Electrophotographic Element Containing a Disazo Pigment

(a) Field of the Invention

The present invention relates to a photosensitive element for use in electrophotography, and more particularly it relates to a novel photosensitive element having a photosensitive layer containing a disazo pigment as effective component.

(b) Description of the Prior Art

There have hitherto been developed various electrophotographic elements comprising an electroconductive support and a photosensitive layer formed thereon containing some azo pigment as effective component. For instance, Japanese Patent Publication No. 16474/1969 discloses an electrophotographic element employing some monoazo pigment, and Japanese Laid-Open Unexamined Patent Application No. 37543/1972 discloses an electrophotographic element employing some disazo pigment. These azo pigments employed in the prior art are admittedly useful materials to serve as effective component of photosensitive elements, but when various requirements for photosensitive elements from the view point of electrophotographic process are taken into consideration, there has, in fact, not been developed any photosensitive element that will meet these requirements satisfactorily. Therefore, it is important to provide a rich variety of pigments useful as effective component, without limiting to azo pigments, so as to permit a wide range of selection according to the intended electrophotographic process. By so doing, it becomes possible to provide a photosensitive element apposite to a specific process. In other words, for the electrophotographic processes, it is desirable that the variety of pigments to serve as effective component of the photosensitive elements should be as rich as possible.

The first object of the present invention is to provide an electrophotographic element containing a novel disazo pigment which can serve as effective component in performing various electrophotographic processes.

The second object of the present invention is to provide an electrophotographic element which permits a wide range of selection of disazo pigment servable as effective component thereof.

The third object of the present invention is to provide a novel electrophotographic element with high sensitivity as well as high flexibility which contains such a disazo pigment as described above.

We have prepared a group of disazo pigments and conducted a series of studies on their application to photosensitive elements. As a result, we found that disazo pigments expressed by the following general formula are servable as excellent effective component of photosensitive elements and have accomplished the present invention on the basis of this finding.

An electrophotographic element according to the present invention is characterized in that a photosensitive layer containing a disazo pigment expressed by the general formula ##STR4## [wherein R.sub.1 represents a member selected from the group consisting of hydrogen atom, methyl radical, ethyl radical, methoxy radical, ethoxy radical, chlorine atom, bromine atom, nitro radical, dimethylamino radical, diethylamino radical and phenyl radical; A represents ##STR5## wherein R.sub.2 represents hydrogen atom or chlorine atom) or ##STR6## and n is an integer of 1 or 2. ] is formed on an electroconductive support.

Concrete examples of the compounds expressed by the foregoing general formula that are useful for the present invention will be shown in the following in terms of structural formula. ##STR7##

These disazo pigments can be easily prepared by effecting, for instance, condensation reaction between 2 moles of ##STR8## and 1 mole of ##STR9## or coupling reaction between 2 moles of ##STR10## and 1 mole of ##STR11## or ##STR12## Details of these processes are disclosed in, for instance, Japanese Laid-Open Unexamined Patent Application No. 3924/1974 or U.S. Pat. No. 4,051,121.

The photosensitive elements according to the present invention contain such disazo pigments as exemplified above, and they can take various forms illustrated in FIGS. 1 through 3 of the appended drawings depending on the mode of application of these pigments. A photosensitive element illustrated in FIG. 1 is one prepared by forming a photosensitive layer 2 of disazo pigment 4 (serving herein as photoconductive substance).about.binder resin 3 type on an electroconductive support 1. A photosensitive element illustrated in FIG. 2 is one prepared by forming a photosensitive layer 2' of disazo pigment 4 (serving herein as charge-carrier generating substance).about.charge-transfer medium (i.e., a mixture of charge-transfer substance and binder resin) 5 type on an electroconductive support 1. A photosensitive element illustrated in FIG. 3 is a modification of the photosensitive element of FIG. 2, and the photosensitive layer 2" thereof is composed of a charge-carrier generating layer 6 consisting essentially of disazo pigment 4 and a charge-transfer medium layer 7.

In the case of the photosensitive element of FIG. 1, the disazo pigment acts as photoconductive substance, and generation and transfer of the charge-carrier necessary for light decay are effected through the medium of pigment particles. In the case of the photosensitive element of FIG. 2, the charge-transfer substance forms a charge-transfer medium together with the binder (and some plasticizer as occasion calls), while the disazo pigment acts as charge-carrier generating substance. This charge-transfer medium is not capable of generating charge-carrier like disazo pigments, but is capable of accepting and transferring charge-carrier generated from disazo pigments. That is, in the case of the photosensitive element of FIG. 2, generation of the charge-carrier necessary for light decay is performed by the disazo pigment, while transfer of the charge-carrier is performed mainly by the charge-transfer medium. A fundamental condition that is additionally required of the charge-transfer medium herein is that the absorption wave-length range of the charge-transfer medium should not overlap mainly the absorption wavelength range of the visible region of the disazo pigment. The reason is that, in order to cause the disazo pigment to generate charge-carrier efficiently, it is necessary to permeate light to the surface of pigment. The foregoing condition, however, does not apply to, for instance, a photosensitive element which is sensitive to a specific wave-length alone. Therefore, the absorption wave-length of the charge-transfer medium and that of the disazo pigment may partially overlap but should not completely overlap each other. Next, in the case of the photosensitive element of FIG. 3, light penetrated the charge-transfer medium layer 7 reaches to the charge-carrier generating layer 6 to cause the disazo pigment of that portion to generate charge-carrier, while the charge-transfer medium layer accepts the pouring of charge-carrier and performs transfer thereof, and the mechanism of effecting transfer of the charge-carrier by means of the charge-transfer medium is the same as in the case of the photosensitive element illustrated in FIG. 2. The disazo pigment herein is also a charge-carrier generating substance.

In order to prepare the photosensitive element of FIG. 1, it will do to coat a dispersion obtained by dispersing fine particles of a disazo pigment in a solution of binder on an electroconductive support and dry thereafter. In order to prepare the photosensitive element of FIG. 2, it will do to disperse fine particles of a disazo pigment in a solution obtained by dissolving a charge-transfer substance and a binder, coat the resulting dispersion on an electroconductive support and dry thereafter. The photosensitive element of FIG. 3 can be prepared either by depositing a disazo pigment on an electroconductive support by vacuum evaporation or by the process comprising dispersing fine particles of a disazo pigment in an appropriate solvent which may contain a binder dissolved therein as occasion demands, coating this dispersion on an electroconductive support and drying, subjecting the thus formed coating film to surface finishing or adjusting the thickness thereof by such means as buffing, etc. as occasion demands, and then coating a solution containing a charge-transfer substance and a binder on the film and drying thereafter. In any case, the disazo pigment for use in the present invention is employed after pulverizing into particle diameter of 5 microns or less, preferably 2 microns or less, by means of a ball mill or the like. The coating is performed by conventional means such as doctor blade, wire bar, etc. The thickness of the photosensitive layer is about 3 to 50 microns, preferably 5 to 20 microns, in the case of the photosensitive elements of FIG. 1 and FIG. 2. In the case of the photosensitive element of FIG. 3, the appropriate thickness of the charge-carrier generating layer is 0.01 to 5 microns, preferably 2 microns or less, while the appropriate thickness of the charge-transfer medium layer is about 3 to 50 microns, preferably 5 to 20 microns. Further, in the photosensitive element of FIG. 1, the appropriate amount of the disazo pigment contained in the photosensitive layer is 30 to 70% by weight, preferably about 50% by weight, relative to the photosensitive layer. (As stated above, in the case of the photosensitive element of FIG. 1, the disazo pigment acts as photoconductive substance and generation and transfer of the charge-carrier necessary for light decay are performed by means of pigment particles. Therefore, it is desirable that contact between pigment particles be continuous from the surface of the photosensitive layer to the support. Accordingly, it is desirable that the content of the pigment in the photosensitive layer be relatively large. However, when the strength and sensitivity of the photosensitive layer are taken into consideration, the appropriate content is about 50% by weight). In the photosensitive element of FIG. 2, the content of the disazo pigment in the photosensitive layer accounts for 1 to 50% by weight, preferably 20% by weight or less, while the content of the charge-transfer substance therein accounts for 10 to 95% by weight, preferably 30 to 90% by weight. The content of the charge-transfer substance in the charge-transfer medium layer in the case of the photosensitive element of FIG. 3 accounts for 10 to 95% by weight, preferably 30 to 90% by weight, like in the case of the photosensitive layer of the photosensitive element of FIG. 2. Further, in preparing any of the photosensitive elements shown in FIGS. 1 through 3, some plasticizer can be employed jointly with the binder.

As the electroconductive support for the photosensitive elements of the present invention, a plate or a foil of a metal such as aluminum, a plastic film deposited with a metal such as aluminum by vacuum evaporation, a paper processed for conductivity, etc. are useful. As the binder for the present invention, condensation resins such as polyamide, polyurethane, polyester, epoxide resin, polyketone, polycarbonate, etc. and vinyl polymers such as polyvinyl ketone, polystyrene, poly-N-vinyl carbazole, polyacrylamide, etc. can be cited, but any other resin will do as long as the insulating property and the adhesive property thereof are sufficient. As the plasticizer applicable to the present invention, halogenated paraffin, polybiphenyl chloride, dimethyl naphthalene, dibutyl phthalate, etc. can be cited. To cite charge-transfer substances suitable for the present invention, as high-molecular substance, there are vinyl polymers such as poly-N-vinyl carbazole, halogenated poly-N-vinyl carbazole, polyvinyl pyrene, polyvinyl indoloquinoxaline, polyvinyl dibenzothiophene, polyvinyl anthracene, polyvinyl acridine, etc. and condensation resins such as pyrene.about.formaldehyde resin, bromopyrene.about.formaldehyde resin, ethyl carbozole.about.formaldehyde resin, chloroethyl carbazole.about.formaldehyde resin, etc., and as low-molecular substance (monomer), there are fluorenone, 2-nitro-9-fluorenone, 2,7-dinitro-9-fluorenone, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 4H-indeno[1,2-b]thiophene-4-one, 2-nitro-4H-indeno[1,2-b]thiophene-4-one, 2,6,8-trinitro-4H-indeno[1,2-b]thiophene-4-one, 8H-indeno[2,1-b]thiophene-8-one, 2-nitro-8H-indeno[2,1-b]thiophene-8-one, 2-bromo-6,8-dinitro-4H-indeno[1,2-b]thiophene, 6,8-dinitro-4H-indeno[1,2-b]thiophene, 2-nitro-dibenzothiophene, 2,8-dinitrodibenzothiophene, 3-nitrodibenzothiophene-5-oxide, 3,7-dinitrodibenzothiophene-5-oxide, 1,3,7-trinitrobenzothiophene-5,5-dioxide, 3-nitrodibenzothiophene-5,5-dioxide, 3,7-dinitrodibenzothiophene-5,5-dioxide, 4-dicyanomethylene-4H-indeno[1,2-b]thiophene, 6,8-dinitro-4H-dicyanomethylene-4H-indeno[1,2-b]thiophene, 1,3,7,9-tetranitrobenzo[c]cinnoline-5-oxide, 2,4,10-trinitrobenzo[c]cinnoline-6-oxide, 2,4,8-trinitrobenzo[c]cinnoline-6-oxide, 2,4,8-trinitrothioxanthone, 2,4,7-trinitro-9,10-phenonthrenequinone, 1,4-naphthoquinone benzo[a]anthracene-7,12-dione, 2,4,7-trinitro-9-dicyanomethylene-fluorene, tetrachlorophthalic anhydride, 1-bromopyrene, 1-methyl pyrene, 1-ethyl pyrene, 1-acetyl pyrene, carbazole, N-ethyl carbazole, N-.beta.-chloroethyl carbazole, N-.beta.-hydroxyethyl carbazole, 2-phenyl indole, 2-phenyl naphthalene, 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, 2,5-bis(4-diethylaminophenyl)-1,3,4-triazole, 1-phenyl-3-(4-diethylaminostyryl)-5-(4-diethylaminophenyl)pyrazoline, 2-phenyl-4-(4-diethylaminophenyl)-5-phenyl oxazole, triphenyl amine, tris(4-diethylaminophenyl)-methane, 3,6-bis(dibenzylamino)-9-ethyl carbazole, etc. These charge-transfer substances are employed either independently or in the form of a mixture of two or more of them.

All the photosensitive elements prepared as above can be further provided with an adhesive layer or a barrier layer, as occasion demands, which is interposed between the electroconductive support and the photosensitive layer. As the material to form this layer, polyamide, nitrocellulose, aluminum oxide, etc. are appropriate, and the thickness of the layer is preferably 1 micron or less.

In order to perform copying by employing a photosensitive element of the present invention, it suffices to charge the element by its photosensitive layer side, expose to light, develop thereafter, and transfer the developed image to a paper or the like as occasion demands.

Photosensitive elements according to the present invention have excellent advantages such that they are generally high in sensitivity and rich in flexibility.

1 part by weight of polyester resin (namely, Polyester Adhesive 49000, the manufacture of Du Pont Inc.), 1 part by weight of disazo pigment No. 1 and 26 parts by weight of tetrahydrofuran were crushed and mixed within a ball mill, and the resulting dispersion was coated, by means of a doctor blade, on a polyester film deposited with aluminum by vacuum evaporation and was dried at 100.degree. C. for 10 minutes, whereby there was obtained a photosensitive element having a 7-micron thick photosensitive layer and assuming the form illustrated in FIG. 1.

Next, the photosensitive layer side of this photosensitive element was positively charged by means of corona discharge of +6 KV for 20 seconds in a commercial testing apparatus for electrostatic copying paper, and then the surface potential Vpo(volt) thereof was measured after standing it for 20 seconds in the dark. Subsequently, the photosensitive layer of this element was exposed to the light of a tungsten lamp so as to attain the surface illumination of 20 luxes, and the amount of exposure E1/2 (in terms of lux.multidot.sec.) was determined from the time of exposure (in terms of second) required for decrease of the surface potential Vpo to half. The results were as follows:

Vpo: 940 V, E1/2: 68.3 lux.multidot.sec.

A variety of photosensitive elements were prepared by applying the same procedure as that in Example 1 except for replacing disazo pigment No. 1 employed therein with the disazo pigments having their serial numbers as shown in the following Table-1 respectively. When the same measurement as in Example 1 was conducted on each of these photosensitive elements, the results were as shown in Table-1 respectively.

10 parts by weight of polyester resin (the same substance as that in Example 1), 10 parts by weight of 2,4,7-trinitro-9-fluorenone, 2 parts by weight of disazo pigment No. 2 and 198 parts by weight of tetrahydrofuran were crushed and mixed within a ball mill, and the resulting dispersion was coated, by means of a doctor blade, on a polyester film deposited with aluminum through vacuum evaporation and was dried at 100.degree. C. for 10 minutes, whereby there was obtained a photosensitive element having a 10-micron thick photosensitive layer and assuming the form illustrated in FIG. 2. Next, this photosensitive element was subjected to the same measurement as in Example 1 to determine the values of Vpo and E1/2 except for applying corona discharge of -6 KV instead of +6 KV. The results were as follows:

Vpo: 680 V, E1/2: 20.4 lux.multidot.sec.

A variety of photosensitive elements assuming the form illustrated in FIG. 2 were prepared by applying the same procedure as that in Example 21 except for replacing disazo pigment No. 2 employed therein with the disazo pigments having their serial numbers as shown in the following Table-2 respectively. When the values of Vpo and E1/2 of each element were determined in the same way as in Example 21, the results were as shown in Table-2.

10 parts by weight of polyester resin (the same substance as that in Example 1), 10 parts by weight of 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, 2 parts by weight of disazo pigment No. 3 and 198 parts by weight of tetrahydrofuran were crushed and mixed within a ball mill, and the resulting dispersion was coated, by means of a doctor blade, on a polyester film deposited with aluminum through vacuum evaporation and was dried at 120.degree. C. for 10 minutes, whereby there was obtained a photosensitive element having a 12-micron thick photosensitive layer and assuming the form illustrated in FIG. 2. Next, this photosensitive element was subjected to the same measurement as in Example 1 to determine the values of Vpo and E1/2. The results were as follows:

Vpo: 950 V, E1/2: 17.8 lux.multidot.sec.

A variety of photosensitive elements assuming the form illustrated in FIG. 2 were prepared by applying the same procedure as that in Example 21 except for replacing disazo pigment No. 3 employed therein with the disazo pigments having their serial numbers as shown in the following Table-3 respectively. When the values of Vpo and E1/2 of each element were determined in the same way as in Example 1, the results were as shown in Table-3.

2 parts by weight of disazo pigment No. 5 and 98 parts by weight of tetrahydrofuran were crushed and mixed within a ball mill, and the resulting dispersion was coated, by means of a doctor blade, on a polyester film deposited with aluminum through vacuum evaporation and was subjected to natural drying, whereby there was formed a 1-micron thick charge-carrier generating layer. On the other hand, 2 parts by weight of 2,4,7-trinitro-9-fluorenon, 2 parts by weight of polycarbonate (namely, Panlite L, the manufacture of TEIJIN Inc.) and 46 parts by weight of tetrahydrofuran were mixed to prepare a dispersion. Then, this dispersion was coated on the foregoing charge-carrier generating layer by means of a doctor blade and was dried at 100.degree. C. for 10 minutes to form a 10-micron thick charge-transfer medium layer, whereby there was obtained a photosensitive element assuming the form illustrated in FIG. 3. When the values of Vpo and E1/2 of the thus obtained element were determined in the same way as in Example 1, the results were as follows:

Vpo: 1020 V, E1/2: 58.3 lux.multidot.sec.

A variety of photosensitive elements assuming the form illustrated in FIG. 3 were prepared by applying the same procedure as that in Example 61 except for replacing disazo pigment No. 5 employed therein with the disazo pigments having their serial numbers as shown in the following Table-4 respectively. The values of Vpo and E1/2 of these photosensitive elements were as shown in Table-4.

2 parts by weight of disazo pigment No. 6 and 98 parts by weight of tetrahydrofuran were crushed and mixed within a ball mill, and the resulting dispersion was coated, by means of a doctor blade, on a polyester film deposited with aluminum through vacuum evaporation and was subjected to natural drying, whereby there was formed a 1-micron thick charge-carrier generating layer. On the other hand, 2 parts by weight of 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, 2 parts by weight of polycarbonate (the same substance as employed in Example 31) and 16 parts by weight of tetrahydrofuran were mixed to prepare a dispersion. Then, this dispersion was coated on the foregoing charge-carrier generating layer by means of a doctor blade and was dried at 420.degree. C. for 10 minutes to form a 14-micron thick charge-transfer medium layer, whereby there was obtained a laminate-type photosensitive element illustrated in FIG. 3. When the thus obtained photosensitive element was subjected to the same measurement as in Example 1 to determine the values of Vpo and E1/2 thereof except for applying corona discharge of -6KV instead of +6KV, the results were as follows:

Vpo: 780V, E1/2: 26.5 lux.multidot.sec.

A variety of photosensitive elements similar to that of Example 81 were prepared by applying the same procedure as that in Example 81 except for replacing disazo pigment No. 6 employed therein with the disazo pigments having their serial numbers as shown in the following Table-5 respectively.

The values Vpo and E1/2 of these photosensitive elements were as shown in Table-5.

FIGS. 1 through 3 illustrate a diagrammatic cross-sectional view each, on an enlarged scale, of various photosensitive elements according to the present invention, in which:

1 . . . an electroconductive support

2, 2', 2" . . . a photosensitive layer

3 . . . a binder

4 . . . a disazo pigment

5 . . . a charge-transfer medium

6 . . . a charge-carrier generating layer

7 . . . a charge-transfer medium layer