm or less, and particularly preferably 3 μm or less.
In particular, from the viewpoint of suppressing the occurrence of ghosts, the thickness of the protective layer is preferably 1 μm or less, more preferably 0.1 μm or more or 3 μm or less, even more preferably 0.5 μm or more or 2 μm or less, and even more preferably 0.8 μm or more or 1.5 μm or less.

Generally, a method for improving the wear resistance of a photoconductor is to increase the thickness of the protective layer, but it has been found that increasing the thickness of the protective layer reduces image characteristics, causing problems such as ghosting. Ghosting is a phenomenon in which an afterimage of the image from the first rotation does not disappear and remains in the image from the second rotation.
In the present electrophotographic photoreceptor, since the inflow current is 15 A/ ^m2 or less, deterioration of the binder resins of the protective layer and the photosensitive layer is suppressed, and the wear resistance of the photoreceptor can be improved. That is, in the present electrophotographic photoreceptor, the wear resistance of the photoreceptor can be improved while maintaining the thickness of the protective layer, and therefore the occurrence of the above-mentioned ghost can be prevented.

In addition, from the viewpoint of suppressing inflow current and balancing between wear resistance and image characteristics, the ratio A/B of the thickness A of the protective layer to the thickness B of the photosensitive layer is preferably 0.002 or more, more preferably 0.005 or more, even more preferably 0.008 or more, and particularly preferably 0.01 or more. On the other hand, the ratio A/B of the thickness A of the protective layer to the thickness B of the photosensitive layer is preferably 0.3 or less, more preferably 0.2 or less, even more preferably 0.1 or less, and particularly preferably 0.06 or less.

<Main photosensitive layer>
The photosensitive layer in the present electrophotographic photoreceptor (the present photosensitive layer) may be a single-layer type photosensitive layer in which a charge generating material (CGM), a hole transporting material (HTM) and an electron transporting material (ETM) are present in the same layer, or may be a laminated type photosensitive layer separated into a charge generating layer and a charge transporting layer.

(Insulating Metal Oxide Particles)
Whether the photosensitive layer is of the single-layer type or the multilayer type, the photosensitive layer preferably contains metal oxide particles (insulating metal oxide particles) having a band gap of 5 eV or more.
It is believed that the inclusion of insulating metal oxide particles in the photosensitive layer of the present invention increases the resistance of the photoreceptor as a whole, making it possible to suppress the aforementioned inflow current and improve wear resistance.

The band gap of the insulating metal oxide particles may be 5 eV or more, preferably 7 eV or more, and more preferably 9 eV or more. On the other hand, it is preferably 13 eV or less, and more preferably 11 eV or less.

Metal oxide particles (insulating metal oxide particles) with a band gap of 5 eV or more can include, for example, silica, alumina, zirconia, etc. Among these, silica and alumina are preferred from the viewpoint of electrical properties and dispersibility, and silica is more preferred. It is known that the band gap of silica particles is generally about 8 to 9 eV.

The insulating metal oxide particles preferably have a low refractive index so as not to reflect light of a wavelength (e.g., 780 nm) irradiated during exposure of the photoreceptor. From this viewpoint, the refractive index of the insulating metal oxide particles is preferably 2.00 or less, and more preferably 1.50 or less. It is known that the refractive index of silica particles is generally about 1.45 to 1.47 in the case of amorphous silica.
From the viewpoint of the stability of the coating solution, a small specific gravity is preferable. From this viewpoint, the specific gravity of the insulating metal oxide particles is preferably 7 or less, more preferably 5 or less, and even more preferably 3 or less. On the other hand, the specific gravity is preferably 1 or more.
Among the insulating metal oxide particles described above, silica particles are particularly preferred from the viewpoints of a low refractive index and a small specific gravity.

The silica particles may be any particles made of silicon dioxide (it is permissible to contain impurities of less than 5% by mass).
The method for producing the silica particles may be either a dry method or a wet method. Among them, the wet method is preferable, and among them, silica particles obtained by a sol-gel method (also referred to as "sol-gel silica") are preferable because they have a sharp particle size distribution.

The insulating metal oxide particles may have any shape, but are preferably spherical from the viewpoint of reducing contamination of the charging roller that comes into contact with the photoreceptor.
From the viewpoint of dispersibility, the insulating metal oxide particles preferably have an average primary particle size of 1 μm or less, more preferably 0.8 μm or less, and even more preferably 0.6 μm or less, while from the viewpoint of reducing inflow current, the average primary particle size is preferably 0.05 μm or more, more preferably 0.1 μm or more, and even more preferably 0.2 μm or more.
The average primary particle size is a value determined from the arithmetic mean value of particle sizes directly observed using a transmission electron microscope (hereinafter also referred to as TEM).

In order to enhance the insulating properties, the insulating metal oxide particles may be surface-treated with, for example, silicone oil, a silane coupling agent, a siloxane compound, or the like to make the surface more hydrophobic.
In addition, the insulating metal oxide particles may be surface-treated with at least one organic compound selected from the group consisting of siloxane compounds, silane coupling agents, polyhydric alcohols, titanium coupling agents, alkanolamines or derivatives thereof, and higher fatty acids or metal salts thereof in order to enhance dispersibility in a binder resin.
Among them, those surface-treated with a silane coupling agent are preferred. Examples of the silane coupling agent include alkoxysilane and silazane, and among them, silazane is preferred from the viewpoint of electrical properties. Among silazanes, hexamethyldisilazane is preferred.

From the viewpoint of suppressing the aforementioned inflow current, the content of the insulating metal oxide particles is preferably 5 parts by weight or more, more preferably 10 parts by weight or more, and even more preferably 15 parts by weight or more, per 100 parts by weight of the photosensitive layer. On the other hand, from the viewpoint of electrical properties, the content is preferably 50 parts by weight or less, more preferably 40 parts by weight or less, and even more preferably 30 parts by weight or less.

From the same viewpoint, the content of the insulating metal oxide particles is preferably 5 parts by mass or more, more preferably 10 parts by mass or more, and even more preferably 15 parts by mass or more, per 100 parts by mass of the binder resin of the photosensitive layer. On the other hand, from the viewpoint of electrical properties, the content is preferably 50 parts by mass or less, more preferably 40 parts by mass or less, and even more preferably 30 parts by mass or less.

From a similar viewpoint, the ratio (mass ratio) of the content of insulating metal oxide particles to the content of charge transport material (total content of hole transport material (HTM) and electron transport material (ETM)) is preferably 0.05 or more, more preferably 0.1 or more, and even more preferably 0.2 or more. On the other hand, it is preferably 1 or less, more preferably 0.8 or less, and even more preferably 0.6 or less.

From a similar viewpoint, the ratio (mass ratio) of the content of insulating metal oxide particles to the content of hole transport material (HTM) is preferably 0.05 or more, more preferably 0.1 or more, and even more preferably 0.2 or more, from the viewpoint of suppressing inflow current. On the other hand, from the viewpoint of preventing contamination of the charging roller, it is preferably 1 or less, more preferably 0.8 or less, and even more preferably 0.6 or less.

<Laminated Photosensitive Layer>
The laminated type photosensitive layer in the present electrophotographic photoreceptor may be a structure in which a charge transport layer (CTL) containing a hole transport material (HTM) is laminated on a charge generation layer (CGL) containing a charge generation material (CGM), or a structure in which the charge generation layer (CGL) is laminated on the charge transport layer (CTL). Among these, a structure in which the charge transport layer (CTL) is laminated on the charge generation layer (CGL) is preferred.
In this case, it is possible to provide layers other than the charge generating layer (CGL) and the charge transport layer (CTL). The charge transport layer (CTL) may further contain an electron transport material (ETM).

In the case of a laminated photosensitive layer, it is preferable that the charge generation layer (CGL) or the charge transport layer (CTL), or both of them, contain the insulating metal oxide particles. In particular, it is preferable that at least the charge transport layer (CTL) contains the insulating metal oxide particles, from the viewpoint of being able to further suppress the inflow current due to the larger layer thickness.

<Charge Generation Layer (CGL)>
The charge generating layer usually contains a charge generating material (CGM) and a binder resin.

(Charge Generation Material (CGM))
Examples of the charge generating material include inorganic photoconductive materials such as selenium and its alloys, cadmium sulfide, and organic photoconductive materials such as organic pigments. Among them, organic photoconductive materials are preferred, and organic pigments are particularly preferred, among which phthalocyanine pigments and azo pigments are more preferred, and phthalocyanine pigments are even more preferred. All of these indicate the skeletal structures of compounds, and include compound groups having these skeletal structures, i.e., derivatives.

When organic pigments are used as charge generating materials, these organic pigment fine particles are usually used in the form of a dispersed layer bound with various binder resins.

Specific examples of the above phthalocyanine pigments include metal-free phthalocyanine, phthalocyanines with metals such as copper, gallium, tin, and titanium, or their oxides and halides coordinated thereto, and phthalocyanine dimers using oxygen atoms as bridging atoms. Particularly preferred are X-type, τ-type metal-free phthalocyanine, A-type (also known as β-type), B-type (also known as α-type), titanyl phthalocyanine (also known as oxytitanium phthalocyanine) and other types of phthalocyanine, which are highly sensitive crystal types, such as D-type (also known as Y-type), which exhibits a clear peak at a diffraction angle 2θ (±0.2°) of 27.1° or 27.3° in powder X-ray diffraction, chlorogallium phthalocyanine such as II-type, and hydroxygallium phthalocyanine such as V-type.

The charge generating material may be used alone or in any combination and ratio of two or more kinds. When two or more kinds of charge generating materials are used in combination, the charge generating materials may be mixed after each other or may be mixed during the manufacturing and processing process of the charge generating material, such as synthesis, pigmentization, and crystallization. Known examples of such processing include acid paste processing, grinding processing, and solvent processing.

The particle size of the charge generating material is usually 1 μm or less, and preferably 0.5 μm or less.
The content of the charge generating material in the photosensitive layer is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, from the viewpoint of sensitivity, and is preferably 50% by mass or less, more preferably 20% by mass or less, from the viewpoint of sensitivity and chargeability.

(Binder resin)
The binder resin used in the charge generating layer can be used without any particular limitation. For example, polyvinyl acetal resins such as polyvinyl butyral resin, polyvinyl formal resin, and partially acetalized polyvinyl butyral resin in which a part of butyral is modified with formal or acetal, etc.; polyarylate resin, polycarbonate resin, polyester resin, phenoxy resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyvinyl acetate resin, polystyrene resin, acrylic resin, methacrylic resin, polyacrylamide resin, polyamide resin, polyurethane resin, epoxy resin, silicone resin, polyvinyl alcohol resin; vinyl chloride-vinyl acetate copolymer, etc. can be mentioned. Among these resins, polyvinyl acetal resin or polyvinyl acetate resin is preferred in terms of pigment dispersibility, adhesion to the conductive support or undercoat layer, and adhesion to the charge transport layer.
These binder resins may be used alone or in any combination of two or more.

(Other ingredients)
The charge generating layer may contain other components as necessary in addition to the charge generating material and the binder resin. For example, known additives such as antioxidants, plasticizers, ultraviolet absorbers, electron-withdrawing compounds, leveling agents, visible light shielding agents, and fillers may be contained for the purpose of improving film-forming properties, flexibility, coating properties, stain resistance, gas resistance, light resistance, etc.

(Mixing ratio)
In the charge generation layer, if the ratio of the charge generation material is too high, the stability of the coating liquid may decrease due to aggregation of the charge generation material, etc., while if the ratio of the charge generation material is too low, the sensitivity of the photoreceptor may decrease. Therefore, the compounding ratio (mass) of the binder resin to the charge generation material is preferably 10 parts by mass or more, and more preferably 30 parts by mass or more, of the charge generation material per 100 parts by mass of the binder resin, and is preferably 1,000 parts by mass or less, and more preferably 500 parts by mass or less. From the viewpoint of film strength, it is more preferably 300 parts by mass or less, and even more preferably 200 parts by mass or less.

(layer thickness)
The thickness of the charge generating layer is preferably 0.1 μm or more, more preferably 0.15 μm or more, and is preferably 10 μm or less, more preferably 0.6 μm or less.

<Charge transport layer (CTL)>
The charge transport layer (CTL) usually contains a hole transport material (HTM) and a binder resin, and may further contain an electron transport material (ETM).

Hole Transport Materials (HTM)
The hole transport material (HTM) is not particularly limited, and examples thereof include electron donating substances such as heterocyclic compounds such as carbazole derivatives, indole derivatives, imidazole derivatives, oxazole derivatives, pyrazole derivatives, thiadiazole derivatives, and benzofuran derivatives, aniline derivatives, hydrazone derivatives, arylamine derivatives, stilbene derivatives, butadiene derivatives, and enamine derivatives, as well as compounds in which a plurality of these compounds are bonded, and polymers having groups made of these compounds in the main chain or side chain.

Among these, carbazole derivatives, arylamine derivatives, stilbene derivatives, butadiene derivatives, enamine derivatives, and combinations of multiple types of these compounds are preferred, with arylamine derivatives and enamine derivatives being more preferred, and enamine derivatives being even more preferred since they are less likely to inject electrons.

Suitable examples of hole transport materials (HTM) include compounds having any of the structures represented by the following general formulas. However, the present invention is not limited to these. Any one of the compounds may be used alone, or two or more of the compounds may be used in any combination.

Among the above-mentioned exemplary compounds, it is preferable to use any one of the following formula (1) or formula (2) or a combination of two or more of these as the hole transport material (HTM), since this allows the effects of the present invention to be more effectively obtained.

Among the hole transport materials (HTM), compounds having two or more nitrogen atoms in one molecule are preferred from the viewpoint of suppressing inflow current. In addition, compounds having a bilaterally symmetric structure are preferred.

The content of the hole transport material (HTM) in the charge transport layer is preferably 10 parts by weight or more, more preferably 20 parts by weight or more, and even more preferably 30 parts by weight or more, per 100 parts by weight of the binder resin. On the other hand, it is preferably 150 parts by weight or less, more preferably 120 parts by weight or less, and even more preferably 100 parts by weight or less.

(Electron transport material (ETM))
The electron transport material (ETM) is not particularly limited, but examples thereof include electron-withdrawing substances such as aromatic nitro compounds such as 2,4,7-trinitrofluorenone, cyano compounds such as tetracyanoquinodimethane, quinone compounds such as diphenoquinone and dinaphthylquinone, and the like, as well as compounds in which a plurality of these compounds are bonded, or polymers having a group consisting of these compounds in the main chain or side chain. However, the present invention is not limited to these, and any known electron transport material can be used. Among these, from the viewpoint of electrical properties, quinone compounds and perylene pigments (perylene derivatives) are preferred, and quinone compounds are more preferred.
Among the quinone compounds, diphenoquinone or dinaphthylquinone is preferable from the viewpoint of electrical properties, and among them, dinaphthylquinone is more preferable.
The above electron transport materials may be used alone or in any combination of two or more.

Specific examples of the electron transport material (ETM) that can be used in the present electrophotographic photoreceptor include the compounds represented by the general formulas (ET1) to (ET3) exemplified in paragraphs 0043 to 0053 of JP-A-2017-09765.
Also included are compounds having any of the following structures:

The content of the electron transport material (ETM) in the charge transport layer is preferably 10 parts by weight or more, more preferably 20 parts by weight or more, and even more preferably 30 parts by weight or more, per 100 parts by weight of the binder resin. On the other hand, it is preferably 150 parts by weight or less, more preferably 120 parts by weight or less, and even more preferably 100 parts by weight or less.

The content of the electron transport material (ETM) in the charge transport layer is preferably 0.1 parts by weight or more, more preferably 0.3 parts by weight or more, and even more preferably 0.5 parts by weight or more, per 100 parts by weight of the hole transport material (HTM) in the charge transport layer. On the other hand, it is preferably 10 parts by weight or less, more preferably 7 parts by weight or less, and even more preferably 5 parts by weight or less.

(Binder resin)
Examples of the binder resin for the charge transport layer include vinyl polymers such as polymethyl methacrylate, polystyrene, and polyvinyl chloride, and copolymers thereof, thermoplastic resins such as polycarbonate, polyarylate, polyester, polyester polycarbonate, polysulfone, phenoxy, epoxy, and silicone resins, and various thermosetting compounds. Among these resins, polycarbonate resins and polyarylate resins are preferred in terms of the light attenuation characteristics and mechanical strength of the photoconductor.

The viscosity average molecular weight (Mv) of the binder resin is usually in the range of 5,000 to 300,000, preferably 10,000 to 200,000, more preferably 15,000 to 150,000, and particularly preferably 20,000 to 80,000. If the viscosity average molecular weight (Mv) is too small, the mechanical strength of the resulting film for use in forming a photoreceptor or the like tends to decrease. If the viscosity average molecular weight (Mv) is too large, the viscosity of the coating liquid increases, making it difficult to coat the film to an appropriate thickness.

(Other ingredients)
The charge transport layer may contain other components as necessary in addition to the hole transport material (HTM), electron transport material (ETM) and binder resin. For example, known additives such as antioxidants, plasticizers, ultraviolet absorbers, electron-withdrawing compounds, leveling agents, visible light shielding agents and fillers may be contained for the purpose of improving film-forming properties, flexibility, coating properties, stain resistance, gas resistance, light resistance, etc.

(layer thickness)
The thickness of the charge transport layer is not particularly limited, but from the viewpoints of electrical characteristics, image stability, and high resolution, it is preferably 5 μm or more and 50 μm or less, more preferably 10 μm or more or 35 μm or less, and even more preferably 15 μm or more or 25 μm or less.

<Single-layer type photosensitive layer>
When the photosensitive layer of the present electrophotographic photoreceptor is a single-layer type photosensitive layer, it usually contains a binder resin, a charge generating material, a hole transport material and an electron transport material in the same layer.

The charge generating material, hole transport material, and electron transport material are the same as those described above for the laminated photosensitive layer.

From the viewpoint of sensitivity, the content of the charge generating material in the single-layer photosensitive layer is preferably 0.1% by mass or more, and more preferably 0.5% by mass or more. From the viewpoint of sensitivity and chargeability, the content is preferably 50% by mass or less, and more preferably 20% by mass or less.

In the single-layer photosensitive layer, the content ratio of the binder resin constituting the photosensitive layer and the hole transport material is preferably 20 parts by mass or more of the hole transport material per 100 parts by mass of the binder resin, more preferably 30 parts by mass or more from the viewpoint of reducing the residual potential, and even more preferably 40 parts by mass or more from the viewpoint of stability and charge mobility during repeated use. On the other hand, from the viewpoint of the thermal stability of the photosensitive layer, it is preferable that the hole transport material is 200 parts by mass or less per 100 parts by mass of the binder resin, and from the viewpoint of compatibility between the hole transport material and the binder resin, it is more preferable that the hole transport material is 150 parts by mass or less, and from the viewpoint of abrasion resistance, it is particularly preferable that the hole transport material is 120 parts by mass or less.

The content ratio of the binder resin and the electron transport material in the single-layer photosensitive layer is preferably 5 parts by mass or more of the electron transport material per 100 parts by mass of the binder resin. From the viewpoint of reducing the residual potential, 10 parts by mass or more is more preferable, and from the viewpoint of stability and charge mobility during repeated use, 20 parts by mass or more is even more preferable. On the other hand, from the viewpoint of the thermal stability of the photosensitive layer, it is preferable that the electron transport material is 100 parts by mass or less per 100 parts by mass of the binder resin. From the viewpoint of compatibility between the electron transport material and the binder resin, 80 parts by mass or less is more preferable, 60 parts by mass or less is even more preferable, and 50 parts by mass or less is particularly preferable.

In the single-layer photosensitive layer, the content ratio (mass ratio) of the electron transport material (ETM) to the hole transport material (HTM) is preferably 0.3 or more, more preferably 0.4 or more, and even more preferably 0.5 or more. On the other hand, it is preferably 1 or less, more preferably 0.9 or less, and even more preferably 0.8 or less.

The content ratio of the binder resin constituting the single-layer photosensitive layer and the charge transport material (electron transport material and hole transport material) is arbitrary, but it is preferable that the charge transport material is 25 parts by mass or more per 100 parts by mass of the binder resin. Among them, from the viewpoint of reducing the residual potential, 35 parts by mass or more is preferable, and from the viewpoint of stability and charge mobility during repeated use, 45 parts by mass or more is more preferable. On the other hand, from the viewpoint of the thermal stability of the photosensitive layer, it is preferable that the charge transport material is 200 parts by mass or less per 100 parts by mass of the binder resin, and from the viewpoint of compatibility between the charge transport material and the binder resin, it is more preferable that the charge transport material is 150 parts by mass or less, even more preferable that the charge transport material is 125 parts by mass or less, and particularly preferable that the charge transport material is 100 parts by mass or less.

<Method of forming each photosensitive layer>
In either the laminated type or the single layer type, the above layers can be formed as follows.
The substance to be contained is dissolved or dispersed in a solvent to obtain a coating solution, which is then applied to a conductive support by a known method such as dip coating, spray coating, nozzle coating, bar coating, roll coating, or blade coating, and the layers are then coated and dried in sequence to form a layer.
However, the method is not limited to this.

There are no particular limitations on the solvent or dispersion medium used to prepare the coating liquid. Specific examples include alcohols such as methanol, ethanol, propanol, and 2-methoxyethanol; ethers such as tetrahydrofuran, 1,4-dioxane, and dimethoxyethane; aromatic hydrocarbons such as benzene, toluene, xylene, and anisole; and chlorinated hydrocarbons such as dichloromethane, chloroform, 1,2-dichloroethane, 1,1,2-trichloroethane, 1,1,1-trichloroethane, tetrachloroethane, 1,2-dichloropropane, and trichloroethylene. These may be used alone or in any combination and ratio.

The amount of the solvent or dispersion medium used is not particularly limited. Taking into consideration the purpose of each layer and the properties of the selected solvent or dispersion medium, it is preferable to appropriately adjust the solid content concentration, viscosity, and other physical properties of the coating liquid so that they fall within the desired range.
The coating film is preferably dried to the touch at room temperature, and then heated and dried for 1 minute to 2 hours at a temperature in the range of 30° C. to 200° C., either stationary or with a fan. The heating temperature may be constant, or the temperature may be changed during drying.

<Main protective layer>
The protective layer of the present electrophotographic photoreceptor (the present protective layer) is a layer containing a cured product obtained by curing a curable compound.

The curable compound may be a photocurable compound, a heat curable compound, a radiation curable compound, etc., but among these, a photocurable compound is preferred from the viewpoint of improving abrasion resistance.

The curable compound is preferably, for example, a compound having a chain polymerizable functional group.
From the viewpoint of reactivity, the compound having a chain polymerizable functional group usually has 2 or more, preferably 3 or more, more preferably 4 or more, and on the other hand, usually has 20 or less, preferably 10 or less, more preferably 6 or less, of the chain polymerizable functional groups.
Examples of the chain polymerizable functional group of the compound having the chain polymerizable functional group include an acryloyl group, a methacryloyl group, a vinyl group, and an epoxy group. Among them, examples of the chain polymerizable functional group capable of radical polymerization include an acryloyl group, a methacryloyl group, and a vinyl group, and from the viewpoint of curing speed, an acryloyl group and a methacryloyl group are preferred.
The compound having a chain-polymerizable functional group is not particularly limited as long as it is a known material, but from the viewpoint of curability, a monomer, oligomer, or polymer having an acryloyl group or a methacryloyl group is preferred.

Preferred compounds are exemplified below.
Examples of monomers having an acryloyl group or a methacryloyl group include trimethylolpropane triacrylate (A-TMPT), trimethylolpropane trimethacrylate, HPA-modified trimethylolpropane triacrylate, EO-modified trimethylolpropane triacrylate, PO-modified trimethylolpropane triacrylate, caprolactone-modified trimethylolpropane triacrylate, HPA-modified trimethylolpropane trimethacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, glycerol triacrylate, and EC. H-modified glycerol triacrylate, EO-modified glycerol triacrylate, PO-modified glycerol triacrylate, tris(acryloxyethyl)isocyanurate, caprolactone-modified tris(acryloxyethyl)isocyanurate, EO-modified tris(acryloxyethyl)isocyanurate, PO-modified tris(acryloxyethyl)isocyanurate, dipentaerythritol hexaacrylate (A-DPH), caprolactone-modified dipentaerythritol hexaacrylate, dipentaerythritol hydroxypentaacrylate, alkyl-modified dipentaerythritol pentaerythritol tetraacrylate, alkyl-modified dipentaerythritol tetraacrylate, alkyl-modified dipentaerythritol triacrylate, dimethylolpropane tetraacrylate, pentaerythritol ethoxy tetraacrylate, EO-modified phosphoric acid triacrylate, 2,2,5,5-tetrahydroxymethylcyclopentanone tetraacrylate, 2-hydroxy-3-acryloyloxypropyl methacrylate, polyethylene glycol diacrylate, polypropylene glycol diacrylate, polytetramethylene glycol diacrylate, EO-modified bisphenol A diacrylate, PO-modified bisphenol A diacrylate, 9,9-bis[4-(2-acryloyloxyethoxy)phenyl]fluorene, tricyclodecane dimethanol diacrylate, decanediol diacrylate, hexanediol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, EO-modified bisphenol A dimethacrylate, PO-modified bisphenol A dimethacrylate, tricyclodecane dimethanol dimethacrylate, decanediol dimethacrylate, hexanediol dimethacrylate, and the like.

As the oligomer or polymer having an acryloyl group or methacryloyl group, known urethane acrylate, ester acrylate, acrylic acrylate, epoxy acrylate, etc. can be used.
Examples of the urethane acrylate include "EBECRYL8301", "EBECRYL1290", "EBECRYL1830", and "KRM8200" (Daicel-Allnex Corporation), "UV1700B", "UV7640B", "UV7605B", "UV6300B", and "UV7550B" (Mitsubishi Chemical Corporation).
Examples of the ester acrylate include "M-7100", "M-7300K", "M-8030", "M-8060", "M-8100", "M-8530", "M-8560", and "M-9050" (Toagosei Co., Ltd.).
Examples of the acrylic acrylate include "8BR-600", "8BR-930MB", "8KX-078", "8KX-089", and "8KX-168" (Taisei Fine Chemical Co., Ltd.).

These may be used alone or in combination of two or more. Among these, it is preferable to contain urethane acrylate from the viewpoint of electrical properties.

In addition to the compound having a chain-polymerizable functional group, the protective layer may contain metal oxide particles or a charge transport material for the purpose of imparting charge transport capability. It may also contain a polymerization initiator to promote the polymerization reaction.

The materials used in this protective layer (metal oxide particles, charge transport material, polymerization initiator) are described in detail below.

(metal oxide particles)
From the viewpoint of imparting charge transport capability to the protective layer and from the viewpoint of improving the mechanical strength, it is preferable that the protective layer contains metal oxide particles having a band gap of 2 eV or more and 4 eV or less (also referred to as "conductive metal oxide particles").

As the conductive metal oxide particles, any metal oxide particles that can be used in an electrophotographic photoreceptor can be used.
More specifically, examples of the conductive metal oxide particles include metal oxide particles containing one type of metal element such as titanium oxide, tin oxide, aluminum oxide, indium oxide, silicon oxide, zirconium oxide, zinc oxide, and iron oxide, and metal oxide particles containing multiple metal elements such as indium tin oxide, calcium titanate, strontium titanate, and barium titanate. These conductive metal oxide particles may be used as a single type of particle or as a mixture of multiple types of particles.
Among these conductive metal oxide particles, from the viewpoint of electron transport properties, titanium oxide, tin oxide, indium tin oxide, aluminum oxide, silicon oxide, and zinc oxide are preferred, and titanium oxide and tin oxide are more preferred. Titanium oxide is particularly preferred.

The crystal type of the titanium oxide particles may be any of rutile, anatase, brookite, and amorphous. In addition, the particles may contain particles with different crystal states, or particles with multiple crystal states.

The conductive metal oxide particles may be subjected to various surface treatments. For example, they may be treated with inorganic substances such as tin oxide, aluminum oxide, antimony oxide, zirconium oxide, silicon oxide, etc., or with organic substances such as stearic acid, polyols, and organic silicon compounds. In particular, when titanium oxide particles are used, it is preferable that the surface is treated with an organic silicon compound. Examples of organic silicon compounds include silicone oils such as dimethylpolysiloxane and methylhydrogenpolysiloxane, organosilanes such as methyldimethoxysilane and diphenyldidimethoxysilane, silazanes such as hexamethyldisilazane, and silane coupling agents such as 3-methacryloyloxypropyltrimethoxysilane, 3-acryloyloxypropyltrimethoxysilane, vinyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, and γ-aminopropyltriethoxysilane. In particular, from the viewpoint of improving the mechanical strength of the protective layer, 3-methacryloyloxypropyltrimethoxysilane, 3-acryloyloxypropyltrimethoxysilane, and vinyltrimethoxysilane, which have chain-polymerizable functional groups, are preferred.

Incidentally, the conductive metal oxide particles may be pretreated with an insulating material such as aluminum oxide, silicon oxide or zirconium oxide before treating the outermost surface with such a treatment agent.
The conductive metal oxide particles may be of one type only, or a mixture of a plurality of types of particles.

The conductive metal oxide particles preferably have an average primary particle size of 500 nm or less, more preferably 1 nm to 100 nm, and even more preferably 5 to 50 nm.
This average primary particle size can be determined from the arithmetic mean value of particle sizes directly observed using a transmission electron microscope (hereinafter also referred to as TEM).

Among the conductive metal oxide particles of the present electrophotographic photoreceptor, specific product names of titanium oxide particles include ultrafine titanium oxide particles that have not been subjected to surface treatment, such as "TTO-55(N)" and "TTO-51(N)" and Al ₂ O The products include ultrafine titanium dioxide particles with _triple coating "TTO-55(A)" and "TTO-55(B)", ultrafine titanium dioxide particles with surface treatment using stearic acid "TTO-55(C)", ultrafine titanium dioxide particles with surface treatment using Al2O3 and organosiloxane "TTO-55(S)", high-purity titanium dioxide "C-EL", titanium dioxide particles produced by sulfuric acid process "R-550", "R-580", "R-630", "R-670", "R-680", "R-780", "A-100", "A-220", "W-10", titanium dioxide particles produced by chlorine process "CR-50", "CR-58", "CR-60", "CR-60-2", "CR-67", conductive titanium dioxide "ET-300W" (all manufactured by Ishihara Sangyo Kaisha, Ltd.), titanium dioxide particles with _Al2O3 , "R-60", "A-110", "A-150", etc. _{The following} are available: "SR-1", "R-GL", "R-5N", "R-5N-2", "R-52N", "RK-1", and "A-SP" coated with SiO ₂ and Al ₂ O ₃ ; "R-GX" and "R-7E" coated with SiO ₂ and Al ₂ O ₃ ; "R-650" coated with ZnO, SiO 2 and Al 2 O 3; and "R-61N" coated with ZrO ₂ and Al ₂ O ₃ (all manufactured by Sakai Chemical Industry Co., Ltd.). In addition, "TR-700" surface-treated with SiO ₂ and Al ₂ O ₃ , "TR-840" and "TA-500" surface-treated with ZnO, SiO ₂ and Al ₂ O ₃ , as well as "TA-100", "TA-200", "TA-300", and other titanium oxide and Al ₂ O 3-untreated steels. Examples of such steels include "TA-400" which has been surface-treated with SiO ₂ and Al ₂ O ₃ (manufactured by Fuji Titanium Industry Co., Ltd. ₎ , "MT-150W" and "MT-500B" which have not been surface-treated, "MT-100SA" and "MT-500SA" which have been surface-treated with SiO ₂ and Al ₂ O ₃ , and "MT-100SAS" and "MT-500SAS" (manufactured by Teika Corporation) which have been surface-treated with SiO 2 , Al 2 O 3 and organosiloxane.

Specific product names for aluminum oxide particles include "Aluminum Oxide C" (manufactured by Nippon Aerosil Co., Ltd.).

Specific product names for silicon oxide particles include "200CF," "R972" (manufactured by Nippon Aerosil Co., Ltd.), and "KEP-30" (manufactured by Nippon Shokubai Co., Ltd.).

Specific product names for tin oxide particles include "SN-100P," "SN-100D" (manufactured by Ishihara Sangyo Kaisha), "SnO2" (manufactured by CIK Nanotech Co., Ltd.), "S-2000," phosphorus-doped tin oxide "SP-2," antimony-doped tin oxide "T-1," and indium-doped tin oxide "E-ITO" (Mitsubishi Materials Corporation).

A specific product name for zinc oxide particles is "MZ-305S" (manufactured by Teika Corporation), but the conductive metal oxide particles that can be used in the present invention are not limited to this.

The content of the conductive metal oxide particles in the protective layer is not particularly limited. From the viewpoint of electrical properties, the content is preferably 10 parts by mass or more, more preferably 20 parts by mass or more, and particularly preferably 30 parts by mass or more, relative to 100 parts by mass of the binder resin. Also, from the viewpoint of maintaining good surface resistance, the content is preferably 300 parts by mass or less, more preferably 200 parts by mass or less, and particularly preferably 120 parts by mass or less.

From the viewpoint of suppressing inflow current, the content ratio (mass ratio) of insulating metal oxide particles in the photosensitive layer to the content of conductive metal oxide particles in the protective layer is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.3 or more. On the other hand, from the viewpoint of electrical properties, it is preferably 0.5 or less, more preferably 0.45 or less, and even more preferably 0.4 or less.

The protective layer may further contain metal oxide particles (insulating metal oxide particles) having a band gap of 5 eV or more. The insulating metal oxide particles contained in the protective layer may be the same as the insulating metal oxide particles contained in the photosensitive layer described above.
It is presumed that by including insulating metal oxide particles in the protective layer in addition to the photosensitive layer, the inflow current can be further suppressed and the abrasion resistance can be further improved.

From the viewpoint of imparting charge transporting ability to the present protective layer, the content of the insulating metal oxide particles in the present protective layer is preferably less than the content of the conductive metal oxide particles in the present protective layer.
The content ratio (mass ratio) of the conductive metal oxide particles to the insulating metal oxide particles in the protective layer is preferably 0.1 or more, more preferably 0.2 or more, and even more preferably 0.3 or more, and is preferably 0.7 or less, more preferably 0.6 or less, and even more preferably 0.5 or less.

(charge transport material)
The charge transport material contained in the protective layer may be the same as the charge transport material used in the photosensitive layer.

The content of the charge transport material in the protective layer is not particularly limited. From the viewpoint of electrical properties, the content is preferably 10 parts by weight or more, more preferably 30 parts by weight or more, and particularly preferably 50 parts by weight or more, relative to 100 parts by weight of the binder resin. Also, from the viewpoint of maintaining good surface resistance, the content is preferably 300 parts by weight or less, more preferably 20 parts by weight or less, and particularly preferably 150 parts by weight or less.

In order to impart charge transporting ability to the protective layer, the protective layer may contain a structure formed by polymerizing a charge transporting material having a chain polymerizable functional group.
Examples of the chain-polymerizable functional group of the charge transport material having a chain-polymerizable functional group include an acryloyl group, a methacryloyl group, a vinyl group, and an epoxy group. Among these, from the viewpoint of curability, an acryloyl group or a methacryloyl group is preferred. Examples of the structure of the charge transport material portion of the charge transport material having a chain-polymerizable functional group include heterocyclic compounds such as carbazole derivatives, indole derivatives, imidazole derivatives, oxazole derivatives, pyrazole derivatives, thiadiazole derivatives, and benzofuran derivatives, aniline derivatives, hydrazone derivatives, arylamine derivatives, stilbene derivatives, butadiene derivatives, and enamine derivatives, as well as compounds in which a plurality of these compounds are bonded, and electron donating materials such as polymers having groups consisting of these compounds in the main chain or side chain. Among these, from the viewpoint of electrical properties, carbazole derivatives, arylamine derivatives, stilbene derivatives, butadiene derivatives, and enamine derivatives, as well as compounds in which a plurality of these compounds are bonded, are preferred.

(Polymerization initiator)
The polymerization initiator includes a thermal polymerization initiator, a photopolymerization initiator, and the like.

Examples of thermal polymerization initiators include peroxide compounds such as 2,5-dimethylhexane-2,5-dihydroperoxide, dicumyl peroxide, benzoyl peroxide, t-butyl peroxide, t-butylcumyl peroxide, t-butyl hydroperoxide, cumene hydroperoxide, and lauroyl peroxide, and azo compounds such as 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(cyclohexanecarbonitrile), 2,2'-azobis(methylisobutyrate), 2,2'-azobis(isobutylamidine hydrochloride), and 4,4'-azobis-4-cyanovaleric acid.

Photopolymerization initiators can be classified into direct cleavage type and hydrogen abstraction type based on the difference in the radical generation mechanism. When a direct cleavage type photopolymerization initiator absorbs light energy, some of the covalent bonds in the molecule are cleaved, generating radicals. On the other hand, when a hydrogen abstraction type photopolymerization initiator absorbs light energy and becomes excited, the molecule abstracts hydrogen from the hydrogen donor, generating radicals.

Direct cleavage type photopolymerization initiators include acetophenone or ketal compounds such as acetophenone, 2-benzoyl-2-propanol, 1-benzoylcyclohexanol, 2,2-diethoxyacetophenone, benzyl dimethyl ketal, and 2-methyl-4'-(methylthio)-2-morpholinopropiophenone; benzoin ether compounds such as benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isobutyl ether, benzoin isopropyl ether, and O-tosylbenzoin; and acylphosphine oxide compounds such as diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide, and lithium phenyl(2,4,6-trimethylbenzoyl)phosphonate.

Examples of hydrogen abstraction type photopolymerization initiators include benzophenone-based compounds such as benzophenone, 4-benzoylbenzoic acid, 2-benzoylbenzoic acid, methyl 2-benzoylbenzoate, methyl benzoylformate, benzyl, p-anisil, 2-benzoylnaphthalene, 4,4'-bis(dimethylamino)benzophenone, 4,4'-dichlorobenzophenone, and 1,4-dibenzoylbenzene, and anthraquinone-based or thioxanthone-based compounds such as 2-ethylanthraquinone, 2-isopropylthioxanthone, 2-chlorothioxanthone, 2,4-dimethylthioxanthone, 2,4-diethylthioxanthone, and 2,4-dichlorothioxanthone. Other examples of photopolymerization initiators include camphorquinone, 1-phenyl-1,2-propanedione-2-(o-ethoxycarbonyl)oxime, acridine-based compounds, triazine-based compounds, and imidazole-based compounds.

In order to efficiently absorb light energy and generate radicals, the photopolymerization initiator preferably has an absorption wavelength in the wavelength region of the light source used for light irradiation. On the other hand, if a component other than the photopolymerization initiator among the compounds contained in the present protective layer has absorption in this wavelength region, the photopolymerization initiator may not be able to absorb sufficient light energy, and the radical generation efficiency may decrease. Since general binder resins, charge transport materials, and metal oxide particles have absorption wavelengths in the ultraviolet (UV) region, this effect is particularly remarkable when the light source used for light irradiation is ultraviolet (UV). From the viewpoint of preventing such defects, it is preferable to contain an acylphosphine oxide-based compound, which has an absorption wavelength on the relatively long wavelength side among photopolymerization initiators. In addition, since the acylphosphine oxide-based compound has a photobleach effect in which the absorption wavelength region changes to the low wavelength side by self-cleavage, it is preferable that light can be transmitted to the inside of the protective layer and has good internal curing properties. In this case, it is even more preferable to use a hydrogen abstraction type initiator in combination from the viewpoint of supplementing the curing properties of the protective layer surface. The content ratio of the hydrogen abstraction initiator relative to the acylphosphine oxide compound is not particularly limited, but from the viewpoint of supplementing surface curing properties, it is preferably 0.1 parts by mass or more relative to 1 part by mass of the acylphosphine oxide compound, and from the viewpoint of maintaining internal curing properties, it is preferably 5 parts by mass or less.

Also, a photopolymerization accelerator can be used alone or in combination with the above photopolymerization initiator. Examples include triethanolamine, methyldiethanolamine, ethyl 4-dimethylaminobenzoate, isoamyl 4-dimethylaminobenzoate, (2-dimethylamino)ethyl benzoate, and 4,4'-dimethylaminobenzophenone.

These polymerization initiators may be used alone or in combination of two or more. The content of the polymerization initiator is 0.5 to 40 parts by mass, preferably 1 to 20 parts by mass, per 100 parts by mass of the total content having radical polymerizability.

(Other materials)
The protective layer may contain other materials as necessary. Examples of the other materials include stabilizers (heat stabilizers, ultraviolet absorbers, light stabilizers, antioxidants, etc.), dispersants, antistatic agents, colorants, lubricants, etc. These may be used alone or in any ratio and combination of two or more.

(Method of forming the protective layer)
Next, the method for forming the protective layer will be described.
The method for forming the protective layer is not particularly limited. For example, the protective layer can be formed by applying a coating liquid in which a binder resin, a charge transport material, metal oxide particles, and other substances are dissolved in a solvent or dispersed in a dispersion medium.

The solvent or dispersion medium used to form this protective layer and the coating method are described below.

[Solvent used in coating solution for forming protective layer]
Examples of organic solvents used in the present protective layer-forming coating liquid include alcohols such as methanol, ethanol, propanol, and 2-methoxyethanol; ethers such as tetrahydrofuran, 1,4-dioxane, and dimethoxyethane; esters such as methyl formate and ethyl acetate; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; aromatic hydrocarbons such as benzene, toluene, xylene, and anisole; chlorinated hydrocarbons such as dichloromethane, chloroform, 1,2-dichloroethane, 1,1,2-trichloroethane, 1,1,1-trichloroethane, tetrachloroethane, 1,2-dichloropropane, and trichloroethylene; nitrogen-containing compounds such as n-butylamine, isopropanolamine, diethylamine, triethanolamine, ethylenediamine, and triethylenediamine; and aprotic polar solvents such as acetonitrile, N-methylpyrrolidone, N,N-dimethylformamide, and dimethylsulfoxide. Mixed solvents of any combination and ratio among these can also be used. In addition, even if the organic solvent does not dissolve the material for the protective layer of the electrophotographic photoreceptor by itself, it can be used if it can dissolve the material by mixing it with the above organic solvent. In general, the use of a mixed solvent can reduce coating unevenness. When using a dip coating method in the coating method described below, it is preferable to select a solvent that does not dissolve the lower layer. From this point of view, it is preferable to include alcohols that have low solubility in polycarbonate and polyarylate, which are suitable for use in the photosensitive layer.

The ratio of the organic solvent and solid content used in the coating solution for forming the protective layer varies depending on the coating method for the coating solution for forming the protective layer, and may be appropriately changed so that a uniform coating film is formed in the coating method to be used.

[Application method]
The method of applying the coating solution for forming the present protective layer is not particularly limited, and examples thereof include spray coating, spiral coating, ring coating, and dip coating.

After forming the coating film by the above coating method, the coating film is dried. In this case, the temperature and time of drying are not important as long as necessary and sufficient drying is obtained. However, if the protective layer is applied by only air drying after the photosensitive layer is applied, it is preferable to perform sufficient drying by the method described above in the [Coating method] for the photosensitive layer.

[Method of curing the protective layer]
The protective layer is formed by applying the coating liquid and then curing it by applying external energy thereto, such as heat, light, or radiation.
The method of applying heat energy is to heat the coating surface side or the support side using air, gas such as nitrogen, steam, or various heat media, infrared rays, or electromagnetic waves. The heating temperature is preferably 100°C or higher and 170°C or lower, and above the lower limit temperature, the reaction speed is sufficient and the reaction proceeds completely. Below the upper limit temperature, the reaction proceeds uniformly and the occurrence of large distortion in the protective layer can be suppressed. In order to proceed uniformly with the curing reaction, it is also effective to heat at a relatively low temperature below 100°C, and then further heat to 100°C or higher to complete the reaction.

As the light energy, a UV irradiation light source such as a high-pressure mercury lamp, a metal halide lamp, an electrodeless lamp bulb, or a light-emitting diode, which mainly have an emission wavelength in the ultraviolet (UV) range, can be used, but a visible light source can also be selected in accordance with the absorption wavelength of the chain polymerizable compound or the photopolymerization initiator.
From the viewpoint of curability, the light irradiation amount is preferably 0.1 J/ ^cm2 or more, more preferably 0.5 J/ ^cm2 or more, and particularly preferably 1 J/cm2 or ^more . From the viewpoint of electrical properties, the light irradiation amount is preferably 150 J/cm2 or ^less , more preferably 100 J/ ^cm2 or less, and particularly preferably 50 J/ ^cm2 or less.
The radiation energy may be an electron beam (EB).

Among these energies, light energy is preferred from the viewpoints of ease of reaction rate control, simplicity of the equipment, and long pot life.

After the protective layer is cured, a heating step may be added from the viewpoint of relieving residual stress, relieving residual radicals, and improving electrical characteristics. The heating temperature is preferably 60°C or higher, more preferably 100°C or higher, and is preferably 200°C or lower, more preferably 150°C or lower.

<Conductive Support>
The conductive support of the present electrophotographic photoreceptor is not particularly limited as long as it supports a layer formed thereon and exhibits electrical conductivity.
The conductive support may be, for example, a metal material such as aluminum, an aluminum alloy, stainless steel, copper, or nickel, a resin material to which a conductive powder such as metal, carbon, or tin oxide is coexisted to impart conductivity, or a resin, glass, or paper to which a conductive material such as aluminum, nickel, or ITO (indium oxide tin oxide alloy) is vapor-deposited or applied on its surface. As a form, a drum, sheet, or belt shape may be used. A conductive material having an appropriate resistance value may be applied onto a conductive support made of a metal material to control the conductivity and surface properties or to cover defects.

When using a metal material such as an aluminum alloy as the conductive support, the metal material may be anodized before use.

The average thickness of the anodized coating is usually 20 μm or less, and preferably 7 μm or less.

When applying an anodized coating to a metal material, it is preferable to perform a sealing treatment. The sealing treatment can be performed by a known method.

The surface of the conductive support may be smooth, or may be roughened by using a special cutting method, by polishing, or by mixing particles of an appropriate particle size into the material constituting the support.
In order to improve adhesion, blocking properties, etc., an undercoat layer, which will be described later, may be provided between the conductive support and the photosensitive layer.

<Undercoat layer>
The present electrophotographic photoreceptor may have an undercoat layer between the present photosensitive layer and the conductive support.

For example, the undercoat layer may be made of a resin or a resin in which organic pigments or particles of metal oxides are dispersed. There are no particular limitations on the organic pigments used in the undercoat layer. For example, the phthalocyanine pigments and azo pigments used as the charge generating material described above may be used.

Examples of metal oxide particles used in the undercoat layer include metal oxide particles containing one type of metal element, such as titanium oxide, aluminum oxide, silicon oxide, zirconium oxide, zinc oxide, and iron oxide, and metal oxide particles containing multiple metal elements, such as calcium titanate, strontium titanate, and barium titanate. The undercoat layer may contain only one type of particle, or multiple types of particles may be mixed in any ratio and combination.

Among the above metal oxide particles, titanium oxide and aluminum oxide are preferred, and titanium oxide is particularly preferred. The titanium oxide particles may have a surface treated with any inorganic or organic substance. The crystal type of the titanium oxide particles may be any of rutile, anatase, brookite, and amorphous. The particles may also include particles in multiple crystal states.

There are no particular limitations on the particle size of the metal oxide particles used in the undercoat layer. In terms of the characteristics of the undercoat layer and the stability of the solution used to form the undercoat layer, the average primary particle size is preferably 10 nm or more, and 100 nm or less, more preferably 50 nm or less.

Here, it is desirable to form the undercoat layer in a form in which the particles are dispersed in a binder resin. The binder resin used in the undercoat layer may be selected from, for example, polyvinyl acetal resins such as polyvinyl butyral resins, polyvinyl formal resins, and partially acetalized polyvinyl butyral resins in which a part of butyral is modified with formal or acetal; insulating resins such as polyarylate resins, polycarbonate resins, polyester resins, phenoxy resins, acrylic resins, methacrylic resins, polyamide resins, polyurethane resins, epoxy resins, silicone resins, polyvinyl alcohol resins, styrene-alkyd resins, silicone-alkyd resins, and phenol-formaldehyde resins. However, the binder resins are not limited to these polymers. In addition, these binder resins may be used alone or in a mixture of two or more kinds, or may be used in a cured form together with a curing agent.
Among these, polyvinyl acetal resins, alcohol-soluble copolymerized polyamides, modified polyamides, etc. are preferred because they exhibit good dispersibility and coatability, and among these, alcohol-soluble copolymerized polyamides are particularly preferred.

The mixing ratio of the particles to the binder resin can be selected as desired. It is preferable to use a ratio in the range of 10% by mass to 500% by mass in terms of the stability and coatability of the dispersion.

The thickness of the undercoat layer can be selected as desired. In consideration of the characteristics of the electrophotographic photoreceptor and the coatability of the dispersion liquid, it is usually preferable to set the thickness to 0.1 μm or more and 20 μm or less. The undercoat layer may also contain known antioxidants, etc.

<Other demographics>
Furthermore, the present electrophotographic photoreceptor may have other layers as necessary in addition to the above-mentioned conductive support, the present photosensitive layer, the present protective layer and the undercoat layer.

(Thickness other than that of the conductive support)
The thickness of the electrophotographic photoreceptor excluding the conductive support, i.e., the thickness of the electrophotographic photoreceptor minus the thickness of the conductive support, is preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more, from the viewpoint of leak resistance, while from the viewpoint of chargeability, it is preferably 50 μm or less, more preferably 40 μm or less, and even more preferably 35 μm or less.

<<Present image forming apparatus>>
The present electrophotographic photoreceptor can be used to configure an image forming apparatus (the present image forming apparatus).

As shown in FIG. 1, the image forming apparatus includes an electrophotographic photoreceptor 1, a charging device 2, an exposure device 3, and a developing device 4, and may further include a transfer device 5, a cleaning device 6, and a fixing device 7 as required.
The electrophotographic photoreceptor 1 is not particularly limited as long as it is the electrophotographic photoreceptor described above. As an example, Fig. 1 shows a drum-shaped photoreceptor in which the above-mentioned photosensitive layer is formed on the surface of a cylindrical conductive support. A charging device 2, an exposure device 3, a developing device 4, a transfer device 5, and a cleaning device 6 are arranged along the outer circumferential surface of the electrophotographic photoreceptor 1.

The charging device 2 charges the electrophotographic photoreceptor 1, uniformly charging the surface of the electrophotographic photoreceptor 1 to a predetermined potential. Examples of typical charging devices include non-contact corona charging devices such as corotrons and scorotrons, and image forming devices of a contact charging type (also called a "contact charging device" or "direct charging device") in which a charging member to which a voltage is applied is brought into contact with the surface of the photoreceptor to charge the surface. Examples of contact charging devices include a charging roller and a charging brush. Note that FIG. 1 shows a roller-type charging device (charging roller) as an example of the charging device 2.
The charging may be performed by a DC voltage or by superimposing an AC voltage on a DC voltage.

There are no particular limitations on the type of exposure device 3, so long as it can expose the electrophotographic photoreceptor 1 to light and form an electrostatic latent image on the photosensitive surface of the electrophotographic photoreceptor 1. Exposure may also be performed using an internal photoreceptor exposure method. Any light may be used for exposure.

The type of toner T is arbitrary, and in addition to powder toner, polymerized toners using methods such as suspension polymerization and emulsion polymerization can be used.

There is no particular limit to the type of transfer device 5, and any device using any method, such as electrostatic transfer methods such as corona transfer, roller transfer, and belt transfer, pressure transfer, and adhesive transfer, can be used. Here, the transfer device 5 is composed of a transfer charger, a transfer roller, a transfer belt, and the like, arranged opposite the electrophotographic photoreceptor 1. The transfer device 5 applies a predetermined voltage value (transfer voltage) with a polarity opposite to the charged potential of the toner T, and transfers the toner image formed on the electrophotographic photoreceptor 1 to recording paper (paper, medium) P.

There are no particular limitations on the cleaning device 6, and any cleaning device can be used, such as a brush cleaner, a magnetic brush cleaner, an electrostatic brush cleaner, a magnetic roller cleaner, or a blade cleaner. The cleaning device 6 scrapes off the residual toner adhering to the photoreceptor 1 with a cleaning member and collects the residual toner. However, if there is little or almost no toner remaining on the photoreceptor surface, the cleaning device 6 may not be necessary.

In the image forming apparatus configured as above, an image is recorded as follows: First, the surface (photosensitive surface) of the photoconductor 1 is charged to a predetermined potential (e.g., 600 V) by the charging device 2. At this time, charging may be performed by a DC voltage, or by superimposing an AC voltage on the DC voltage.
Next, the charged photosensitive surface of the photoreceptor 1 is exposed by an exposure device 3 according to the image to be recorded, and an electrostatic latent image is formed on the photosensitive surface. Then, the electrostatic latent image formed on the photosensitive surface of the photoreceptor 1 is developed by a development device 4.

In the developing device 4, the toner T supplied by the supply roller 43 is formed into a thin layer by a regulating member (developing blade) 45, and is frictionally charged to a predetermined polarity (here, positive polarity, which is the same polarity as the charging potential of the photosensitive member 1), and is transported while being carried by the developing roller 44 and brought into contact with the surface of the photosensitive member 1.
When the charged toner T carried by the developing roller 44 comes into contact with the surface of the photoreceptor 1, a toner image corresponding to the electrostatic latent image is formed on the photoreceptor surface of the photoreceptor 1. This toner image is then transferred to the recording paper P by the transfer device 5. Thereafter, the toner remaining on the photoreceptor surface of the photoreceptor 1 without being transferred is removed by the cleaning device 6.

After the toner image is transferred onto the recording paper P, the recording paper P passes through a fixing device 7 where the toner image is thermally fixed onto the recording paper P, thereby obtaining a final image.
In addition to the above-mentioned configuration, the image forming apparatus may be configured to perform, for example, a charge removal process.

In addition, this image forming apparatus may be further modified, for example, to perform processes such as a pre-exposure process and an auxiliary charging process, to perform offset printing, or even to be configured as a full-color tandem system using multiple types of toner.

<<This electrophotographic cartridge>>
The electrophotographic photoreceptor 1 can be combined with one or more of a charging device 2, an exposure device 3, a developing device 4, a transfer device 5, a cleaning device 6 and a fixing device 7 to form an integrated cartridge (referred to as "the electrophotographic cartridge").

The electrophotographic cartridge can be configured to be detachable from the main body of an electrophotographic device such as a copier or laser beam printer. In that case, for example, if the electrophotographic photoreceptor 1 or other components deteriorate, the electrophotographic photoreceptor cartridge can be removed from the main body of the image forming device and a new electrophotographic photoreceptor cartridge can be attached to the main body of the image forming device, making it easy to maintain and manage the image forming device.

<<Terminology explanation>>
In the present invention, when expressed as "X to Y" (X and Y are arbitrary numbers), unless otherwise specified, it includes the meaning of "X or more and Y or less", as well as "preferably larger than X" or "preferably smaller than Y".
Furthermore, when it is expressed as "X or more" (X is any number) or "Y or less" (Y is any number), it also includes the intention that "it is preferably greater than X" or "it is preferably less than Y".

The present invention will be further described by the following examples, which are not intended to limit the invention in any manner.

<Preparation of Coating Solution for Forming Undercoat Layer>
(Coating solution A for forming undercoat layer)
Titanium oxide particles in which the surface of rutile titanium oxide ("TTO55N" manufactured by Ishihara Sangyo Kaisha, Ltd., average primary particle size 40 nm) had been treated with methyldimethoxysilane ("TSL8117" manufactured by Toshiba Silicones Co., Ltd.) and a copolymeric polyamide having the following composition ratio were mixed with stirring while heating in a mixed solvent (methanol/1-propanol/toluene in a mass ratio of 7/1/2) to dissolve the copolymeric polyamide. Subsequently, ultrasonic dispersion treatment was performed for 1 hour using an ultrasonic oscillator with an output of 1200 W, and further filtration was performed to obtain coating solution A for forming an undercoat layer in which the mass ratio of surface-treated titanium oxide/copolymeric polyamide was 3/1 and the solid content concentration was 18.0 mass%.
Composition ratio (molar ratio) of copolymerized polyamide: ε-caprolactam/bis(4-amino-3-methylcyclohexyl)methane/hexamethylenediamine/decamethylenedicarboxylic acid/octadecamethylenedicarboxylic acid=75/9.5/3/9.5/3

<Production of Coating Solution for Forming Charge Generating Layer>
(Coating Solution B for Forming Charge Generating Layer)
The coating liquid for forming the charge generating layer was prepared as follows.
As a charge generating material, 20 parts of D-type (Y-type) oxytitanium phthalocyanine A and 280 parts of 1,2-dimethoxyethane were mixed and ground in a sand grind mill for 1 hour to carry out a fine particle dispersion treatment. Next, this fine particle treatment liquid was mixed with a binder liquid obtained by dissolving 10 parts of polyvinyl butyral (manufactured by Denki Kagaku Kogyo Co., Ltd., "Denka Butyral"#6000C) in a mixed liquid of 255 parts of 1,2-dimethoxyethane and 85 parts of 4-methoxy-4-methyl-2-pentanone, and 230 parts of 1,2-dimethoxyethane to prepare a charge generating layer forming coating liquid B1.

On the other hand, 20 parts of A-type (β-type) oxytitanium phthalocyanine B was mixed with 280 parts of 1,2-dimethoxyethane as a charge generating material and ground in a sand grind mill for 4 hours to carry out a fine particle dispersion treatment. Next, this fine particle treatment liquid was mixed with a binder liquid obtained by dissolving 10 parts of polyvinyl butyral (Denka Butyral #6000C, manufactured by Denki Kagaku Kogyo Co., Ltd.) in a mixture of 255 parts of 1,2-dimethoxyethane and 85 parts of 4-methoxy-4-methyl-2-pentanone, and 230 parts of 1,2-dimethoxyethane to prepare a charge generating layer forming coating liquid B2.

The charge generating layer forming coating liquid B1 and the charge generating layer forming coating liquid B2 were mixed in a mass ratio of 7:3 to prepare the charge generating layer forming coating liquid B used in this example.

<Preparation of Coating Solution for Forming Charge Transport Layer>
(Coating solution C1 for forming charge transport layer)
66.64 parts of a polycarbonate resin represented by the following repeating structure (resin X1, viscosity average molecular weight 50,000), 33.33 parts of HTM1 as a charge transport material, and 0.03 parts of a leveling agent silicone oil (KF96-10CS, manufactured by Shin-Etsu Chemical Co., Ltd.) were dissolved in a mixed solvent of tetrahydrofuran and toluene at a ratio of 8:2 with heating and stirring to obtain a coating solution C1 for forming a charge transport layer having a solid content concentration of 18% by mass.

Resin X1 is a resin having the following chemical structure:

HTM1 is a compound with the following chemical structure:

(Coating solution C2 for forming charge transport layer)
Silica (silicon dioxide) particles (KE-S30HG, manufactured by Nippon Shokubai Co., Ltd., average primary particle size 0.25 μm, specific gravity 2.2, spherical) were surface-treated with hexamethyldisilazane, and the surface-treated silica particles were placed in a tetrahydrofuran solvent and subjected to ultrasonic dispersion for 4 hours to obtain a surface-treated silica particle slurry.
Next, 66.64 parts of the resin X1, 33.33 parts of HTM1 as a charge transport material, and 0.03 parts of silicone oil (KF96-10CS, manufactured by Shin-Etsu Chemical Co., Ltd.) as a leveling agent were dissolved in a mixed solvent of tetrahydrofuran and anisole at a ratio of 9:1 with heating and stirring to prepare an intermediate liquid. The surface-treated silica particle slurry was added to this intermediate liquid so that the number of parts of the surface-treated silica particles was 0.3 times the number of parts of HTM1. After that, the mixture was heated and stirred to obtain a charge transport layer forming coating liquid C2 with a solid content concentration of 18% by mass.
The concentration of silica particles in the surface-treated silica particle slurry and the mixing ratio of tetrahydrofuran to anisole in the intermediate liquid were determined so that the mixing ratio of tetrahydrofuran to anisole in the entire liquid after adding the surface-treated silica particle slurry to the intermediate liquid would be 9:1.

(Charge transport layer forming coating liquid C3)
A coating solution C3 for forming a charge transport layer was obtained in the same manner as in the coating solution C2 for forming a charge transport layer, except that the amount of the surface-treated silica particle slurry used in the coating solution C2 for forming a charge transport layer was 0.1 times the number of parts of HTM1.

<Production of Coating Solution for Forming Protective Layer>
(Protective Layer Forming Coating Solution S1)
Rutile-type white titanium oxide having an average primary particle size of 40 nm (TTO55N, manufactured by Ishihara Sangyo Kaisha, Ltd.) and 7 parts by mass of 3-methacryloxypropyltrimethoxysilane per 100 parts by mass of the titanium oxide were stirred in a super mixer by shear force until the temperature inside the mixer reached 150° C., thereby performing a surface treatment of the titanium oxide.
Next, 1000 g of raw material slurry prepared by mixing 250 g of this surface-treated titanium oxide with 750 g of methanol was dispersed for 30 minutes in an Ultra Apex Mill (UAM-015, manufactured by Kotobuki Industries Co., Ltd.) having a mill volume of approximately 0.15 L using zirconia beads (YTZ, manufactured by Nikkato Corporation) with a diameter of approximately 50 μm as dispersion medium at a rotor peripheral speed of 9 m/sec and a liquid flow rate of 2.8 g/sec in a circulating state to produce a surface-treated titanium oxide dispersion.

The surface-treated titanium oxide dispersion liquid was mixed with a urethane acrylate oligomer (UV6300B, manufactured by Mitsubishi Chemical Corporation) previously dissolved in a mixed solvent of methanol/1-propanol/toluene, and benzophenone and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (Omnirad TPO H, manufactured by IGM RESINS) as a polymerization initiator to obtain a protective layer forming coating solution S1 having a ratio of urethane acrylate oligomer/surface-treated titanium oxide/benzophenone/2,4,6-trimethylbenzoyl-diphenylphosphine oxide = 100/55/1/2 (mass ratio), a solvent composition of methanol/1-propanol/toluene = 7/1/2, and a solids concentration of 18.0%.

<Production of photoconductor drum>
[Example 1]
The coating solution A for forming an undercoat layer prepared in the above-mentioned Coating Solution Production Example was applied by a dip coating method to a cylinder (conductive support) made of an aluminum alloy and having a roughly machined surface, the cylinder having an outer diameter of 30 mm, a length of 357.4 mm and a thickness of 0.75 mm, and then air-dried to form an undercoat layer with a thickness of 1.5 μm.
Next, the charge generating layer forming coating solution B was applied onto the undercoat layer by dip coating and air-dried to form a charge generating layer having a thickness of 0.4 μm.
Finally, the charge transport layer forming coating solution C1 was applied onto the charge generating layer by dip coating and dried at 125° C. for 24 minutes to form a charge transport layer having a thickness of 24 μm, thereby obtaining a photoreceptor before being coated with a protective layer.

Next, the protective layer forming coating solution S1 was ring-coated on the photoconductor before the protective layer was applied, and the photoconductor was dried at room temperature for 20 minutes. Then, the photoconductor was rotated at 60 rpm in a nitrogen atmosphere (oxygen concentration 1% or less) and irradiated with a metal halide lamp at an illuminance of 140 mW/ ^cm2 for 2 minutes to form a protective layer having a thickness of 1.0 μm after curing. The photoconductor was then heated at 125° C. for 10 minutes to obtain the photoconductor of Example 1.

[Example 2]
A photoreceptor of Example 2 was obtained in the same manner as in Example 1, except that the coating liquid C2 for forming the charge transport layer was used instead of the coating liquid C1 for forming the charge transport layer in Example 1, and the thickness of the charge transport layer was set to 19 μm.

[Example 3]
A photoreceptor of Example 3 was obtained in the same manner as in Example 1, except that the coating liquid C3 for forming the charge transport layer was used instead of the coating liquid C1 for forming the charge transport layer in Example 1, and the thickness of the charge transport layer was set to 19 μm.

[Comparative Example 1]
A photoreceptor of Comparative Example 1 was obtained in the same manner as in Example 1, except that the thickness of the charge transport layer was changed to 19 μm and no protective layer was formed.

[Comparative Example 2]
A photoreceptor of Comparative Example 2 was obtained in the same manner as in Example 1, except that the thickness of the charge transport layer was changed to 19 μm.

[Comparative Example 3]
A photoreceptor of Comparative Example 3 was obtained in the same manner as in Example 1, except that the thickness of the charge transport layer was changed to 19 μm and the thickness of the protective layer was changed to 3 μm.

<Method of measuring inflow current>
Aluminum foil was applied to one side of 3M Scotch brand conductive double-sided tape, which was then cut into a 1.2 cm x 1.2 cm square, and the adhesive side on which the aluminum foil was not applied was applied to the surface (the surface opposite the conductive support) at a position 250 mm from the end of the photoreceptor obtained in each of Examples 1 to 3 and Comparative Examples 1 to 3 to form an electrode. On top of the electrode, a brass electrode consisting of a cylinder with a diameter of 1.0 cm and a cylinder with a diameter of 0.3 cm connected to each other was placed with the 1.0 cm cylindrical surface facing down and fixed with insulating tape. The surface of the 1.0 cm diameter cylinder in contact with the aluminum foil was processed so that it would contact the curved surface of the photoreceptor.

As shown in Figure 2, the electrodes were connected to a high-voltage power supply (Trek, 610D), and the photoconductor was connected to earth. An ammeter was then connected between the photoconductor and earth. In this state, a voltage of -3.4 kV was applied to the photoconductor from the high-voltage power supply. The current value was averaged from 10 to 15 seconds after application, and this was divided by the area of the electrodes on the photoconductor surface (1.2 cm x 1.2 cm) to obtain the inflow current (A/ ^m2 ). The results are shown in Table 1.
When measuring the inflow current for a positively charged photoreceptor, a voltage of +3.4 kV is applied to the photoreceptor by a high voltage power supply, whereby the inflow current (A/m ² ) can be measured.

<Evaluation of photoconductor life (1)>
The photoreceptors obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were mounted in photoreceptor cartridges of a commercially available contact charging type monochrome copier, and 20,000 sheets were printed under an environment of 25°C temperature and 50% relative humidity, followed by 20,000 sheets under an environment of 32°C temperature and 80% relative humidity, followed by 20,000 sheets under an environment of 10°C temperature and 15% relative humidity, and then further under an environment of 25°C temperature and 50% relative humidity until the bare surface of the bare tube of each photoreceptor was exposed. The number of continuous prints at which the end of the photoreceptor was worn down after printing and the bare surface of the bare tube was exposed was taken as the photoreceptor life. The results are shown in Table 1. In Table 1, K means 1000.

<Evaluation of Image Characteristics (Ghost Occurrence)>
The photoconductors obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were mounted in the photoconductor cartridges of the copying machine, and 5,000 sheets were continuously printed in an environment of a temperature of 32° C. and a relative humidity of 80%.
Next, the photoconductor after 5,000 continuous printing was used to print the following print pattern under an environment of 32°C temperature and 80% relative humidity. The print pattern had characters written above the print medium, and halftone (0.5% print density) solid print written below the characters. The strength of the characters printed on the first rotation at the position above the halftone in the print pattern on the second rotation was visually evaluated. The results are shown in Table 1.

= Image characteristic evaluation criteria =
◯ (very good): No characters were printed at all on the second rotation.
△ (good): Characters are printed weakly on the second rotation, but are acceptable.
× (poor): Characters are printed strongly on the second rotation, which is unacceptable.

<Evaluation of photoconductor life (2)>
A photoconductor having a protective layer thickness of 1.0 μm (photoconductors obtained in Examples 1 to 3 and Comparative Example 2) was mounted in the photoconductor cartridge of the copier, and continuous printing was performed, and the film thickness of the entire photoconductor (hereinafter referred to as photoconductor film thickness) was measured every 30,000 sheets. The continuous printing was performed under an environment of a temperature of 25° C. and a relative humidity of 50% from 0 to 20,000 sheets, a temperature of 32° C. and a relative humidity of 80% from 20,001 to 40,000 sheets, a temperature of 10° C. and a relative humidity of 15% from 40,001 to 60,000 sheets, and a temperature of 25° C. and a relative humidity of 50% from the 60,001st sheet onward.
A graph was created in which the photoreceptor film thickness was plotted on the vertical axis and the number of continuous prints on the horizontal axis, and the number of prints required until the protective layer was completely worn away was calculated from the behavior of the decrease in film thickness. The results are shown in Table 2. In Table 2, K means 1000.

<Consideration>
From the results of the above examples and the results of tests conducted by the inventors thus far, it has been found that by setting the inflow current to 15 A/ ^m2 or less when a voltage of -3.4 kV is applied, it is possible to improve the wear resistance, and in particular, to suppress wear at the ends of the photoconductor when used in a contact charging type image forming apparatus.
It was also found that an effective method for reducing the inflow current to 15 A/m ² or less when a voltage of -3.4 kV is applied is to increase the thickness of the photosensitive layer, specifically to make it 20 μm or more.
Furthermore, it was found that another effective method for reducing the inflow current to 15 A/m2 or ^less when a voltage of -3.4 kV is applied is to make the photosensitive layer contain insulating metal oxide particles with a band gap of 5 eV or more.
Furthermore, the results in Table 2 show that the abrasion resistance of the protective layer can be increased by improving the photosensitive layer, which is the intermediate layer, rather than by improving the protective layer, which is the outermost layer.

The above examples are for a negatively charged (laminated structure) photoreceptor, but a positively charged (single layer structure) photoreceptor would likely produce similar results, since the photoreceptor surface is charged for image formation and is not a complete insulator. In fact, it is believed that a positively charged photoreceptor would be more likely to benefit from the effects of the present invention. This is because a positively charged photoreceptor contains an electron transport material in the photosensitive layer, which makes it easier for the photosensitive layer to transport electrons, i.e., to pass current, and is therefore more likely to increase the inflow current and wear out.

In addition, in the above examples, it was confirmed that the inflow of current could be suppressed and wear resistance could be improved by including silica particles, i.e., silicon dioxide particles, in the photosensitive layer. This is presumably because silica particles are insulating particles, which increases the resistance of the photosensitive body as a whole and suppresses the inflow of current. Based on this mechanism, it is believed that the same effect can be obtained when metal oxide particles having insulating properties with a band gap of 5 eV or more are used instead of silica particles.

Claims (11)

An electrophotographic photoreceptor having a photosensitive layer and a protective layer containing a cured product obtained by curing a curable compound, which are successively provided on a conductive support,
the photosensitive layer contains metal oxide particles having a band gap of 5 eV or more;
An electrophotographic photoreceptor for a contact charging type image forming apparatus, which has a current flow of 1 A/m ² or more and 15 A/m ² or less when a voltage of −3.4 kV is applied thereto. The electrophotographic photoreceptor according to claim 1, wherein the thickness of the photosensitive layer is 10 μm or more. The electrophotographic photoreceptor according to claim 1 or 2, wherein the protective layer has a thickness of 1 μm or less. The electrophotographic photoreceptor according to any one of claims 1 to 3, wherein the ratio A/B of the thickness A of the protective layer to the thickness B of the photosensitive layer is 0.002 or more and 0.3 or less. 5. The electrophotographic photoreceptor according to claim 1, wherein the metal oxide particles having a band gap of 5 eV or more are silica particles. 6. The electrophotographic photoreceptor according to claim 1 , wherein the protective layer contains metal oxide particles having a band gap of 2 to 4 eV. 7. The electrophotographic photoreceptor according to claim 6, wherein a content ratio (mass ratio) of the metal oxide particles having a band gap of 5 eV or more in the photosensitive layer to a content of the metal oxide particles having a band gap of 2 to 4 eV in the protective layer is 0.1 to 0.5. The electrophotographic photoreceptor according to any one of claims 1 to 7 , wherein the photosensitive layer contains a hole transport material represented by the following formula (1) or (2), or a combination of two or more of these:

9. The electrophotographic photoreceptor according to claim 1, wherein the curable compound is a photocurable compound. An electrophotographic photoreceptor cartridge comprising the electrophotographic photoreceptor according to any one of claims 1 to 9 . An image forming apparatus comprising the electrophotographic photoreceptor according to any one of claims 1 to 9 .

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