1.  Field of the Invention
The present invention relates to an electrophotographic photoreceptor.  In addition, the present invention relates to an electrophotographic image forming method and apparatus using the photoreceptor.  Further, the present invention relates to a process cartridge for electrophotographic image forming apparatus, which includes the photoreceptor. 
2.  Discussion of the Background
Recently the growth of information processing system using electrophotography is remarkable.  In particular, digital copiers capable of recording digital information using light after converting information to digital signals have been drastically improving in recording qualities and reliability.  In addition, the technique has been applied to full color laser printers and copiers by being combined with a high speed recording technique.  In the light of this background, a need exists for a photoreceptor which not only produces high quality images but also has high durability. 
As the photoreceptor used for such laser printers and digital copiers, photoreceptors using an organic photosensitive material have been widely used because of having advantages such as good productivity and low pollution. 
Specific examples of the organic photoreceptors include the photoreceptors including one of the following photosensitive layers:
(1) organic photoconductive resin layers typified by a poly-N-vinylcarbazole resin;
(2) charge transfer complex type photosensitive layers typified by a combination of poly-N-vinylcarbazole (PVK) with 2,4,7-trinitrofluorenon (TNF);
(3) pigment dispersion type photosensitive layers typified by a combination of phthalocyanine and a binder resin; and
(4) functionally-separated photosensitive layer typified by a combination of a charge generation material and a charge transport material. 
Among these photoreceptors, the functionally-separated photoreceptors attract considerable attention now. 
The electrophotographic image forming methods typically include the following processes:
(1) charging an electrophotographic photoreceptor in a dark place (charging process);
(2) irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image thereon (light irradiating process);
(3) developing the latent image with a developer including a toner mainly constituted of a colorant and a binder to form a toner image thereon (developing process);
(4) optionally transferring the toner image onto an intermediate transfer medium (first transfer process);
(5) transferring the toner image onto a receiving material such as a receiving paper ((second) transfer process);
(6) heating the toner image to fix the toner image on the receiving material (fixing process); and
(7) cleaning the surface of the photoreceptor (cleaning process). 
The mechanism of forming an electrostatic latent image in the functionally-separated photosensitive layer, which has a charge generation layer and a charge transport layer formed on the charge generation layer, is as follows:
(1) when the photosensitive layer is exposed to light after being charged, light passes through the transparent charge transport layer and then reaches the charge generation layer;
(2) the charge generation material included in the charge generation layer absorbs the light and generates a charge carrier such as electrons and positive holes;
(3) the charge carrier is injected into the charge transport layer and transported through the charge transport layer, which is caused by the electric field formed by the charge on the photosensitive layer;
(4) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image on the photosensitive layer. 
However, photosensitive layers of organic photoreceptors are easily abraded when the photoreceptors are repeatedly used.  When a photosensitive layer is largely abraded, various image quality problems such that the potential of the charged photoreceptor decreases; the photosensitivity thereof deteriorates; background fouling occurs in the resultant images; and image density decreases tend to occur.  Therefore the abrasion of photoreceptors have been a big problem to be solved. 
In addition, currently electrophotographic image forming apparatus become smaller and smaller in size.  Therefore the diameter of photoreceptors used for such miniaturized image forming apparatus also becomes smaller and smaller.  Accordingly a need exists for a photoreceptor having good durability. 
In attempting to impart good durability to an organic photoreceptor, the following methods have been proposed:
(1) a protective layer having lubricating property is formed as an outermost layer of the photoreceptor;
(2) a crosslinked protective layer is formed as an outermost layer of the photoreceptor; and
(3) a protective layer including a filler is formed as an outermost layer of the photoreceptor. 
In particular, the method (3) is effective.  However, when an electrical insulating filler is included in a protective layer, the resistance of the protective layer increase, resulting in increase of residual potential of the resultant photoreceptor.  The increase of residual potential is mainly caused by increase of the resistance of the protective layer and increase of the number of charge trap sites therein due to addition of the filler having high insulating property.  In contrast, when an electroconductive filler is used, the resistance of the protective layer decreases, and thereby the residual potential of the protective layer hardly increase.  However, a problem tends to occur such that the resultant images have unclear outlines, i.e., blurred images are produced. 
In attempting to solve such a blurred image problem, a method is proposed in which a photoreceptor having a protective layer including an electroconductive filler is heated by a drum heater to decrease moisture in the photoreceptor.  By heating the photoreceptor, the formation of blurred images can be avoided but the diameter of the photoreceptor becomes large because a drum heater has to be provided in the photoreceptor.  Therefore, this technique cannot be used for small-size photoreceptors typically provided in current small image forming apparatus.  In other words, a photoreceptor having a small-diameter and good durability has not yet been developed.  In addition, when a drum heater is provided, the resultant image forming apparatus becomes large in size.  Further, the image forming apparatus have other drawbacks such that it is needed for the image forming apparatus to continuously work the heater, resulting in increase of power consumption, and it takes a long warm-up time. 
When a photoreceptor has a high residual potential due to use of a filler having high insulating property, the potential of a lighted area of the photoreceptor increases, resulting in deterioration of image density and half toner reproducibility of the resultant images.  In order to avoid such problems, the potential of dark areas needs to be increased.  However, when the dark-area potential is increased, the electric field strength is increased, and therefore not only undesired images such as background fouling are produced, but also the life of the photoreceptor is shortened. 
In attempting to avoid increase of residual potential, methods in which a photoconductive protective layer is formed have been disclosed in Japanese Patent Publications Nos. ( hereinafter JPPs) 44-834, 43-16198 and 49-10258.  However, imagewise light is absorbed by the protective layer, and therefore the quantity of light which reaches the photosensitive layer decreases, resulting in decrease of the photosensitivity of the photoreceptor.  Therefore, this method is of little effect. 
Japanese Laid-Open Patent Publication No. ( hereinafter JOP) 57-30846 discloses a method in which a metal or a metal oxide having an average particle diameter not greater than 0.3 xcexcm is included as a filler in a protective layer to prepare a transparent protective layer, resulting in prevention of increase of residual potential.  However, its effect of preventing increase of residual potential is not insufficient, and therefore the problem cannot be solved. 
This is because the increase of residual potential is caused by charge trapping due to the added filler and uneven dispersion of the filler rather than deterioration of charge generation efficiency.  Even when a filler having an average particle diameter not greater than 0.3 xcexcm is used, the transparency of the resultant protective layer decreases if the filler aggregates.  On the contrary, when a filler having an average particle diameter not less than 0.3 xcexcm is used, a transparent protective layer can be formed if the filler is uniformly dispersed. 
In addition, JOP 4-281461 discloses a method in which a charge transport material is included in a protective layer together with a filler in attempting to prepare a photoreceptor capable of preventing increase of residual potential while having a good mechanical strength.  To include a charge transport material in a protective layer improves the charge mobility and therefore the increase of residual potential can be improved to some extent.  However, when a filler is added, residual potential is remarkably increased, which is caused by the increase of resistance of the protective layer and the number of charge trap sites in the protective layer.  Therefore, there is a limit to improvement of the increase of residual potential only by increasing the charge mobility.  Accordingly, the demand for a photoreceptor having good durability cannot be satisfied only by this method. 
In addition, in attempting to improve the increase of residual potential, a method in which a Lewis acid is included in a protective layer (JOP 53-133444); a method in which an organic proton acid is included in a protective layer (JOP 55-157748); and a method in which an electron accepting material is included in a protective layer (JOP 2-4275), have been disclosed. 
It is considered that these methods are aimed to improve the charge injection at the interface between the protective layer and the charge transport layer, and portions having a low resistance are formed in the protective layer, such that the charge can reach the surface of the protective layer, resulting in decrease of residual potential.  The residual potential increase problem can be improved to some extent by these methods, but the increase of residual potential of a durable photoreceptor having a protective layer including a filler cannot be avoided by these methods because the reason for the residual potential increase problem occurred for such a durable photoreceptor is different from the reason for the residual potential increase problem improved by these methods mentioned above.  In addition, when an organic acid is included in a protective layer in combination with a filler, the dispersion of the filler in the protective layer tends to deteriorate and therefore the resultant images are blurred.  Thus, these methods produce adverse effects. 
JOP 2000-66434 discloses a method in which a wax having an acid value not greater than 5 mgKOH/g is included in a protective layer.  However, the effect of adding a filler in the outermost layer is not described in JOP 2000-66434.  In addition, as can be understood from the below-mentioned description, a satisfactory effect cannot be exerted by this method against a photoreceptor including a filler in the outermost layer to improve its durability. 
Japanese Patent No.  2,884,812 (i.e., JOP 4-295855) discloses a method that a layer including a graft copolymer resin having an acid value of from 30 to 260 mgKOH/g is formed on the surface of a photosensitive layer.  Increase of residual potential can be prevented.  However, this technique intends to improve the lubrication property and releasability of the surface of the photoreceptor, and in addition the photoreceptor does not include a filler in the outermost layer, which is different from the photoreceptor of the present invention.  Further, the durability of the photoreceptor is not discussed therein. 
Namely, the outermost layer consists of a graft polymer, and therefore the construction of the outermost layer is different from the outermost layer of the photoreceptor of the present invention, in which a filler is included to improve the durability thereof. 
In addition, JOP 9-281718 discloses a photoreceptor having a photosensitive layer which includes a titanylphthalocyanine, a polycarbonate and a resin having an acid value of from 1 to 50 mgKOH/g. This technique intends to heighten the xcex3-properties of a single-layer photoreceptor.  Therefore, the photoreceptor is different from the photoreceptor of the present invention, in which a protective layer including a filler is formed on a photosensitive layer.  Namely, the durability of the photoreceptor is not discussed therein. 
On the other hand, blurred images are also produced due to ozone and NOx.  It is known that antioxidants are effective at improving the blurred images of this kind.  JOP 8-292585 discloses a method in which at least one antioxidant selected from hindered phenol derivatives and hindered amine derivatives is included in the photoreceptor to reduce the chance that blurred images are produced.  It is know that this method is effective at improving the blurred images caused by reactive gasses such as ozone and NOx.  However, residual potential of the resultant photoreceptor increases.  In particular, when a filler is included in an outermost layer, residual potential seriously increases, resulting in deterioration of initial image qualities.  Therefore this technique cannot be used for a photoreceptor having a protective layer including a filler because residual potential thereof seriously increases and high quality images cannot be produced. 
Thus, a photoreceptor having a protective layer including a filler is known.  In addition, a method in which an acid is added to the protective layer to reduce residual potential is also known.  However, a technique by which serious increase of residual potential due to addition of a filler in a protective layer can be improved has not been discovered.  Therefore there is a desire for a photoreceptor capable of producing high quality images while having good durability. 
In order to produce high quality images on photoreceptors in which a filler is included in the outermost layer to improve their durability, it is needed to prevent formation of blurred images and increase of residual potential.  In addition, it is also important that charges in photoreceptors linearly move toward the surface of the photoreceptors without being obstructed by the filler included therein.  Therefore, it is needed that the filler in the protective layer is well dispersed therein.  When the filler included in a protective layer agglomerates, movement of the charges injected into the protective layer from the photosensitive layer are obstructed by the filler when the charges move toward the surface of the protective layer.  Therefore a toner image formed of scattered toner particles is formed, resulting in deterioration of resolution of the toner image. 
In addition, when imagewise light irradiates such a protective layer including an agglomerated filler, the light is scattered by the filler, resulting in deterioration of light-transmittance, and thereby resolution of the resultant image deteriorates. 
Further, the dispersion of a filler included in a protective layer largely influences the abrasion resistance of the photoreceptor.  When a filler seriously agglomerates (i.e., a filler is poorly dispersed), not only the abrasion resistance of the resultant photoreceptor deteriorates but also uneven abrasion tends to occur.  Therefore a desired durability cannot be imparted to the resultant photoreceptor. 
Therefore, in order to provide a photoreceptor in which a filler is included in a protective layer to improve the durability of the photoreceptor and which can produce high quality images, it is important not only to prevent occurrence of blurred images and increase of residual potential but also to improve dispersion of the filler in the protective layer.  Namely, it is important not to form aggregates of the filler used, i.e., to prepare a coating liquid in which the filler is well dispersed. 
However, a solution by which these problems are solved at the same time has not been discovered.  Namely, when a filler is included in an outermost layer of a photoreceptor to improve its durability, blurred images tend to be produced and residual potential tends to increase, and therefore a problem in that high quality images cannot be produced still remains.  As mentioned above, a drum heater has to be provided in an image forming apparatus to prevent such a blurred image problem.  However, a drum heater cannot be provided in a small-sized photoreceptor, which is especially desired to have good durability.  Therefore, there is no small-size photoreceptor having good durability and capable of producing high quality images.  To install a drum heater is an obstruction to a small-size image forming apparatus and having low electric power consumption. 
Accordingly, an object of the present invention is to provide a durable photoreceptor which can stably produce high quality images without blurring wile preventing increase of residual potential even when repeatedly used for a long period of time. 
Another object of the present invention is to provide an image forming method by which high quality images can be stably produced at a high speed for a long period of time without frequently changing the photoreceptor. 
A further object of the present invention is to provide a small-size image forming apparatus and a process cartridge by which high quality images can be stably produced at a high speed for a long period of time without frequently changing the photoreceptor. 
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophotographic photoreceptor including at least an electroconductive substrate, one or more photosensitive layers formed overlying the substrate and optionally a protective layer formed overlying the photosensitive layer, wherein the outermost layer includes at least a filler, a binder resin and an organic compound having an acid value of from 10 to 700 mgKOH/g.
The outermost layer is defined as the layer furthest away from the substrate.  The photosensitive layer can be the outermost layer.  xe2x80x9cOverlyingxe2x80x9d can include direct contact and allow for intermediate layers. 
A layer such as an undercoat layer may be formed between the substrate and the photosensitive layer.  In addition, a layer such as intermediate layer may be formed between the photosensitive layer and the protective layer. 
It is preferable that the organic compound having an acid value in the specified range has at least one hydrophilic group.  More preferably the hydrophilic group is a carboxyl group.  The organic compound is preferably an unsaturated polycarboxylic acid type wetting dispersant.  The number average molecular weight of the organic compound is preferably from 300 to 30,000. 
The photoreceptor preferably satisfies the following relationship:
0.1xe2x89xa6(Axc3x97B/C)xe2x89xa620 
wherein A represents the content of the organic compound having an acid value of from 10 to 700 mgKOH/g in the outermost layer in units of grams; B represents the acid value of the organic compound in units of mgKOH/g; and C represents the content of the filler in the outermost layer in units of grams. 
The filler preferably includes an inorganic pigment.  The inorganic pigment is preferably a metal oxide, which preferably has a specific resistance (i.e., resistivity) not less than 1010 xcexa9xc2x7cm.  The pH of the metal oxide at its isoelectric point is preferably not less than 5. 
It is preferable that the surface of the inorganic pigment is subjected to a treatment preferably using a material selected from the group consisting of titanate coupling agents and aluminum coupling agents.  The ratio (Ws/Wf) of a weight (Ws) of the surface treating agent to a weight (Wf) of the filler is from 0.02 to 0.30.  The average primary particle diameter of the filler is preferably form 0.01 to 0.9 xcexcm. 
The content of the filler is preferably from 0.1 to 50% by weight base on total solid components of the outermost layer. 
The binder resin preferably includes a resin selected from the group consisting of polycarbonate resins and polyarylate resins.  In addition, a charge transport polymer can be used as the binder resin. 
The outermost layer preferably includes an antioxidant, which is preferably a compound having both a hindered phenol structure and a hindered amine structure therein. 
In another aspect of the present invention, a coating liquid for an outermost layer of a photoreceptor is provided which includes a filler, a binder resin, a compound having an acid value of from 10 to 700 mgKOH/g, and a mixture of two or more organic solvents.  It is preferable that the coating liquid is dispersed by a ball mill containing alumina balls as a dispersion element. 
In yet another aspect of the present invention, a method for preparing a photoreceptor is provided in which the coating liquid mentioned above is coated by a spray coating method.  The outermost layer is preferably formed by performing spray coating at least twice. 
In a further aspect of the present invention, an image forming method is provided which includes the steps of charging a photoreceptor, irradiating the photoreceptor with light to form an electrostatic latent image thereon, developing the latent image with a developer to form a toner image, and transferring the toner image on a receiving material, wherein the photoreceptor is the photoreceptor mentioned above. 
In a still further aspect of the present invention, an image forming apparatus is provided which includes a photoreceptor, a charger configured to charge the photoreceptor, an image irradiator configured to irradiate the photoreceptor with light to form an electrostatic latent image on the photoreceptor, an image developer configured to develop the latent image with a developer to form a toner image thereon, and a transferer configured to transfer the toner image on a receiving material optionally via an intermediate transfer medium, wherein the photoreceptor is the photoreceptor of the present invention. 
Preferably the image irradiator irradiates the photoreceptor using a laser diode or a light emitting diode.  In addition, the charger is preferably a charging roller, and the charger is preferably a proximity charging roller, which charges the photoreceptor while a small gap is formed between the charger and the photoreceptor and a DC voltage overlapped with an AC voltage is applied. 
The toner preferably includes a lubricant, and/or the image forming apparatus further has a lubricant applying device configured to apply a lubricant, such as zinc stearate and fluorine-containing compounds, to the surface of the photoreceptor. 
The image forming apparatus may be a tandem type image forming apparatus having four photoreceptors in which cyan, magenta, yellow and black images are formed on the respective photoreceptor in parallel to form a full color image.  In this case, an intermediate transfer medium is preferably used. 
In a still further aspect of the invention, a process cartridge is provided which includes at least a photoreceptor and a housing, wherein the photoreceptor is the photoreceptor of the present invention. 
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.