The present invention relates to an electrostatic latent image bearing member for use in electrophotography. In addition, the present invention relates to an image forming apparatus and a process cartridge using the electrostatic latent image bearing member.

In image forming apparatuses using electrophotography (such as copiers, printers, facsimiles), an image is typically formed as follows:

1. (1) a uniformly charged photoreceptor (i.e., electrostatic latent image bearing member) is irradiated by a light containing image information to form an electrostatic latent image thereon;
2. (2) a developing means supplies a toner to the electrostatic latent image to form a toner image on the photoreceptor;
3. (3) the toner image formed on the photoreceptor is transferred onto a recording medium (e.g., recording paper);
4. (4) a fixing means fixes the toner image onto the recording medium upon application of heat and pressure thereto; and
5. (5) residual toner particles remaining on the surface of the photoreceptor are removed with a cleaning blade and collected.

In such electrophotographic image forming apparatuses, organic photoreceptors including an organic photoconductive material are widely used. Organic photoreceptors have the following advantages:

1. (1) capable of using materials responsive to various light (e.g., visible light, infrared light) irradiators, which are easily developed;
2. (2) capable of using environment-friendly materials; and
3. (3) low manufacturing cost.

On the other hand, organic photoreceptors have poor mechanical strength, and therefore photosensitive layers thereof are abraded after long repeated use. When a specific amount of the photosensitive layer is abraded, the electrical property of the photoreceptor changes, and therefore a proper image forming process cannot be performed. The photoreceptor is abraded due to the friction between the photoreceptor and all image forming members (such as developing means, transfer means) which are in contact with the photoreceptor in an image forming process.

Various attempts have been made to prevent the photoreceptor from being abraded so as to lengthen the life thereof. For example, Japanese Patent No. (hereinafter referred to as JP) 3258397 discloses a photoreceptor having a protective layer including a hardened silicone resin containing a colloidal silica. It is described therein that such a protective layer has good abrasion resistance. However, fogging and blurring tend to appear in produced images after long repeated use because such a photoreceptor has insufficient electrophotographic property. Such a photoreceptor cannot satisfy the recent demands for a long-life photoreceptor having good durability.

JP 3640444 discloses a resin manufacturing method in which an organosilicon polymer is hardened in the presence of an organosilicon-modified positive hole transport compound. JP 3267519 discloses a photoreceptor having an outermost layer including a resin prepared by the above method. Such a photoreceptor tends to produce blurred images, and therefore an image-blurring-preventing mechanism such as a drum heater needs to be mounted on the machine used, resulting in upsizing of the machine and increasing the manufacturing cost. In addition, residual potential of the irradiated portion of the photoreceptor is hardly reduced, and therefore image density tends to decrease when the photoreceptor is particularly used for low potential developing processes.

Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 2000-171990 discloses a photoreceptor having a resin layer including a hardened siloxane resin having a charge transport group, which has a three-dimensional network structure. In such a photoreceptor, cracks tend to appear on the layer due to volume contraction of the resin, especially when low-priced and easy-to-handle commercially available coating agents are used in combination. In addition, residual potential of the irradiated portion of the photoreceptor depends on the layer thickness. Moreover, image density tends to decrease when the photoreceptor is used for low potential developing processes. When the content of the charge transport group increases, the layer strength decreases, and therefore durability of the photoreceptor deteriorates. Such a photoreceptor tends to produce blurred images after long repeated use. It is difficult to easily obtain a photoreceptor in low cost which can produce high quality images for a long period of time.

JP-A 2003-186223 discloses a photoreceptor having a protective layer including a charge transport material having at least one hydroxyl group, a three-dimensional cross-linked resin, and a particulate conductive material. It is described therein that such a photoreceptor has good abrasion resistance, and residual potential can be decreased to some extent. However, the particulate conductive material decreases volume resistance of the protective layer, and therefore blurred images tend to be produced due to blurred electrostatic latent images, especially under high temperature and high humidity conditions. Since the charge transport material may be a constitutional unit of the three-dimensional structure, as the amount of the charge transport material included in the protective layer increases, the effect of the molecular structure thereof (i.e., the number and the binding site of hydroxyl group) on abrasion resistance of the protective layer increases. In some cases, the resultant photoreceptor has insufficient abrasion resistance.

JP-A 2004-117766 discloses a photoreceptor having a protective layer including a urethane resin which is obtained by cross-linking plural polyols and a polyisocyanate. It is described therein that such a photoreceptor has good abrasion resistance. When an underlying layer (i.e., a recording layer) of the protective layer includes a polycarbonate, the adhesion between the protective layer and the underlying layer is not always sufficient. In this case, the protective layer tends to peel off from the edge of the photoreceptor or the portion on which scratches were made by carriers and paper powders, and therefore the underlying layer is exposed. Since a portion at which the underlying layer is exposed has charging property and light attenuation property different from those of an unexposed portion, abnormal images such as color unevenness tend to be produced.

When the thickness of the protective layer decreases due to abrasion, the protective layer easily peels off and disappears, resulting in reducing the life of the photoreceptor. In order to improve durability of the photoreceptor, the protective layer needs to have a large thickness. In this case, residual potential of the irradiated portion of the photoreceptor increases. When the residual potential is too high, potential gradation of the irradiated portion of the photoreceptor tends to deteriorate, and image density tends to decrease.

By the way, spherical polymerization toners come into practical use so as to respond to recent demands for producing high quality images. It is generally known that spherical polymerization toners remaining on a photoreceptor are difficult to remove with a cleaning blade made of a urethane rubber, compared to conventional pulverization toners. In attempting to solve this problem, a technique in which a contact pressure of the cleaning blade is increased to remove toner particles is proposed. However, this technique accelerates abrasion of the photoreceptor and promotes peeling of the protective layer. Because of these reasons, a need exists for a photoreceptor having a durable protective layer which hardly peels off, which can be used for electrophotographic image forming processes using a polymerization toner.

Accordingly, an object of the present invention is to provide an electrostatic latent image bearing member having good abrasion resistance, electrophotographic property, and durability.

Another object of the present invention is to provide an image forming apparatus and a process cartridge which can stably produce high quality images for a long period of time.

These and other objects of the present invention, either individually or in combinations thereof, as hereinafter will become more readily apparent can be attained by an electrostatic latent image bearing member, comprising:

• a substrate; and
• a photosensitive layer located overlying the substrate,

These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, wherein:

• FIGs. 1 to 6 are schematic views illustrating cross-sections of embodiments of the electrostatic latent image bearing member of the present invention.
• FIG. 7 is a schematic view illustrating an embodiment of the image forming apparatus of the present invention.
• FIG. 8 is a schematic view illustrating an embodiment of a cleaning unit including a lubricant applicator for use in the image forming apparatus of the present invention.
• FIG. 9 is a schematic view illustrating another embodiment of the image forming apparatus of the present invention.
• FIG. 10 is a schematic view illustrating another embodiment of the image forming apparatus of the present invention.
• FIG. 11 is a schematic view illustrating another embodiment of the image forming apparatus of the present invention.
• FIG. 12 is a schematic view illustrating an embodiment of the image forming unit of the image forming apparatus illustrated in FIG. 11.
• FIG. 13 is a schematic view illustrating an embodiment of the process cartridge of the present invention.
• FIG. 14 is an infrared absorption spectrum of a charge transport polyol for use in the electrostatic latent image bearing member of the present invention.

Generally, the present invention provides an electrostatic latent image bearing member including a substrate and a photosensitive layer located overlying the substrate, wherein an outermost layer of the electrostatic latent image bearing member includes a cross-linked resin formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups including a reactive charge transport material having the formula (1) and an isocyanate compound including an aromatic isocyanate compound having an isocyanate group and an aromatic ring.

The first embodiment of the electrostatic latent image bearing member of the present invention includes a substrate and a single-layered photosensitive layer overlaid on the substrate, and optionally includes a protective layer, an intermediate layer, etc.

The second embodiment of the electrostatic latent image bearing member of the present invention includes a substrate and a multi-layered photosensitive layer including at least a charge generation layer and a charge transport layer overlaid on the substrate in this order, and optionally includes a protective layer, an intermediate layer, etc. In the second embodiment of the electrostatic latent image bearing member, the charge transport layer and the charge generation layer may be overlaid on the substrate in this order.

The outermost layer of the single-layered photosensitive layer is the photosensitive layer or a protective layer overlaid on the photosensitive layer. The outermost layer of the multi-layered photosensitive layer is the charge transport layer or a protective layer overlaid on the charge transport layer. When the charge transport layer and the charge generation layer are overlaid on the substrate in this order, the outermost layer is the charge generation layer or a protective layer overlaid on the charge generation layer. Within the context of the present invention, if a first layer is stated to be "overlaid" on, or "overlying" a second layer, the first layer may be in direct contact with the second layer, or there may be one or more intervening layers between the first and second layer, with the second layer being closer to the substrate than the first layer.

FIG. 1 is a cross section of an embodiment of the electrostatic latent image bearing member of the present invention. This electrostatic latent image bearing member includes a substrate 201 and a single-layered photosensitive layer 202 overlaid on the substrate 201. FIG. 2 is a cross section of another embodiment of the electrostatic latent image bearing member of the present invention, further including a protective layer 206 overlaid on the photosensitive layer 202.

FIGs. 3 to 6 are cross sections of other embodiments of the electrostatic latent image bearing member of the present invention. The electrostatic latent image bearing member illustrated in FIG. 3 includes a substrate 201, a charge generation layer (CGL) 203, and a charge transport layer (CTL) 204, wherein the layers 203 and 204 are overlaid on the substrate 201 in this order. In this case, the charge generation layer 203 and the charge transport layer 204 form a photosensitive layer 202. The electrostatic latent image bearing member illustrated in FIG. 4 further includes an undercoat layer 205 located between the substrate 201 and the charge generation layer 203. The electrostatic latent image bearing member illustrated in FIG. 5 further includes a protective layer 206 overlaid on the charge transport layer 204. The electrostatic latent image bearing member illustrated in FIG. 6 further includes an intermediate layer 207 located between the undercoat layer 205 and the charge generation layer 203. As long as the electrostatic latent image bearing member includes the substrate 201 and the photosensitive layer 202, the electrostatic latent image bearing member can optionally include other layers. The photosensitive layer may be either single-layered or multi-layered.

The outermost layer of the electrostatic latent image bearing member includes a cross-linked resin formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups including a reactive charge transport material having the formula (1) and an isocyanate compound including an aromatic isocyanate compound having an isocyanate group and an aromatic ring.

The aromatic isocyanate compound preferably has at least 2, and more preferably at least 3, isocyanate groups per molecule.

A cross-linked resin formed from a cross-linking reaction between a reactive charge transport material having 2 or more hydroxyl groups and an isocyanate compound is a polyurethane resin having urethane bonds. A polyurethane resin which is formed from a cross-linking reaction between a polyfunctional isocyanate compound and a polyol compound has a three-dimensional network structure, and therefore the polyurethane resin has good abrasion resistance and is preferably used as a binder resin. When the reactive charge transport material is used as the polyol compound, a large amount of the isocyanate compound is needed. Therefore, the properties of the isocyanate compound largely influence on those of the resultant electrostatic latent image bearing member.

When an isocyanate compound having no aromatic ring (i.e., an aliphatic isocyanate compound) represented by HDI (hexamethylene diisocyanate) is used as the isocyanate compound, the resultant electrostatic latent image bearing member can be practically used for a low-speed or a short-term electrophotographic image forming process. In contrast, such an electrostatic latent image bearing member cannot be practically used for a high-speed or a long-term electrophotographic image forming process because the potential of the irradiated portion increases and abnormal images (such as deterioration of image density) are produced.

The above phenomenon has become apparent by subjecting electrostatic latent image bearing members to a continuous electrostatic fatigue loading test, using a test machine used for cylindrical electrostatic latent image bearing members. For example, it is clear from the test that an electrostatic latent image bearing member prepared using an aliphatic isocyanate compound largely increase the residual potential of the irradiated portion thereof immediately after an electrostatic fatigue is loaded thereto for 120 minutes. Such an electrostatic latent image bearing member cannot be practically used for high-speed machines.

In contrast, an electrostatic latent image bearing member prepared using an aromatic isocyanate compound can largely reduce the residual potential of the irradiated portion thereof. The reason is uncertain, but is considered as follows.

In theory, a polyurethane resin is formed by cross-linking equal numbers of hydroxyl groups and isocyanate groups. When a reactive charge transport material having hydroxyl groups is used, an isocyanate compound having the same number of isocyanate groups as the hydroxyl groups is needed. For example, when the resin includes the reactive charge transport material in an amount of 25% by weight, the resin also includes the isocyanate compound in an amount of 25% by weight, although it depends on the OH equivalent (i.e., the ratio of the molecular weight to the number of hydroxyl group) of the reactive charge transport material and the amount of NCO groups included in the isocyanate compound.

When the outermost layer is prepared using an aliphatic isocyanate compound, few pi-electrons are present in the layer. In other words, the aliphatic isocyanate compound merely contributes to transporting charges, and thereby the resultant outermost layer has poor charge transport ability. When such an electrostatic latent image bearing member is subjected to a continuous electrostatic fatigue loading test just like repeatedly subjected to electrophotographic image forming process, charges are prevented from being transported due to the poor charge transport ability of the outermost layer. As a result, the charges are accumulated and increase the residual potential.

In contrast, an aromatic isocyanate compound used for the present invention has an aromatic ring having a lot of pi-electrons. It is clear from the fact that a charge transport material has charge transport ability owing to the spread of pi-electrons therein, pi-eleetron largely influences on charge transport ability. Since the outermost layer of the electrostatic latent image bearing member of the present invention is prepared using an aromatic isocyanate compound, the outermost layer includes a large amount of pi-electrons. Therefore, the outermost layer has good charge transport ability and the residual potential is reduced.

When an aromatic isocyanate compound having only one isocyanate group is used, the aromatic isocyanate compound forms the end portion suspended from the polyol in the resin. In order that the resultant polyurethane resin has a network structure, a polyfunctional isocyanate compound may be used in combination. If the polyfunctional isocyanate compound is an aliphatic isocyanate compound, the resultant resin has few pi-electrons, resulting in deterioration of charge transport ability. Therefore, the aromatic isocyanate compound for use in the present invention preferably has at least 2, and more preferably at least 3, isocyanate groups. In this case, a three-dimensional network structure is formed, and therefore the resultant resin has high durability.

It is preferable that the isocyanate group of the aromatic isocyanate compound is bound to the aromatic ring thereof through the intermediary of an alkylene group.

Further, it is preferable that the aromatic isocyanate compound is an adduct of a diisocyanate and a polyol.

The aromatic ring has an almost plane structure owing to the presence of conjugate double bonds. Therefore, the molecule has low flexibility in forming molecular conformations and the movement of the isocyanate group of the aromatic isocyanate compound is limited so that the isocyanate group hardly reacts with a hydroxyl group.

As a result, there is a possibility that some hydroxyl groups and isocyanate groups remain unreacted. In this case, the cross-linking density decreases and the unreacted functional groups cause electrostatic side effects, resulting in deterioration of abrasion resistance and electrostatic property of the resultant electrostatic latent image bearing member.

When the isocyanate group of the aromatic isocyanate compound is bound to the aromatic ring thereof through the intermediary of an alkylene group, the molecule has high flexibility in forming molecular conformations and the movement of the isocyanate group is not limited very much. This is because the alkylene group can freely rotates owing to the presence of sigma bonds. In this case, a cross-linking structure can be easily formed.

When the aromatic isocyanate compound is an adduct of a diisocyanate and a polyol, the polyol portion has high flexibility in forming molecular conformations, and therefore a cross-linking structure can be easily formed. The resultant electrostatic latent image bearing member has good abrasion resistance and electrostatic properties.

In addition, it is also preferable that the isocyanate group of the aromatic isocyanate compound is directly bound to the aromatic ring thereof.

In this case, the urethane bond formed from a cross-linking reaction between a polyol and the isocyanate compound is directly bound to the aromatic ring. Generally, urethane bond is considered to trap positive holes, and thereby charges are prevented from being transported and the residual potential increases. When the aromatic ring is adjacent to the urethane bond, the urethane bond tends not to trap positive holes due to the effect of the steric hindrance of the aromatic ring. The resultant electrostatic latent image bearing member has good electrostatic property which can reduce the residual potential.

In the present invention, "aromatic isocyanate compounds" include compounds having an isocyanate group and an aromatic ring, such as the above-mentioned compound in which the aromatic ring and the isocyanate group are bound together through the intermediary of an alkylene group, and are not limited to aromatic compounds to which an isocyanate group is directly bound.

Specific examples of the aromatic isocyanate compounds include, but are not limited to, tolylene diisocyanate (TDI), diphenylmethane diisocynate (MDI) and polymer thereof (polymeric MDI), xylene diisocyanate (XDI), and adduct of TDI,MDI, or XDI and trimethylolpropane.

Specific examples of commercially available aromatic isocyanate compounds include, but are not limited to, BURNOCK® D500, D750, and D800 (from Dainippon Ink and Chemicals, Incorporated); and COSMONATE® T series, M series, and ND, and TAKENATE® 500 and D-110N (from Mitsui Takeda Chemicals, Inc.).

Among these, xylene diisocyanates and adducts thereof are preferably used because of having a structure in which an aromatic ring is bound to an isocyanate group through the intermediary of a methylene group, which can easily form a cross-linking structure. BURNOCK® D750, which is an adduct of tolylene diisocyanate, is also preferably used because the residual potential can be reduced even after the electric fatigue is loaded thereto.

The aromatic isocyanate compound preferably includes the isocyanate group in an amount of from 3 to 50% by weight, and more preferably 10 to 50% by weight, based on total weight of the aromatic isocyanate compound.

As the amount of the isocyanate group increases, the number of the cross-linking point increases, i.e., the cross-linking density increases, and therefore abrasion resistance of the resultant electrostatic latent image bearing member increases. When the amount of isocyanate group is too small, the ratio of the isocyanate group to the hydroxyl group is too large, and therefore the cross-linking density decreases. As a result, the image bearing member has poor abrasion resistance. When the amount of isocyanate group is too large, the isocyanate compound has too large a reactivity, and therefore the isocyanate compound tends to react in the coating liquid. Thereby, the life of the coating liquid shortens, the handling performance thereof decreases, and the amount of organic liquid wastes increases.

An aliphatic polyisocyanate compound can be used in combination with the aromatic isocyanate compound unless abrasion resistance and electrophotographic properties of the resultant electrostatic latent image bearing member deteriorate.

Specific examples of the aliphatic polyisocyanates include, but are not limited to, chain isocyanates (e.g., tetramethylene diisocyanate, hexamethylene diisocyanate, 2,6-diisocyanatomethyl caproate), alicyclic polyisocyanates (e.g., isophorone diisocyanate, cyclohexylmethane diisocyanate), isocyanurates, the above-mentioned polyisocyanates blocked with phenol derivatives, oxime and caprolactam, etc., and trimers consisting essentially of an isocyanate compound (e.g., hexamethylene diisocyanate trimer).

In addition, adducts of (i) trimethylolpropane and (ii) an aliphatic polyisocyanate (e.g., hexamethylene diisocyanate) or an alicyclic polyisocyanate (e.g., isophorone diisocyanate) can be preferably used.

Specific examples of such isocyanate compounds include, but are not limited to, an adduct of trimethylolpropane and hexamethylene diisocyanate having the following formula (II):

Specific examples of commercially available compounds having the formula (II) include SUMIDUR HT (manufactured by Sumika Bayer Urethane Co., Ltd.), etc.

In addition, polyisocyanates having a charge generation molecular skeleton and polyisocyanates having a charge transport molecular skeleton can be used.

Next, the reactive charge transport material (this material may be hereinafter referred to as charge transport polyol) having hydroxyl groups will be explained in detail.

The outermost layer of the electrostatic latent image bearing member includes a cross-linked resin formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups including a reactive charge transport material having the following formula (1) and an isocyanate compound including an aromatic isocyanate compound having an isocyanate group and an aromatic ring: wherein Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, and X represents an organic residue group comprising a hydrocarbon bond having 2 to 4 valences, which has a charge transport molecular structure; and n represents an integer of from 2 to 4; or

wherein Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms, wherein 2 carbon atoms are each bound to a hydroxyl group; X represents an organic residue group comprising a hydrocarbon bond having 1 to 4 valences, which has a charge transport molecular structure; and n represents an integer of from 1 to 4.

Specific examples of the unsubstituted alkyl groups having 1 to 4 carbon atoms include, but are not limited to, methyl group, ethyl group, propyl group, butyl group, isopropyl group, isobutyl group, etc.

Specific examples of the unsubstituted alkoxy groups having 1 to 4 carbon atoms include, but are not limited to, alkoxy groups including the above unsubstituted alkyl groups having 1 to 4 carbon atoms such as methoxy group, ethoxy group, propoxy group, butoxy group, isopropoxy group, and isobutyloxy group.

Specific examples of the substituent groups include, but are not limited to, halogen atom, nitro group, nitrile group, alkoxy groups (e.g., methoxy group, ethoxy group), aryloxy groups (e.g., phenoxy group), aryl groups (e.g., phenyl group, naphthyl group), aralkyl groups (e.g., benzyl group, phenethyl group), etc.

Specific examples of the unsubstituted alkyl groups having 2 to 6 carbon atoms include, but are not limited to, ethyl group, propyl group, butyl group, pentyl group, hexyl group, isopropyl group, isobutyl group, etc.

Specific examples of the unsubstituted alkoxy groups having 2 to 6 carbon atoms include, but are not limited to, alkoxy groups including the above unsubstituted alkyl groups having 2 to 6 carbon atoms such as ethoxy group, propoxy group, butoxy group, pentyloxy group, hexyloxy group, isopropoxy group, and isobutyloxy group.

Specific examples of the substituent groups include, but are not limited to, halogen atom, nitro group, nitrile group, alkoxy groups (e.g., methoxy group, ethoxy group), aryloxy groups (e.g., phenoxy group), aryl groups (e.g., phenyl group, naphthyl group), aralkyl groups (e.g., benzyl group, phenethyl group), etc.

In the formula (1), X represents an organic residue group comprising a hydrocarbon bond, which has an electron donating or electron accepting charge transport molecular structure. When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, X has 2 to 4 valences. When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms and 2 carbon atoms of which are each bound to a hydroxyl group, X has 1 to 4 valences.

Specific examples of the electron donating charge transport molecular structures include, but are not limited to, positive-hole transport compounds such as triphenylamine derivatives, oxazole derivatives, oxadiazole derivatives, imidazole derivatives, 9-(p-diethylaminostyrylanthracene), 1,1-bis-(4-dibenzylaminophenyl)propane, styrylanthracene, styrylpyrazoline, phenylhydrazone, &agr;-phenylstilbene derivatives, thiazole derivatives, triazole derivatives, phenazine derivatives, acridine derivatives, benzofuran derivatives, benzimidazole derivatives, and thiophene derivatives.

Specific examples of the electron accepting charge transport molecular structures include, but are not limited to, electron transport materials such as chloranil, bromanil, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenon, 2,4,5,7-tetranitro-9-fluorenon, 2,4,5,7-tetranitroxanthone, 2,4,8-trinitrothioxanthone, 2,6,8-trinitro-4H-indeno[1,2-b]thiophene-4-one, and 1,3,7-trinitrodibenzothiophene-5,5-dioxide.

Among these, positive-hole transport materials having nitrogen atom (e.g., triarylamine structure) are preferably used because of having good charge transport ability.

Such a cross-linked resin can impart a good combination of charge transport ability and abrasion resistance to the resultant electrostatic latent image bearing member. In particular, the electrostatic latent image bearing member has the following advantages:

1. (1) good abrasion resistance because of good abrasion resistance of the resin;
2. (2) sensitivity does not deteriorate;
3. (3) residual potential can be reduced;
4. (4) fogged images, blurred images, and images having uneven image density are not produced; and
5. (5) image density does not decrease.

Such a durable electrostatic latent image bearing member having good electrophotographic property can stably produce high quality images for a long period of time. When the layer (i.e., recording layer) located underlying the outermost layer (i.e., protective layer) includes a polycarbonate, adhesion between the protective layer and the underlying layer improves.

In particular, it is preferable that Y is a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms, wherein 2 adjacent carbon atoms are each bound to a hydroxyl group. In this case, the resultant electrostatic latent image bearing member has better abrasion resistance.

When the 2 carbon atoms, each bound to a hydroxyl group, are adjacent to each other, each of the hydroxyl groups independently cross-links with an independent isocyanate. As a result, each of the 2 resultant urethane bonds is bound to an independent carbon atom being adjacent to each other (i.e., C-C bond). In this case, the charge transport molecular structure of the charge transport polyol is not present in the main chain thereof, and is suspended from the main chain. For this reason, steric strain of the charge transport polyol hardly occurs. Since the main chain of the polyurethane chain includes the minimum amount of carbon atoms, network structures are densely formed, and therefore abrasion resistance of the electrostatic latent image bearing member improves. The above-mentioned charge transport polyol can impart a good combination of electrophotographic property and abrasion resistance to the electrostatic latent image bearing member without deteriorating charge transport ability.

The charge transport polyol mentioned above may have the following formula (2): wherein R represents a substituted or unsubstituted alkylene group or oxyalkylene group having 1 to 4 carbon atoms; X represents an organic residue group comprising a hydrocarbon bond having 1 to 4 valences, which has a charge transport molecular structure; and n represents an integer of from 1 to 4.

Specific examples of the unsubstituted alkylene groups having 1 to 4 carbon atoms include, but are not limited to, divalent groups such as methyl group, ethyl group, propyl group, and butyl group.

Specific examples of the unsubstituted oxyalkylene groups having 1 to 4 carbon atoms include, but are not limited to, oxyalkylene groups derived from the above substituted or unsubstituted alkylene groups.

Specific examples of the substituent groups include, but are not limited to, halogen atom, nitro group, nitrile group, alkoxy groups (e.g., methoxy group, ethoxy group), aryloxy groups (e.g., phenoxy group), aryl groups (e.g., phenyl group, naphthyl group), aralkyl groups (e.g., benzyl group, phenethyl group), etc.

When 2 adjacent carbon atoms, each of which is bound to an independent hydroxyl group, are present on the end of the molecule of the charge transport polyol, the resultant electrostatic latent image bearing member has better abrasion resistance. This is because these 2 hydroxyl groups can form a conformation in which steric hindrance thereof is minimized, and therefore the hydroxyl groups can easily react. As a result, a very small amount of unreacted hydroxyl group may remain after being subjected to the cross-linking reaction, and therefore an outermost layer having high cross-linking density can be formed without deteriorating the electrophotographic property of the resultant electrostatic latent image bearing member. Thus, an electrostatic latent image bearing member having good abrasion resistance and electrophotographic property can be obtained.

It is preferable that X results from a charge transport molecular structure having the following formula (3):

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, X has 2 to 4 valences. In this case, at least two of A₁, A₂, and A₃ are bound to Y, wherein any two of which bound to Y each, independently, represent a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the other represents a substituted or unsubstituted aryl group, aralkyl group, or alkyl group.

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms and 2 carbon atoms of which are each bound to a hydroxyl group, X has 1 to 4 valences. In this case, at least one of A₁, A₂, and A₃ is bound to Y in the formula (1) or R in the formula (2), wherein any one of which bound to Y or R represents a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the others each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group.

It is also preferable that X results from a charge transport molecular structure having the following formula (4):

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, X has 2 to 4 valences. In this case, at least two of R₁, R₂, and Ar₂ are bound to Y, wherein any two of which bound to Y each, independently, represent a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the other represents a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and Ar₁ represents a substituted or unsubstituted arylene group.

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms and 2 carbon atoms of which are each bound to a hydroxyl group, X has 1 to 4 valences. In this case, at least one of R₁, R₂, and Ar₂ is bound to Y in the formula (1) or R in the formula (2), wherein any one of which bound to Y or R represents a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the others each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and Ar₁ represents a substituted or unsubstituted arylene group.

It is also preferable that X results from a charge transport molecular structure having the following formula (5):

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, X has 2 to 4 valences. In this case, at least two of Ar₄, Ar₅, R₄, and R₅ are bound to Y, wherein any two of which bound to Y each, independently, represent a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the others each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and Ar₃ represents a substituted or unsubstituted arylene group.

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms and 2 carbon atoms of which are each bound to a hydroxyl group, X has 1 to 4 valences. In this case, at least one of Ar₄, Ar₅, R₄, and R₅ is bound to Y in the formula (1) or R in the formula (2), wherein any one of which bound to Y or R represents a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the others each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and Ar₃ represents a substituted or unsubstituted arylene group.

It is also preferable that X results from a charge transport molecular structure having the following formula (6):

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, X has 2 to 4 valences. In this case, at least two of biphenylyl, R₆, and R₇ are bound to Y; when at least one of R₆ and R₇ is bound to Y, any one of R₆ and R₇ bound to Y represents a substituted or unsubstituted arylene group, aralkylene group, or alkylene group; when at least one of R₆ and R₇ is not bound to Y, any one of R₆ and R₇ not bound to Y represents a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and when biphenylyl is bound to Y, biphenylyl represents a biphenylidene group.

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms and 2 carbon atoms of which are each bound to a hydroxyl group, X has 1 to 4 valences. In this case, at least one of biphenylyl, R₆, and R₇ is bound to Y in the formula (1) or R in the formula (2); when R₆ or R₇ is bound to Y or R, any one of R₆ and R₇ bound to Y or R represents a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the other represents a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and when biphenylyl is bound to Y or R, biphenylyl represents a biphenylidene group, and R₆ and R₇ each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group.

It is also preferable that X results from a charge transport molecular structure having the following formula (7):

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 1 to 4 carbon atoms and one hydroxyl group, X has 2 to 4 valences. In this case, at least two of A₄, A₅, A₇, and A₈ are bound to Y, and any two of which bound to Y each, independently, represent a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the others each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and A₆ represents a substituted or unsubstituted arylene group.

When Y represents a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms and 2 carbon atoms of which are each bound to a hydroxyl group, X has 1 to 4 valences. In this case, at least one of A₄, A₅, A₇, and A₈ is bound to Y in the formula (1) or R in the formula (2), wherein any one of which bound to Y or R represents a substituted or unsubstituted arylene group, aralkylene group, or alkylene group, and the others each, independently, represent a substituted or unsubstituted aryl group, aralkyl group, or alkyl group; and A₆ represents a substituted or unsubstituted arylene group.

In the above formulae (3) to (7), specific examples of the unsubstituted aryl groups represented by A₁, A₂, A₃, R₁, R₂, Ar₂, Ar₄, Ar₅, R₄, R₅, R₆, R₇, A₄, A₅, A₇, or A₈ which is bound to neither Y nor R include, but are not limited to, phenyl group, naphthyl group, biphenylyl group, triphenylenyl group, etc.

In the above formulae (3) to (7), specific examples of the unsubstituted aralkyl groups represented by A₁, A₂, A₃, R₁, R₂, Ar₂, Ar₄, Ar₅, R₄, R₅, R₆, R₇, A₄, A₅, A₇, or A₈ which is bound to neither Y nor R include, but are not limited to, benzyl group, etc.

In the above formulae (3) to (7), specific examples of the unsubstituted alkyl groups represented by A₁, A₂, A₃, R₁, R₂, Ar₂, Ar₄, Ar₅, R₄, R₅, R₆, R₇, A₄, A₅, A₇, or As which is bound to neither Y nor R include, but are not limited to, methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, etc.

In the above formulae (3) to (7), specific examples of the unsubstituted arylene groups, aralkylene groups, or alkylene groups which are bound to either Y or R include, but are not limited to, divalent groups of the above aryl groups, aralkyl groups, and alkyl groups.

Specific examples of the substituent groups of the above functional groups include, but are not limited to, halogen atom, nitro group, nitrile group, alkoxy groups (e.g., methoxy group, ethoxy group), aryloxy groups (e.g., phenoxy group), aryl groups (e.g., phenyl group, naphthyl group), aralkyl groups (e.g., benzyl group, phenethyl group), etc.

In the formula (3), any one of A₁, A₂, and A₃ bound to Y or R may represent, for example, stilbenylidene group, &agr;-phenylstilbenylidene group, etc.

In the formula (7), specific examples of the unsubstituted arylene groups represented by A₆ include, but are not limited to, divalent groups of the above aryl groups such as phenyl group, naphthyl group, biphenylyl group, and triphenylenyl group.

Next, specific examples of suitable charge transport polyols of the present invention will be explained in detail.

A cross-linked resin, formed from a cross-linking reaction between (i) a charge transport polyol having 2 or more hydroxyl groups and (ii) an isocyanate compound, is a polyurethane resin having urethane bonds.

A polyurethane resin which is formed from a cross-linking reaction between a polyfunctional isocyanate compound and a polyol compound has a three-dimensional network structure, and therefore the polyurethane resin has good abrasion resistance and is preferably used as a binder resin. Some reactive charge transport materials have a disadvantage in forming three-dimensional network structure. For example, the following reactive charge transport materials (D1-1) to (D1-5) having only one hydroxyl group are not preferably used for the present invention. These reactive charge transport materials are different in structure from the charge transport polyol of the present invention.

Each of the reactive charge transport materials (D1-1) to (D1-5) has only one reactive hydroxyl group. When such a reactive charge transport material is reacted with a polyfunctional isocyanate compound, the reaction product has a structure such that the reactive charge transport material unit is suspended from the end of the main skeleton (i.e., the reactive charge transport material unit forms a pendant group).

In this case, the reaction product no longer has a polymer structure. It is difficult for the reactive charge transport materials (D1-1) to (D1-5) to form a strong resin layer (i.e., polymer structure) without using some other polyol in combination. Even if another polyol is used in combination, it is inevitable that the reactive charge transport material unit is suspended from the end of the main skeleton and inhibits formation of a three-dimensional network structure in the resultant polyurethane. As a result, abrasion resistance of the electrostatic latent image bearing member largely deteriorates. When the content of the reactive charge transport material is decreased and the content of the other polyol is increased in order to prevent the above problem, other problems such as deterioration of charge transport ability and photosensitivity of the outermost layer and increase of residual potential occur. It is difficult for the electrostatic latent image bearing member to have a good balance between abrasion resistance and electrical property of the outermost layer thereof.

The following reactive charge transport materials (D2-1) to (D2-6) having 2 or more hydroxyl groups, each of which is present on different end of the molecule, are used in the present invention.

When the reactive charge transport materials (D2-1) to (D2-6) are used, the charge transport molecular structure of the reactive charge transport material is sandwiched with plural urethane bonds. In other words, the charge transport molecular structure of the reactive charge transport material is present in the main chain of the polyurethane chain. Therefore, steric strain tends to occur due to the secondary structure of the polyurethane chain in the resultant cross-linked resin. The steric strain tends to weaken pi-electron conjugated system of the charge transport molecular structure, and therefore problems such as increase of ionized potential and deterioration of charge transport ability tend to occur. As a result, the sensitivity of the resultant electrostatic latent image bearing member deteriorates and the residual potential increases in some cases.

The following charge transport polyols (D3-1) to (D3-7) of the present invention are more preferably used because the above problems can be solved.

The charge transport polyols (i.e., reactive charge transport materials) (D3-1) to (D3-7) hardly cause the above-mentioned problems and can impart good abrasion resistance to the resultant electrostatic latent image bearing member.

For example, the charge transport polyol (D3-1) has a structure in which (i) a substituted or unsubstituted alkyl group or alkoxy group having 2 to 6 carbon atoms, wherein 2 carbon atoms each are bound to a hydroxyl group, and (ii) an organic residue group comprising a hydrocarbon bond having 1 to 4 valences, which has a charge transport molecular structure, are bound together. In this case, the charge transport molecular structure is suspended from the polyurethane chain (i.e., the charge transport molecular structure forms a pendant group) while forming 2 or more cross-links therebetween. The charge transport molecular structure is not present in the main chains of the plural polyurethane chains.

For this reason, the charge transport molecular structure is hardly influenced by the secondary structure of the polyurethane chain, and therefore the steric strain thereof hardly occurs. As a result, the resultant electrostatic latent image bearing member has good charge transport ability, sensitivity, resistance to residual potential increase, and abrasion resistance.

The charge transport polyols (D3-2) to (D3-6) furthermore prevent increase of residual potential.

This is because these charge transport polyols hardly cause steric strain when urethane bonds are formed, and therefore the charge transport molecular structure easily exerts the effect thereof. Stilbene compounds and &agr;-phenylstilbene compounds have by nature good charge transport ability. The charge transport polyols (D3-2) to (D3-6), which have a stilbene or &agr;-phenylstilbene structure having an alkyl or alkoxy group having 2 to 4 hydroxyl groups, have very good charge transport ability.

The charge transport polyol (D3-7) also forms a cross-linked resin having a three-dimensional network structure, and therefore the resultant electrostatic latent image bearing member has good abrasion resistance and electrophotographic properties.

The outermost layer of the electrostatic latent image bearing member of the present invention includes a cross-linked resin formed from a polyol, as mentioned above. Another polyol can optionally be used in combination other than the charge transport polyol having the formula (1). Specific examples of such polyols include, but are not limited to, diols and polyols having 3 or more valences.

Specific examples of the diols include, but are not limited to, alkylene glycols (e.g., ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol), alkylene ether glycols (e.g., diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, polytetramethylene ether glycol), alicyclic diols (e.g., 1,4-cyclohexanedimethanol, hydrogenated bisphenol A), bisphenols (e.g., bisphenol A, bisphenol F, bisphenol S), alkylene oxide (e.g., ethylene oxide, propylene oxide, butylene oxide) adducts of the above alicyclic diols, alkylene oxide (e.g., ethylene oxide, propylene oxide, butylene oxide) adducts of the above bisphenols, etc.

Specific examples of the polyols having 3 or more valences include, but are not limited to, aliphatic polyols (e.g., glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, sorbitol), phenols having 3 or more valences (e.g., phenol novolac, cresol novolac), alkylene oxide adducts of the above phenols having 3 or more valences, etc.

Among these, trimethylolpropane and a polyol having a styrene-acrylic copolymer skeleton having a hydroxyethyl group, represented by the following formula (I), are preferably used: wherein k is 28, m is 42, and n is 30. The compound (I) has a number average molecular weight of not less than 1,000 and a weight average molecular weight of about 31,000. Specific examples of commercially available compounds having the formula (I) include, but are not limited to, a styrene-acrylic copolymer LZR-170 (manufactured by Fujikura Kasei Co., Ltd), etc.

In addition, polyols having a polyether skeleton, polyols having a polyester skeleton, polyols having an acrylic skeleton, polyols having an epoxy skeleton, polyols having a polycarbonate skeleton, polyols having a charge generation molecular skeleton, and polyols having a charge transport molecular skeleton can be used.

These polyols can be used alone or in combination.

When plural polyol are used in combination, at least one polyol preferably has a ratio of the molecular weight to the number of the hydroxyl group (i.e., OH equivalent), of not less than 30 and less than 150, and more preferably not less than 40 and less than 120.

When the OH equivalent satisfies the above range, the outermost layer has good abrasion resistance. In other words, as a content of a polyol having small OH equivalent increases, the cross-linking density increases, and therefore dense three-dimensional structure can be formed in the outermost layer.

The content of the polyol having an OH equivalent of not less than 30 and less than 150 is preferably from 10 to 90 % by weight based on the total weight of the polyols.

When the content is too small, abrasion resistance of the resultant electrostatic latent image bearing member is poor. When the content is too large, cross-linking density increases, and therefore abrasion resistance of the resultant electrostatic latent image bearing member improves. However, too large an amount of functional group increases the reactivity of the polyol, and therefore the storage stability of the coating liquid thereof deteriorates and the life thereof is shortened. In this case, various problems tend to occur in the manufacturing process, and a large amount of organic waste liquid may be produced. In addition, the amount of cross-linking point increases in the product, and therefore volume contraction becomes larger. As a result, fractures and cissings tend to appear on the resultant layer.

Moreover, at least one polyol preferably has an OH equivalent of not less than 150 and less than 1,500.

In this case, the coating liquid thereof is well coated and the resultant outermost layer has good abrasion resistance. The coating liquid also has good storage stability (i.e., preservability).

This is because such a polyol satisfying the above OH equivalent range has relatively a large molecular weight so that the coating liquid has an appropriate viscosity. Therefore, the polyol having a small OH equivalent, a polyisocyanate, and the charge transport polyol of the present invention can be uniformly mixed. As a result, the wet coated layer has good leveling property and uniformity.

The weight ratio (i.e., D/R) of the charge transport polyol unit (D) to the cross-linked resin (R) is preferably from 1/10 to 15/10, and more preferably from 3/10 to 10/10.

When the weight ratio is too small, charge transport ability of the resultant electrostatic latent image bearing member deteriorates, and therefore residual potential increases. In contrast, when the ratio is too large, the content of the binder resin component is too small, and therefore formation of three-dimensional network structure is distributed, resulting in deterioration of abrasion resistance.

The outermost layer optionally includes various additives so as to improve smoothness and chemical stability thereof, if desired.

The outermost layer is formed on the photosensitive layer by known coating methods such as a dip coating method, a spray coating method, a blade coating method, and a knife coating method. Among these, the dip coating method and the spray coating method are preferably used in terms of mass productivity and coating quality.

The outermost layer preferably has a thickness of from 0.5 to 50 µm, more preferably from 1 to 40 µm, and much more preferably from 2 to 20 µm.

When the thickness is too small, resistance to abrasion and flaws is too small, resulting in deterioration of durability. When the thickness is too large, residual potential tends to increase.

A multi-layered photosensitive layer includes a charge generation layer (CGL) and a charge transport layer (CTL). The CGL and the CTL are typically overlaid on the substrate in this order.

When the charge generation layer is the outermost layer of the electrostatic latent image bearing member, the charge generation layer includes at least a cross-linked resin (i.e., binder resin) formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups including at least the reactive charge transport material having the formula (1) and an isocyanate compound including at least an aromatic isocyanate compound having an isocyanate group and an aromatic ring, and optionally includes other components.

When the charge generation layer is not the outermost layer of the electrostatic latent image bearing member, the charge generation layer includes at least a charge generation material, and optionally includes other components such as a binder resin. Any known charge generation materials, both inorganic materials and organic materials, can be used.

Specific examples of the inorganic charge generation materials include, but are not limited to, crystalline selenium, amorphous selenium, selenium-tellurium compounds, selenium-tellurium-halogen compounds, selenium-arsenic compounds, etc.

Specific examples of the organic charge generation materials include, but are not limited to, phthalocyanine pigments (e.g., metal phthalocyanine, metal-free phthalocyanine), azulenium salt pigments, squaric acid methyne pigments, azo pigments having a carbazole skeleton, azo pigments having a triphenylamine skeleton, azo pigments having a diphenylamine skeleton, azo pigments having a dibenzothiophene skeleton, azo pigments having a fluorenone skeleton, azo pigments having an oxadiazole skeleton, azo pigments having a bisstilbene skeleton, azo pigments having a distyryloxadiazole skeleton, azo pigments having a distyrylcarbazole skeleton, perylene pigments, anthraquinone and polycyclic quinone pigments, quinonimine pigments, diphenylmethane and triphenylmethane pigments, benzoquinone and naphthoquinone pigments, cyanine and azomethine pigments, indigoid pigments, bisbenzimidazole pigments, etc. These charge generation materials can be used alone or in combination.

When the charge generation layer is not the outermost layer of the electrostatic latent image bearing member, any known resins can be used as a binder resin. Specific examples of the binder resins include, but are not limited to, polyamide resins, polyurethane resins, epoxy resins, polyketone resins, polycarbonate resins, silicone resins, acrylic resins, polyvinyl butyral resins, polyvinyl formal resins, polyvinyl ketone resins, polystyrene resins, poly-N-vinylcarbazole resins, polyacrylic amide resins, etc. These binder resins can be used alone or in combination.

The charge generation layer may optionally include a charge transport material. In addition to the above-mentioned binder resins, charge transport polymer materials can be used as a binder resin of the charge generation layer.

The charge generation layer is typically formed by a vacuum thin layer manufacturing method or a casting method using a liquid dispersion.

Specific examples of the vacuum thin layer manufacturing methods include, but are not limited to, a glow discharge polymerization method, a vacuum deposition method, a CVD method, a sputtering method, a reactive sputtering method, an ion plating method, an accelerate ion injection method, etc. The vacuum thin layer manufacturing method can well form a layer of the above inorganic and organic charge generation materials.

Specific examples of the casting methods include any known coating methods such as a dip coating method, a spray coating method, and a bead coating method, using a charge generation layer coating liquid.

The charge generation layer coating liquid can be prepared by dispersing or dissolving a charge generation material and a binder resin in an organic solvent.

Specific examples of the organic solvents for use in the charge generation layer coating liquid include, but are not limited to, acetone, methyl ethyl ketone, methyl isopropyl ketone, cyclohexanone, benzene, toluene, xylene, chloroform, dichloromethane, dichloroethane, dichloropropane, trichloroethane, trichloroethylene, tetrachloroethane, tetrahydrofuran, dioxolane, dioxane, methanol, ethanol, isopropyl alcohol, butanol, ethyl acetate, butyl acetate, dimethyl sulfoxide, methyl cellosolve, ethyl cellosolve, propyl cellosolve, etc. These can be used alone or in combination.

Among these, solvents having a boiling point of from 40 to 80 °C such as tetrahydrofuran, methyl ethyl ketone, dichloromethane, methanol, and ethanol, are preferably used because these can be easily removed.

The charge generation material can be dispersed in the organic solvent by dispersing methods using a dispersion medium such as a ball mill, a bead mill, a sand mill, and a vibration mill, or high-speed liquid collision dispersing methods.

Electrophotographic property, especially photosensitivity, of the charge generation layer depends on the thickness thereof. Generally speaking, as the thickness of the charge generation layer increases, photosensitivity thereof improves. The thickness of the charge generation layer is preferably determined according to the requirements of the specification of the image forming apparatus used. In order to satisfy the requirements for a photoreceptor used for electrophotography, the charge generation layer preferably has a thickness of from 0.01 to 5 µm, and more preferably from 0.05 to 2 µm.

When the charge transport layer is the outermost layer of the electrostatic latent image bearing member, the charge transport layer includes at least a cross-linked resin (i.e., binder resin) formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups including at least the reactive charge transport material having the formula (1) and an isocyanate compound including at least an aromatic isocyanate compound having an isocyanate group and an aromatic ring, and optionally includes other components.

When the charge transport layer is not the outermost layer and a protective layer is formed thereon, the charge transport layer is not required to have abrasion resistance. The charge transport layer has functions of keeping a charge and transporting a charge generated in the charge generation layer so as to be bound to the keeping charge kept. In order to keep a charge, the charge transport layer is required to have high electrical resistance. In order to achieve high surface potential with the charge kept, the charge transport layer is required to have a small dielectric constant and good charge transport ability.

The charge transport layer includes at least a charge transport material mentioned below, and optionally includes other components such as a binder resin.

Specific examples of the charge transport materials include, but are not limited to, electron transport materials, positive-hole transport materials, polymeric charge transport materials, etc.

Specific examples of the electron transport materials (i.e., electron accepting materials) include, but are not limited to, chloranil, bromanil, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenone, 2,4,5,7 -tetranitro-9- fluorenone, 2,4,5,7-tetranitroxanthone, 2,4,8-trinitrothioxanthone, 2,6,8-trinitro-4H-indeno[1,2-b]thiophene-4-one, 1,3,7-trinitrodibenzothiophene-5,5-dioxide, etc. These can be used alone or in combination.

Specific examples of the positive-hole transport materials (i.e., electron donating materials) include, but are not limited to, oxazole derivatives, oxadiazole derivatives, imidazole derivatives, triphenylamine derivatives, 9-(p-diethylaminostyrylanthracene), 1,1-bis-(4-dibenzylaminophenyl)propane, styrylanthracene, styrylpyrazoline, phenylhydrazones, &agr;-phenylstilbene derivatives, thiazole derivatives, triazole derivatives, phenazine derivatives, acridine derivatives, benzofuran derivatives, benzimidazole derivatives, thiophene derivatives, etc. These can be used alone or in combination.

Specific examples of the polymeric charge transport materials include, but are not limited to, the following compounds:

1. (1) polymers having a carbazole ring (e.g., poly-N-vinylcarbazole, compounds disclosed in JP-As 50-82056 , 54-9632 , 54-11737 , 04-175337 , 04-183719 , and 06-234841 );
2. (2) polymers having a hydrazone structure (e.g., compounds disclosed in JP-As 57-78402 , 61-20953 , 61-296358 , 01-134456 , 01-179164 , 03-180851 , 03-180852 , 03-50555 , 05-310904 , and 06-234840 );
3. (3) polysilylene polymers (e.g., compounds disclosed in JP-As 63-285552 , 01-88461 , 04-264130 , 04-264131 , 04-264132 , 04-264133 , and 04-289867 );
4. (4) polymers having a triarylamine structure (e.g., N,N-bis(4-methylphenyl)-4-amino polystyrene, compounds disclosed in JP-As 01-134457 , 02-282264 , 02-304456 , 04-133065 , 04-133066 , 05-40350 , and 05-202135 ); and
5. (5) other polymers (e.g., formaldehyde condensate of nitropyrene, compounds disclosed in JP-As 51-73888 , 56-150749 , 06-234836 , and 06-234837 ).

In addition, polycarbonate resins having a triarylamine structure, polyurethane resins having a triarylamine structure, polyester resins having a triarylamine structure, polyether resins having a triarylamine structure, and compounds disclosed in JP-As 64-1728 , 64-13061 , 64-19049 , 04-11627 , 04-225014 , 04-230767 , 04-320420 , 05-232727 , 07-56374 , 09-127713 , 09-222740 , 09-265197 , 09-211877 , and 09-304956 can be used as the polymeric charge transport material.

In addition to the above polymers, any known copolymers, block polymers, graft polymers, star polymers, and cross-linked polymers having an electron donating group (e.g., a polymer disclosed in JP-A 03-109406 ) can be used as electron donating polymers.

Specific examples of the binder resins for use in the charge transport layer include, but are not limited to, polycarbonate resins, polyester resins, methacrylic resins, acrylic resins, polyethylene resins, polyvinyl chloride resins, polyvinyl acetate resins, polystyrene resins, phenol resins, epoxy resins, polyurethane resins, polyvinylidene chloride resins, alkyd resins, silicone resins, polyvinyl carbazole resins, polyvinyl butyral resins, polyvinyl formal resins, polyacrylate resins, polyacrylamide resins, phenoxy resins, etc. These can be used alone or in combination.

The charge transport layer may include a copolymer of a cross-linked binder resin with a cross-linked charge transport material.

The charge transport layer can be formed by applying a coating liquid, in which a charge transport material and a binder resin are dissolved in an organic solvent, onto the charge generation layer, followed by drying. The charge transport layer optionally includes a plasticizer, an oxidation inhibitor, a leveling agent, etc., other than the charge transport material and the binder resin, if desired.

The charge transport layer preferably has a thickness of from 5 to 100 µm. In order to satisfy the recent demands for producing high quality images having a resolution of 1200 dpi or more, the charge transport layer is required to be as thin as possible, and therefore the charge transport layer more preferably has a thickness of from 5 to 30 µm.

When the single-layered photosensitive layer is the outermost layer of the electrostatic latent image bearing member, the single-layered photosensitive layer includes at least a cross-linked resin (i.e., binder resin) formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups including at least the reactive charge transport material having the formula (1) and an isocyanate compound including at least an aromatic isocyanate compound having and isocyanate group and an aromatic ring, and optionally includes other components.

When the single-layered photosensitive layer is not the outermost layer and a protective layer is formed thereon, the single-layered photosensitive layer is not required to have abrasion resistance. Therefore, it is not necessary for the single-layered photosensitive layer to include a cross-linked resin (i.e., binder resin) formed from a cross-linking reaction between the charge transport polyol of the present invention and an isocyanate compound.

When the single-layered photosensitive layer is not the outermost layer, the single-layered photosensitive layer includes at least a charge transport material (such as the above-mentioned positive-hole transport materials, electron transport materials, and polymeric charge transport materials) and a binder resin, and optionally includes other components.

The single-layered photosensitive layer can be formed by applying a coating liquid, in which a charge generation material and a thermosetting binder resin and a charge transport material (which may have a cross-linking group) are dissolved in an organic solvent, onto the substrate, followed by drying (i.e., a casting method). The single-layered photosensitive layer optionally includes a plasticizer, if desired.

The single-layered photosensitive layer preferably has a thickness of from 5 to 100 µm, and more preferably from 5 to 50 µm. When the thickness is too small, chargeability of the resultant electrostatic latent image bearing member deteriorates. When the thickness is too large, sensitivity of the resultant electrostatic latent image bearing member deteriorates.

Suitable materials for use in the substrate include materials having conductivity, and are not particularly limited.

For example, conductive materials and conductive-treated insulating materials are preferably used. Specific examples of such materials include, but are not limited to, metals (e.g., Al, Ni, Fe, Cu, Au) and alloys thereof; insulating substrates (e.g., polyesters, polycarbonates, polyimides, glasses), on the surface of which a thin layer of a metal (e.g., Al, Ag, Au) or a conductive material (e.g., In₂O₃, SnO₂) is formed; resin substrates in which a carbon black, a graphite, a metal (e.g., Al, Cu, Ni) powder, conductive glass powder, etc. are uniformly dispersed in a resin, so as to impart conductivity to the resin; etc.

The shape and size of the substrate are not particularly limited. For example, platy substrates, cylindrical substrates, and belt substrates can be used. Belt substrates have a drawback such that a driving roller and a driven roller have to be arranged inside the belt, resulting in complication and upsizing of the machine. In contrast, belt substrates have an advantage of being flexibly arranged in the machine. When an electrostatic latent image bearing member has a protective layer, cracks may appear on the surface thereof, because the protective layer has insufficient flexibility in some cases. Such an electrostatic latent image bearing member tends to produce images having grainy background fouling. For these reasons, cylindrical substrates are most preferably used.

The electrostatic latent image bearing member of the present invention optionally includes an undercoat layer located between the substrate and the photosensitive layer, if desired. The undercoat layer is formed for the purposes of improving adhesion between the layers, preventing occurrence of moiré, improving coating property of the upper layer, decreasing residual potential, etc.

The undercoat layer generally includes a resin as a main component. It is preferable that the resin is insoluble in typical organic solvents because photosensitive layers are coated thereon using organic solvents. Specific examples of such resins include, but are not limited to, water-soluble resins (e.g., polyvinyl alcohols, casein, sodium polyacrylates), alcohol-soluble resins (e.g., copolymerized nylons, methoxymethylated nylons), indurative resins (e.g., polyurethanes, melamine resins, alkyd-melamine resins, epoxy resins) which can form a three-dimensional network structure, etc.

The undercoat layer optionally includes a fine powder of metal oxides (e.g., titanium oxide, silica, alumina, zirconium oxide, tin oxide, indium oxide), metal sulfides, metal nitrides, etc. The undercoat layer can be formed by a typical coating method using a solvent.

In addition, metal oxide layers formed by sol-gel method using silane coupling agents, titanium coupling agents, chromium coupling agents, etc.; Al₂O₃ layers formed by anodic oxidation; and layers of organic materials (e.g., poly-para-xylylene (i.e., parylene)) or inorganic materials (e.g., SnO₂, TiO₂, ITO, CeO₂) formed by a vacuum thin-layer manufacturing method can be used as the undercoat layer.

The undercoat layer preferably has a thickness of from 0.1 to 10 µm, and more preferably from 1 to 5 µm, but the thickness is not limited thereto.

The electrostatic latent image bearing member of the present invention optionally includes an intermediate layer on the substrate, in order to improve adhesion between other layers and charge blocking property. The intermediate layer typically includes a resin as a main component. It is preferable that the resin is insoluble in typical organic solvents because photosensitive layers are coated thereon using organic solvents.

Specific examples of such resins include, but are not limited to, water-soluble resins (e.g., polyvinyl alcohols, casein, sodium polyacrylates), alcohol-soluble resins (e.g., copolymerized nylons, methoxymethylated nylons), indurative resins (e.g., polyurethanes, melamine resins, alkyd-melamine resins, epoxy resins) which can form a three-dimensional network structure, etc.

Next, the image forming apparatus and image forming method of the present invention will be explained in detail.

The image forming apparatus of the present invention comprises:

• an electrostatic latent image bearing member comprising:
  • a substrate; and
  • a photosensitive layer located overlying the substrate,
  wherein an outermost layer of the electrostatic latent image bearing member comprises a cross-linked resin formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups comprising a reactive charge transport material having the following formula (1) and an isocyanate compound comprising an aromatic isocyanate compound having an isocyanate group and an aromatic ring;
• an electrostatic latent image forming means for forming an electrostatic latent image on the electrostatic latent image bearing member;
• a developing means for developing the electrostatic latent image with a toner to form a toner image;
• a transfer means for transferring the toner image onto a recording medium; and
• a fixing means for fixing the transferred image onto the recording medium.

The image forming apparatus of the present invention optionally includes other means, such as a discharging means, a cleaning means, a recycle means, a controlling means, etc., if desired.

For example, a cleaning means which contacts the surface of the electrostatic latent image bearing member so as to remove residual toner particles remaining thereon is preferably used.

The image forming method of the present invention comprises:

• forming an electrostatic latent image on an electrostatic latent image bearing member comprising:
  • a substrate; and
  • a photosensitive layer located overlying the substrate,
  wherein an outermost layer of the electrostatic latent image bearing member comprises a cross-linked resin formed from a cross-linking reaction between a polyol having 2 or more hydroxyl groups comprising a reactive charge transport material having the following formula (1) and an isocyanate compound comprising an aromatic isocyanate compound having an isocyanate group and an aromatic ring (i.e., electrostatic latent image forming process);
• developing the electrostatic latent image with a toner to form a toner image (i.e., developing process);
• transferring the toner image onto a recording medium (i.e., transfer process); and
• fixing the transferred image onto the recording medium (i.e., fixing process).

The image forming method of the present invention optionally includes other processes, such as a discharging process, a cleaning process, a recycle process, a controlling process, etc., if desired.

The image forming method of the present invention is preferably performed using the image forming apparatus of the present invention. Namely, the electrostatic latent image forming process can be performed with the electrostatic latent image forming means, the developing process can be performed with the developing means, the transfer process can be performed with the transfer means, the fixing process can be performed with the fixing means, and the other processes can be performed with the corresponding means.

Each of the image forming processes and image forming means will be explained in detail below.

In the electrostatic latent image forming process, an electrostatic latent image is formed on a charged electrostatic latent image bearing member by irradiation of light. As the electrostatic latent image bearing member, the electrostatic latent image bearing member of the present invention is used. The electrostatic latent image forming means includes a charger and an irradiator.

The electrostatic latent image bearing member can be charged by applying a voltage to the surface thereof, using the charger. Specific examples of the chargers include, but are not limited to, contact chargers including a conductive or semi-conductive roller, brushes, films, rubber blades, etc.; non-contact chargers using corona discharge such as corotron and scorotron; non-contact chargers including a roller having means for forming a gap (such as a gap tape) on the ends thereof, so as not to be in contact with the electrostatic latent image bearing member; etc.

The configuration of the charging member may be a roller, a magnetic brush, a fur brush, etc., and is not particularly limited. The magnetic brush-type charging member includes, for example, ferrite particles (such as Zn-Cu ferrites), a non-magnetic conductive sleeve supporting the ferrite particles, and a magnet roll arranged in the non-magnetic conductive sleeve. The fur brush-type charging member includes, for example, charging members in which a fur treated with a carbon, copper sulfide, a metal, or a metal oxide to have conductivity is wound around or attached to a metal or a cored bar treated to have conductivity.

Among these, the contact chargers and non-contact chargers having a means for forming a gap are preferably used, because these chargers produce less ozone. It is more preferable that contact and non-contact chargers charging the electrostatic latent image bearing member by applying a DC voltage overlapped with an AC voltage thereto are used.

When a charger is a non-contact charging roller located close to the electrostatic latent image bearing member with a gap therebetween, and the electrostatic latent image bearing member is charged by applying a DC voltage overlapped with an AC voltage to the non-contact charging roller, charging inconsistency and poor chargeability resulted from contaminations of the charging roller can be reduced. Such an image forming apparatus is preferably used because of being free from maintenance.

The charged electrostatic latent image bearing member can be irradiated with light containing image information using the irradiator.

Specific examples of the irradiators include, but are not limited to, emit optical irradiators, rod lens array irradiators, laser optical irradiators, liquid crystal shutter irradiators, etc.

In the present invention, the electrostatic latent member can be irradiated from the back side thereof.

In the developing process, the electrostatic latent image is developed with a toner to form a toner image. The toner includes the toners and developers mentioned later.

The formation of the toner image is performed with the developing means by developing the electrostatic latent image with a toner or a developer.

Suitable developing means include any known developing means, and are not particularly limited. For example, developing devices containing a toner or a developer, which can directly or indirectly supply the toner or the developer to an electrostatic latent image, are preferably used.

The developing device may be either or both of a dry developing device or a wet developing device in the present invention. Moreover, the developing device may be either or both of a single-colored developing device or a multi-colored developing device in the present invention. For example, a developing device including an agitator configured to agitate the toner or the developer so as to be friction-charged and a rotatable magnet roller is preferably used.

In the developing device, the toner and the carrier mentioned later are mixed and agitated. The toner is charged while agitated, and held in a magnetic brush which is formed on the surface of a rotating magnetic roller. Because the magnetic roller is located near the electrostatic latent image bearing member, a part of the toner held in the magnetic brush, which is formed on the surface of the rotating magnetic roller, is moved to the surface of the electrostatic latent image bearing member due to the electric force. As a result, the electrostatic latent image is developed with the toner to form a toner image on the electrostatic latent image bearing member.

The developer contained in the developing device may be either or both of a one-component developer or a two-component developer.

In the transfer process, the toner image is transferred onto a recording medium. It is preferable that the toner image is firstly transferred onto an intermediate transfer medium, and then secondly transferred onto the recording medium. It is more preferable that the toner image is a multiple toner image which is formed with two or more full-color toner images, and the multiple toner image is firstly transferred onto the intermediate transfer medium (i.e., primary transfer process), and then secondly transferred onto the recording medium (i.e., secondary transfer process).

The toner image is charged with a transfer charger and then transferred with the transfer means. The transfer means preferably includes a primary transfer means for transferring a toner image onto an intermediate transfer medium to form a multiple toner image, and a secondary transfer means for transferring the multiple toner image onto a recording medium.

Namely, it is preferable that plural single-colored toner images are independently formed on an independent electrostatic latent image bearing member, and then each of the single-colored toner images is transferred onto the intermediate transfer medium one by one to form a multiple toner image (i.e., primary transfer process), and then the multiple toner image is transferred onto the recording medium (i.e., secondary transfer process). As the intermediate transfer medium, any known transfer media can be used, and is not particularly limited. For example, transfer belts are preferably used.

The intermediate transfer medium preferably has a static friction coefficient of from 0.1 to 0.6, and more preferably from 0.3 to 0.5.

The intermediate transfer medium preferably has a volume resistivity of not less than several &OHgr;·cm and not greater than 10³ &OHgr;·cm. In this case, the intermediate transfer medium is hardly charged, and the charge supplied with a charge supplying means hardly remains on the intermediate transfer medium, and therefore occurrence of uneven transfer in the secondary transfer process can be prevented. In addition, it becomes easier to apply a transfer bias in the secondary transfer process.

Any known materials can be used for the intermediate transfer medium, and are not particularly limited. Specific examples of the materials used for the intermediate transfer medium include, but are not limited to, the following materials.

1. (1) A single-layered belt made of a material having a high Young's modulus (i.e., modulus of elongation). Specific examples of the materials having a high Young's modulus include, but are not limited to, PC (polycarbonate), PVDF (polyvinylidene fluoride), PAT (polyalkylene terephthalate), blended materials of PC and PAT, blended materials of ETFE (ethylene-tetrafluoroethylene copolymer) and PC, blended materials of ETFE and PAT, blended materials of PC and PAT, thermosetting resins in which a carbon black is dispersed therein, etc. Such a single-layered belt having a high Young's modulus deforms a little even if a stress is applied thereto, when an image is formed. In particular, occurrence of registration drift can be prevented when a color image is formed.
2. (2) A two-layered belt including the above single-layered belt having a high Young's modulus and a surface layer, and a three-layered belt further including an intermediate layer. When such a two-layered or three-layered belt is used, occurrence of hollow defects in line images, which is caused due to the hardness of the single-layered belt, can be prevented.
3. (3) A Belt made of a rubber or an elastomer having a low Young's modulus. When such a belt is used, hollow defects hardly occur in line images because the belt is soft. Since the belt has a width larger than those of the driving roller and the driven roller, meandering of the belt is prevented, using elasticity of the projected portions of the belt. The manufacturing cost can be reduced because a rib and a meandering preventing apparatus are not needed.

Conventional intermediate transfer belts are made of fluorocarbon resins, polycarbonate resins, polyimide resins, etc. Elastic belts in which all layers or a part of the belt is made of an elastic material are used for the intermediate transfer belts recently.

When a resin belt is used in transferring full-color images, the following problem tends to occur.

A typical full-color image includes four single-colored toner layers. When the single-colored toner layers are transferred from an electrostatic latent image bearing member onto an intermediate transfer belt (i.e., primary transfer), and then transferred from the intermediate transfer belt onto a recording medium (i.e., secondary transfer), the toner particles receive a pressure, and therefore cohesion among the toner particles increases. As a result, line images with hollow defects and solid images with edge defects tend to be produced. This is because the resin belt has a high hardness and cannot deform according to the deformation of the toner layers. Therefore, the toner layers are easily compressed and hollow defects tend to occur.

On the other hand, demands for forming full-color images on various kinds of papers (e.g., Japanese papers, papers having convexes and convexities) have increased recently. However, papers having poor smoothness tend to form voids between the paper and a toner when the toner is transferred thereon, resulting in occurrence of transfer defect. If the secondary transfer pressure is increased so as to improve adhesion between the paper and the toner, cohesion among the toner particles increases, and therefore the above-mentioned hollow defects tend to occur.

The elastic belt receives attention because the belt can deform according to the toner layer and the papers having poor smoothness, in the transfer process. The elastic belt can deform following concavities and convexities locally formed on the paper, and therefore the toner and the paper are firmly attached to each other without application of excessive pressure to the toner layer. As a result, a uniform transfer image without hollow defects in characters can be formed on the paper having poor smoothness.

Specific examples of the materials used for the elastic belt include, but are not limited to, polycarbonate resins; fluorocarbon resins (e.g., ETFE, PVDF); styrene resins (i.e., homopolymers and copolymers of styrene or styrene substitutions) such as polystyrene resins, chloro polystyrene resins, poly-&agr;-methylstyrene resins, styrene-butadiene copolymers, styrene-vinyl chloride copolymers, styrene-vinyl acetate copolymers, styrene-maleic acid copolymers, styrene-acrylate copolymers (e.g., styrene-methyl acrylate copolymers, styrene-ethyl acrylate copolymers, styrene-butyl acrylate copolymers, styrene-octyl acrylate copolymers, styrene-phenyl acrylate copolymers), styrene-methacrylate copolymers (e.g., styrene-methyl methacrylate copolymers, styrene-ethyl methacrylate copolymers, styrene-phenyl methacrylate copolymers), styrene-methyl &agr;-chroloacrylate copolymers, and styrene-acrylonitrile-acrylate copolymers; and other resins such as methyl methacrylate resins, butyl methacrylate resins, ethyl acrylate resins, butyl acrylate resins, modified acrylic resins (e.g., silicone-modified acrylic resins, vinyl chloride-modified acrylic resins, acrylic urethane resins), polyvinyl chloride resins, styrene-vinyl acetate copolymers, vinyl chloride-vinyl acetate copolymers, rosin-modified maleic acid resins, phenol resins, epoxy resins, polyester resins, polyester polyurethane resins, polyethylenes, polypropylenes, polybutadienes, polyvinylidene chloride, ionmer resins, polyurethane resins, silicone resins, ketone resins, ethylene-ethyl acrylate copolymers, xylene resins, polyvinyl butyral resins, polyamide resins, modified polyphenyleneoxide resins, etc. These can be used alone or in combination.

Specific examples of the elastic rubber and elastomer include, but are not limited to, butyl rubbers, fluorocarbon rubbers, acrylic rubbers, EPDM, NBR, acrylonitrile-butadiene- styrene rubbers, natural rubbers, isoprene rubbers, styrene-butadiene rubbers, butadiene rubbers, ethylene-propylene rubbers, ethylene-propylene terpolymers, chloroprene rubbers, chlorosulfonated polyethylenes, chlorinated polyethylenes, urethane rubbers, syndyotactic 1,2-polybutadiene, epichlorohydrin rubbers, silicone rubbers, polysulfide rubbers, polynorbornene rubbers, hydrogenated nitrile rubbers, thermoplastic elastomers (e.g., polystyrene-based elastomers, polyolefin-based elastomers, polyvinyl chloride-based elastomers, polyurethane-based elastomers, polyamide-based elastomers, polyurea-based elastomers, polyester-based elastomers, fluorocarbon resin-based elastomers), etc. These can be used alone or in combination.

The intermediate transfer medium can include a conductive agent to control the volume resistivity thereof.

Specific examples of the conductive agents include, but are not limited to, carbon black, graphite, metal (e.g., aluminum, nickel) powders, conductive metal oxides (e.g., tin oxide, titanium oxide, antimony oxide, indium oxide, potassium titanate, antimony oxide-tin oxide combined oxide (ATO), indium oxide-tin oxide combined oxide (ITO)), etc. The conductive metal oxides can be covered with a particulate insulative material such as barium sulfate, magnesium silicate, and calcium carbonate.

Materials used for the outermost layer of the intermediate transfer medium are required to prevent contamination of the elastic materials to the electrostatic latent image bearing member, to decrease surface friction resistance so as to reduce toner adhesion and to improve cleanability and secondary transferability.

For example, a material in which at least one particulate material (e.g., a fluorocarbon resin, a fluorine compound, a carbon fluoride, a titanium dioxide, a silicon carbide), which can decrease surface energy and improve lubricity, is dispersed in at least one resin selected from a polyurethane resin, a polyester resin, and an epoxy resin, can be used. The particulate materials can be used alone or in combination. In addition, the particulate materials can be used in combination with the same material having a different particle diameter. Fluorocarbon rubbers can form a layer thereof on the intermediate transfer medium by application of heat. In this case, the surface energy decreases because a large amount of fluorine atoms are present on the surface.

A method of preparing the belt is not particularly limited. For example, the above-mentioned belts can be prepared by a centrifugal molding method in which constituent materials are poured into a rotating cylindrical mold; a spray coating method in which a coating liquid is sprayed; a dipping method in which a cylindrical mold is dipped into a constituent liquid and then raised up; a cast molding method in which constituent materials are poured into an inner mold or an outer mold; and a method in which a compound is wound around a cylindrical mold and then subject to vulcanization polishing. These methods are typically used in combination when a belt is prepared.

In order to prevent the elongation of the elastic belt, a method in which a rubber layer is formed on a resin-cored layer which hardly elongates, a method in which an elongation inhibitor is put into a core layer, etc., have been proposed. However, the method of preparing the belt is not particularly limited.

Specific examples of the materials used for the elongation inhibitors include, but are not limited to, natural fibers (e.g., cotton, silk), synthesized fibers (e.g., polyester fibers, nylon fibers, acrylic fibers, polyolefin fibers, polyvinyl alcohol fibers, polyvinyl chloride fibers, polyvinylidene chloride fibers, polyurethane fibers, polyacetal fibers, polyfluoroethylene fibers, phenol fibers), inorganic fibers (e.g., carbon fibers, glass fibers, boron fibers), metal fibers (e.g., iron fibers, copper fibers), etc. These can be used alone or in combination. Textiles and threads of the above-materials can also be used.

Any type of threads can be used. For example, a thread can be prepared by twisting one or more filaments. The thread may be a blended thread in which the above plural fibers are mixed. Of course, the thread can be conductive-treated. Any type of textiles (e.g., knitted fabrics) can be used. Of course, the textile can be conductive-treated.

A method of preparing the core layer is not particularly limited. For example, the above-mentioned core layer can be prepared by a method in which a metal mold is covered with a cylindrical textile, and then a cover layer is formed thereon; a method in which a cylindrical textile is dipped into a liquid rubber, etc. to form a cover layer on one side or both sides thereof; and a method in which a thread is spirally wound around a metal mold with a given pitch, and then a cover layer is formed thereon.

The elastic layer preferably has a thickness of less than about 1 mm. When the elastic layer has too large a thickness, cracks tend to appear on the surface thereof because the surface largely expands and contracts, even though it depends on the hardness of the elastic layer. When the surface largely expands and contracts, the produced images also expand and contract.

The transfer means (i.e., the primary transfer means and the secondary transfer means) preferably comprises a transfer device configured to attract the toner image from the electrostatic latent image bearing member to the recording medium. The number of the transfer member can be one or more.

Specific examples of the transfer devices include, but are not limited to, corona transfer devices, transfer belts, transfer rollers, pressure transfer rollers, adhesion transfer members, etc.

Any known media on which an unfixed image can be transferred can be used as the recording medium. Specific examples of the recording media include, but are not limited to, plain papers, overhead projection PET sheets, etc.

In the fixing process, the toner image transferred onto the recording medium is fixed with the fixing means. When the toner image is a full-color toner image, each single-colored toner image can be independently fixed on the recording medium one by one. Of course, a multiple toner image, in which all of the single-colored toner images are superimposed, can be fixed on the recording medium.

As the fixing means, heat pressing means are preferably used, but are not limited thereto.

The heat pressing means typically includes a combination of a heat roller and a pressing roller; and a combination of a heat roller, a pressing roller, and an endless belt; etc. Heating temperature of the heat pressing means is preferably from 80 to 200 °C. In the present invention, any known light fixing devices can be used in combination with the heat fixing device, or instead of the heat fixing device.

In the discharging process, which is optionally performed, a discharging bias is applied to the electrostatic latent image bearing member so as to remove the charge therefrom with the discharging means.

As the discharging means, any known discharging device which can apply a discharging bias to the electrostatic latent image bearing member can be used, and is not particularly limited. For example, discharging lamps are preferably used.

In the cleaning process, which is optionally performed, residual toner particles remaining on the electrostatic latent image bearing member are removed with the cleaning means.

As the cleaning means, any known cleaning devices which can remove residual toner particles from the electrostatic latent image bearing member can be used, and is not particularly limited. Specific examples of usable cleaning devices include, but are not limited to, magnet brush cleaners, electrostatic brush cleaners, magnetic roller cleaners, blade cleaners, web cleaners, etc.

The image forming apparatus of the present invention preferably includes a lubricant applicator configured to apply a lubricant to the surface of the electrostatic latent image bearing member.

As the lubricant, metal soaps are preferably used. The metal soap is preferably selected from zinc stearate, aluminum stearate, and calcium stearate.

In the recycling process, which is optionally performed, the toner particles removed with the cleaning means are collected and transported to the developing means with the recycling device.

As the recycling device, any known transport means can be used, and is not particularly limited.

In the controlling process, which is optionally performed, each image forming process is controlled with the controlling means.

Specific examples of the controlling means include sequencers, computers, etc., but are not limited thereto.

Next, the image forming apparatus of the present invention will be explained, referring to drawings.

FIG. 7 is a schematic view illustrating an embodiment of the image forming apparatus of the present invention. The image forming apparatus illustrated in FIG. 7 includes the cylindrical electrostatic latent image bearing member (hereinafter referred to as photoreceptor) 10 of the present invention, a discharging lamp 2, a charger 3, an eraser 4, a light irradiator 5, a developing unit 6, a pre-transfer charger 7, a pair of registration rollers 8, a transfer charger 110, a separation charger 111, a separation pick 112, a pre-cleaning charger 113, a cleaning brush 114, and a cleaning blade 115.

The shape of the photoreceptor 10 is not limited in a cylindrical form, and sheet-shaped photoreceptors and endless belt-shaped photoreceptors can also be used.

As the chargers, any known charging means such as corotron, scorotron, solid state chargers, contact charging rollers, and non-contact charging rollers (i.e., a gap is formed between the photoreceptor and the charging roller using a gap forming means, such as a gap tape or a step formed on the ends thereof, so that the roller is located close to the photoreceptor) can be used.

The non-contact chargers have the following advantages:

1. (1) uneven charging hardly occurs;
2. (2) chargeability hardly deteriorates even if the charging roller is contaminated; and
3. (3) maintenance-free.

However, the non-contact chargers have a drawback such that high voltage needs to be applied thereto. This is hazardous to the surface of the photoreceptor. The outermost layer (i.e., a charge transport layer or a protective layer) including a polymer is significantly abraded, and therefore the life of the photoreceptor shortens, resulting in increase in cost and maintenance frequency.

When only a DC (direct current) is applied to the non-contact charger, discharging is unstably performed, resulting in producing image unevenness. It is preferable that an AC (alternate current) is overlapped with the DC.

The electrostatic latent image bearing member (i.e., photoreceptor) of the present invention can be stably charged with the non-contact charger without being abraded. In addition, residual potential of the irradiated portion thereof can be decreased, and blurred images are hardly produced. Therefore, the electrostatic latent image bearing member of the present invention can stably produce high quality images even after a long repeated use.

As a transfer means, the above chargers can be used. As illustrated in FIG. 7, the transfer charger 110 and the separation charger 11 1 are preferably used in combination as the transfer means.

Suitable light sources for use in the light irradiator 5 and the discharging lamp 2 include illuminants such as fluorescent lamps, tungsten lamps, halogen lamps, mercury lamps, sodium lamps, light emitting diodes (LEDs), laser diodes (LDs), and electroluminescence lamps (ELs), but are not limited thereto. In addition, in order to obtain light having a desired wavelength range, filters such as sharp-cut filters, band pass filters, near-infrared filters, dichroic filters, interference filters, and color temperature converting filters can be used.

The above-mentioned light sources can be used not only for the processes mentioned above and illustrated in FIG. 7, but also for other processes using light irradiation, such as the transfer process, the discharging process, and the cleaning process including light irradiation and pre-exposure process.

When the toner image formed on the photoreceptor 10 by the developing unit 6 is transferred onto a recording medium 9, all toner particles of the toner image are not transferred, and some toner particles remain on the photoreceptor 10. If the next image forming process is performed before such residual toner particles are removed, an electrostatic latent image is not sufficiently formed. The residual toner particles are typically removed from the photoreceptor 10 using a cleaning means such as the cleaning brush 114, the cleaning blade 115, and the combination thereof. As the cleaning brush 114, any known brushes such as fur brushes and magnetic fur brushes can be used.

The cleaning blade 115 is made of an elastic material having a low friction coefficient such as urethane resins, silicone resins, fluorocarbon resins, urethane elastomers, silicone elastomers, and fluorocarbon elastomers. Among these, urethane elastomers including a thermosetting urethane resin are preferably used because of having good resistance to abrasion, ozone, and contamination. In this application, rubbers are considered as elastomers.

The cleaning blade 115 preferably has a JIS-A hardness of from 65 to 85, a thickness of from 0.8 to 3.0 mm, and an extended portion of from 3 to 15 mm. Other conditions such as contact pressure, contact angle, and contact length can be determined as desired.

Since the cleaning means contacts the photoreceptor, the cleaning means gives mechanical impact to the photoreceptor and abrades the surface thereof, while removing residual toner particles. The photoreceptor of the present invention has a protective layer having good abrasion resistance, and therefore high quality images can be stably produced even if the cleaning means contacts the photoreceptor.

The image forming apparatus of the present invention may optionally include a lubricant applicator (not shown in FIG. 7) configured to apply a lubricant to the surface of the photoreceptor. It is known that spherical toners, which are considered to have an advantage in producing high quality images and are practically used recently, are difficult to be removed with a cleaning blade, compared to conventional pulverized toners. When a spherical toner is used, the contact pressure of the cleaning blade is increased or a urethane rubber blade having a high hardness is used to improve cleanability of the spherical toner.

In this case, the cleaning blade tends to give much larger impact to the surface of the photoreceptor, and therefore the surface of the photoreceptor is abraded much more when the spherical toner is used. Since the photoreceptor of the present invention has excellent abrasion resistance, the protective layer is hardly abraded even if a large impact is applied thereto. However, a blade noise, which is considered to occur due to high friction coefficient of the cleaning blade, and abrasion of the blade edge may occur.

These problems can be solved by constantly applying a lubricant to the surface of the photoreceptor using a lubricant applicator, so as to decrease friction coefficient of the surface of the photoreceptor to the cleaning blade for a long period of time.

FIG. 8 is a schematic view illustrating an embodiment of a cleaning unit including a lubricant applicator.

A cleaning unit 117 includes a cleaning brush 114', a cleaning blade 115', and a lubricant bar 116. The lubricant bar 116 contacts the cleaning brush 114' under pressure. The cleaning brush 114' rotates to scrape the lubricant off, and the scraped lubricant adhered to the brush is applied to the surface of the photoreceptor.

The lubricant need not be solid. Any known lubricants which can be applied to the surface of the photoreceptor and satisfy electrophotographic property, such as liquid lubricants, powder lubricants, and half-kneaded lubricants can be used, and are not particularly limited.

Specific examples of the lubricants include, but are not limited to, metal soaps (e.g., zinc stearate, barium stearate, aluminum stearate, calcium stearate), waxes (e.g., carnauba waxes, lanolin, haze waxes), lubricant oils (e.g., silicone oils), etc. Among these, zinc stearate, aluminum stearate, and calcium stearate are preferably used because these can be easily converted into a bar and have high lubrication property.

When a lubricant applicator is included in a cleaning unit as illustrated in FIG. 8, there are advantages that the layout around the photoreceptor is easily designed, and the image forming apparatus can be simplified. In contrast, there are disadvantages that a large amount of the lubricant is mixed with removed toner particles and therefore these toner particles cannot be recycled, and cleaning efficiency of the cleaning brush decreases. These disadvantages can be improved when an application unit including a lubricant applicator is independently arranged from a cleaning unit. In this case, the application unit is preferably arranged on a downstream side from the cleaning unit. When plural application units are arranged, each of the application units can simultaneously or successively work so as to improve lubricant application efficiency and to control the amount of consumed lubricant.

FIG. 9 is a schematic view illustrating another embodiment of the image forming apparatus of the present invention.

A photoreceptor 122 is the electrostatic latent image bearing member of the present invention. The photoreceptor 122 is tightly stretched with a drive roller 123 and rollers 124 and 128, and driven by the drive roller 123. The photoreceptor 122 is charged with a charger 220, and then irradiated with light by a light irradiator 121 to form an electrostatic latent image thereon. The electrostatic latent image is developed with a developing device (not shown), and then transferred onto a recording medium using a transfer charger 125. Residual toner particles remaining on the photoreceptor 122 are removed using a cleaning brush 126. The photoreceptor 122 is discharged with a discharging lamp 127.

FIG. 10 is a schematic view illustrating an embodiment of the full-color image forming apparatus of the present invention.

A photoreceptor 156 is the electrostatic latent image bearing member of the present invention. The photoreceptor 156 is driven to rotate in the counterclockwise direction. The surface of the photoreceptor 156 is uniformly charged with a charger 153 using corotron, scorotron, etc., and is exposed to a laser light beam L emitted by a laser optical device (not shown) to form an electrostatic latent image on the photoreceptor 156. The laser light beam scanning is performed based on single-color information (yellow, magenta, cyan, and black) separated from an original full-color image. Thus, single-color images (yellow, magenta, cyan, and black) are formed on the photoreceptor 156.

On the left side of the photoreceptor 156, a revolver developing unit 250 is arranged. The revolver developing unit 250 includes a yellow developing device, a magenta developing device, a cyan developing device, and a black developing device inside a rotating cylinder, and rotates each developing device to transport the developing device to a developing point facing the photoreceptor 156. The yellow developing device, the magenta developing device, the cyan developing device, and the black developing device develop the electrostatic latent image with a yellow toner, a magenta toner, a cyan toner, and a black toner, respectively. Namely, the electrostatic latent images corresponding to yellow, magenta, cyan, and black images, which are formed one by one by on the photoreceptor 156, are developed one by one by the respective developing devices, and a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are formed.

An intermediate transfer unit is arranged on a downstream side from the developing point relative to the rotating direction of the photorecep