ELECTROPHOTOGRAPHIC PHOTORECEPTOR

Abstract: An electrophotographic photoreceptor according to this invention has an electrically conductive support, a charge-generation layer a charge-transport layer and a protective layer which consists essentially of a thermo-setting silicone resin and a polyvinyl acetal resin. The electrophotographic photoreceptor according to this invention has the improved durability in the repeated copying operation due to the inclusion of the polyvinyl acetal resin into the protective layer.

Description: This invention relates to an electrophotographic photoreceptor. More specifically, it relates to a photoreceptor having an improved durability in the repeated copying operation. An electrophotographic photoreceptor has an electrically conductive support and a photosensitive layer formed thereon which includes an inorganic or organic photoconductor. Recently, double layer photoreceptors with the combination of a charge-generation layer and a charge-transport layer have been known to have higher sensitivity and a part of them have been practically employed. Especially, the photoreceptors in which an organic material is used as a charge-transporting medium have high charge acceptance in addition to the higher sensitivity and so they have been highly interested. The photoreceptor is subjected to the repeated copying operation which includes charging by corona charging device, exposing, developing, transferring and cleaning steps and is required to have an excellent durability in the repeated copying operation. When the above-mentioned prior photoreceptors, especially the photoreceptors having the organic charge-transporting medium are subjected to the repeated copying operation ( for example, several thousands to several ten thousands times), however, an abrasion and cracks are observed in the photoreceptor due to the practical loads such as the development with the toner, the friction with paper and/or cleaning means and therefore the printing-resistance is actually limited. The above-mentioned phenomena are mainly caused by the low surface strength of the charge-transport layer. An attempt for increasing the surface strength of the charge-transport layer by the selection of a suitable polymeric binder which is generally included together with a charge-transporting material in the charge-transport layer resulted in failure since a large amount of the charge-transporting material is doped therein. A method for providing a protective layer on the charge-transport layer to improve the surface strength of the photoreceptor has been proposed. In this method, the protective layer is formed by coating a solution in which a thermo-setting silicone resin is dissolved on the charge-transport layer and then setting on heating. However, this protective layer has problems such as the occurance of cracks and cuts as well as an ease separation from the charge-transport layer since the silicone resin protective layer could unsatisfactorily adhere to the charge-transport layer. Further, the protective layer may partially peel off when subjected to the pressure of the cleaning means and the like. A method for providing an adhesive layer between the protective layer and the charge-transport layer to improve the adhesive strength therebetween has been also proposed. This method is not practical since it offers undesirable effects such as an increase of the residual potential and the development of fog by the presence of the adhesive layer. US-A-4 148 637 discloses a photosensitive material for use in electrophotography which comprises a conductive support having a photoconductive layer coated with a protective layer. The protective layer is composed of a film - forming resin, such as polyvinyl butyral or polyvinyl acetate and a silane - coupling agent. Such a photoreceptor has a pure surface strength. Now, there is a strong request for increasing the adhesive strength between the protective layer and the charge-transport layer without any undesirable effects so as to provide an electrophotographic photoreceptor having an improved durability in the repeated copying operation. An object of this invention is to provide an electrophotographic photoreceptor having an improved durability in the repeated copying operation by improving the adhesive strength between the protective layer and the charge-transport layer. According to the invention, this object is achieved by an electrophotographic photoreceptor having an electrically conductive support, a photoconductive layer and a protective layer comprising a polyvinyl acetal resin and an organic silicon compound, characterized in that the photoconductive layer comprises a charge-generation layer and a charge-transport layer, and the protective layer consists essentially of a thermosetting silicone resin as said organic silicon compound and 0.5 to 30 % by weight of said polyvinyl acetal resin based on the total weight of the protective layer. The electrically conductive support is made of a metal material such as aluminum, stainless steel, copper and nickel. Alternatively, the support may be made of an insulating material such as plastic film or paper carrying an electrically conductive layer thereon. The electrically conductive layer includes an electrically conductive substance such as aluminum, copper, palladium, tin oxide and indium oxide. The charge-generation layer in which a photoconductor is included is formed on the support by vapor-depositing or sputtering the photoconductor. The photoconductor may be an inorganic or organic photoconductor. Representative photoconductors include selenium, its alloy, cadmium sulfide, zinc oxide and organic dyes such as phthalocyanine, perylene, indigo, quinacridone, bis-azo compound and their derivatives. Alternatively, the charge-generation layer may be formed on the support by coating a solution in which the photoconductor and optionally a polymeric binder are dispersed. The charge-generation layer has generally a thickness of 0.1 to 1 µm, preferably a thickness of 0.15 to 0.6 µm. A barrier layer may be provided between the support and the charge-generation layer. A representative barrier layer is made of a metal oxide such as aluminum oxide or a resin such as polyamide, polyurethane, cellulose and casein. The charge-transport layer in which a charge-transporting material is included is coated on the charge-generation layer by coating a solution in which the charge-transporting material and optionally the polymeric binder are dispersed. The known charge-transporting materials can be used. Representative charge-transporting agents include heterocyclic compounds such as indole, carbazole, imidazole, oxazole, thiazole, oxadiazole, pyrazole, pyrazoline, thiadiazole, benzoxazole, benzothiazole, benzimidazole and the like; aromatic hydrocarbons such as benzene, naphthalene, anthracene, fluorene, perylene, pyrene, phenylanthracene, styryl anthracene and the like; their substituted derivatives having any substituents such as alkyl, alkoxy, amino or substituted amino groups; the other derivatives such as triarylalkane, triarylamino, chalcone derivatives, hydrazine derivatives, hydrazones and the like; and their polymers such as polyvinyl carbazole, polystyryl anthracene and the like. The known polymeric binders can be used. Representative polymeric binders include homopolymers or copolymers of vinyl compounds such as styrene, vinyl chloride, acrylic or methacrylic esters and the like, phenoxy resin, polyvinyl acetal, polyvinyl butyral, polyester, polycarbonate, cellulose ester, silicone resin, urethane resin, unsaturated polyester and the like, as well as their partially cross-linked cured material. The charge-transport layer may include known additives such as anti-oxidants, sensitizers and the like. The charge-transport layer has generally a thickness of 5 to 40 µm, preferably a thickness of 10 to 30 µm. The protective layer consisting essentially of the thermo-setting silicone resin and the polyvinyl acetal resin is coated on the charge-transport layer. The thermo-setting silicone resin which is included in the protective layer is prepared by subjecting a silane compound to hydrolysis and condensation. In the preparation of the silicone resin, one or more silane compounds selected from dialkoxy dialkyl silane, trialkoxy alkyl silane and tetraalkoxy silane are preferably used since these silane compounds have high reactivities so as to easily set on heating and the resultant protective layer shows very high surface strength. The alkyl or alkoxy group in the silane compound means lower ( generally C₁ ₋ ₄ ) alkyl or alkoxy group. Although a mixture of silane compounds is used in the preparation of the silicone resin, the mixture of trialkoxy alkyl silane and tetraalkoxy silane in which the content of the tetraalkoxy silane is more than 50 % by weight is preferable. The molecular weight of the silicone resin before thermosetting is generally in a range of several hundreds to several hundred thousands. The polyvinyl acetal resin which is included in the protective layer is prepared by subjecting a polyvinyl alcohol resin obtained by partial hydrolysis of polyvinyl acetate to acetal formation. The preferable degree of acetal formation is more than 40 %. Representative polyvinyl acetal resins includes polyvinyl butyral, polyvinyl formal, polyvinyl acetacetal and polyvinyl propylacetal resin, among which the polyvinyl butyral resin is preferred. The polyvinyl acetal resin is included in the protective layer in an amount of 0.5 to 30 % by weight, preferably 3 to 20% by weight based on the total weight of the protective layer. If the amount is below the above range, the increase of the adhesive strength is not satisfactory, while if it is above this range, the natural surface strength is impaired. The protective layer may include a filler for further improving the surface strength such as colloidal silica and/or known additives, in addition to the silicone resin and the polyvinyl acetal resin. The protective layer is formed by dissolving a composition consisting essentially of the thermosetting silicone resin and the polyvinyl acetal resin in a suitable solvent, for example, alcohols such as isobutanol and isopropanol or esters such as ethyl acetate, methyl acetate and methylcellosolve acetate so as to prepare a coating solution, coating the coating solution on the charge-transport layer and then setting on heating. The protective layer has a thickness of 0.1 to 5 µm, preferably a thickness of 0.5 to 2 µm. The electrophotographic photoreceptor according to this invention can be widely applied in the electrophotographic field, for example, in copying machines, printers having laser, CRT or LED as the optical source and the like. Examples The following examples will further describe various preferred embodiments of this invention and includes comparative examples. Parts are by weight unless otherwise specified. Comparative Example 1 Ten parts of bis-azo compound having the following formula: 5 parts of phenoxy resin (PKHH, manufactured by Union Carbide Corp.) and 5 parts of polyvinyl butyral resin ( BH-3, manufactured by Sekisui Chemical Co., Ltd.) were dispersed in 100 parts of tetrahydrofuran with a sand grinder to prepare a coating solution. A cylinder made of planished aluminum was immersed in the thus-prepared solution so that the dry thickness of the charge-generation layer was 0.4 µm. Thus, a charge-generation layer was formed on the support. While, 100 parts of hydrazone compound having the following formula: and 100 parts of polycarbonate resin (Novalex® 7030A, manufactured by MITSUBISHI CHEMICAL INDUSTRIES LTD.) were dissolved in 1000 parts of tetrahydrofuran to prepare a coating solution. The above cylinder was immersed in the thus-prepared solution so that the dry thickness of the charge-transport layer was 20 µm. Thus, a photoreceptor without a protective layer ( sample No. A ) was prepared. Comparative Example 2 A protective layer was coated on the charge-transport layer of the photoreceptor( sample No. A ) by immersing in a coating solution so that the dry thickness of the protective layer was 1 µm and then heating at 130°C for 30 minutes to thermo-setting. The coating solution used was prepared by diluting a silicone resin ( Tosgard 510, mainly containing the condensate obtained after hydrolyzing a mixture of trialkoxy alkyl silane and tetraalkoxy silane, manufactured by TOSHIBA SILICONE CO., LTD.) with isopropanol until the solid matter concentration was 5 %. Thus, a photoreceptor with a protective layer consisting of the silicone resin and a 1 µm thickness ( sample No. B ) was prepared. Example 1 A protective layer was coated on the charge-transport layer of the photoreceptor ( sample No. A ) in the same manner as described in Comparative Example 2, provided that the coating solution was changed. The coating solution used was prepared by diluting the same silicone resin ( Tosgard 510, manufactured by TOSHIBA SILICONE CO., LTD.) with isopropanol until the solid matter concentration was 5 % and adding and dissolving a polyvinyl butyral resin ( Eslex® BL-S, manufactured by Sekisui Chemical Co., Ltd.) in an amount of 5 grams per 1000 grams of the resultant diluted solution. Thus, a photoreceptor with a protective layer consisting essentially of the silicone resin and the polyvinyl butyral resin and having a 1 µm thickness (sample No. C) was prepared. Example 2 A protective layer was coated on the charge-transport layer of the photoreceptor ( sample No. A ) in the same manner as described in Comparative Example 2, provided that the coating solution was changed. The coating solution used was prepared by diluting a silicone resin ( X-12-22, mainly containing the condensate obtained after hydrolyzing trialkoxy alkyl silane, manufactured by Shin-Etsu Chemical Co., Ltd.) with isopropanol until the solid matter concentration was 5 % and adding and dissolving a polyvinyl butyral resin ( Eslex® BL-S, manufactured by Sekisui Chemical Co., Ltd.) in an amount of 5 grams per 1000 grams of the resultant diluted solution. Thus, a photoreceptor with a protective layer consisting essentially of the silicone resin and the polyvinyl butyral resin and having a 0.7 µm thickness ( sample No. D ) was prepared. Comparative Example 3 A photoreceptor ( sample No. E ) was prepared in the same manner as described in Example 2, provided that the addition of the polyvinyl butyral resin was omitted. Example 3 i) The surface strength of each of the photoreceptors ( sample Nos. A to C ) was tested using the pencil. The surface of the photoreceptor ( sample No. B ) was injured by the pencil with the hardness B. On the other hand, the surface of the photoreceptor ( sample No. A or C ) was injured only by the pencil with the hardness more than 4H. As the above results, it was found that the photoreceptor according to this invention has a high surface strength. ii) The adhesive property of the protective layer in each of the photoreceptors was tested by putting a commercial adhesive tape thereon and then peeling it. There are cracks on the protective layer of the photoreceptor ( sample No. B or E ) and the protective layer easily peeled off. On the other hand, a separation of the protective layer from the charge-transport layer in the photoreceptor ( sample No. C and D ) was not observed. As the above results, it was found that the protective layer of the photoreceptor according to this invention firmly adheres. iii) Each of the photoreceptors ( sample Nos. A to C ) was subjected to the 100,000 times copying operations using a commercial copying machine to test the durability. In the photoreceptor ( sample No. A ) a gradual lowering in the density and the surface potential was observed and therefore the resultant copies were not clear. And the thickness of the photosensitive layer was reduced to 6 µm. In the photoreceptor ( sample No. B ) a partial separation of the protective layer was observed and therefore the resultant copies, locally, were not clear. In the photoreceptor ( sample No. C ) a separation of the protective layer and cracks thereon as well as a lowering in the thickness of the photosensitive layer were not observed and therefore the resultant copies could be usually clear. As the above results, it was found that the photoreceptor according to this invention has a high durability in the repeated copying operation. The electrophotographic photoreceptor according to this invention has a protective layer with a high surface strength and a high adhesive strength and therefore the electrophotographic photoreceptor according to this invention has an improved durability in the repeated copying operation.

Claims: An electrophotographic photoreceptor having an electrically conductive support, a photoconductive layer and a protective layer comprising a polyvinyl acetal resin and an organic silicon compound, characterized in that the photoconductive layer comprises a charge-generation layer and a charge-transport layer, and the protective layer consists essentially of a thermosetting silicone resin as said organic silicon compound and 0.5 to 30 % by weight of said polyvinyl acetal resin based on the total weight of the protective layer. The photoreceptor according to claim 1, characterized in that the polyvinyl acetal resin is prepared by subjecting a polyvinyl alcohol resin to acetal formation. The photoreceptor according to claim 2, characterized in that the degree of acetal formation in the polyvinyl acetal resin is more than 40 %. The photoreceptor according to any one of claims 1 to 3, wherein the polyvinyl acetal resin is a polyvinyl butyral, polyvinyl formal, polyvinyl acetacetal or polyvinyl propylacetal resin. The photoreceptor according to claim 4, characterized in that the polyvinyl acetal resin is a polyvinyl butyral resin. The photoreceptor according to claim 1, characterized in that the polyvinyl acetal resin is contained in an amount of 3 to 20 % by weight based on the total weight of the protective layer. The photoreceptor according to claim 1, characterized in that the thermo-setting silicone resin is prepared by subjecting one or more silane compounds selected from dialkoxy dialkyl silane, trialkoxy alkyl silane or tetraalkoxy silane to hydrolysis and condensation. The photoreceptor according to claim 1, characterized in that the protective layer further comprises a filler and/or additives. The photoreceptor according to claim 1, characterized in that the protective layer is formed by dissolving a composition consisting essentially of a thermosetting silicone resin and a polyvinyl acetal resin in a solvent so as to prepare a coating solution, coating the coating solution on the charge-transport layer and then setting by heating. The photoreceptor according to claim 1, characterized in that the protective layer has a thickness of 0.1 to 5 µm. The photoreceptor according to claim 10, characterized in that the protective layer has a thickness of 0.5 to 2 µm. The photoreceptor according to claim 1, characterized in that the support is made of a metal material or an insulating material carrying an electrically conductive layer thereon. The photoreceptor according to claim 1, characterized in that the charge-generation layer includes a photoconductor and optionally a polymeric binder. The photoreceptor according to claim 1, characterized in that the charge-transport layer includes a charge-transporting material and, optionally, a polymeric binder and additives. The photoreceptor according to claim 1, characterized in that a barrier layer is provided between the support and the charge-generation layer.

Inventors: FURUYA, KOHKI A-101 MITSUBISHIKASEI-SAKURADAI-APAT, NOZOMI, MAMORU, OTSUKA, SHIGENORI
Applicants: MITSUBISHI KASEI CORPORATION
Application year: 1986
Main classification: G03G5
IPC codes: G03G5