Electrophotographic photosensitive element

Abstract: An electrophotographic photosensitive element comprising an electrically conductive substrate having formed, first, a charge transfer layer, second, a charge generating layer, and, third, a surface protective layer. The charge transfer layer contains a butadiene derivative and/or a hydrazone compound. The charge generating layer contains a hydrazone compound as a charge transfer material in addition to a charge generating material. The charge generating layer is formed by coating the charge transfer layer with an alcoholic solvent in which is dissolved the coating composition of the charge generating layer. The surface protective layer is formed on the surface of the charge generating layer.

Description: Field Of The Invention This invention relates to an electrophotographic photosensitive element which is suitably used for an image-forming apparatus such as a copying machine. BACKGROUND OF THE INVENTION Recently, a photosensitive element has been proposed which has a wide variation in function design, particularly such as a lamination type positive-charged electrophotographic photosensitive element which and has a charge generating layer (CGL) comprising a charge generating material which generates electrostatic charges by. light irradiation and a charge transfer layer which contains a charge transfer material for transferring the charges formed in the charge generating layer. In such a lamination type photosensitive element, electrostatic charges (holes) formed by the action of light near the surface of the charge generating layer travel through the charge generating layer and into the charge transfer layer to form electrostatic latent images. An example of such an electrophotographic photosensitive element is disclosed in JP-A-2-222960. (The term "JP-A" as herein used means an "unexamined published Japanese patent application".) The electrophotographic photosensitive element comprises an electrically conductive substrate upon which is laminated, first, a charge transfer layer and, second, a charge generating layer, wherein the charge transfer layer contains a butadiene derivative represented by general formula (A); wherein A₁ to A₄ each represents an aryl group which may have a substituent and a hydrazone compound such as, e.g., 4-(N,N-diethylamino)benzaldehyde-N,N-diphenylhydrazone and 4-(N,N-dimethylamino) benzaldehyde-N,N-diphenylhydrazone, and the charge generating layer is formed by coating the conductive substrate with a charge transfer material dissolved in an alcohol series solvent. The hydrazone compound has been used before as a charge transfer material. But the use of the hydrazone compound with the butadiene derivative shown by formula (I) described above has the following advantages. (a) Since each of the hydrazone compounds has a far lower melting point than the butadiene derivative and has good compatibility with a binder resin, the hydrazone compound functions as a plasticizer and stabilizes the compatible state of the butadiene derivative. Accordingly, the crystallization of the butadiene derivative in the coating composition of the charge generating layer can be prevented along with the occurrence of cracks. (b) Each of the hydrazone compounds has a solubility in the range of 0.1 to 2% in an alcoholic solvent. The hydrazone compounds themselves have a charge transfer faculty. When an alcoholic solvent is used as a solvent for the coating composition coating for a charge transfer layer, hydrazone compound dissolves and diffuses into the charge generating layer. By this means, the injection of charges from the charge generating layer into the charge transfer layer is smoothly conducted. Thus, the sensitivity of the photosensitive element is thereby improved. In the conventional electrophotographic photosensitive element as described above, when the amount of the hydrazone compound contained in the charge transfer layer is increased by increasing the amount of the hydrazone compound diffusing into the charge generating layer, the sensitivity of the photosensitive element is thereby increased. But, not only is there a limit on the amount of the hydrazone compound that can be dissolved in the charge generating layer, there is also the problem that when too much hydrazone compound is added to the charge transfer layer, the glass transition point of the charge transfer layer is lowered, thereby reducing the mechanical strength of the layer. Alternatively, it is also possible to increase the dissolving amount of the hydrazone into the charge generating layer without increasing the content of the hydrazone compound in the charge transfer layer by using a solvent in which the hydrazone is more soluble than in an alcoholic solvent. However, when such a solvent is used, a crystallization (eutectic crystallization) occurs by a so-called solvent shock and resultant cracking causes swelling of the charge transfer layer and chapping of the interface, which results in a defective image. SUMMARY OF THE INVENTION The present invention has been made to solve the problems discussed above. The object of this invention is to provide an electrophotographic photosensitive element having a long life and that is both highly stable and highly sensitive. It has now been discovered that the object can be attained by the present invention as shown below. According to this invention, there is provided an electrophotographic photosensitive element comprising an electrically conductive substrate having formed, first, a charge transfer layer, second, a charge generating layer, and, third, a surface protective layer, wherein said charge transfer layer contains a butadiene derivative represented by formula (I): wherein Ar₁, Ar₂, Ar₃ and Ar₄ each represents an aryl group with a substituent and/or a hydrazone compound represented by formula (II): wherein R₁, R₂, and R₃ each represents an alkyl group having from 1 to 5 carbon atoms or an alkoxy group having from 1 to 2 carbon atoms; R₄ and R₅ each represents an alkyl group having from 1 to 5 carbon atoms or a phenyl group; and p₁, p₂ and p₃ each represents an integer of from 0 to 2 and said charge generating layer contains the hydrazone compound represented by the above formula (II) as a charge transfer material in addition to a charge generating material and a binder resin, and wherein the content of said hydrazone compound to said binder resin contained in said charge generating layer is between 0.1 and 100 parts hydrazone to 100 parts binder resin. The charge generating layer is formed by coating the charge transfer layer with an alcoholic solvent in which is dissolved the coating composition of the charge generating layer. A surface protective layer is formed on the surface of the charge generating layer, which serves as a protective layer to increase the abrasion resistance. It also lengthens the life of the electrophotographic photosensitive element and decreases the thickness required of the charge generating layer. Since the charge generating layer of the electrophotographic photosensitive element also contains the hydrazone compound shown by aforesaid formula (II), which is a charge transfer material, the charges (holes) generated by the action of light near the surface of the charge generating layer are quickly transferred to the charge transfer layer, thereby increasing the sensitivity. It is therefore unnecessary in this invention to incorporate large amounts of the hydrazone compound into the charge transfer layer to increase the sensitivity. There is thus no need to risk causing the lowering of the glass transition point or the reduction of the mechanical strength of the charge transfer layer by adding too much of the hydrazone compound. It is also unnecessary to use a solvent in which the hydrazone compound is soluble during the formation of the charge generating layer. Also, in this invention, since the thickness of the charge generating layer can be reduced, the probability of having such structural traps as thermal carriers and holes becomes very low, so that a stable photosensitive element can be designed. DETAILED DESCRIPTION OF THE INVENTION The charge transfer layer in this invention is formed by coating a conductive substrate with a coating composition comprising a charge transfer material, a binder resin, and a solvent and allowing the wet layer to dry. Similarly, the charge generating layer is formed by coating a charge transfer layer coated conductive substrate with a coating composition comprising a charge generating material, a charge transfer material, a binder resin, and a binder on the charge transfer layer. The layer is then dried. Also, the surface protective layer is formed by coating the already twice layered conductive substrate with a coating composition comprising an electric conductivity imparting agent and a binder on the charge generating layer and setting or hardening the layer. The charge transfer material being incorporated into the charge transfer layer is composed of the butadiene derivative represented by the formula (I) and/or the hydrazone compound represented by the formula (II). Preferred examples of Ar₁ to Ar₄ in the formula (I) include wherein R' represents a methyl group or an ethyl group. Specific examples of the butadiene derivatives include those disclosed in JP-A-62-30255. Preferred examples include Compounds (III) and (IV) shown below. The Compound (III) is more preferred as the butadiene derivative used in the present invention. Since the compound (III) has a plane structured electron system and also has a structure in which two phenyl groups are bonded to the 1-position of the butadiene and two 4-N,N-diethylaminophenyl groups are bonded to the 4-position of the butadiene, there are advantages in that the compound is largely polarized and has very excellent charge transfer capabilities. But, there are disadvantages in that the compound is not very soluble. Accordingly, when such a butadiene derivative is used together with the hydrazone compound, the disadvantages of the butadiene derivative are minimized and an electrophotographic photosensitive element having a high sensitivity is attained. R₁, R₂ and R₃ in the formula (II) each preferably represents a methyl group, an ethyl group, a methoxy group or an ethoxy group. As the preferred hydrazone compounds represented by the formula (II), 4-(N,N-diethyl-amino)benzaldehyde-N,N-diphenylhydrazone or 4-(N,N-dimethylamino)benzaldehyde-N,N-diphenylhydrazone can be used. These compounds have a relatively high solubility in alcoholic solvents as compared with other charge transfer materials and also have the nearest oxidation potential to the oxidation potential of the butadiene derivative shown by the formula (I). If there is a pronounced difference in oxidation potential between two charge transfer materials, trapped charges occurs. The ratio by weight of the hydrazone compound to the butadiene derivative, is preferably between 10 and 300 parts, more preferably between 40 to 200 parts, of the hydrazone compound to 100 parts of the butadiene derivative. If the content of the hydrazone compound is larger than this range, the sensitivity of the electrophotographic photosensitive element is not improved and if the content is less than this range, it is difficult for charge transfer to occur between the charge transfer layer and the charge generating layer. This difficulty of charge transfer deteriorates the sensitivity and also causes crystallization and cracking to occur in the charge transfer layer. Thus, the presence of the hydrazone compound outside this range is undesirable in this invention. The compounds of formula (I) and/or formula (II) as charge transfer materials can also be used in combination with other conventionally known charge transfer materials. Examples of such conventionally known charge transfer materials which may be used are fluorenone series compounds such as tetracyanoethylene, 2,4,7-trinitro-9-fluorenone, etc.; nitro compounds such as 2,4,8-trinitrothioxanthone, dinitroanthracene, etc.; acid anhydrides such as succinic anhydride, maleic anhydride, dibromomaleic anhydride, etc,; oxadiazole series compounds such as 2,5-di-(4-dimethylaminophenyl)-1,3,4-oxadiazole, etc.; styryl series compounds such as 9-(4-diethylaminostyryl) anthracene, etc.; carbazole series compounds such as polyvinylcarbazole, etc.; pyrazoline series compounds such as 1-phenyl-3-(p-dimethylaminophenyl)pyrazoline, etc.; amine derivatives such as 4,4',4"-tris(N,N-diphenylamino)triphenylamine, 4,4'-bis[N-phenyl-N-(3-methylphenyl)amino]diphenyl, etc.; conjugated unsaturated compounds; hydrazole series compounds; nitrogen containing cyclic compounds such as indole series compounds, oxazole series compounds, iso-oxazole series compounds, thiazole series compounds, thiadiazole series compounds, imidazole series compounds, pyrazole series compounds, triazole series compounds, etc.; and condensed polycyclic compounds. These charge transfer materials may be used singly or as mixtures thereof. The ratio of the total content of the charge transfer materials to the binder resin for the charge transfer layer is preferably between 20 and 200 parts, more preferably between 50 to 200 parts, of charge transfer materials to 100 parts of binder resin of the charge transfer layer. If the total amount of the charge transfer materials is below this range, the charge transfer faculty becomes insufficient. If the total amount is above this range, the mechanical strength of the charge transfer layer is reduced. Examples of the solvent which can be used for mixing the charge transfer material(s) with the binder resin are alcohols such as methanol, ethanol, propanol, isopropanol, butanol, etc.; cellosolves such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, etc.; esters such as ethyl acetate, methyl acetate, etc.; ketones such as acetone, methyl ethyl ketone, cyclohexanone, etc.; aliphatic hydrocarbons such as n-hexane, octane, cyclohexane, etc.; aromatic hydrocarbons such as benzen, toluene, xylene, etc.; halogenated hydrocarbons such as dichloroethane, carbon tetrachloride, methylene chloride, chlorobenzene, etc.; ethers such as dimethyl ether, diethyl ether, tetrahydrofuran, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, etc.; dimethylformamide; and dimethyl sulfoxide. They may be used singly or as a mixture thereof. Examples of the binder resin for the charge transfer layer are olefinic polymers such as styrenic polymers, acrylic polymers, styrene-acrylic copolymers, polyethylene, an ethylene-vinyl acetate copolymer, chlorinated polyethylene, polypropylene, ionomer, polyvinyl chloride, a vinyl chloride vinyl acetate copolymer, polyester, alkyd resins, polyamide, polyurethane, epoxy resins, polycarbonate, polyarylate, polysulfone, diallylphthalate resins, silicone resins, ketone resins, polyvinylbutyral resins, polyether resins, phenol resins, melamine resins, benzoguanamine resins, and photosetting resins such as epoxy acrylate, urethane acrylate, polyester acrylate, etc. These resins can be used singly or as a mixture thereof. The coating composition composed of the charge transfer material (including the butadiene derivative (I) and the hydrazone compound (II), the binder resin, and the solvent described above is coated on an electrically conductive substrate at a thickness of from 10 to 40µm, and particularly from 15 to 30µm and dried. The electrically conductive substrate can be a sheet form material or a drum form material each having an electric conductivity. As the substrates, there are various materials having an electric conductivity. For example, there are metals such as aluminum, aluminum alloys, copper, tin, platinum, gold, silver, vanadium, molybdenum, chromium, cadmium, titanium, nickel, palladium, indium, stainless steels, brass, etc., and plastics and glasses each having an electrically conductive layer of the metal, indium oxide, tin oxide, etc., formed thereon by a means such as a vapor deposition, etc. In the conductive substrates, the materials having an oxide surface, in particular, alumite-treated aluminum, and more particularly alumite-treated aluminum having the thickness of the alumite-treated layer of from 5 to 12 µm and having the surface roughness of 1.5S or less is preferred for increasing the adhesion with the photosensitive layer formed thereon. In addition, for further increasing the adhesion between the conductive substrate and the photosensitive layer, the conductive substrate can be treated with a surface treating agent such as a silane coupling agent, a titanium coupling agent, etc. Examples of the charge generating material being incorporated in the charge generating layer are selenium, selenium-tellurium, selenium-arsenic, amorphous silicone, pyrylium salts, azoic compounds, disazoic compounds, phthalocyanine compounds, ansanthrone series compounds, perylene series compounds, indigo series compounds, triphenylmethane series compounds, multi-rings condenced series quinone compounds, toluidine series compounds, pyrazoline series compounds, quinacridone series compounds, and pyrrolopyrrole series compounds. These charge generating materials can be used singly or as a mixture thereof. From the above charge generating materials, the most preferred materials are those containing phthalocyanine series compounds having various crystal types such as α-type, β-type and γ-type, for example, aluminum phthalocyanine, copper phthalocyanine, and, in particular, metal free phthalocyanine, oxotitianyl phthalocyanine, etc. In this invention, as a charge generating material, the use of a combination of the N-type pigment and the P-type pigment is preferred. The function of the P-type pigment is as follows. When the electrophotographic photosensitive element is positively charged by corona discharging, etc., thermal holes existing in the P-type pigment are injected into the charge transfer layer and negative space charges are generated in the charge generating layer. The negative space charges increase the electric field of the light carriers in the charge generating layer to increase the generating efficiency of light carriers. Also, when the electrophotographic photosensitive element is exposed to light, both positive and negative charges are generated by the N-type pigment which absorbs light in the range of 550 to 600 nm. The positive charges are transferred through the charge generating layer by the P-type pigment which has a hole transferring property up to the interface between the charge generating layer and the charge transfer layer, after which the charges are injected into the charge transfer layer. On the other hand, the negative charges are neutralized with positive charges, which are induced on the surface layer of the electrophotographic photosensitive element by electrostatic charging. By this means, electrostatic latent images are formed at the light-exposed portions. As described above, by incorporating the N-type pigment and the P-type pigment in the charge generating layer, the electric field for forming light carriers can be increased. The sensitivity of the photosensitive element can also be increased by improving the hole transferring property in the charge generating layer. The weight ratio of N-type pigment and the P-type pigment (N-type pigment/P-type pigment, hereinafter referred to as "N/P ratio") is in the range of from 40/60 to 90/10. If the N/P ratio is over 90/10, the content of the P-type pigment in the layer is relatively less, so that there is less increase in the electric field, a weakening of the hole transferring property, and a deterioration of sensitivity. On the other hand, if there is an N/P ratio of less than 40/60, because the content of the N-type pigment is therefore less, therefore, the sensitivity and copying properties of a red original deteriorate. Examples of the N-type pigment are perylene series compounds, ansanthrone series compounds, azoic compounds, methane, diphenylmethane, xanthene, and acridine, each having an amino group or a derivative thereof as a substituent. Among these materials listed above, the ansanthrone series compounds are suitably used owing to their efficiency in the formation of light carriers. Examples of the P-type pigment are azo pigments, anthraquinone pigments, triphenylmethane pigments, nitro pigments, xanthene pigments, azine pigments, and quinoline pigments each having a sulfon group or a carboxyl group, and phthalocyanine series compounds. In these compounds, phthalocyanine series compounds which are non-toxic and excellent in workability, in particular, metal free phthalocyanine and oxotitanyl phthalocyanine are preferably used. The same hydrazone compounds shown by formula (II) above are used for the charge generating layer as well as the the charge transfer layer. Prefferred examples of the hydrazone compounds incorporated into the charge generating layer are the same as those preferably incorporated into the charge transfer layer in the present invention. Hydrazine compounds incorporated into the charge transfer layer and the charge generating layer may be the same or differnt. With the increase of the content of hydrazone in the charge generating layer, the hole transferring property is improved, and the sensitivity tends to increase, but, at the same time, thermal carriers are increased, which means that the stability necessary for repeated use is generally lowered. The contribution which the hydrazone compound gives to the photosensitive element of the invention differs according to the kinds and the structures of materials (i.e., the charge transferring agent, the charge transfer agent, the solvent, and other additives) being used. Therefore, the optimum amount of the hydrazone compound for the sensitivity and the repeating stability required in each formulation may be selected. In the construction of this invention, the ratio by weight of the content of the hydrazone compound contained in the charge generating layer to the binder resin in the layer is between 0.1 and 100 parts hydrazone, preferably between 5 and 50 parts, to 100 parts of the binder resin. If the content of the hydrazone is over 100 parts, the solubility of the hydrazone compound becomes insufficient and the mechanical strength of the layer deteriorates, while if there is less than 0.1 part hydrazone, the desired addition effect of the hydrazone compound is not substantially obtained. As the binder resin for the charge generating layer, the same binder resins described above for the charge transfer layer can be used. Particularly, as binder compounds, polyvinyl acetal is suitably used in this invention since the binder is excellent in dispersibility for the components of the layer. Also, a coating composition which contains the binder is excellent in storage stability. However, since polyvinyl acetal contains a large amount of hydroxyl groups, the compound has a high hygroscopicity, which causes a decrease in resistance to surrounding conditions. Also, the hydroxyl groups act as traps for charge carriers (holes) generated at light exposure. This trapping characteristic of the hydroxyl groups lowers the sensitivity of the photosensitive element. Also, as described above, polyvinyl acetal, with a large amount of hydroxyl groups, has a high solubility in an organic solvent such as an alcohol. When a surface protective layer forms on the charge generating layer, polyvinyl acetal in the charge generating layer is to a great extent swelled or dissolved by an organic solvent contained in the coating composition for the surface protective layer. This swelling or dissolving of the surface protective layer makes the interface indistinct between the charge generating layer and the surface protective layer. This in turn has a pronounced negative effect on the sensitivity characteristics, etc., of the photosensitive element. Additionally, it lowers the mechanical strength of the surface protective layer. Thus, when polyvinyl acetal is used as the binder resin for the charge generating layer, an acetylacetone complex salt (metal acetylacetonate) is preferably added. Since the acetylacetone complex salt is hydrolyzed by drying the coating composition of the charge generating layer to cause a condensation reaction with the hydroxyl groups in the polyvinyl acetal, the amount of the hydroxyl groups remaining in the layer formed is decreased. As the acetylacetone complex salt which can be used in this invention, there are various chelate compounds selected from the group of (mono)acetyl acetonate complex salts composed of acetylacetone and a metal atom, a bisacetyl acetonate complex salt, a trisacetyl acetonate complex salt, and a tetracisacetyl acetonate complex salt. In these complex salts, the complex salts represented by the following formulae (IV) or (V) are preferably used in this invention. M(C₅H₇O₂) n M(C₅H₇O₂) n-m Q 1 m wherein M represents a trivalent or tetravalent metal; Q¹ represents an alkyl group or an alkoxy group; n represents 3 when M is a trivalent metal or 4 when M is 4; and m represents an integer of 2 or lower. In addition, examples of M in the above formulae (V) and (VI) are aluminum and zirconium. The acetylacetone complex salt is usually supplied as a solid, such as a powder, which is advantageous in terms of storage stability. But since it takes a long time to uniformly disperse the complex salt in polyvinyl acetal, it is preferable to use the acetylacetone complex salt in a solution state (e.g., a solution of the acetylacetone complex salt dissolved in alcohol and water). As the alcohol which is used together with water to form a solution of the acetylacetone complex salt, there are, for example, ethanol, methanol, isopropanol, butanol, β-oxyethyl methyl ether (methyl-cellosolve), β-oxyethyl ether (ethylcellosolve), β-oxyethyl propyl ether (propylcellosolve), and butyl-β-oxyethyl ether (butylcellosolve). Of these alcohols, butanol and butylcellosolve which are safe to use and have a low volatility are suitably used. There is no particular restriction in this invention on the concentration of the acetylacetone complex salt in a solution of the acetylacetone complex salt in an alcohol and water (hereinafter, referred to as an acetylacetone complex salt solution). The concentration of the acethylacetone complex salt is preferably in the range between 0.05 and 0.5 mol/liter. If the concentration of the acetylacetone complex salt is higher than 0.5 mol/liter, it takes a long time to dissolve the total amount of the acetylacetone complex salt and it takes time to prepare the solution of the complex salt. Also with a concentration higher than 0.5 mol/liter, uneven coating is liable to occur, which causes unevenness and lengthwise stripes on the specific layer formed, which results in the formation of defective images. Such unevenness also affects the sensitivity characteristics, the mechanical strength of the layer, the resistance to surrounding conditions, etc. Also, if the concentration of the acetylacetone complex salt is less than 0.05 mol/liter, a large amount of the acetylacetone complex salt solution must be added to the coating composition of the binder resin to sufficiently lower the amount of the hydroxyl groups of polyvinyl acetal remaining in the charge generating layer. This lowers the viscosity of the coating composition of the binder resin and deteriorates the coating and film-forming properties of the coating composition. It also increases the drying time for the coated layer. On the other hand, there is no particular restriction on the concentration of water in the acetylacetone complex salt solution in this invention but it is preferred that the concentration of water is in the range of from 1 to 10 mol/liter. If the concentration of water is larger than 10 mol/liter, the acetylacetone complex salt is hydrolyzed, whereby the amount of the hydroxyl groups remaining in the charge generating layer cannot be sufficiently decreased and also when a pigment, etc., is used, its dispersibility is decreased. Also, if the concentration of water is less than 1 mol/liter, the addition effect of water is not sufficiently obtained, it is difficult to dissolve the total amount of the acetylacetone complex salt in the solution, and it takes too much time to prepare the solution. Plus, when the water concentration is low, uneven coating, defective images, uneven sensitivity, decreased layer strength, and lowered resistance to surrounding conditions results. In addition, although there is no specific proportional relation between the concentration of the acetylacetone complex salt and the concentration of water in the acetylacetone complex salt solution, because of the relation between the concentration and the polarity of the acetylacetone complex salt, in order to keep the solution stable, it is desirable that a solution which contains a large amount of the acetylacetone complex salt also contains a large amount of water. There is no particular restriction on the compounding ratio of the acetylacetone complex salt solution with the coating composition for the charge generating layer, but it is preferable that the concentration of the acetylacetone complex salt in the solution be adjusted so that the acetylacetone complex salt is combined with the coating composition in an amount of from 0.01 to 2.0 equivalent to the hydroxyl groups of the polyvinyl acetal contained in the coating composition. If the compounding ratio of the acetylacetone complex salt to the hydroxyl groups of the polyvinyl acetal is larger than 2.0 equivalent, each of the characteristics is improved but the stability is decreased with repeated use. Also, if the compounding ratio of the acetylacetone complex salt to the hydroxyl groups of the polyvinyl acetal is less than 0.01 equivalent, the added effect of the acetylacetone complex salt is not sufficiently obtained and large amounts of hydroxyl groups remain in the charge generating layer, whereby the lowering of the sensitivity, the deterioration of the resistance to surrounding conditions, and the resistance to an organic solvent are not sufficiently prevented. As the solvent for the coating composition of the charge generating layer, alcohols such as methanol, ethanol, propanol, isopropanol, n-butanol, etc. can be used. The reason of using these particular alcohols is because the hydrazone compound contained in the charge transfer layer has a solubility of from about 0.1 to 2% with these particular alcoholic solvents. Thus, part of the hydrazone compound contained in the charge transfer layer is dissolved while the coating composition for the charge generating layer is applied. The dissolved hydrazone diffuses into the charge generating layer formed. Thus, the occurrence of an electric barrier at the interface between the charge generating layer and the charge transfer layer can be prevented. By properly selecting the alcoholic solvent, the occurrence of the electric barrier at the interface between the charge generating layer and the charge transfer layer can be effectively prevented. Choosing such a solvent also results in the prevention of cracks by solvate shock, the crystallization of the charge transfer material, the dissolving off of the binder resin, etc. To produce the above-mentioned effects, the use of n-butanol is particularly preferred. The ratio of the charge generating material to the binder in the charge generating layer is in the range preferably between 5 and 500 parts of charge generating material (more preferably between 10 and 250 parts) to 100 parts of binder resin. If the content of the charge generating material is less than 5 parts, the charge generating faculty is less, while if the content is over 500 parts, the adhesion between layers is lowered. The thickness of the charge generating layer is suitably from 0.01 to 3 µm, and particularly suitable from about 0.1 to 2µm. As the coating composition for the surface protective layer, a conventionally known coating composition such as a silicone resin series coating composition, an alkyd resin series coating composition, etc., can be used and in these resins, a silicone resin series coating composition forming a silicone resin film having excellent electric characteristics can be most preferably used as the coating composition for the surface protective layer in this invention. The silicone resin series coating composition is a coating composition prepared by dissolving or dispersing in a solvent the hydrolyzed product(s) (so-called organosiloxane oligomer) of silane series compounds, e.g., organosilanes such as tetraalkoxysilane, trialkoxyalkylsilane, dialkoxydialkylsilane, diphenyldiethoxysilane, diphenyldimethoxysilane, diphenylmethylethoxysilane, phenyltriethoxysilane, phenyltrimethoxysilane, etc., and organohalosilanes such as trichloroalkylsilane, dichlorodialkylsilane, etc., singly or as a mixture thereof, or the initial condensation reaction product(s) of the hydrolyzed product(s) as a non-volatile solid component. As the alkoxy group and alkyl group of the silane compound, there are groups having from 1 to about 4 carbon atoms, such as methoxy, ethoxy, isopropoxy, t-butoxy, glycidoxy, methyl, ethyl, etc. As the electrical conductivity imparting agent described above, there are fine particles of a metal oxide such as tin oxide, titanium oxide, indium oxide, antimony oxide, etc., or a solid solution of tin oxide and antimony oxide. In particular, the fine particles of the solid solution of tin oxide and antimony oxide are preferably used in this invention. The coating composition for the surface protective layer and the electrical conductivity imparting agent can be mixed by using a conventionally known means such as a ball mill, an ultrasonic dispersing apparatus, etc. The thickness of the surface protective layer is preferably in the range of from 0.01 to 3 µm. If the thickness of the surface protective layer is less than 0.01µm, the sensitivity of the electrophotographic photosensitive element is lowered, while if the thickness is over 3 µm, there is a possibility of reducing the stability during repeated use. The charge transfer layer, the charge generating layer, and the surface protective layer may contain, if necessary, sensitizers such as terphenyl, halonaphthoquinones, acenaphthylenes, etc.; fluorene series compounds such as 9-(N,N-diphenylhydrazino) fluorene, 9-carbazolyliminoflurene, etc.; antioxidants such as hindered phenols, hindered amines, etc.; ultraviolet absorbants such as benzotriazole, etc.; plasticizers; radical uptaking agents; etc. At the preparation of the coating compositions for the charge transfer layer, the charge generating layer, and the surface protective layer, a leveling agent such as a silicone oil, etc., a surface active agent, a thickener, etc., may be used together. Also, each coating composition can be prepared by a conventional method using, for example, a mixer, a ball mill, a paint shaker, a sand mill, an attritor, a ultrasonic dispersing means, etc. The invention is explained in more detail by the following examples. Examples 1 to 9 and Comparative Examples 1 to 4 Preparation of Coating Composition for Charge Transfer Layer The following coating composition for a charge transfer layer was prepared. 4-(N,N-Diethylamino)benzaldehyde-N,N-diphenylhydrazone (hereinafter, CT agent A) 100 parts Polyarylate (U-100, trade name, made by Unitika Ltd.) 100 Dichloromethane 900 Preparation of Coating Composition for Charge Generating Layer An n-butyl alcohol solution containing 0.2 mol/liter of tetracisacetylacetonate zirconium Zr(C₅H₇O₂)₄ (made by Nihon Kagaku Sangyo Co., Ltd.) and 3.0 mol/liter of water was prepared. Also, one or two kinds of charge generating materials selected from dibromoansanthrone (made by Imperial Chemical Industries Limited, hereinafter, is referred to as CC agent A), metal free phthalocyanine (made by BASF A.G., hereinafter, is referred to as CG agent B), and oxotitanyl phthalocyanine (made by Sanyo Colour Work., LTD, hereinafter, is referred to as CG agent C), and the CT agent A as a charge transfer agent were added to 100 parts of polyvinyl butyral (3000K, trade name, made by Electro Chemical Industry Co., Ltd) at the ratios shown in Table 1 below and after adding thereto 2000 parts of alcohol series solvents shown in Table 1 and the above-prepared solution such that tetracisacetylacetonate zirconium was contained in an amount of 0.25 equivalent to the hydroxyl groups in the polyvinyl butyral, the resulting mixture was mixed by stirring in a ball mill for 2 hours to provide a coating composition for a charge generating layer. Preparation of Coating Composition for Surface Protective Layer By mixing 50 parts of an antimony-doped tin oxide fine powder (made by Sumitomo Cement Co., Ltd.) as an electrical conductivity imparting agent with 100 parts of a silicone resin (Tosguard 520, trade name, made by Toshiba Silicone Co., Ltd.), a coating composition for a surface protective layer was prepared. Preparation of Photosensitive Element The coating composition for a charge transfer layer was coated on an aluminum substrate by dip coating and dried for 30 minutes at 90°C to form a charge transfer layer having the thickness shown in Table 1 below. Then, the coating composition for a charge generating layer was coated on the charge transfer layer by dip coating and dried for 30 minutes at 110°C to form a charge generating layer having a thickness of 0.5µm. Furthermore, the coating composition for a surface protective layer was coated on the charge generating layer by dip coating and dried for 60 minutes at 110°C to form a surface protective layer having a thickness of 2.0µm. Thus, 13 kinds of electrophotographic photosensitive elements were prepared. Examples 10 to 21 and Comparative Examples 5 and 6 By following the same procedures as Examples 1 to 9 described above except that each charge transfer layer and each charge generating layer having the thicknesses shown in Table 2 below were formed using one or two kinds of charge transfer materials being contained in the charge transfer layer selected from the CT agent A described above, 4-(N,N-dimethylamino)benzaldehyde-N,N-diphenylhydrazone (hereinafter referred to as CT agent B), and 1,1-bis(4-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene (hereinafter referred to as CT agent C) at the ratios shown in Table 2 below, using the CG agent A and the CG agent C as the charge generating agent being contained in the charge generating agent at the ratios shown in Table 2, and using the CT agent A or the CT agent B as the charge transfer agent being contained in the charge generating layer at the ratio shown in Table 2, 12 kinds of electrophotographic photosensitive elements were prepared. Examples 22 and 23, Comparative Examples 7 to 13 By following the same procedure as Examples 1 to 9 described above except that the CT agent A and CT agent C were used at the ratio shown in Table 3 as the charge transfer material contained in the charge transfer layer, the CT agent A and CT agent C were used at the ratio shown in Table 3 as the charge transfer material being contained in the charge generating layer, and the thicknesses of the charge transfer layer and the charge generating layer were changed as shown in Table 3, 9 kinds of electrophotographic photosensitive elements were prepared. Evaluation Test For determining the charging characteristics and the sensitivity characteristics of the electrophotographic photosensitive elements obtained in the examples and comparative examples, the following tests were conducted. (a) Measurement of Surface Potential Each electrophotographic photosensitive element was positively charged using a drum-form sensitivity test machine (Gentec Cynthia 30M Type, made by Gentec) and the charged potential (V) was measured. (b) Measurement of Potential After Light Exposure A halogen lamp was used such that the exposure intensity thereof was adjusted to be 0.1 mW/cm² per unit area of the photosensitive element, the photosensitive element was exposed to the halogen lamp for 0.3 second, and the potential (V) after the light exposure was measured. (c) Evaluation of the Appearance of Charge Transfer Layer and Charge Generating Layer. It was visually observed whether or not there was abnormality in the appearance of the charge transfer layer and the charge generating layer. (d) Determination of Repeating Stability Whether or not the reduction of the potential after 300 cycles was within 50 V was measured. (e) Measurement of Glass Transition Temperature (Tg). The glass transition temperature (°C) of the electrophotographic photosensitive element was measured using a thermal analysis apparatus DT-30 made by Shimazu Seisakusho Ltd. The results of the evaluation tests are shown in Table 1 to Table 3. In addition, in the evaluation of the appearance of the charge transfer layer and the charge generating layer, the sample in which no abnormality was observed in the appearance of both the charge transfer layer and charge generating layer was shown by A and the sample in which the abnormality was observed in the appearance of at least one of the charge transfer layer and the charge generating layer was shown by B. Also, in the evaluation of the repeating stability, the sample in which the lowering of the potential after 300 cycles was less than 20 V was shown by A, the sample in which the lowering of the potential after 300 cycles was from 20 V to 50 V was shown by B, and the samples in which the lowering of the potential after 300 cycles was 50 V or higher was shown by C. Also, in the tables, IPA represents isopropyl alcohol, n-BuOH represents n-butyl alcohol, MIBK represents methyl isobutyl ketone, and CH₃CN represents acetonitrile. As is clear from the results shown in Table 1, it can be seen that the electrophotographic photosensitive elements in Comparative Examples 3 and 4 are inferior in sensitivity since the charge generating layers do not contain the hydrazone compound as a charge transfer material. Also, the photosensitive elements in Comparative Examples 1 and 2 are inferior in sensitivity and repeating stability since the thickness of the charge transfer layer is too thin or too thick. On the other hand, the photosensitive elements in Examples 1 to 9 have a greatly improved sensitivity without causing cracks and crystallization since each charge generating layer contains the specific hydrazone compound. Table 2 and Table 3 show almost the same matters. Also, the glass transition temperature of the photosensitive element in Comparative Example 13 is 54°C, which is 20°C lower than the glass transition temperature (74°C) of the photosensitive element in Example 23 having the same compositions of the charge transfer layer and the charge generating layer except for the CT agent A in both layers, which shows that the mechanical strength of the photosensitive element in Comparative Example 13 is greatly lowered. This is based on the content of the charge transfer material (CT agent A) in the charge transfer layer of the photosensitive element in Comparative Example 13. In addition, in Comparative Examples 9 and 10, since the CT agent C (butadiene derivative) was not dissolved in the coating composition for the charge generating layer during preparation of the coating composition for the charge generating layer, it was impossible to prepare electrophotographic photosensitive elements. In Comparative Example 11, the crystallization by a solvent shock occurred and in Comparative Example 12, cracks occurred in the charge transfer layer. This is most likely caused by using acetonitrile or methyl isobutyl ketone as a solvent which has a higher dissolving power for the CT agent A (4- (N,N-diethylamino)benzaldehyde-N,N-diphenylhydrazone) than when an alcoholic solvent is used. (In addition, the solubility of the CT agent A in acetonitrile is 4.40% and the solubility of the CT agent A in methyl isobutyl ketone is 12.8%). Furthermore, the electrophotographic photosensitive element having a charge transfer layer containing the CT agent A and the CT agent C and a charge generating layer containing the N type pigment (CG agent A) and the P type pigment (CG agent B or CG agent C), with the charge generating layer being formed using an alcoholic solvent, shows a low potential after light exposure and has an excellent sensitivity as compared with other photosensitive elements. In addition, when a hydrazone compound (CT agent A or CT agent B) is incorporated in the charge generating layer, the sensitivity is even more improved. As described above, in the electrophotographic photosensitive element of this invention, the charge generating layer contains the hydrazone compound shown by formula (II) described above and the thickness of the charge generating layer is thin, the transfer of charges from the charge generating layer to the charge transfer layer is smoothly conducted and the photosensitive element has a high performance stability without causing thermal carriers and traps. Also, by forming the surface protective layer, the abrasion resistance is increased, whereby the life of the photosensitive element is prolonged so that the sensitivity is increased and images having a high quality can be obtained.

Claims: An electrophotographic photosensitive element comprising an electrically conductive substrate having formed thereon a charge transfer layer, a charge generating layer, and a surface protective layer in this order, wherein said charge transfer layer comprises a binder resin, a butadiene derivative represented by formula (I) wherein Ar₁, Ar₂, Ar₃ and Ar₄ each represents an aryl group which may have a substituent and/or a hydrazone compound represented by formula (II) wherein R₁, R₂, and R₃ each represents an alkyl group having from 1 to 5 carbon atoms or an alkoxy group having from 1 to 2 carbon atoms; R₄ and R₅ each represents an alkyl group having from 1 to 5 carbon atoms or a phenyl group; and p₁, p₂ and p₃ each represents an integer from 0 to 2, and said charge generating layer comprises a binder resin, a charge generating material and said hydrazone compound represented by the above formula (II), and the content of said hydrazone compound to said binder resin contained in said charge generating layer is between 0.1 and 100 parts hydrazone to 100 parts binder resin. An electrophotographic photosensitive element according to claim 1, wherein said hydrazone compound (II) is selected from the group consisting of 4-(N,N-diethyl-amino)benzaldehyde-N,N-diphenylhydrazone and 4-(N,N-dimethylamino)benzaldehyde-N,N-diphenyl-hydrazone. An electrophotographic photosensitive element according to claim 1, wherein said butadiene derivative (I) is An electrophotographic photosensitive element according to claim 1, wherein said charge generating layer further comprises a combination of an N-type pigment and a P-type pigment.

Inventors: HANATANI YASUYUKI, KAWAHARA AKIHIKO, MIZUTA YASUFUMI, NAKATANI KANAME, SAKUMA TADASHI, TANAKA NARIAKI, YOSHIDA TAKESHI, HANATANI, YASUYUKI, KAWAHARA, AKIHIKO, MIZUTA, YASUFUMI, NAKATANI, KANAME, SAKUMA, TADASHI, TANAKA, NARIAKI, YOSHIDA, TAKESHI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1990
Main classification: G03G5
IPC codes: G03G5