Liquid spray-coating method and electrophotographic photoreceptor formed by the method

Abstract

 A liquid spray-coating method which comprises circulating a liquid from a liquid tank (2) to the liquid tank by a pressurizer (3) through a liquid supplier (6) and a spray coating device (7); and spraying the liquid to an object to coat the liquid on the object, wherein the liquid is circulated through a liquid feeding route and a liquid circulating route when the spraying is stopped.

Description

Field of the Invention

[0001] The present invention relates to a liquid spray-coating method, and to an electrophotographic photoreceptor formed by the method which can be widely used for electrophotographic applications such as copiers, laser beam printers, CRT printers and photoengraving systems.

Discussion of the Background

[0002] Conventionally, inorganic photoconductive materials such as selenium and zinc oxide have been used for photosensitive layers of electrophotographic photoreceptors.

[0003] Recently, organic photoconductive materials for use in the photosensitive layers have been investigated, and further a functionally separated photoreceptor including two layers was mainly investigated. One layer is a charge generation layer absorbing light and generating a charge carrier, and the other is a charge transport layer transporting the charge carrier.

[0004] The functionally separated photoreceptor comprises a combination of an organic compound efficiently generating charge and an organic compound efficiently transporting the charge. Thus, the photoreceptor has high sensitivity and is in practical use.

[0005] In addition, due to requirements of digitization, high-quality image and high durability, many photoreceptors such as a photoreceptor having an undercoat layer between the electroconductive substrate and the photosensitive layer and a photoreceptor having a protective layer on the photosensitive layer have been invented.

[0006] Many photoreceptors having high chargeability, preventing moire and image deformation, and having high durability and environmental resistance have been invented by providing undercoat layers and protective layers having a thickness of from submicrons to a few microns and including fine particles, either organic or inorganic, uniformly dispersed in the layers on the photoreceptors.

[0007] These inventions contribute to the progress of image forming apparatus using various electrophotographic processes typified by a copier.

[0008] Such layers are typically coated on a cylindrical drum substrate or an endless belt-shaped substrate, etc. to form organic photoreceptors. As the coating methods, a dip coating method, a spray coating method, a nozzle coating method, etc. are known.

[0009] Hereinafter, the above-mentioned coating methods will be explained.

Dip coating method

[0010] The dip coating method is industrially used most and has been subject of extensive research. However, the method has a disadvantage that much coating liquid is consumed when coating an endless belt-shaped substrate which has a large diameter and a cylindrical drum substrate which is long in many cases. In addition, a low molecular-weight compound in a layer previously coated on a substrate tends to be dissolved out when coating a functionally separated photoreceptor because of dipping the substrate into the coating liquid and drawing up the substrate therefrom, resulting in quality deterioration of the resultant photoreceptor. Further, the compound dissolved out in the coating liquid often contaminates the coating liquid and largely affects the coating method.

[0011] Particularly, when forming a protective layer on a functionally separated photoreceptor having a charge transport layer on a charge generation layer by the dip coating method, a serious problem of deterioration of basic charge transportability occurs because a low molecular-weight charge transport compound in the charge transport layer is dissolved out in the liquid.

Nozzle coating method

[0012] The nozzle coating method is advantageous for coating an endless belt-shaped substrate which has a large diameter and a cylindrical drum substrate which is long in many cases
because the consumption of the coating liquid is small. However, a part of the substrate doubly coated at the beginning and the end of coating becomes thick, and technical difficulty to solve the problem is high.

[0013] In addition, the method is not suitable for industrial use because mechanical precision required for the coating apparatus is unrealistically high.

Spray coating method

[0014] The spray coating method is also advantageous for coating an endless belt-shaped substrate which has a large diameter and a cylindrical drum substrate which is long in many cases
because the consumption of the coating liquid is small. Further, the method does not have the problem of thickening a part of the substrate doubly coated at the beginning and the end of coating, and is industrially used on occasion.

[0015] In addition, even when coating a conventional cylindrical drum substrate, a low molecular-weight compound does not tend to dissolve out from a layer previously coated and quality deterioration of the resultant photoreceptor can be prevented. Particularly, the method is advantageous for coating a protective layer on a functionally separated photoreceptor having a charge transport layer on the charge generation layer.

[0016] However, usually a solvent having a slow evaporation speed is used in a conventional spray coating method in many cases to make the coated surface smooth. Therefore, it generally takes time to dry the coated substrate to the touch in the coater even after the spray is stopped. In addition, the spray often stops when a coated subject is transferred.

[0017] In particular, when the coating liquid is a fine-particle dispersion liquid including at least a solvent and fine particles, the fine particles tend to sink in the spray coating apparatus and clog the spray gun when the spraying is stopped, resulting in deterioration of stable coating and coating irregularity.

[0018] This is a case for general spray coating methods, and Japanese Laid-Open Patent Publication No. 9-75794 discloses an air spray coating method and an apparatus supplying and transporting a coating liquid without retaining the liquid in the spray coating apparatus (a spray gun).

[0019] However, it is typically required in coating an undercoat layer and a protective layer of an electrophotographic photoreceptor that a thin and uniformly dispersed layer having a thickness of from about submicrons to a few microns should be formed for their specific electric properties. Therefore, when an electrophotographic photoreceptor is formed by spray coating, it is necessary to precisely transport small flow rate of the liquid to the spray coating apparatus.

[0020] With such a small flow rate, the sedimentation of the fine particles in the spray coating device and the liquid transport route cannot be sufficiently prevented when the spraying is stopped in such a coating apparatus as has a circular route in which the coating liquid simply returns to the liquid tank from the coating device. Therefore, the spray gun tends to be clogged.

[0021] In addition, Japanese Laid-Open Patent Publication No. 58-174264 discloses a spray coating apparatus circulating a coating liquid. The apparatus has the following circulation routes:

(1) a coating liquid tank;

(2) a pump;

(3) a coating-liquid feeding pipe;

(4) a spray gun;

(5) a coating-liquid returning pipe; and

(1) the coating-liquid tank,

wherein a bypass pipe is connected to the route between the pump (2) and the coating-liquid feeding pipe (3) and to the route between the coating-liquid returning pipe (5) and the coating-liquid tank (1) to prevent the precipitaion of the liquid, and the reservoir liquid in the tank is forcibly agitated by the divided coating liquid. However, this apparatus makes coating with a large flow rate, and when such a flow rate of the liquid as can sufficiently prevent the sedimentation of the fine particles in the coating device and the transport route when the spraying is stopped is transported, the transported liquid is too much to form a thin and uniformly dispersed layer having a thickness of from about submicrons to a few microns required for an electrophotographic photoreceptor.

[0022] Further, when a small flow rate of the liquid is precisely transported to the spray coating device by the spray coating method to form an electrophotographic photoreceptor, the sedimentation of the fine particles tends to occur in the fine-particles dispersion-liquid supplier as well.

[0023] Particularly, when a syringe pump capable of precisely transporting small flow rate of the liquid to the spray coating device is used as a fine-particle dispersion-liquid supplier instead of liquid suppliers liable to bring pulsation such as a diaphragm pump and a gear pump, the dispersion liquid is retained in the cylinder and the sedimentation of the fine particles tends to go on because it is effective on production efficiency to stop the syringe pump as well when the spraying is stopped.
Further, there also is a case where the rheology of the liquid changes.

[0024] In this case, since the liquid capacity (cylinder capacity) of the liquid supplier is far larger than that of the spray coating device, the adverse effect of the fine-particle sedimentation in the liquid supplier is more serious than that in the spray coating device.

[0025] Namely, when a dispersed layer is formed by spraying with the liquid in which the fine-particle sedimentation went on,
the supernatant liquid including less fine particles is sprayed to form the dispersed layer. Consequently, the fine-particle content in the solid contents included in the dispersion liquid differs from that in the formed dispersion layer, which causes a considerable adverse effect on the electric properties, durability, etc. of the resultant electrophotographic photoreceptor.

SUMMARY OF THE INVENTION

[0026] Accordingly, an object of the present invention is to provide a coating method and a coating apparatus capable of forming a coated layer having a uniform composition.

[0027] Another object of the present invention is to provide a coating method and a coating apparatus capable of forming a coated layer having a uniform composition without a clogging of the spray coating device when the coating liquid is a liquid dispersing fine particles. In other words, a method and an apparatus forming a coated layer having a uniform composition are provided, by a circulation-type spray coating method in which the liquid constantly flows in the liquid supplier, the spray coating device and the liquid feeding route when the spraying is stopped to prevent the heterogeneous liquid composition.

[0028] Yet another object of the present invention is to provide an electrophotographic photoreceptor having a layer including a uniform composition using the above-mentioned method and apparatus.

[0029] Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a method for manufacturing an electrophotographic photoreceptor using a circulation-type spray coating apparatus including at least a liquid supplier, a spray coating device atomizing and spraying the liquid, a liquid feeding route in which the liquid is transported from a liquid tank through the liquid supplier to the spray coating device, a liquid circulating route in which the liquid returns to the liquid tank from the spray coating device and a liquid pressurizer, wherein the liquid constantly flows in the liquid supplier, the spray coating device and the liquid feeding route.

[0030] 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.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the detailed description when considered in connection with the accompanying drawings in which like reference characters designate like corresponding parts throughout and wherein:

Fig. 1 is a schematic view illustrating an outline of the method for manufacturing an electrophotographic photoreceptor in the present invention;

Fig. 2 is a schematic view illustrating an embodiment of the liquid supplier of the present invention;

Fig. 3. is a schematic view:illustrating another embodiment of the liquid supplier of the present invention;

Fig. 4 is a schematic view illustrating yet other three embodiments of the liquid supplier of the present invention;

Fig. 5 is a schematic view illustrating an embodiment of the spray coating device of the present invention;

Fig. 6 is a schematic view illustrating a cross section of the photosensitive layer of a functionally separated electrophotographic photoreceptor having an undercoat layer.

Fig. 7 is a schematic view illustrating a cross section of the photosensitive layer of a functionally separated electrophotographic photoreceptor having a protective layer; and

Fig. 8 is a schematic view illustrating a cross section of the photosensitive layer of a functionally separated electrophotographic photoreceptor having an undercoat layer and a protective layer.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Generally, the present invention provides a manufacturing method and an apparatus preferably used for coating such a liquid as loses its homogeneity due to the retention, particularly the method and the apparatus are preferably used for spray coating of a fine-particle dispersion liquid. The method and the apparatus of the present invention prevent the spray gun from clogging, and equalize the content ratio of the fine particles of the solid content included in the dispersion liquid and that of the formed dispersed layer.
In addition, the method and the apparatus can form a thin and uniform coated layer having a thickness of from about sub
µm to some µm.

[0033] The manufacturing method and the apparatus of the present invention are particularly suitable for forming a photosensitive layer of an electrophotographic photoreceptor. However, the present invention is not limited hereto, and applicable for the fields besides the electrophotographies.

[0034] Therefore, hereinafter, a case where a fine-particle dispersion liquid is used as a coating liquid will be explained, however, the present invention is not limited to this case. Besides the fine-particle dispersion liquid, dye-solution liquids, resin-solution liquids, emulsion liquids and the like liquids can be used as the coating liquids.

[0035] A syringe pump can be preferably used as the liquid supplier for use in the present invention. The syringe pump includes at least a cylinder, a piston rod having a piston member capable of being freely inserted into the cylinder and a means of driving the piston rod, and precisely supplies a small flow rate of the liquid to the above-mentioned spray coating device by driving the piston rod when the spraying is performed. The syringe pump preferably has a liquid-flow duct in the cylinder or in the piston rod. Particularly, the piston rod preferably includes the liquid-flow duct, for example, it is preferable to use a syringe pump having a piston rod including at least an entrance and an exit for the liquid circulation, and the liquid-flow duct, in which the fine-particle dispersion liquid can constantly flow without staying in the cylinder when the spraying is stopped.

[0036] The spray coating device for use in the present invention is an air spray gun including at least a coating nozzle and an air cap, in which the pressure-fed liquid to the coating nozzle is atomized by compressed air discharge from the air cap. In addition, the coating nozzle has at least a liquid-feed opening and an exit opening for the liquid circulation besides a discharge opening for the atomized liquid, and the liquid can constantly flow without staying in the spray gun when the spraying is stopped.

[0037] In the liquid feeding and circulating route of the present invention including a liquid feeding route in which the liquid is transported to the spray coating device through the liquid supplier from a liquid tank; a circulating route in which the liquid returns to the liquid tank from the spray coating device; and a pressurizer for circulating the liquid, the liquid feeding route between the liquid tank and the liquid supplier and the circulating route between the spray coating device and the liquid tank respectively have one liquid-pressure blocker or more which blocks the liquid pressure given by the pressurizer. When the circulation is stopped by blocking the liquid pressure when the spraying is performed, the liquid pressure between the liquid supplier and the spray coating device is separated from the liquid pressure given by the pressurizer and equals the pressure given by the liquid supplier. Thus, the liquid feeding and circulating route of the present invention can spray by the pressure of the liquid supplier.

[0038] In the present invention, "the liquid supplier" mainly gives the discharging pressure of the spray coating device, and "the pressurizer for circulation" mainly controls the liquid circulation of the whole coating apparatus. In addition, the liquid supplier is preferably located between the pressurizer for circulation and the spray coating device.

[0039] When the flow rate of the dispersion liquid by the liquid pressure of the pressurizer for the liquid circulation is Fa (cc/min.) and that of the fine-particle dispersion liquid supplier is Fb (cc/min.), the following relationship is preferably satisfied:

[0040] When manufacturing an electrophotographic photoreceptor, the Fa is preferably from 300 to 1,500 cc/min, and the Fb is preferably from 5 to 100 cc/min.

[0041] The details of the present invention will be explained, referring to the drawings. However, the drawing is an example, and the present invention is not limited to the embodiments.

[0042] First, the operation when the spraying is stopped will be explained.

[0043] Fig. 1 is a schematic view illustrating an outline of the method for manufacturing an electrophotographic photoreceptor in the present invention.

[0044] A fine-particle dispersion liquid 2 put and agitated in a coating liquid tank 1 is transported by the pressure of a pressurizer for circulation 3 to a liquid supplier 6 and a spray coating device 7 through a liquid feeding route 4, and then returned to the coating liquid tank 1 through a circulation route 8. At this point, liquid-pressure blockers 5 located in the liquid feeding route 4 and the circulation route 8 are disengaged. As the pressurizer for circulation, gear pumps, diaphragm pumps and the like pumps are preferably used, which can give sufficient flow rate to prevent the sedimentation of the fine particles.

[0045] Particularly, the details of the liquid supplier 6 when the spraying is stopped will be explained, referring to Fig. 2.

[0046] Fig. 2 is a schematic view illustrating an embodiment of the liquid supplier of the present invention. The liquid supplier is a syringe pump including a cylinder 11; a piston member 17; and a piston rod 12, and the piston rod 12 includes a circulation-liquid entrance 14, a circulation-liquid exit 15 and a circulation-liquid flow duct 16.

[0047] The fine-particle dispersion liquid 2 pressurized by the pressurizer for circulation 3 in Fig. 1 enters the circulation-liquid entrance 14 of the piston rod 12 connected to the liquid feeding route 4 and is supplied inside the cylinder 11 from the circulation-liquid exit 15 through the circulation-liquid flow duct 16 penetrating the piston rod 12.

[0048] Then, after filling the inside of the cylinder 11, the fine-particle dispersion liquid 2 is transported to the spray coating device 7 through the liquid feeding route 4 connected to the upper part of the cylinder. When the spraying is stopped, the liquid supplier 6 stops as well and the piston rod 12 stays waiting in the bottom dead center.

[0049] The capacity of the cylinder 11 is preferably larger than the quantity of the dispersion liquid required for spray coating at a time. However, the capacity which is larger than necessary is not preferable because of being a cause of deteriorating the preciseness of the liquid feeding when the spraying is performed.

[0050] Because of being the above-mentioned structure, the fine-particle dispersion liquid 2 can flow without staying in the cylinder 11 when the spraying is stopped. Therefore, the pressurizer for circulation 3 can give a liquid pressure such that sufficient circulating flow-rate can be obtained, regardless of the strength of the liquid pressure for spray coating. In addition, the sedimentation of the fine particles in the spray coating device 7 and the cylinder 11 can be prevented.

[0051] The circulating direction of the fine-particle dispersion liquid 2 is not limited thereto.

[0052] Namely, the liquid feeding route 4 from the pressurizer for circulation 3 may be connected to the upper part of the
cylinder, and the fine-particle dispersion liquid 2 may be supplied inside the cylinder 11 through the route and transported to the spray coating device 7 through the circulation-liquid flow duct 16 penetrating the piston rod 12 after filling the inside of the cylinder 11. In this case, the numeral 15 is a circulation-liquid entrance, and the numeral 14 is an exit.

[0053] Fig 3. and Fig. 4 are schematic views illustrating other embodiments of the liquid supplier of the present invention.

[0054] Fig. 3 is an example in which circulation-liquid flow ducts are formed inside a piston rod 12 and a piston member 17, which enable the liquid to flow uniformly inside the cylinder when the spraying is stopped. Fig. 4 shows three examples in which circulation-liquid entrances 14 are formed on the wall of cylinders 11. A in Fig. 4 is an example in which the liquid entrances is formed in the top dead center, B in Fig. 4 is an example in which the entrance is formed in the bottom dead center and C in Fig. 4 shows an example in which plural entrances are formed. The plural entrances are preferable for uniformity of the liquid flow inside the cylinder when the spraying is stopped.

[0055] The details of the spray coating device 7 will be explained when the spraying is stopped, referring to Fig. 5.

[0056] Fig. 5 is a schematic view illustrating an embodiment of the spray coating device of the present invention.

[0057] The fine-particle dispersion liquid 2 is supplied to the coating nozzle 21 through the liquid feeding route 4 connected to a liquid-feed opening 24. Then the dispersion liquid 2 fills the inside of the coating nozzle 21, and returns to the coating liquid tank 1 through the circulation route 8 connected to a circulation exit 25.

[0058] When the spraying is stopped, an atomized-liquid discharger 23 is closed by a needle valve 26. The coating nozzle 21 may be a single piece with a spray gun 27, or may be detachably equipped with the spray gun 27 by a threaded type method.

[0059] In addition, the needle valve 26 is preferably structured such that the liquid leak is prevented by a gasket (not shown) at the rear end of the coating nozzle (the opposite side of the atomized-liquid discharger 23).

[0060] Next, the operation when the spraying is performed will be explained.

[0061] When the pressurizer for circulation 3 is stopped and the liquid circulation is blocked by the one or more liquid-pressure blockers 5 such as valves located on the liquid feeding route 4 and the circulation route 8 respectively, the liquid supplier 6 and the spray coating device 7 become independent of the circulating system.

[0062] At this point, the preferable operation order of the one or more liquid-pressure blockers 5 such as valves located on the liquid feeding route 4 and the circulation route 8 respectively is as follows:

the liquid-pressure blockers 5 such as valves located on the liquid feeding route 4 in the upstream of the circulation are activated first to block the liquid pressure; and

the liquid-pressure blockers 5 such as valves located on the circulation route 8 in the downstream of the circulation are activated.

[0063] Thus, the residual pressure given by the pressurizer for circulation 3 retained between the one or more liquid-pressure blockers 5 located on the liquid feeding route 4 and the circulation route 8 respectively is released.

[0064] The liquid-pressure blockers 5 on the liquid feeding route 4 are preferably located between the pressurizer for circulation 3 and the liquid supplier 6 in view of the liquid-pressure stability.

[0065] In succession, in the liquid supplier 6 in Fig. 2, the piston rod 12 which stayed waiting in the bottom dead center begins to go up by the driving force of a stepping motor 13b through a ball screw 13a. In the cylinder 11, the liquid pressure having very little fluctuation is given to the dispersion liquid 2 filled and supplied in the cylinder when the liquid is circulated. Thus, the small flow rate of the dispersion liquid 2 can be precisely transported to the spray coating device 7 by fixing the lifting speed of the piston rod 12.

[0066] Because of these consecutive operations, the liquid pressure between the liquid-pressure blockers 5 located on the liquid feeding route 4 and the circulation route 8, i.e., the liquid pressure in and between the liquid supplier 6 and the spray coating device 7 equals the liquid pressure given by the liquid supplier.

[0067] Further, in the spray coating device 7 in Fig. 5, the needle valve 26 which closed the atomized liquid discharger 23 opens, and a small quantity of the liquid having very little fluctuation can be discharged from the atomized liquid discharger 23 by the liquid pressure for spray coating given by the liquid supplier 6 to the dispersion liquid 2, regardless of the circulating liquid flow-rate.

[0068] The discharged dispersion liquid 2 is sprayed by the air for atomization supplied to an air cap 22 through another route (not shown). Thus, a thin and uniform dispersed layer having a thickness of from about submicrons to a few microns can be formed by spray coating.

[0069] When the spraying is stopped, the liquid-pressure blockers located on the liquid feeding route 4 and the circulation route 8 are released again, and the pressurizer for circulation 3 begins the circulation. At this point, in the liquid supplier in Fig. 2, the piston rod 12 returns to the bottom dead center and stays waiting.

[0070] As for the relationship between the circulating flow rate when the spraying is stopped and the liquid feeding flow rate when the spraying is performed, when the circulation flow rate when the spraying is stopped is Fa and the liquid feeding flow rate when the spraying is performed is Fb, Fa has to be sufficiently large such that the sedimentation of the fine particles in the liquid supplier 6 and the spray coating device 7 may not be formed, and Fb has to be sufficiently small such that a thin and uniform dispersed layer having a thickness of from about submicrons to a few microns can be formed. In the method for manufacturing an electrophotographic photoreceptor, functional separation to make Fa and Fb different from each other is important such that the following relationship can be satisfied:

[0071] The present invention is an embodiment of the method for manufacturing an electrophotographic photoreceptor.

[0072] In addition, in a continuous long-time manufacturing process, the following relationship between Fa and Fb is particularly preferable:

[0073] Next, cases where the electrophotographic photoreceptors of the present invention are manufactured will be explained, referring to Figs. 6 to 8.

[0074] Fig. 6 is a schematic view illustrating a cross section of a functionally separated electrophotographic photoreceptor, in which an undercoat layer 32, a charge generation layer (CGL) 33, a charge transport layer (CTL) 34 are layered in this order on an electroconductive substrate 31.

[0075] Fig. 7 is a schematic view illustrating a cross section of a functionally separated electrophotographic photoreceptor, in which a charge generation layer (CGL) 33, a charge transport layer (CTL) 34 and a protective layer 35 are layered in this order on an electroconductive substrate 31.

[0076] Fig. 8 is a schematic view illustrating a cross section of a functionally separated electrophotographic photoreceptor, in which an undercoat layer 32, a charge generation layer (CGL) 33, a charge transport layer (CTL) 34 and a protective layer 35 are layered in this order on an electroconductive substrate 31.

[0077] The layered order of the charge generation layer (CGL) 33 and the charge transport layer (CTL) 34 may be reversible. Further, these layers may be a single layer including a charge generation material and a charge transport material.

[0078] As the electroconductive substrate, various known substrates can be used. For example, a metallic drum made of aluminium, copper, nickel, stainless, steel and the like, and substrates such as plastic films, plastic drums, glass drums and papers whose surfaces are applied with an electroconductive treatment can be used. Specific examples of the substrates applied with an electroconductive treatment include such as substrates laminated with a metallic foil; substrates evaporated or sputtered with a metal, an electroconductive oxide and the like; and substrates coated with electroconductive materials such as metallic powders, carbon black, copper iodide and tin oxide, which are optionally coated together with a binder resin. Among these substrates, metallic drums, plastic films applied with an electroconductive treatment and plastic drums can be preferably used.

[0079] The undercoat layer 32 typically includes a resin as a main component. Considering that the photosensitive layer is coated on the undercoat layer using a solvent, the resin preferably has good solvent resistance against the organic solvent used for coating the photosensitive layer.

[0080] Specific examples of such resins include watersoluble resins such as polyvinylalcohol, casein, polyacrylic natrium; alcohol-soluble resins such as nylon copolymer and nylon methoxymethylate; and cured resins forming three-dimensional network structures such as polyurethane, melamine resins, alkyd-melamine resins and epoxy resins.

[0081] In addition, fine particles of a metal oxide such as titanium oxide, silica, alumina, zirconium oxide, tin oxide and indium oxide or of a metal sulfide and metal nitride, or of these fine particles applied with electroconductive treatments may be optionally included in the undercoat layer. Or, organic fine particles such as fluorocarbon resins, silicone resins, acryl resins, melamine resins can be used.

[0082] The undercoat layer 32 can be formed by a dip coating method, a spray coating method, a bead coating method, etc. The thus formed undercoat layer 32 preferably has a thickness of form about 0.1 to 15 µm, and more preferably from 0.3 to 5 µm.

[0083] In case of spray coating, the manufacturing method of the present invention can be used to form the undercoat layer 32.

[0084] Namely, the fine-particle dispersion liquid 2 including at least a solvent and fine particles for use in the present invention is formed by selecting from the above-mentioned fine particles for the undercoat layer 32 and the group of resin materials, and properly dispersing the materials with a ball mill, an attritor and a sand mill, etc.

[0085] Next, the charge generation layer 33 will be explained.

[0086] The charge generation layer 33 includes a charge generation material as a main component, and a binder resin can be optionally used in the charge generation layer. As the charge generation material, inorganic materials or organic materials can be used.

[0087] Specific examples of the inorganic materials include crystallized selenium, amorphous selenium, selenium-tellurium, selenium-tellurium-halogen, selenium-arsenic compound, amorphous silicon, etc. As for the amorphous silicon, amorphous silicon formed from a dangling bond terminated with a hydrogen atom or a halogen atom, and amorphous silicon formed from a doped boron atom, a phosphorous atom, etc. are preferably used.

[0088] On the other hand, as the organic material, known materials can be used. For examples, phthalocyanine pigments such as metal phthalocyanine and metal-free phthalocyanine, azulenium salt pigments, squaric acid methine pigments, azo pigments having carbazole skeletons, azo pigments having triphenylamine skeletons, azo pigments having diphenylamine skeletons, azo pigments having dibenzothiophene skeletons, azo pigments having fluorenone skeletons, azo pigments having oxadiazole skeletons, azo pigments having bisstilbene skeletons, azo pigments having distyryloxadiazole skeletons, azo pigments having distyrylcarbazole skeletons, perylene pigments, anthraquinone or polycyclic quinone pigments, quinoneimine pigments, diphenylmethane and triphenylmethane pigments, benzoquinone and naphthoquinone pigments, cyanine and azomethine pigments, indigoid pigments, bisbenzimidazole pigments and the like pigments can be used.

[0089] These charge generation materials may be used alone or in combination.

[0090] Specific examples of the binder resins optionally used for the charge generation layer 33 include polyamide, polyurethane, epoxy resins, polyketone, polycarbonate, silicone resins, acryl resins, polyvinylbutyral, polyvinylformal, polyvinylketone, polystyrene, poly-N-vinylcarbazole, polyacrylamide and the like resins. These binder resins can be used alone or in combination.

[0091] In addition, a charge transport material may be optionally included in the charge generation layer. Besides the above-mentioned binder resins, a high-molecular weight charge transport material may be used as the binder resin for the charge generation layer 33.

[0092] As a method for forming the charge generation layer 33, a vacuum thin-film manufacturing method, a casting method from liquid-solution dispersion methods, etc. can be used.

[0093] Specific examples of the vacuum thin-film manufacturing method include a vacuum-deposition method, a glow-discharge polymerization method, an ion-plating method, a sputtering method, a reactive sputtering method, CVD method, etc.

[0094] In order to form the charge generation layer 33 by the casting method, the above-mentioned materials are optionally dispersed with a binder resin by a ball mill, an attritor, a sand mill, etc. using a solvent such as tetrahydrofuran, cyclohexane, dioxane and butanone; and the dispersion liquid properly diluted is coated to form the charge generation layer.

[0095] A dip coating method, a spray coating method, a bead coating method, etc. can be used for coating the charge generation layer.

[0096] The thus formed charge generation layer 33 preferably has a thickness of from about 0.01 to 5 µm, and more preferably from 0.05 to 2 µm.

[0097] In case of using the spray coating method, the manufacturing method of the present invention can be used for forming the charge generation layer 33.

[0098] Namely, the fine-particle dispersion liquid 2 including at least a solvent and fine particles for use in the present invention is formed by selecting from the above-mentioned fine particles of the charge generation materials and the group of resin materials as the components, and properly dispersing the materials with a ball mill, an attritor and a sand mill, etc.

[0099] Next, the charge transport layer 34 will be explained.

[0100] The charge transport layer 34 includes a charge transport material and a binder resin. The charge transport material and the binder resin are properly dissolved or dispersed in a solvent, and this is coated and dried to form the charge transport layer. Optionally, a plasticizer, an antioxidant, a leveling agent, etc. may be included in the charge transport layer besides the charge transport material and the binder resin.

[0101] As the charge transport material, positive-hole transport materials and electron transport materials are available.

[0102] As the electron transport materials, known materials can be used. Specific examples of the electron transport materials include electron acceptors such as chloranil, bromanil, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenon,2,4,5,7-7-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-dioxides.

[0103] These electron transport materials can be used alone in combination.

[0104] As the positive-hole transport material, the following known electron-releasing materials can be used. For example, oxazole derivatives, oxadiazole derivatives, imidazole derivatives, triphenylamine derivatives, 9-(p-diethylaminostyrylanthracene),1,1-bis-(4-dibenzilaminophenyl)propane, styrylanthracene, styrylpyrazoline, phenylhydrazone, α-phenylstilbene derivatives, thiazole derivatives, triazole derivatives, phenazine derivatives, acridine derivatives, benzofuran derivatives, benzimidazole derivatives and thiophene derivatives, etc. can be used. These positive-hole transport materials can be used alone in combination.

[0105] In addition, as a high-molecular-weight charge transport material, the following materials are available when classified by their structures.

(a) Polymers having carbazole rings

[0106] For example, poly-N-vinylcarbazole, and the compounds disclosed in Japanese Laid-Open Patent Publications Nos. 50-82056, 54-9632, 54-11737,4-175337,4-183719 and 6-234841 are available.

(b) Polymers having hydrazone structures

[0107] For example, the compounds disclosed in Japanese Laid-Open Publications Nos. 57-78402, 61-20953, 61-296358, 1-134456, 1-179164, 3-180851, 3-180852, 3-50555, 5-310904 and 6-234840 are available.

(c) Polysilylene polymers

[0108] For example, the compounds disclosed in Japanese Laid-Open Publications Nos. 63-285552, 1-88461, 4-264130, 4-264131,4-264132, 4-264133 and 4-289867 are available.

(d) Polymers having triarylamine structures

[0109] For example, N,N-bis(4-methylphenyl)-4-aminopolystyrene, and the compounds disclosed in Japanese Laid-Open Publications Nos. 1-134457, 2-282264, 2-304456, 4-133065, 4-133066, 5-40350 and 5-202135 are available.

(e) Other polymers

[0110] For example, formaldehyde condensation polymer of nitropyrene, and the compounds disclosed in Japanese Laid-Open Publications Nos. 51-73888, 56-150749, 6-234836 and 6-234837 are available.

[0111] Specific examples of the binder resins for use in the charge transport layer 34 include vinyl polymers such as polyvinylbutyral, polyvinylacetal, polyester, polycarbonate, polystyrene, polyestercarbonate, polysulfone, polyimide, polymethylmethacrylate and poly vinyl chloride, and their copolymers; and resins such as phenoxy resins, epoxy resins and silicone resins, or partially crosslinked resins thereof. These resins can be used alone or in combination.

[0112] The charge transport layer preferably has a thickness of from about 5 to 100 µm, and more preferably from 10 to 30 µm. An antioxidant and a plasticizer used for a rubber, a plastic, a fat, etc. can be included in the charge transport layer 34.

[0113] A leveling agent can be included in the charge transport layer 34. As the leveling agent, silicone oils such as a dimethyl silicone oil and a methyl phenyl silicone oil; and a polymer or an oligomer having a perfluoroalkyl group can be used. The content of the leveling agent is preferably from'0 to 1 part by weight per 100 parts by weight of the binder resin.

[0114] Fine particles can be included in the charge transport layer 34. Specific examples of the fine particles include inorganic fine particles such as titanium oxide, silica, tin oxide, aluminum oxide, zirconium oxide, indium oxide, silicon nitride, calcium oxide, zinc oxide and barium sulfate; or organic fine particles such as fluorocarbon resins, silicone resins, acryl resins and melamine resins.

[0115] A dip coating method, a spray coating method, a bead coating method, etc. can be used for coating the charge transport layer.

[0116] When a dispersion liquid including fine particles is coated by the spray coating method, the manufacturing method of the present invention can be used for forming the charge transport layer 34.

[0117] Namely, the fine-particle dispersion liquid 2 including at least a solvent and fine particles for use in the present invention is formed by selecting from the above-mentioned fine particles of the charge transport materials and the group of resin materials as the components, and properly dispersing the materials with a ball mill, an attritor and a sand mill, etc.

[0118] Next, the protective layer 35 will be explained.

[0119] The protective layer includes fine particles and a binder resin, and a charge transport material can be optionally included in the protective layer.

[0120] Specific examples of the fine particles include inorganic fine particles such as titanium oxide, silica, tin oxide, aluminum oxide, zirconium oxide, indium oxide, silicon nitride, calcium oxide, zinc oxide and barium sulfate; or organic fine particles such as fluorocarbon resins, silicone resins, acryl resins and melamine resins.

[0121] The surface of these fine particles may be treated with an inorganic or an organic substance for the purpose of increasing the dispersibility. Specific examples of the surface treatment include a water-repellent treatment such as a silane-coupling agent treatment, a fluorochemical silane-coupling agent treatment and a higher fatty-acid treatment. As the inorganic substance treatment, a filler whose surface is treated with alumina, zirconia, tin oxide and silica can be used.

[0122] Specific examples of the binder resins include vinyl polymers such as polyvinylbutyral, polyvinylacetal, polyester, polycarbonate, polystyrene, polyestercarbonate, polysulfone, polyimide, polymethylmethacrylate and poly vinyl chloride, and their copolymers; and resins such as phenoxy resins, epoxy resins and silicone resins, or partially crosslinked resins thereof. These resins can be used alone or in combination.

[0123] The same charge transport material which can be included in the charge transport layer 34 can be optionally included in the protective layer.

[0124] The protective layer 35 can be formed by a dip coating method, a spray coating method, a bead coating method, etc. The thus formed protective layer 35 preferably has a thickness of from about 0.1 to 20 µm, and more preferably from 0.5 to 10 µm.

[0125] In case of using the spray coating, the manufacturing method of the present invention can be used for forming the protective layer 35.

[0126] Namely, the fine-particle dispersion liquid 2 including at least a solvent and fine particles for use in the present invention is formed by selecting from the above-mentioned fine particles for the protective layer 35 and the group of resin materials as the components, and properly dispersing the materials with a ball mill, an attritor and a sand mill, etc.

[0127] Various solvents may be used for the coating liquids forming the above-mentioned each layer 32 to 35 of an electrophotographic photoreceptor. Specific examples of the solvent include ethers such as diethylether, dimethoxymethane, tetrahydrofuran and 1,2-dimethoxyethane; carbon hydrides such as toluene and xylene; ketones such as acetone, methyl ethyl ketone and cyclohexanone; esters such as methyl acetate and ethyl acetate; and alcohols such as methanol, ethanol and propanol. These solvents can be used alone or in combination.

[0128] Having generally described this invention, further understanding can be obtained by reference to certain specific examples which are provided herein for the purpose of illustration only and are not intended to be limiting. In the descriptions in the following examples, the numbers represent weight ratios in parts, unless otherwise specified.

EXAMPLES

[0129] The following dispersion liquids having the respective components were prepared to form an electrophotographic photoreceptor.

Dispersion liquid A for coating an undercoat layer

[0130] The following materials were dispersed with a ball mill for 100 hrs. to prepare a dispersion liquid A.

Alkyd resin (tradename Bekkosol 1307-60-EL from Dainippon Ink & Chemicals, Inc.)	5
Melamine resin (tradename Super Bekkamin G-821-60 from Dainippon Ink & Chemicals, Inc.)	5
Titanium oxide (tradename CREL from Ishihara Sangyo Kaisha Ltd.)	10
Methyl ethyl ketone	50
Cyclohexanone	30

Dispersion liquid B for coating a charge generation layer

[0131] The following materials were dispersed with a ball mill for 72 hrs. to prepare a dispersion liquid B.

Oxotitaniumphthalocyanine pigment	2
Polyvinylbutyral (tradename XYHL from UCC)	0.2
Tetrahydrofuran	132
Cyclohexanone	88

Dispersion liquid C1 for coating a charge transport layer

[0132] The following materials were dispersed with a ball mill for 72 hrs. to prepare a dispersion liquid C1.

Liquid solution C2 for coating a charge transport layer

[0133] The following materials were dissolved to prepare a liquid solution C2.

Tetrahydrofuran liquid solution including 200 parts of tetrahydrofuran, 135 parts of cyclohexanone and 1 % of silicone oil (tradename KF50 from Shin-Etsu Silicone Co., Ltd.)	1
Charge transport material having the Formula (I)	6
Polycarbonate resin (tradename Z polyca from Teijin Chemicals Ltd. having a viscosity-average molecular weight of 50,000)	10

Dispersion liquid D for coating a protective layer

[0134] The following materials were dispersed with a ball mill for 24 hrs. to prepare a dispersion liquid D.

Titanium oxide (tradename CREL from Ishihara Sangyo Kaisha Ltd.)	2
Polycarbonate resin (tradename Z polyca from Teijin Chemicals Ltd. having a viscosity-average molecular weight of 50,000)	6
Tetrahydrofuran	190
Cyclohexanone	70

Example 1

[0135] By the circulation-type spray coating method of the present invention shown in Fig. 1, the dispersion liquid A was coated by spraying on a rotating aluminium cylinder having a diameter of 30 mm to form an undercoat layer 32. When the spraying is performed, the spray coating device 7 was located 100 mm apart from the rotating aluminium cylinder and sprayed the cylinder reciprocating in the axial direction of the cylinder at a predetermined speed. The flow rate of the liquid from the dispersion liquid supplier 6 to the spray coating device 7 was 10 cc/min. when the spraying is performed.

[0136] After the spray was stopped, the cylinder was dried to the touch while rotating for about 20 min. Then, the rotation was stopped, and the cylinder was taken out to be dried in a drier. The thickness of the undercoat layer 32 was 5 µm after dried.

[0137] While the cylinder was dried to the touch, and was taken off and put on after the spray was stopped, the dispersion liquid was circulated by the method of the present invention. The flow rate of the liquid circulated by the pressurizer for liquid circulation 3 was 800 cc/min.

[0138] Next, another aluminium cylinder having a diameter of 30 mm was set to be sprayed, which was 30 min. after the spray for the first cylinder was stopped.

[0139] These operations were repeated 10 times to prepare 10 sample rolls.

[0140] Then, spray and stop were repeated 10 times respectively for forming a charge generation layer 33 with the dispersion liquid B, forming a charge transport layer with the liquid solution C2 and forming a protective layer 35 with the dispersion liquid D in this order to prepare 10 photoreceptors. The tenth prepared photoreceptor was a photoreceptor sample of Example 1.

[0141] The flow rate of the liquid from the dispersion liquid supplier 6 to the spray coating device 7 was 4 cc/min. when forming the charge generation layer 33 with the dispersion liquid B, 42 cc/min. when forming the charge transport layer 34 with the liquid solution C2 and 15 cc/min. when forming the protective layer 35. The thickness of the charge generation layer 33 was 0.1 µm, that of the charge transport layer was 20 µm and that of the protective layer was 3 µm after dried.

[0142] When changing each liquid, the cylinder 11 in the dispersion liquid supplier 6 and the coating nozzle 21 in the spray coating device 7 were disassembled to find no deposition formed by sedimentation of the dispersed fine particles. In addition, no clogging was found at the tip of the coating nozzle 21.

[0143] In addition, each liquid discharged from the spray coating device 7 was collected in a bottle when starting to spray the tenth photoreceptor. Each collected liquid was a liquid sample of Example 1.

[0144] The concentration of the fine particles in each liquid was determined as follows:

(1) the weight of each liquid was measured;

(2) dissolved components such as resins were sufficiently removed with tetrahydrofuran;

(3) fine particles were separated with a suction filter;

(4) solvent components were dried; and then

(5) the weight of the fine particles was measured.

[0145] As a reference, the concentration of the fine particles in each dispersion liquid after the liquid was prepared was measured by the same method mentioned above. The results are shown in Table 1. The fine-particle concentrations of the liquid samples of Example 1 scarcely changes compared with those of the references. This is appraisable as an alternative property proving that the reproducibility of the content ratio of the fine particles in the formed layers is good, compared with that of the fine particles in the solid contents included in the dispersion liquids.

Comparative Example 1

[0146] The procedure for preparation of 10 photoreceptors in Example 1 was repeated except that the liquid circulation was stopped with the liquid-pressure blockers 5 to retain the dispersion liquid in the liquid supplier 6 and the spray coating device 7 when the spray was stopped. The tenth prepared photoreceptor was a photoreceptor sample of Comparative Example 1.

[0147] When the cylinder 11 in the dispersion liquid supplier 6 and the coating nozzle 21 in the spray coating device 7 were disassembled in changing each liquid, a deposition formed by sedimentation of the dispersed fine particles was observed. In addition, a clogging was found at the tip of the coating nozzle 21.

[0148] In addition, each liquid discharged from the spray coating device 7 was collected in a bottle when starting to spray the tenth photoreceptor. Each collected liquid was a liquid sample of Comparative Example 1. The procedure for determination of the fine-particle concentration of the liquids in Example 1 was repeated to determine that of the liquids of Comparative Example 1. The results are shown in Table 1. Compared with those of the liquid samples in Example 1, the fine-particle concentrations of the liquids decrease. This is appraisable as an alternative property proving that the reproducibility of the content ratio of the fine particles in the formed layers is poor, compared with that of the fine particles in the solid contents included in the dispersion liquids.

Table 1

Fine-Particle Concentration (wt. %)
	Liquid samples when prepared	Liquid samples in Example 1	Liquid samples in Comparative Example 1
Dispersion liquid A	9.98 %	9.97 %	6.52 %
Dispersion liquid B	0.91 %	0.90 %	0.62 %
Dispersion liquid D	0.75 %	0.74 %	0.15 %

Example 2

[0149] The procedure for preparation of 10 photoreceptors in Example 1 was repeated except for using the dispersion liquid C1 instead of the dispersion liquid C2. The tenth prepared photoreceptor was a photoreceptor sample of Example 2.

[0150] When changing each liquid, the cylinder 11 in the dispersion liquid supplier 6 and the coating nozzle 21 in the spray coating device 7 were disassembled to find no deposition formed by sedimentation of the dispersed fine particles.
In addition, no clogging was found at the tip of the coating nozzle 21. In addition, each liquid discharged from the spray coating device 7 was collected in a bottle when starting to spray the tenth photoreceptor. Each collected liquid was a liquid sample of Example 2. The procedure for determination of the fine-particle concentration of the liquids in Example 1 was repeated to determine that of the liquids of Example 2. The results are shown in Table 2. The fine-particle concentrations of the liquid samples of Example 2 scarcely changes compared with those of the references. This is appraisable as an alternative property proving that the reproducibility of the content ratio of the fine particles in the formed layers is good, compared with that of the fine particles in the solid contents included in the dispersion liquids.

Comparative Example 2

[0151] The procedure for preparation of 10 photoreceptors in Example 2 was repeated except that the liquid circulation was stopped with the liquid-pressure blockers 5 to retain the dispersion liquid in the liquid supplier 6 and the spray coating device 7 when the spray was stopped. The tenth prepared photoreceptor was a photoreceptor sample of Comparative Example 2.

[0152] When the cylinder 11 in the dispersion liquid supplier 6 and the coating nozzle 21 in the spray coating device 7 were disassembled in changing each liquid, a deposition formed by sedimentation of the dispersed fine particles was observed. In addition, a clogging was found at the tip of the coating nozzle 21.

[0153] In addition, each liquid discharged from the spray coating device 7 was collected in a bottle when starting to spray the tenth photoreceptor. Each collected liquid was a liquid sample of Comparative Example 2. The procedure for determination of the fine-particle concentration of the liquids in Example 1 was repeated to determine that of the liquids of Comparative Example 2. The results are shown in Table 2. Compared with those of the liquid samples in Example 2, the fine-particle concentrations of the liquids decrease. This is appraisable as an alternative property proving that the reproducibility of the content ratio of the fine particles in the formed layers is poor, compared with that of the fine particles in the solid contents included in the dispersion liquids.

Table 2

Fine-Particle Concentration (wt. %)
	Liquid samples when prepared	Liquid samples in Example 2	Liquid samples in Comparative Example 2
Dispersion liquid A	9.98 %	9.97 %	5.66 %
Dispersion liquid B	0.91 %	0.88 %	0.68 %
Dispersion liquid C1	0.44 %	0.43 %	0.32 %

[0154] Next, each photoreceptor sample prepared in Examples 1 and 2,and Comparative Examples 1 and 2 was equipped with a copier model No. MF200 from Ricoh Company, Ltd., and the initial copy image was evaluated using a test chart. Further, the copy image after a durability test in which 10,000 copies were produced was evaluated.

[0155] The results are shown in Table 3.

[0156] Both the initial copy images and the copy images after the durability test were good when using the photoreceptors prepared in Examples 1 and 2.

[0157] However, when using the photoreceptors prepared in Comparative Examples 1 and 2, even the initial copy images had low and irregular image density. In addition, the copy images after the durability test had lower image density, and background fouling as well.

Table 3

Image Evaluation Results
	Initial	After durability test in which 10,000 copies were produced
Example 1	Good	Good
Comparative Example 1	Low and irregular image density	Lower image density and background fouling
Example 2	Good	Good
Comparative Example 2	Low and irregular image density	Lower image density and background fouling

[0158] This document claims priority and contains subject matter related to Japanese Patent Applications Nos. 2001-003860, 2001-131076 and 2001-368911, filed on January 11, 2001, April 27, 2001 and December 3, 2001 respectively, incorporated herein by reference.

Claims

1. A liquid spray-coating method comprising:

circulating a liquid from a liquid tank to the liquid tank by a pressurizer through a liquid supplier and a spray coating device; and

spraying the liquid to an object to coat the liquid on the object,

   wherein the liquid is circulated through a liquid feeding route and a liquid circulating route when the spraying is stopped.

2. The liquid spray-coating method of Claim 1, wherein the liquid is circulated in a direction of from the pressurizer to the spray coating device via the liquid supplier when the spraying is stopped.

3. The liquid spray-coating method of Claim 1, wherein the liquid is circulated in a direction of from the spray coating device to the pressurizer via the liquid supplier when the spraying is stopped.

4. The liquid spray-coating method of any one of Claims 1 to 3, wherein the liquid supplier comprises a syringe pump.

5. The liquid spray-coating method of Claim 4, wherein the syringe pump comprises:

a cylinder;

a piston rod comprising:

a piston member capable of being freely moving in the cylinder;

a drive unit for driving the piston rod;

a liquid entrance from which the liquid is fed into the cylinder; and

a liquid exit from which the liquid in the cylinder is discharged,

   wherein the liquid is supplied to the spray coating device by driving the piston rod when the spraying is performed.

6. The liquid spray-coating method of Claim 5, wherein the piston rod further comprises:

a liquid flow-duct through which the liquid fed from the liquid entrance flows to the liquid exit,

   wherein the liquid flows in the cylinder when the spraying is stopped.

7. The liquid spray-coating!method of any one of Claims 1 to 6, wherein the spray coating device is a circulation-type air spray gun comprising:

a nozzle comprising:

a liquid discharger;

a liquid-feeding opening;

an exit for circulating the liquid; and

an air cap,

   wherein the air cap discharges compressed air to discharge the liquid fed from the liquid discharger upon application of pressure while atomizing the liquid, and
   wherein the liquid flows from the liquid-feeding opening to the exit through the spray gun when the spraying is stopped.

8. The liquid spray-coating method of any one of Claims 1 to 7, wherein the liquid feeding route comprises a first liquid-pressure blocker between the liquid tank and the liquid supplier, and the circulating route comprises a second liquid-pressure blocker between the spray coating device and the liquid tank, and
   wherein the spraying is performed while stopping the circulation of the liquid by blocking the liquid pressure by activating the first and second liquid-pressure blockers.

9. The liquid spray-coating method of any one of Claims 1 to 8, wherein when the liquid is flown from the pressurizer to the liquid supplier in a flow rate of Fa cc/min by the pressurizer and is flown from the liquid supplier to the spray coating device in a flow rate of Fb cc/min by the liquid supplier, the following relationship is satisfied:

10. A liquid spray-coating apparatus comprising:

a liquid tank configured to contain a liquid;

a spray coating device configured to spray the liquid;

a liquid supplier configured to supply the coating liquid to the spray coating device;

a liquid feeding route configured to feed the liquid from the tank to the spray coating device via the liquid supplier;

a liquid circulating route configured to feed the liquid from the spray coating device to the liquid tank; and

a pressurizer configured to circulate the liquid from the tank to the tank via the liquid supplier and the spray coating device, through the liquid feeding route and the liquid circulating route.

11. The liquid spray-coating apparatus of Claim 10, wherein the liquid supplier is a syringe pump.

12. The liquid spray-coating apparatus of Claim 11, wherein the syringe pump comprises:

a cylinder;

a piston rod comprising:

a piston member capable of being freely moving in the cylinder;

a drive unit for driving the piston rod;

a liquid entrance from which the liquid is fed into the cylinder; and

a liquid exit from which the liquid in the cylinder is discharged,

   wherein the liquid is supplied to the spray coating device by driving the piston rod when the spraying is performed.

13. The liquid spray-coating apparatus of Claim 12, wherein the piston rod further comprises:

a liquid flow-duct through which the liquid fed from the liquid entrance flows to the liquid exit,

   wherein the liquid flows in the cylinder when the spraying is stopped.

14. The liquid spray-coating apparatus of any one of Claims 10 to 13, wherein the spray coating device is a circulation-type air spray gun comprising:

a nozzle comprising:

a liquid discharger;

a liquid-feeding opening;

an exit for circulating the liquid; and

an air cap,

   wherein the air cap discharges compressed air to discharge the liquid fed from the liquid discharger upon application of pressure while atomizing the liquid, and
   wherein the liquid flows from the liquid-feeding opening to the exit through the spray gun when the spraying is stopped.

15. The liquid spray-coating apparatus of any one of Claims 10 to 14, wherein the liquid feeding route comprises a first liquid-pressure blocker between the liquid tank and the liquid supplier, and the circulating route comprises a second liquid-pressure blocker between the spray coating device and the liquid tank, and
   wherein the spraying is performed while stopping the circulation of the liquid by blocking the liquid pressure by activating the first and second liquid-pressure blockers.

16. An electrophotographic photoreceptor comprising an electroconductive substrate and at least one of an undercoat layer, a charge generation layer, a charge transport layer and a protective layer, said at least one layer containing fine particles and having been produced by the liquid spray-coating method of any one of Claims 1 to 9.

Drawing