Photoconductive phthalocyanine pigments, electrophotographic elements containing them, and a method of use.

0001This invention relates to electrophotographic elements as claimed in claim 1 containing photoconductive titanyl phthalocyanine pigments having fluorine substituted on a least one of the aromatic rings, and to a method of forming electrophotographic images with such elements as claimed in claim 7.

0002Electrophotographic imaging processes employ a photoconductive element comprising an insulating photoconductive layer disposed on an electrically conductive support. Photoconductive layers have in the past contained various inorganic photoconductors such as amorphous selenium, cadmium sulfide or lead oxide or organic photoconductors such as polyvinyl carbazole, aryl amine compounds and chloroindium phthalocyanine chloride. Such photoconductive elements are for instance described in European Patent Application 0157492. In this application, the element comprises a charge-generation layer containing a photoconductive material and a charge transport layer containing a specific electron transporter. While many of the common photoconductors have good electrophotographic sensitivity in the wavelength range from ultraviolet to red, their sensitivity in the near infrared range above about 700 nm is not sufficiently high for use with diode lasers which currently are of great importance in recording techniques.

0003In lieu of the more commonly used photoconductors it has been suggested that certain metal phthalocyanines which absorb in the near infrared region be employed. For example, U.S. Patent 4,426,434 suggests a photoreceptor having an evaporated film of chloroaluminium phthalocyanine or chloroaluminium monochlorophthalocyanine. Also, U.S. Patent 4,458,004 describes fluorophthalocyanines free from metal or containing a metal, a metal oxide such as titanium oxide, or a metal halide ; these fluorophthalocyanines, however, are used to prepare an optical information recording medium having high laser-beam absorption efficiency, which is not an electrophotographic medium. It is said that such compounds have high photosensitivity in the near infrared region. Unfortunately, such compounds, as well as many other phthalocyanines, evidently are not readily dispersible in coating formulations and, to deposit them as photoconductive layers, the expensive procedure of vacuum evaporation must be used.

0004The present invention provides photoconductive elements including an electrically conductive substrate and a photoconductive layer containing a phthalocyanine pigment which not only has high electrophotographic sensitivity in the near infrared region but, in addition, are readily dispersible in polymeric binders and have extremely low granularity when dispersed in the polymeric binder of a photoconductive layer. Photoconductive elements can be prepared with these phthalocyanines by the relatively inexpensive procedure of dispersion coating. The resulting near infrared-sensitive photoconductive elements provide excellent image resolution because of the small particle size and dispersability of the photoconductive phthalocyanine compounds. Also, because they can be deposited on a substrate by dispersion coating, it is possible to make single layer and inverted multilayer photoconductive elements with the phthalocyanine pigments of the invention.

0005The pigments of the invention are photoconductive titanyl phthalocyanines having fluorine substituted in one or more of their carbocyclic rings, which are obtainable by the acid-pasting method as defined hereinbelow. The near infrared sensitive photoconductive elements of the invention comprise an electrically conductive substrate and a photoconductive layer comprising a binder resin and a phthalocyanine as described, and especially such an element wherein the phthalocyanine of the photoconductive layer is deposited by dispersion coating.

0006The method of the invention comprises electrostatically charging a photoconductive element of the type described, exposing it to a pattern of near infrared radiation and developing the resulting charge pattern.

0007The photoconductive titanyl fluorophthalocyanine pigments used in the photoconductive layer of the electrophotographic element of the invention can be prepared by the reaction of a fluorine-substituted phthalonitrile with a titanium chloride in a hot solvent, as illustrated below, wherein the letter n represents an integer from 1 to 4. <chemistry id="chem0001" num="0001"><img file="EP0244780B1_D0001.tif" /></chemistry>

0008Since the fluorine substituted phthalonitrile molecules which have fewer than four fluorine substituents can combine with each other in different spatial configurations, the reaction product will be a mixture of isomeric titanyl fluorophthalocyanines. When tetrafluoro phthalonitrile is the starting material, only one isomer forms, i.e., titanylhexadecyl fluorophthalocyanine.

0009Although the maximum benefits of the invention have been found with pigments of formula I, wherein the phthalocyanine is synthesized from a single fluoro-substituted phthalonitrile, the invention also includes fluorine substituted titanyl phthalocyanines in which not all of the aromatic rings have fluorine substituents or do not have an equal number of fluorine substituents.

0010Non-uniformly substituted compounds of this type can be made by reacting titanium trichloride with two or more phthalonitriles, one of which is a fluorophthalonitrile as illustrated above, and the other or others of which are of the formula <chemistry id="chem0002" num="0002"><img file="EP0244780B1_D0002.tif" /></chemistry> wherein R can be hydrogen, a halogen such as fluorine, chlorine or bromine, iodine or a hydrocarbon radical such as lower alkyl, e.g., methyl, ethyl, isopropyl and t-butyl, and n is an integer from 1 to 4.

0011When the titanyl phthalocyanine having fluorine substitution on the aromatic rings is made in this manner by the reaction of two or more phthalonitriles the reaction product will contain a mixture of substituted oxotitanium phthalocyanines. This mixture will include phthalocyanines in which as few as one of the aromatic rings is substituted with fluorine. Thus, the invention includes phthalocyanines having fluorine substituents equal in number on each aromatic ring as well as, photoconductive fluorine-substituted-titanyl phthalocyanines which are non-uniformly substituted, all having valuable qualities such as good dispersability in coating compositions.

0012Before using the fluoro titanylphthalocyanine pigment as a near infrared sensitive electrophotographic charge generation compound, it is subjected to a treatment which purifies it, modifies its crystalline form and reduces the particle size.

0013The resulting pigment is photoconductive. By this is meant that in the dark the pigment is electrically insulating but when exposed to actinic radiation in the near infrared region, it becomes electrically conductive. More specifically, the dark resistivity is greater than 10¹¹ ohm cms at 25°C and is rapidly reduced several orders of magnitude when exposed to actinic radiation in the near infrared region (700-900 nm) at an intensity of 5 to 30 ergs/cm²/sec. Typically, a charged photoconductor discharges by at least 50 volts within 10 seconds when exposed to such radiation. For the more sensitive pigments of the invention, e.g., the tetrafluoro species, the conductivity upon exposure to near infrared radiation is such that a photoconductive element containing the pigment in its charge generation layer can be discharged from -500 volts to -100 volts in one second at an exposure of 7 ergs per cm² and a wavelength of 830 nm.

0014One useful procedure for rendering the pigment photoconductive is called "acid-pasting". It involves dissolving the pigment, after extraction purification with a solvent such as dimethylformamide, in cold concentrated mineral acid, preferably sulfuric acid. The solution is poured into ice water to reprecipitate the phthalocyanine compound. The precipitate is washed free of acid with water, then with methanol and then is dried. The resulting purified pigment when dispersed in a binder resin has a substantially smaller particle size than the crude pigment and is highly sensitive to radiation in the near infrared region.

0015The photoconductive elements of the invention comprise an electrically conductive substrate having disposed on it a photoconductive layer containing a titanyl fluoro phthalocyanine compound I. The phthalocyanine compound can be deposited on the conductive substrate as the sole component of the photoconductive layer or in admixture with other components such as a resin binder, a coating aid, other photoconductive compounds, chemical sensitizers and spectral sensitizing dyes and pigments.

0016Although the photoconductive elements of the invention can have only a single photoconductive layer, the preferred photoconductive elements are multilayer (also called multiactive) elements having a charge generation layer and at least one charge transport layer. The configurations and general principles of multilayer elements have been described in the prior art, for example, in the patent to Berwick et al, U.S. 4,175,960. When employed in multilayer photoconductive elements in accordance with the present invention, the titanyl fluoro phthalocyanine compound is placed in the charge generation layer.

0017The photoconductive elements of the invention are used according to the method of the invention to make electrophotographic images, as in diode laser recording. In the method, the photoconductive element is electrostatically charged, e.g., by corona treatment, to a voltage, for example, of 500 to 600 volts negative or positive. Then it is exposed to a pattern of near infrared irradiation, causing charge to dissipate in the exposed regions. The resulting charge pattern on the photoconductive element is developed by contact with a positively or negatively charged toner. Suitable developers include liquid and dry developers which are well known in the art. Such liquid developers comprise dispersion of pigmented polymeric toner particles in a volatile insulating liquid such as an isoparaffinic hydrocarbon. Useful dry developers include both single component and two-component developers, the latter comprising, for example, a mixture of magnetic carrier particles and powdered toner particles composed of a pigmented thermoplastic resin.

0018The preparation of a specific titanyl fluorophthalocyanine, namely, the compound titanyl 2, 9, 16, 23-tetrafluorophthalocyanine and its isomers, will illustrate how compounds of the invention and their precursors can be made.

0019The titanyl tetrafluorophthalocyanine compound can be prepared by the reaction of 4-fluorophthalonitrile with titanium trichloride in 1-chloronaphthalene at 210°-215°C. The synthesis of 4-fluorophthalonitrile as the precursor for the polyfluorinated phthalocyanine starts with 3,4-dimethylaniline and is shown in the following series of reactions: <chemistry id="chem0003" num="0003"><img file="EP0244780B1_D0003.tif" /></chemistry>

0020Diazotization (i) and conversion (ii) to the tetrafluoroborate followed by pyrolysis in benzene of that derivative (Balz-Schieman reaction) gives 3,4-dimethylfluorobenzene. This is oxidized by aqueous potassium permanganate to 4-fluorophthalic acid, monopotassium salt. Vacuum distillation of the latter in the presence of sulfuric acid yields 4-fluorophthalic anhydride. Treatment of the anhydride with urea in boiling chlorobenzene yields the imide which is converted with concentrated ammonium hydroxide to the diamide. Finally, dehydration of the diamide with thionyl chloride in dimethylformamide gives 4-fluorophthalonitrile.

0021The following example describes the use of 4-fluorophthalonitrile, prepared as described above, to prepare a polyfluoro phthalocyanine compound of the invention.

Example 1:

Preparation of Titanyl Tetrafluorophthalocyanine

00224-Fluorophthalonitrile (38.7 g, 0.267 mole) and 20.7 g (0.134 mole) of titanium trichloride was suspended in 200 ml 1-chloronaphthalene and heated to 210°-215°C (oil bath) and maintained for 2 1/2 hours at this temperature. The reaction mixture was cooled slightly, the dark solid was collected, washed with acetone and methanol. After drying, the dark blue solid (34 g) was slurried twice in refluxing dimethylformamide, filtered hot each time and washed with acetone yielding a pigment having an x-ray diffractogram peak (2Θ) of 7.4°.

0023The next example illustrates the acid pasting procedure which purifies, reduces the particle size and modifies the crystalline form of the phthalocyanine pigments.

Example 2 -

Acid Pasting of Titanyl Fluorophthalocyanine

0024The blue solid of Example 1 was dissolved in 300 ml of concentrated sulfuric acid with cooling, stirred for one hour at room temperature, and filtered through a medium frit Buchner funnel. The acid filtrate was added to 2 liters of ice and water mixture with stirring. The bright blue solid that separated was collected, washed free of acid with water, then reslurried in 500 ml of boiling water. The phthalocyanine was collected by hot filtration and air-dried to give 26.4 g of pigment having an x-ray diffractogram peak (2Θ) of 6.8°.

0025Calcd. for C₃₂H₁₂F₄N₈OTi (648.4): C, 59.2; H, 1.88; F, 11.7; N, 17.3; Ti, 7.38. Found: C, 59.0; H, 1.7; F, 11.3; N, 17.3; Ti, 6.9.

0026The photoconductive elements of the invention preferably are made from the 6.8° form of the pigment, as made in Example 2, or from a mixture of the 6.8° and 7.4° forms. Such a mixture can be obtained if the 6.8° form is dispersed in an organic solvent.

0027To prepare a photoconductive element of the invention from a titanyl fluorophthalocyanine pigment, the pigment is dispersed in a solution of a binder resin such as a polycarbonate resin. To achieve homogeneity the dispersion is milled for an extended time, e.g., for several days, in a ball mill or other homogenizing apparatus. After diluting the milled dispersion to a suitable viscosity for coating and after blending in a coating aid such as a silicone surfactant, the dispersion is coated on a conductive substrate. The latter, for example, can be a nickel-coated polyethylene terephthalate film. The coating is dried, e.g., by heating mildly for a few minutes.

0028The resulting charge generation layer is overcoated with a dispersion of a charge transport compound or compounds in a binder resin. This coating is then dried to provide a relatively thicker charge transport layer.

0029The next example describes the preparation of multilayer photoconductive elements containing phthalocyanine pigments of Examples 1 and 2.

Example 3 -

Multilayer Photoconductive Element Containing Titanyl Fluorophthalocyanine Pigment

0030Pigments prepared as described in Examples 1 and 2, which are referred to as [(4-F)₄Pc]TiO, were incorporated in the following formulation: <dl id="dl0001"><dt>1.75 g</dt><dd>High molecular weight polycarbonate binder resin poly(oxycarbonyloxy-1,4-phenylene (1-methylethylidene)-1,4-phenylene) in solution in a solvent mixture of 1,1,2-trichloromethane (40 g) and dichloromethane (26 g)</dd><dt>0.18 g</dt><dd>[(4-F)₄Pc]TiO</dd><dt>9.56 g</dt><dd>Dichloromethane solvent</dd><dt>2 drops</dt><dd>1% Solution of silicone surfactant in dichloromethane (DC510 surfactant of Dow Chemical Co.)</dd></dl>

0031To form a coating dispersion this formulation was placed in a glass container with 30 g of zirconia beads, shaken for three hours in a paint shaker and separated from the beads. The dispersion was then coated as a charge generation layer on a subbed, nickelized conductive poly(ethylene terephthalate) film support. The coated film was cured for 30 minutes in an 80°C oven, yielding a dried charge generation layer of 1.5 µm thickness. This thin layer was then overcoated with a thicker layer of an organic charge transport layer containing 35 percent 1,1-bis(4-di-<u style="single">p</u>-tolylaminophenyl) cyclohexane and 65 percent of a polyester binder in a solvent mixture consisting of 60 parts by weight dichloromethane and 40 parts 1,1,2-trichloroethane, the polyester being poly(4,4'-[2-norbornylidene]bisphenylene terephthalate-co-azelate) 60/40. The resulting multilayer element was cured for two hours in a 60°C oven.

0032Tests were made of multilayer elements prepared as in Example 3 using the crude pigment of Example 1 and the acid pasted-pigment of Example 2. Results are given in the following table. <tables id="tabl0001" num="0001"><img file="EP0244780B1_D0004.tif" /></tables>

0033The data show that the acid-pasting treatment of Example 2 yields a pigment of very small particle size and improves the electrical properties of the element.

0034When corona treated to a negative charge, the element containing the non-acid-pasted pigment (Example 1) was measured to be only about -220 volts (V_o). The element containing the acid-pasted pigment (Example 2) discharged from -500 to -100 volts when exposed to radiation of 830 nm at 7.0 ergs/cm². Dark decay was only 9.4 volts/sec.

Example 4 -

Titanyl Hexadecylfluorophthalocyanine (Fl6Pc)TiO

0035The reaction of tetrafluoro phthalonitrile with TiCl₃ by the procedure of Example 1 yields the pigment, titanyl hexadecylfluorophthalocyanine. The absorption spectrum of this compound shows a λ-max at 728 nm, which is hypsochromatically shifted about 100 nm from the λ-max of 826 nm exhibited by the titanyl tetrafluorophthalocyanine compound of Example 1. Although the electrical and dark decay characteristics of the tetrafluoro pigment were superior to those of the hexadecylfluoro species, the acid-pasted hexcadecylfluoro pigment is nevertheless useful as a near infra-red sensitive photoconductor and its dispersability in binder resins is good.

0036The examples have illustrated the preparation of a multilayer photoconductive element of the invention in which the charge transport layer is coated over the charge generation layer which contains a titanyl fluorophthalocyanine pigment and is contiguous to the conductive substrate. Also within the scope of the invention are "inverted" multiactive photoconductive elements in which a charge transport layer or layers are contiguous to the conductive support and a charge generation layer is coated over the charge transport layer or layers. This configuration is advantageous when it is desired to charge the photoconductive element positively instead of negatively. It is also advantageous in reducing the formation of white spots in the image which are caused by electrical breakdown in the development stage. Further, it is useful in improving the uniformity of charge across the surface of the photoconductive element or, in other words, improving the electrical granularity of the element.

0037The next example, describes such an inverted element in which a charge generating layer containing a titanyl tetrafluorophthalocyanine compound is coated over the charge transport layer.

Example 5 -

Inverted Multilayer Photoconductive Element

0038The pigment, titanyl tetrafluorophthalocyanine, (3.0 g) of Example 2 and 9.0 g of poly(oxycarbonyloxy-1,4-phenylenebicyclo[2.2.1] hept-2-ylidene-1,4-phenylene) binder resin were dispersed in 81 g of 1,1,2-trichloroethane solvent. The dispersion was milled for three days in a grinding mill. The milled dispersion was then diluted with 360 g of dichloromethane and filtered to provide pigment concentrate. This concentrate was incorporated in a dispersion for the charge generating layer having the following composition: <dl id="dl0002"><dt>4.0 g</dt><dd>Polycarbonate binder resin</dd><dt>2.0 g</dt><dd>Tri-p-tolylamine photoconductor</dd><dt>32.0 g</dt><dd>Dichloromethane coating solvent</dd><dt>11.0 g</dt><dd>1,1,2-Trichloroethane coating solvent</dd><dt>151.0 g</dt><dd>Phthalocyanine pigment concentrate</dd><dt>0.03 g</dt><dd>Silicone surfactant coating aid (Dow DC510, trademark by and available from Dow Chemical Co.)</dd></dl>

0039For the charge transport layer a liquid dispersion of the following formulation was prepared: <dl id="dl0003"><dt>172.5 g</dt><dd>Polycarbonate binder resin (Lexan 145 polycarbonate, trademark by and available from General Electric which is identified as poly(oxycarbonyloxy-1,4-phenylene (-1-methylethylidene)-1,4-phenylene)</dd><dt>7.5 g</dt><dd>Poly(ethylene-co-neopentylene terephthalate) polyester binder resin</dd><dt>120.5 g</dt><dd>Tri-p-tolylamine photoconductor</dd><dt>1680.0 g</dt><dd>Dichloromethane</dd><dt>420.0 g</dt><dd>1,1,2-trichloroethane</dd><dt>0.3 g</dt><dd>Silicone surfactant coating aid (Dow DC510)</dd></dl>

0040The inverse multilayer element was prepared by coating the indicated charge transport layer liquid dispersion on a nickel-coated subbed poly(ethylene terephthalate) film support at (1.5 g/ft²) 16.15 g/m² dry coverage and drying the coating. The charge generating layer was then formed by coating the indicated charge generating layer liquid dispersion over the charge transport layer at (0.2 g/ft²) 2.15 g/m² dry coverage and drying the coating. The inverse layer element of this example when tested in comparison with an element having the same pigment but the standard layer configuration had less electrical breakdown (as shown by reduction in image white spots after liquid development) and lower electrical granularity.

0041Although the examples have described pigments made from 4-fluoro and tetrafluoro phthalonitriles, any of the fluorine-substituted phthalonitriles are suitable precursors. Hence compounds of the invention include all of those represented by formula I wherein n is an integer of 1, 2, 3 or 4. For those in which n is less than 4, the fluorine can be substituted at any position on the phthalonitrile ring. All of the resulting titanyl fluorophthalonitrile pigments of the invention have near infra-red sensitivity and dispersability so that photoconductive elements can be made from them by dispersion coating.

0042The patent to Scozzafava et al, U.S. 4,514,481 discloses electrophotographic layers that contain an electron donor and an electron transporting compound such as certain 4<u style="single">H</u>-thiopyran-1,1-dioxides. It has been found, in accordance with the present invention, that these electron transporting compounds are unexpectedly beneficial when used in a multilayer photoconductive element of the present invention which can be positively charged for imaging. These elements have high sensitivity in the near infrared region and low dark decay, making them especially useful in laser or LED-based printers.

0043One disadvantage of the inverted layer configuration of multilayer photoconductor is that the thin charge generation layer is on top of the element and may be subject to abrasion as the element is used repeatedly in a series of imaging cycles. This, of course, shortens the life of the photoconductive element. By using an electron transporting compound, as described herein, the charge transport layer can be the top layer which is coated over the charge generation layer. This protects the thinner and more critically important charge generation layer from abrasion as the element is reused, and yet the element can be positively charged.

0044The following examples illustrate electrophotographic elements containing an electron transporting compound.

Example 6

-

Photoconductive Element Containing a 4 H-Thiopyran-4-one-1,1-dioxide

0045A photoconductive element was prepared substantially as in Example 3 except that the charge transport layer contained 30 percent by weight of the compound 4-dicyanomethylene-2,6-diphenyl-4<u style="single">H</u>-thiopyran-4-one-1,1-dioxide in the polyester binder. The charge generation layer contained titanyl tetrafluorophthalocyanine in a polycarbonate binder as in Example 3. The conductive layer was an aluminium plate.

Example 7

0046Another multilayer element was prepared as in Example 6 except that the electron transporting compound was 4-dicyanomethylene-2,6-di-<u style="single">m</u>-tolyl-4<u style="single">H</u>-thiopyran-1,1-dioxide and the conductive layer was a nickel-coated polymeric film.

Comparative Example

-

Aggregate Charge Generation Layer

0047A multilayer element like that of Example 6 was prepared except that, instead of titanyl fluorophthalocyanine, an aggregate photoconductor as in Berwick et al U.S. 4,175,960 was used in the charge generation layer.

0048The photoconductive elements of Example 6 and 7 and the comparative example were charged to 500 volts positive and exposed to radiation sufficient to discharge the element to 250 volts. The exposure for Examples 6 and 7 was at 830 nm and for the comparative example was at 680 nm, that being within the region of sensitivity of the aggregate photoconductor. The relative quantities of exposure required to discharge each element to 250 volts are listed in the table below, an arbitrary value of 100 being assigned to the quantity of exposure for the comparative example. The table also shows the residual voltage (V_toe) of each element after full irradiation. <tables id="tabl0002" num="0002"><img file="EP0244780B1_D0005.tif" /></tables>

Example 8

00490.5 grams of the pigment, prepared as described in Example 2 [(4-F)₄P_c]TiO is dispersed with extended milling in 20.5 grams of a polyester (Vylon 200-Toyobo Chemical Co.) prepared from terephthalic and isophthalic acids and neopentyl glycol and ethylene glycol which previously was dissolved in a mixture of 20 grams of dichloromethane and 100 grams of trichloroethane and cast into a film. This film exhibits an absorption maximum at 820 nm.

Example 9

0050As a comparison, titanyl tetrachlorophthalocyanine is treated in the same manner as described in Example 2 and formed into a film as described in Example 8. This film exhibits an absorption maximum at 720 nm.

0051The results show that when the thiopyrandioxide electron transporting compounds were used with a titanyl fluorophthalocyanine charge generator in accordance with the invention (Examples 6 and 7), the exposure required to discharge the photoconductor from V_o = 500 volts to 250 volts was about one eighth the exposure required for the same degree of discharge of the aggregate-type photoconductor used with the same class of electron transporting compound. The table also shows that upon complete radiation the elements of the invention discharged to voltages of 40 and 44 while the residual voltage for the element having a different charge generation layer was discharged only to 124 volts. These results demonstrate an unexpected cooperation of the titanyl fluorophthalocyanine layer and the electron transporting layer in the multilayer photoconductive elements of the invention.

0052When using an electron transport compound in a multilayer element of the invention having a conductive layers which is electron injecting, e.g., nickel, it may be desirable to place a thin barrier layer between the conductive layer and the charge generation layer to prevent dark discharge. The barrier layer can be a non-conductive polymer such as a polyester or other materials disclosed in U.S. 4,175,960.

0053One of the advantages of the phthalocyanine pigments of the invention is that because of their sensitivity to radiation in the near infrared region of the spectrum they can be used in photoconductive elements which record the radiation of diode lasers. They are not limited to that use, however. They can be used in admixture with charge generation compounds having sensitivity, e.g., to visible light to provide photoconductive elements that have a broad range of sensitivity including the visible light range and near infrared. Examples 8 and 9 show the advantageous effect of fluorine containing titanyl phthalocyanines when compared with the corresponding chloro-containing compound. The fluoro compound is much more sensitive into the infrared and at the wavelength emitted by diode lasers.

0054The charge generation layer which contains the phthalocyanine pigments can have a thickness within a wide range depending upon the degree of photosensitivity desired. Thickness affects photosensitivity in two opposite ways. As thickness increases a greater proportion of actinic radiation is absorbed by the layer, but there is a greater likelihood of a charge carrier being trapped and thus not contributing to image formation. These two factors must be balanced in selecting an appropriate thickness. A thickness in the range of about 0.05 µm to 6.0 µm is preferred for maximum photosensitivity. At thicknesses much below 0.05 µm the radiation absorption is inadequate and at thicknesses much above 6.0 µm, the trapping of charge carriers becomes excessive.

0055As indicated in the examples and as disclosed in U.S. 4,175,960, the charge transport layer in a multilayer element is thicker than the charge generation layer. Its thickness can be of the order of 3 to 40 µm, preferably, 5 to 30. It can be several times thicker than the charge generation layer, e.g., 2 to 20 times thicker.

0056Although the examples have illustrated specific charge transport layer materials, the charge transport layer can be any monomeric or polymeric organic photoconductors or inorganic materials which can transport charge carriers generated in the charge generation layer. Combinations may be used. Most charge transport materials preferentially accept and transport either positive charges (holes) or negative charges (electrons), although materials are known which will transport both positive and negative charges.

0057Various p-type organic charge transport materials can be used. in the charge transport layer in accordance with the present invention. Representative p-type organic photoconductive materials include: <ul id="ul0001" list-style="none"><li>1. Carbazoles including carbazole, N-ethyl carbazole, N-isopropyl carbazole, N-phenyl carbazole, halogenated carbazoles, various polymeric carbazole materials such as poly(vinyl carbazole) halogenated poly(vinyl carbazole), and the like.</li><li>2. Arylamines including monoarylamines, diarylamines, triarylamines, as well as polymeric arylamines. Examples of arylamine photoconductors include the non-polymeric triphenylamines illustrated in Klupfel et U.S. Patent No. 3,180,730 issued April 27, 1965; the polymeric triarylamines described in Fox U.S. Patent No. 3,240,597 issued March 15, 1966; the triarylamines having at least one of the aryl radicals substituted by either a vinyl radical or a vinylene radical having at least one active hydrogen-containing group, as described in Brantly et al U.S. Patent 3,567,450 issued March 2, 1971; the triarylamines in which at least one of the aryl radicals is substituted by an active hydrogen-containing group, as described in Brantly et al U.S. Patent No. 3,658,520 issued April 25, 1972; and tritolylamine.</li><li>3. Polyarylalkanes of the type described in Noe et al U.S. Patent 3,274,000 issued September 20, 1966; Wilson U.S. Patent No. 3,542,547 issued November 24, 1970 and in Rule et al U.S. Patent No. 3,615,402 issued October 26, 1971. Preferred polyarylalkane photoconductors can be represented by the formula: <chemistry id="chem0004" num="0004"><img file="EP0244780B1_D0006.tif" /></chemistry> wherein    D and G, which may be the same or different, represent aryl groups and J and E, which may be the same or different, represent a hydrogen atom, an alkyl group, or an aryl group, at least one of D, E and G containing an amino substituent. An especially useful polyarylalkane photoconductor which may be employed as the charge transport material is a polyarylalkane having the formula noted above wherein J and E represent a hydrogen atom, an aryl group, or an alkyl group and D and G represent substituted aryl groups having as a substituent thereof a group represented by the formula: <chemistry id="chem0005" num="0005"><img file="EP0244780B1_D0007.tif" /></chemistry> wherein:    R represents an unsubstituted aryl group such as phenyl or an alkyl substituted aryl such as a tolyl group. Additional information concerning certain of these latter polyarylalkanes may be found in Rule et al U.S. Patent No. 4,127,412 issued November 28, 1978.</li><li>4. Strong Lewis bases such as aromatic compounds, including aromatically unsaturated heterocyclic materials which are free of strong electron withdrawing groups. Such aromatic Lewis bases include tetraphenylpyrene, 1-methylpyrene, perylene, chrysene, anthracene, tetraphene, 2-phenyl naphthalene, azapyrene, fluorene, fluorenone, 1-ethylpyrene, acetyl pyrene, 2,3-benzochrysenes, 3,4-benzopyrene, 1,4-bromopyrene, phenylindole, polyvinyl carbazole, polyvinyl pyrene, polyvinyl tetracene, polyvinyl perylene, and polyvinyl tetraphene.</li><li>5. Other useful p-type charge-transport materials include the p-type organic photoconductors, including metallo-organo materials, known to be useful in electrophotographic processes, such as any of the organic photoconductors described in <u style="single">Research Disclosure</u>, Vol. 109, May 1973, pages 61-67, paragraph IV (A) (2) through (13) which are p-type photoconductors. (<u style="single">Research Disclosure</u> is published by Industrial Opportunities Limited, Havant, Hampshire, P09 lEF, United Kingdom.)</li></ul>

0058Representative of n-type charge-transport materials are strong Lewis acids such as organic and metallo-organic compounds containing one or more aromatic, including aromatically unsaturated heterocyclic compounds having an electron withdrawing substituent. These are considered useful because of their electron accepting capability. Typical electron withdrawing substituents include cyano and nitro groups; sulfonate groups; halogens such as fluorine chlorine, bromine, and iodine; ketone groups; ester groups; acid anhydride groups; and other acid groups such as carboxyl and quinone groups. Examples of such n-type aromatic Lewis acids having electron withdrawing substituents include phthalic anhydride, tetrachlorophthalic anhydride, benzil, mellitic anhydride, S-tricyanobenzene, picryl chloride, 2,4-dinitrochlorobenzene, 2,4-dinitrobromobenzene, 4-nitrobiphenyl, 4,4′-dinitrobiphenyl, 2,4,6-tri- nitroanisole, trichlorotrinitrobenzene, trinitro-o-toluene, 4,6-dichloro-1,3-dinitrobenzene, 4,6-dibromo-1,3-dinitrobenzene, p-dinitrobenzene, chloranil, bromanil, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitrofluorenone, trinitroanthracene, dinitroacridene, tetracyanopyrene, dinitroanthraquinone, and mixtures thereof.

0059Other useful n-type charge transport materials are conventional n-type organic photoconductors, for example, complexes of 2,4,7-trinitro-9-fluorenone and poly(vinyl carbazole). Still other useful n-type organic and metallo-organo photoconductive materials include any of the n-type organic photoconductive materials known to be useful in electrophotographic processes such as any of those described in <u style="single">Research Disclosure</u>, Vol. 109, May 1973, pages 61-67, paragraph IV(A) (2) through (13).

0060A single charge transport layer or more than one can be employed. Where a single charge transport layer is employed it can be either a p-type or an n-type material.

0061Where it is intended that the charge generation layer be exposed to actinic radiation through the charge transport layer, it is preferred that the charge transport layer have little or no absorption in the region of the electromagnetic spectrum to which the charge generation layer responds, thus permitting the maximum amount of actinic radiation to reach the charge generation layer. Where the charge transport layer is not in the path of exposure, this consideration does not apply.

0062When solvent coating the layers, a film-forming polymeric binder can be employed. The binder may, if it is electrically insulating, help to provide the element with electrical insulating characteristics. It also is useful (a) in coating the layer, (b) in adhering the layer to an adjacent layer, and (c) when it is a top layer, in providing a smooth, easy to clean, wear resistant surface.

0063When a polymeric binder is employed in either the charge generation or charge transport layer, the optimum ratio of charge generation or charge transport material to binder may vary widely depending on the particular binder and charge transport materials. In general, useful results are obtained wherein when the amount of active charge generation or charge transport material contained within the layer varies within the range of from about 2 to about 90 weight percent based on the dry weight of the layer.

0064Representative materials which may be employed as binders in the charge generation and charge transport layers are film-forming polymers having a fairly high dielectric strength and good electrically insulating properties. Such binders include, for example, styrene-butadiene copolymers; polyvinyl toluene-styrene copolymers; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; vinylidene chloride-vinyl chloride copolymers; poly(vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; vinyl acetatevinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); nitrated polystyrene; polymethylstyrene; isobutylene polymers; polyesters, such as poly[ethylene-co-alkylenebis(alkyleneoxyaryl)phenylenedicarboxylate]; phenolformaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly[ethylene-co-isopropylidene-2,2-bis(ethyleneoxyphenylene)terephthalate]; copolymers of vinyl haloacrylates and vinyl acetate such as poly(vinyl-m-bromobenzoate-co-vinyl acetate); and chlorinated poly(olefins), such as chlorinated poly(ethylene).

0065Binder polymers should provide little or no interference with the transport of charge carriers through the layer. Examples of binder polymers which are especially useful in p-type charge transport layers include styrene-containing polymers, bisphenol A polycarbonates, phenol-formaldehyde resins, polyesters such as poly[ethylene-co-isopropylidene-2,2-bis(ethyleneoxy-phenylene)]terephthalate, and copolymers of vinyl haloacrylates and vinylacetate such as poly(vinyl-m-bromobenzoate-co-vinyl acetate).

0066The charge generation and charge transport layers can also contain other addenda such as leveling agents, surfactants and plasticizers to enhance or improve various physical properties of the layer. In addition, various addenda to modify the electrophotographic response of the element can be incorporated in the charge transport layer. For example, contrast control materials, such as certain hole-trapping agents and certain easily oxidized dyes can be incorporated. Such contrast control materials are described in <u style="single">Research Disclosure</u>, Vol. 122, June 1974, p. 33, in an article entitled "Additives For Contrast Control In Organic Photoconductor Compositions and Elements".

0067When the charge generation layer or the charge transport layer is solvent coated, the components are dissolved or dispersed in a liquid together with the binder, if one is employed, and other addenda. Useful liquids include aromatic hydrocarbons such as benzene, naphthalene, toluene, xylene and mesitylene; ketones such as acetone and butanone; halogenated hydrocarbons such as methylene chloride, chloroform and ethylene chloride; ethers including cyclic ethers such as tetrahydrofuran and ethyl ether; and mixtures thereof.

0068A variety of electrically conducting supports can be employed in the elements of this invention, such as for example, paper (at a relative humidity above 20 percent); aluminium-paper laminates; metal foils such as aluminium foil, zinc foil, etc.; metal plates such as aluminium, copper, zinc brass and galvanized plates; vapor deposited metal layers such as silver, chromium, nickel, aluminium and the like coated on paper or conventional photographic film bases such as poly(ethylene terephthalate), cellulose acetate, polystyrene, etc. Such conducting materials as chromium, nickel, etc. can be vacuum deposited on transparent film supports in sufficiently thin layers to allow electrophotographic elements prepared therewith to be exposed from either side. An especially useful conducting support can be prepared by coating a support material such as poly(ethylene terephthalate) with a conducting layer containing a semiconductor dispersed in a resin. Such conducting layers both with and without electrical barrier layers are described in U.S. Patent 3,245,833 by Trevoy, issued April 12, 1966. Other useful conducting layers include compositions consisting essentially of an intimate mixture of at least one protective inorganic oxide and from 30 to 70 percent by weight of at least one conducting metal, e.g., a vacuum-deposited cermet conducting layer as described in Rasch U.S. Patent No. 3,880,657 issued April 29, 1973. Likewise, a suitable conducting coating can be prepared from the sodium salt of a carboxyester lactone of maleic anhydride and a vinyl acetate polymer. Such conducting layers and methods for their optimum preparation and use are disclosed in U.S. Patent 3,007,901 by Minsk issued November 7, 1961 and 3,262,807 by Sterman et al issued July 26, 1966.

0069The various layers of the element can be coated directly on the conducting substrate. In some cases, it may be desirable to use one or more intermediate subbing layers over the conducting substrate to improve adhesion between the conducting substrate and overlying layers or to act as an electrical barrier layer as described in Dessauer U.S. Patent 2,940,348. Such subbing layers, if used, typically have a dry thickness in the range of about 0.01 to about 5 µm. Typical subbing layer materials include film-forming polymers such as cellulose nitrate, polyesters, copolymers of poly(vinyl pyrrolidone) and vinylacetate, and various vinylidene chloride-containing polymers including two, three and four component polymers prepared from a polymerizable blend of monomers or prepolymers containing at least 60 percent by weight of vinylidene chloride. Representative vinylidene chloride-containing polymers include vinylidene chloride-methyl methacrylate-itaconic acid terpolymers as disclosed in U.S. Patent No. 3,143,421. Various vinylidene chloride containing hydrosol tetrapolymers which may be used include tetrapolymers of vinylidene chloride, methyl acrylate, acrylonitrile and acrylic acid as disclosed in U.S. Patent No. 3,640,780. Other useful vinylidene chloride-containing copolymers include poly(vinylidene chloride-methyl acrylate), poly(vinylidene chloride-methacrylonitrile), poly(vinylidene chloride-acrylonitrile), and poly(vinylidene chloride-acrylonitrile-methyl acrylate). Other subbing materials include the so-called tergels described in Nadeau et al U.S. Patent 3,501,301 and the vinylidine chloride terpolymers described in Nadeau U.S. patent 3,228,770.

0070One especially useful subbing layer is a hydrophobic film-forming polymer or copolymer free from any acid-containing group, such as a carboxyl group, prepared from a blend of monomers or pre-polymers, each of said monomers or prepolymers containing one or more polymerizable ethylenically unsaturated groups. Useful materials include many of the above-mentioned copolymers, and in addition, the following: copolymers of polyvinylpyrrolidone and vinyl acetate, poly(vinylidene chloride-methyl methacrylate), and the like.

0071Optional overcoat layers can also be used if desired. For example, to improve surface hardness and resistance to abrasion, the surface layer of the element of the invention can be coated with one or more electrically insulating, organic polymer coatings or electrically insulating, inorganic coatings. A number of such coatings are well known in the art. Typical overcoats are described for example, in <u style="single">Research Disclosure</u>, "Electrophotographic Elements, Materials and Processes", Vol. 109, p. 63, Paragraph V, May 1973.

0072While the novel photoconductive elements are useful in electrophotography, they can also be used in other arts, such as in solar cells, where photoconductive elements are employed.