Electrophotographic elements containing preferred pigment particle size distribution

Abstract: An electrophotographic element comprising an electrically conductive support, a barrier layer and a photoconductive layer containing a polymeric binder having dispersed therein charge generating pigments (a) selected from the group consisting of titanyl fluorophthalocyanines and cocrystalline mixtures of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine and (b) having a particle size from 0.05 to 0.7 µm, up to 30% of the particles are smaller than 0.15 µm and less than 8% of such particles are larger than 0.6 µm.

Description: Field of the Invention This invention relates to electrophotography. Background of the Invention Multiactive electrophotographic elements are known. They are useful in electrophotographic copiers and printers. One type comprises, in the following order, a conducting support, a barrier layer, a charge generating layer and a charge transport layer. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves in nonirradiated areas a charge pattern known as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles. Electrophotographic elements in which both the charge generating function and the charge transport function are combined in the single layer are also known. Such elements essentially produce images in the same way as the above described multiactive electrophotographic elements. U.S. 5,238,766 discloses an electrophotographic element comprising a tityanyl fluorophthalocyanine pigment. EP 0 460 615 A 1 discloses such elements comprising a mixture of titanyl phthalocyanine and titanyl fluorophthalocyanine pigments having a particle size that does not exceed 0.5 micrometers. Hiro teaches, in US Patent 4,980,254, that for maximum speed in charge generation layers pigment particles should have size below 0.5 µm in an amount of 80% or more in weight or number of the total pigment particles. The working examples therein show that at least 77.2% by weight of the pigment particles are no greater than 0.1 µm, and at least 14% were no greater than 0.02 µm. It is known that titanylphthalocyanines and titanyl fluorophthalocyanines pigments perform the charge generating function in electrophotographic elements. In view of the teachings of Hiro it was surprising to discover that certain titanyl pthalocyanine pigments having sizes that fall within the teachings of Hiro exhibited slower speeds than these same pigments having larger pigment size distributions. Moreover we discovered increased size (>0.6µm) of these pigments produced greater speeds than taught by Hiro. However when the larger pigments are used in imaging processes requiring relatively high surface potential, especially in discharge area development, so called "breakdown spots" occur. Breakdown spots, as used herein, is different from the more common background. Background is the effect of randomly distributed non-image forming toner particles in the non-image areas on electrophotographic copies. This results in a decrease in the reflection of the paper. Background usually involves wrong sign toner particles. Breakdown spots are strongly associated with specific areas of a photoconductor. Breakdown spots consist of clusters of toner particles in non-image areas. Summary of the Invention The present invention provides an electrophotographic element comprising an electrically conductive support, a barrier layer and a photoconductive layer containing a polymeric binder having dispersed therein charge generating pigments (a) selected from the group consisting of titanyl fluorophthalocyanines and cocrystalline mixtures of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine wherein the cocrystalline mixtures have a distinct crystallogram exhibiting major peaks of the Bragg angle 2-theta with respect to X-rays of Cu ka at a wavelength of 1.541Å at 7.5, 10.2,12.7, 13.2, 15.1, 16.1, 17.2, 18.5, 22.4, 24.2, 25.3, 28.7 all +/- 0.2, for a wide range of weight ratio of the starting phthalocyanines and (b) have a particle size from 0.05 to 0.7 µm, characterized in that up to 30% of the particles of the pigments are smaller than 0.15 µm and less than 8% of such particles are larger than 0.6 µm. This invention makes possible electrophotographic elements having a good balance of speed and minimized breakdown. Details of the Invention The useful titanyl fluorophthalocyanines have the general structure: where each of k, l, m, and n is independently an integer from 0 to 4 and at least one of k, l, m, and n is an integer from 1 to 4. Titanyl tetrafluorophthalocyanines are particularly useful. The cocrystalline mixtures of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine have a distinct crystallogram exhibiting major peaks of the Bragg angle 2-theta with respect to X-rays of Cu ka at a wavelength of 1.541Å at 7.5, 10.2, 12.7, 13.2, 15.1, 16.1, 17.2, 18.5, 22.4, 24.2, 25.3, 28.7 all +/- 0.2 for a wide range of weight ratio of the starting phthalocyanines. The following preparations (1-4) discloses methods of making representative charge generating pigments used in this invention. Preparation 1: Unsubstituted Titanyl Phthalocyanine Phthalonitrile (1100 g) and titanium tetrachloride (813 g) were suspended in 6800 ml of 1-chloronaphthalene and heated to 215-220 °C and maintained for 2.5 hours at this temperature. The reaction mixture was cooled to 140 °C, and the dark solid was collected and washed with acetone, and methanoL After drying, the dark blue solid (1090 g) was slurried twice in refluxing 10 liters of distilled water for two hours, filtered hot each time, and washed with acetone to yield crude titanyl phthalocyanine. The x-ray diffraction spectrum exhibits major peaks of the Bragg angle at 7.5, 8.3, 10.5, 12.7, 14.2, 14.6, 18.9, 22.1, 24.3, 26.1, 29.9 (all +/- 0.2 degree). Preparation 2: Crude Titanyl 4 -Fluorophthalocyanine 4-Fluorophthalonitrile (38.7 g, 0.267 mole) and titanium tetrachloride (20.7 g, 0.134 mole) were suspended in 200 ml of 1-chloronaphthalene and heated to 210-215 °C and maintained for 2.5 hours at this temperature. The reaction mixture was cooled slightly, and the dark solid was collected and 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 to yield crude titanyl tetrafluorophthalocyanine. The x-ray diffraction spectrum exhibits major peaks of the Bragg angle at 7.3, 10.6, 11.5, 11.8, 15.7, 16.6,17.0, 18.2, 22.1, 23.2, 24.3, 27.0, 31.2 (all +/- 0.2 degree) Preparation 3: Dichloromethane-Treated Titanyl Fluorophthalocyanine Crude titanyl fluorophthalocyanine was dissolved in concentrated sulfuric acid (10 liters) over 2 hours. The temperature of the solution was maintained at about 20 °C. The solution was filtered through a coarse sintered glass funnel, precipitated rapidly (50 minutes) into water kept between 6-32 °C. The pigment was allowed to settle and the water was decanted, then the pigment was again dispersed in water. This was repeated for a total of 15 times. The pH of the final wash water was 2. The pigment was dispersed in water, dichloromethane (DCM) was added, the dichloromethane was distilled off, and the water was decanted. This was repeated until a pH neutral filtrate was obtained. The pigment was then redispersed in DCM and filtered through a fine sintered glass funnel, washed with DCM, then with acetone and dried. An X-ray diffraction pattern of the resultant high crystallinity titanyl fluorophthalocyanine powder exhibits major peaks of the Bragg angle at 7.2, 11.8, 15.9, 23.3, 24.5, 27.1 (all +/-0.2 degree). The sample of the pigment was titrated for residual acid and was found to be substantially free of acid (less than 0.05 weight/weight percent). Preparation 4: Cocrystalline Mixture of Unsubstituted Titanyl Phthalocyanine and Titanyl Fluorophthalocyanine 75:25 7.5 gram of crude titanyl phthalocyanine, and 2.5 gram of crude titanyl fluorophthalocyanine were mixed in a 16 oz. jar with 300 g of 3 mm steel beads. The pigment sample was thus milled using a Sweco Vibro Energy grinding mill manufactured by Sweco, Inc. of Florence, Kentucky for three days. The pigment particles were completely fused, coating the stainless steel beads. 200 g of dichloromethane were added to the jar. The mixture was further milled for 48 hours. Then the beads separated , and the pigment filtered, and washed with dichloromethane, and dried. The X-ray diffraction spectrum of the dry-milled material exhibits three major broad peaks of the Bragg angle at 7.2, 15.4, and 25,5 (all +/- 0.2 degree), depicting a very noncrystalline mixture. After the dichloromethane mixture the X-ray diffraction spectrum of the material exhibits major peaks of the Bragg angle at 7.5, 10.2, 12.7, 13.2, 15.1, 16.1, 17.2, 18.5, 22.4, 24.2, 25.3, 28.7. (all +/- 0.2 degree). The layers of the multiactive electrophotographic elements can be made using well known solvent coating techniques. Such techniques are well known in this art. Indeed the methods are described in many published patents referred to herein. Several patents can be cited in the prior art, U.S. 3,245,833 and 3,428,451 to Trevoy, 3,932,179 to Perez-Albuerne, 4,082,551 to Steklenski et al., 4,410,614 to Lelental et al., and 4,485,161 to Scozzafava et al. The dispersion of the binder and pigments formed by mixing and dispersing the pigment with an organic polymer using a sand mill, ball mill, roll mill, attritor, or Sweco mill. The pigment particle size distribution can be measured using various techniques known in the art, such as Dynamic Light Scattering, some times referred to as Quasi-elastic Light Scattering (QELS). In particular a Microtac Ultrafine Particle Analyzer marketed by Leeds & Northrup can be used. The instrument measures the volume distribution of particles with no assumptions about the distribution whether it be broad or narrow, single mode, or multimode. The Ultrafine Particle Analyzer (UPA) can measure the size distribution of particles which can be suspended in liquid. The range of the instrument is 0.0054 to 6 micrometers. The thickness of the charge generating layer is 0.05 to about 6 µm, preferably 0.001 to 1 µm. As those skilled in the art appreciate, as layer thickness increases, a greater proportion of incident radiation is absorbed by a layer, but the likelihood increases of trapping a charge carrier which then does not contribute to image formation. Thus, an optimum thickness of a given such layer can constitute a balance between these competing effects. The weight ratio of the charge generating pigment to the binder is in the range of from about 5:1 to 1:5, preferably from about 2:1 to 1:4. In preparing the electrophotographic elements of the invention, the dispersion from the milling step, referred to above, of the photoconductive layer (in single layer elements) or charge generation layer (in multiactive layer elements), including any desired addenda, are dissolved or dispersed together in a liquid to form a coating composition which is then coated over an appropriate underlayer, for example, a support, barrier layer or electrically conductive layer. The liquid is then allowed or caused to evaporate from the mixture to form the permanent photoconductive layer or charge generation layer. The pigments can be mixed with the solvent solution of polymeric binder immediately or can be stored for some period of time before making up the coating composition. The polymeric binder used in the preparation of the coating composition can be any of the many different binders that are useful in the preparation of electrophotographic layers. The polymeric binder is a film-forming polymer having a fairly high dielectric strength. In a preferred embodiment of the invention, the polymeric binder also has good electrically insulating properties. The binder should provide little or no interference with the generation and transport of charges in the layer. The binder can also be selected to provide additional functions. For example, adhering a layer to an adjacent layer; or, as a top layer, providing a smooth, easy to clean, wear-resistant surface. Representative binders are film-forming polymers having a fairly high dielectric strength and good electrically insulating properties. Such binders include, for example, styrene-butadiene copolymers; vinyl toluene-styrene copolymers; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; vinylidene chloride-vinylchloride copolymers; poly(-vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); nitrated polystyrene; poly(methylstyrene); isobutylene polymers; polyesters, such as poly{ethylene-coakylenebis(alkyleneoxyaryl) phenylenedicarboxylate}; phenol-formaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly{ ethylene-coisopeopyliden-2,2-bis(ethylenoxyphenylene)-terephthalate}; copolymers of vinyl haloacrylates and vinyl acetate such as poly(vinyl-m-bromobenzoate-covinyl acetate); chlorinated poly(olefins), such as chlorinated poly(ethylene); cellulose derivatives such as cellulose acetate, cellulose acetate butyrate and ethyl cellulose; and polyimides, such as poly{ 1,1,3-trimethyl-3-(4'-phenyl)-5-indane pyromellitimide}. Examples of binder polymers which are particularly desirable from the viewpoint of minimizing interference with the generation or transport of charges include: bisphenol A polycarbonates and polyesters such as poly[(4,4'-norbonylidene)diphenylene terephthalate-co-azelate]. Polyester ionomers are useful as well. Examples of such polyester ionomers include: poly[l,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthlate-co-5-sodiosulfoisophthlate (95/5)]; poly [1,4-cyclohexylenedimethylene -co -2,2' -oxydiethylene (46/54) isophthlate-co-5-sodiosulfoisophthlate (90/10)]; poly[1,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthalate-co-5-sodiosulfoisophthalate (85/15)]; poly[1,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthalate-co-5-sodiosulfoisophthalate (80/20)]; poly[1,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthalate-co-5-sodiosulfoisophthalate (75/25)]; poly[1,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthalate-co-5-lithiosulfoisophthalate (90/10)]; poly[1,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthalate-cotriphenylmethylphosphoniumsulfoisophthalate (90/10)]; poly{ 1,4-cyclohexylenedimethylene -co -2,2' -oxydiethylene (46/54) isophthalate-co-5-(4-sulfophenoxy)isophthlate (90/10)}; poly{ 1,4-cyclohexyloxydiethylene terephthalate-co-5-(4-sulfophenoxy)isophthalate (70/30)}; and poly[1,4-cyclohexylenedimethylene-co-2,2'-oxydiethylene (46/54) isophthalate-co-4,4'-dicarboxyphenylmethylphenyl phosphonium p-toluenesulfonate (90/10)]. A wide variety of organic solvents are useful in forming the pigment polymer dispersion subjected to milling. Solvents include, for example, aromatic hydrocarbons such as benzene, toluene, xylene and mesitylene; ketones such as acetone, butanone and 4-methyl-2-pentanone; halogenated hydrocarbons such as dichloromethane, trichloroethane, methylene chloride, chloroform and ethylene chloride; ethers including ethyl ether and cyclic ethers such as dioxane and tetrahydrofuran; other solvents such as acetonitrile and dimethylsulfoxide; and mixtures of such solvents. The amount of solvent used is typically in the range of from about 2 to about 100 parts of solvent per part of binder by weight, and preferably in the range of from about 10 to 50 parts of solvent per part of binder by weight. In the coating composition, the optimum ratio of pigment to binder or pigment and charge transport material to binder can vary widely, depending on the particular materials employed. In general, useful results are obtained when the total concentration of both pigment and charge transport material in a layer is within the range of from about 0.01 to about 90 weight percent, based on the dry weight of the layer. In a preferred embodiment of a single active layer electrophotographic element of the invention, the coating composition contains from about 10 to about 70 weight percent of an charge transport agent and from 0.01 to about 20 weight percent of titanyl fluorophthalocyanine pigment of the invention. In a preferred embodiment of a multiple active layer electrophotographic element of the invention, the coating composition contains from about 0 to about 50 weight percent of a charge-transport agent and from 0.01 to about 80 weight percent of titanyl fluorophthalocyanine pigment of the invention. One or more hole donor agents can also be added to the single layer element or the charge generating layer of multilayer elements. Such agents include 1,1-bis(4-di-p-tolylaminophenyl) cyclohexane, as taught in U.S. Pat. No. 4,127,412, incorporated herein by reference, tri-p-tolylamine, and the like. Coating aids, such as levelers, surfactants, crosslinking agents, colorants, plasticizers, and the like can also be added. The quantity of each of the respective additives present in a coating composition can vary, depending upon results desired and user preferences. The barrier layer is used to prevent holes from being injected from the conducting layer into the layer carrying the charge generating function. When such injection occurs, surface charges on the electrophotographic element are dissipated in non-exposed areas of the surface, i.e., in dark areas not exposed to actinic radiation. Barrier layers are well know in the art. conducting layers with barriers layers are described in U.S. Patents 3,245,833; 2,901,348; 3,573,906; 3,640,708; 3,932,179 and 4,082,551. The barrier layer is coated directly on an electrically-conductive support. Either the support material that is electrically-conductive or a non-conductive substrate coated with a conductive layer such as vacuum deposited nickel. The support can be fabricated in any suitable configuration, for example, as a sheet, a drum, or an endless belt. Anodized aluminum substrates can also serve as combined substrate and barrier layer. Polyamides are used as barrier layers. Also, the aforementioned polyester ionomers are useful as barrier layers. Examples of electrically-conductive supports include paper (at a relative humidity above 20 percent); aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, etc.; metal plates or drums, such as aluminum, copper, zinc, brass, and galvanized plates or drums; vapor deposited metal layers such as silver, chromium, nickel, aluminum, and the like coated on paper or on conventional photographic film bases such as cellulose acetate, poly(ethylene terephthalate), 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 of such elements. 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 Trevoy, U.S. Pat. No. 3,245,833, issued April 12, 1966. Other useful conducting layers are disclosed in U.S. Pat. No. 3,880,657, U.S. Pat. No. 3,007,901, and U.S. Pat. No. 3,262,807. The single layer photoconductive element or the charge transport layer in multilayer elements can be comprised of any material, organic or inorganic, which is capable of transporting positive charge carriers generated in the charge generation layer. Most charge transport materials preferentially accept and transport either positive charges (holes) or negative charges (electrons), although there are materials known which will transport both positive and negative charges. Transport materials which exhibit a preference for conduction of positive charge carriers are referred to as p-type transport materials whereas those which exhibit a preference for the conduction of negative charges are referred to as n-type. Various p-type organic charge transport materials can be used in the charge transport layer in accordance with the present invention. Any of a variety of organic photoconductive materials which are capable of transporting positive charge carriers may be employed. Many such materials are disclosed in the patent literature already cited herein. Representative p-type organic photoconductive materials include carbazole materials arylamines (3,3'-(4-p-tolylaminophenyl)-1-phenylpropane, 1,1 -bis(4-di-p-tolylaminophenyl) cyclohexane, and tritolylamine) and polyarylalkane materials. Polymeric binders useful for the charge generation layer or photoconductor layer can also be used in producing a charge transport layer. The charge transport layer can be solvent coated or can be produced in some other manner, for example, by vacuum deposition. The layers used in the elements provided by the invention optionally contain other addenda such as leveling agents, surfactant, plasticizer, sensitizes, contrast control agents, and release agents, as is well known in the art. Various electrically conductive layers or supports can be employed in electrophotographic elements of the invention, for example, paper (at a relative humidity above 20 percent) aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, and the like; metal plates such as aluminum, copper, zinc, brass and galvanized plates; vapor deposited metal layers such as silver, chromium, vanadium, gold, nickel, aluminum and the like; and semiconductive layers such as cuprous iodide and indium tin oxide. The metal or semiconductive layers can be 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 so prepared to be exposed from either side. Of course the charge transport and charge generation functions maybe combined in a single layer. In that case the layer could contain both the above described charge generation materials and the charge transport materials. Electrophotographic elements of the invention can include various additional layers known to be useful in electrophotographic elements in general, for example, subbing layers, overcoat layers, barrier layers, and screening layers. The following examples illustrate useful embodiments of electrophotographic elements provided by the invention. Example 1 A sample of the dichloromethane-treated titanyl fluorophthalocyanine pigment of preparation 3 (9.6 g) was mixed with 2.4g of a polyvinylbutyral polymer sold under the trademark BN-18 by Wacker Chemical Company, 3 mm diameter stainless steel shots (600 g), dichloromethane (150 g) in a 9 ounce jar, and milled in a Sweco Vibro Energy grinding mill manufactured by Sweco, Inc. of Florence, Kentucky, for three days. The steel shot was then removed and rinsed with 105 g of dichloromethane, and 45 g of 1,1,2 trichloroethane, which was added back into the pigment dispersion. The resulting pigment dispersion was added to a solution of the BN-18 polyvinyl butyral binder (4.4 g), 1-bis{4-(di-4-tolylamino)phenyl}cyclohexane(2.3 g) dichloromethane (179.6 g), 1,1,2 trichloroethane (141.2 g), and 0.12g of a siloxane surfactant sold under the Trademark DC-510, by Dow Corning, USA. The dispersion was then filtered through an 8 micrometer filter and coated onto the conductive film support using the hopper coating machine at a dry coverage of 0.05g/ft 2 (0.05g/0.092903 m 2 ). The conductive film support was first coated with a barrier layer solution made of a polyamide (sold under the tradename amilan CM8000 by Toray Chemical Company) in an ethanol/1,1,2-trichloroethane 60:40 mixture. The barrier layer thickness was about 0.5 micron. particle size distribution was measured for the dispersion. The results are shown in Table 1. Comparative Example 1 A dispersion was prepared using the conditions of example 1, except that the binder was a polyester formed from 4,4'(2-norbornylidene) diphenol and a 40/60 molar ratio of terephthalic/azelaic acids. Particle size distribution for that dispersion is shown in table 1. Electrophotographic speed results are also shown in Table 1. Comparative Example 2 A dispersion was prepared using the conditions of example 1, except that the binder was a polycarbonate of bisphenol A sold under the tradename Lexan, by general Electric Corporation. Particle size distribution is shown in Table 1. Electrophotographic speed results are also shown in Table 1. Particle Size Distribution in Micrometers Dispersion 0.00-0.049 0.05-0.15 0.16-0.20 0.21-0.30 0.31-0.40 0.41-0.5 0.51-0.60 0.6-0.7 0.7-2.0 total E 50 ergs/cm 2 Comparative Example 1 0.00 0.00 0.00 4.74 22.78 20.71 21.66 15.93 14.18 100 2.00 Comparative Example 2 0.00 0.00 0.00 0.00 9.27 13.15 20.03 23.01 34.54 100 1.55 Example 1 0.00 0.00 7.06 24.30 38.32 15.61 9.32 5.39 0.00 100 2.10 From table 1 it can be seen that the dispersion with more large particles, comparative example 2, is about 25 percent faster than the other two. Breakdown Evaluation Breakdown spots are evaluated using conventional background measurement such as the modified GS measurement of Edinger (RMSGS) (J.R, Edinger, Jr., J. Imaging Sci., 31:177-183 (1987). The GS algorithm was derived by Dooley and Shaw (J. Appl. Photogr. Eng., 5:190-196 (1979)) as an expression for graininess: GS = (4.74x10 6 nd 4 ) 1/2 wherein d is the toner particle diameter in µm and n is the number of particles per square millimeter. The particle diameter (d) is determined as the average diameter taken over all the particles detected. Therefore, as the average particle diameter increases, GS also increases. The GS number has been shown to correlate well with observers' impression of background. However the results become questionable when a significant change in particle size distribution (PSD) occurs. To overcome that problem, Edinger has reexpressed the GS equation as (one square mm field assumed): RMSGS = (CD 1 4 +CD 2 4 +CD 3 4 +....CD n 4 ) 1/2 wherein C is a constant of 4.74 x 10 -6 . The toner particle distribution on paper is evaluated with image analyzers which count and size the individual toner particles. Background is the effect of randomly distributed, non-image-forming toner particles in the nominally white areas on electrophotographic copies or prints. Background results in a decrease in the reflectance of the paper. On the other hand breakdown spots are clusters of toner particles that result in the appearance of black dots, adding to the effect of conventional background. To image analyzers, breakdown spots are just larger toner particles. They are counted and sized the same way. Thus image analyzers results can be evaluated for breakdown with the RSMGS number. H. C. Kan (unpublished results, Eastman Kodak Company) has derived the index: RMSGS.ALL = (CD 1 4 +CD 2 4 +CD 3 4 +....CD n 4 ) 1/2 to designate a measurement that acknowledges the presence of breakdown spots. C is the constant 4.74 x 10 -6 and Dn is larger than the largest individual toner particle in the toner particle size distribution, and represents the diameter of the largest cluster of toner particles making up the largest breakdown spot. For example for a typical toner used in a laser printer, such as Hewlett Packard Laserjet 3 and 4 series, where the average toner particle is around 9 micrometers, we consider Dn to be larger than 21 µm. Thus we can define the contribution of breakdown to the overall background as: RMSGS.BD = (RMSGS.ALL)x{(RMSGS, >21µ )/(RMSGS, <21µ)} wherein (RMSGS, >21µ) and (RMSGS, <21µ) represent respectively the RMSGS measured for particles with diameters above 21µ, and for particles with diameters below 21µ. The smaller the number of breakdown spots, and the smaller their sizes, the lower the RMSGS.BD. With no breakdown spots, the RMSGS.BD will be Zero. The RMSGS.BD is also known as the H. C. Kan index. To evaluate breakdown spots the dispersion of example 1, and comparative examples 1 & 2 were coated on 10 inch long (254 mm), 30 mm aluminum drum substrate at 0.5 micron. The substrate was first coated with a barrier layer solution made of a polyamide (sold under the tradename amilan CM8000 by Toray Chemical Company) in an ethanol/1,1,2-trichloroethane 60:40 mixture. The barrier layer thickness was about 0.5 micron. A charge transport layer was coated on top of the coated charge generation dispersion at several thicknesses, from 20 micrometers to about 45 micrometers. The coated drums were evaluated for image quality using a Hewlett Packard Laserjet 4 printer. A white page sample (no image) was printed and evaluated for RGMGS.BD as described above. The results are shown in table 2. For comparative examples 1 and 2, it can be seen that the RMSGS.BD is as high as 0.50 for drums coated at a thickness lower than 25 micrometers. It takes a charge transport layer thickness of at least 34 t0 36 micrometers to bring the RSGMS.BD below 0.10. It should be noted that both of these comparative examples use a charge generation dispersion with at least 50% of the pigment particles having sizes above 0.40 micrometers. On the other hand the RMSGS.BD index for example 1 is below 0.10 even with charge transport layer thickness around 20 micrometers. The charge generation dispersion of this example have less than 15% of its pigment particles with sizes above 0.40 micron. Dispersion Element Thickness (micron) Breakdown Index Comparative Example 1 24 0.51 Comparative Example 1 27 0.551 Comparative Example 1 31 0.639 Comparative Example 1 33 0.201 Comparative Example 1 36 0.066 Comparative Example 2 22 0.48 Comparative Example 2 23 0.56 Comparative Example 2 27 0.193 Comparative Example 2 34 0.057 Comparative Example 2 37 0.083 Comparative Example 2 41 0.078 Comparative Example 2 43 0.064 Example 1 20 0.023 Example 1 24 0.066 Example 1 26 0.02 Example 1 32 0.057 Example 1 34 0.052 Comparative Example 3 A 01-HD attritor (1400 mL tank capacity) made by Union Process Company, was loaded with 700 cc stainless steel 3 mm (1/8") spheres media, 23.68 g of the pigment of preparation 2, 5.92 g of a polyvinyl butyral sold under the trademark BN-18 by Wacker Chemical Company, 222.24 g of dichloromethane, and 148.16 g of 1,1,2-trichloroethane. The media height was leveled with the liquid. The mixture was milled at 400 rpm for two hours. Then the mill was lowered to 100 rpm. A premixed solution consisting of 21.95 g of SLEC-BMS binder, 730.36 g of dichloromethane, and 247.69 g of 1,1,2-trichloroethane was added to the attritor. The mixture was milled at 100 rpm for 15 minutes. Then a mixture of 259.28 g of dichloromethane, and 111.12 g of 1,1,2-trichloroethane was added. Mixing is continued for another five minutes, before the dispersion is discharged through a screen, and diluted to 1.5% solid. The particle size distribution for the sample is shown in table 3. Electrophotographic speed results are also shown in table 3. Comparative Example 4 This dispersion was prepared in the same conditions as in comparative example 3, except that the media height was 10% above the liquid level. The milling was done for 8 hours at 200 rpm. Particle size distribution is shown in table 3. Electrophotographic speed (E 50 ) results are also shown in Table 3. Comparative Example 5 This dispersion was prepared as in comparative example 4, except that milling was for 8 hours at 400 rpm. The media height was 10% below the solution liquid level. Particle size distribution is shown in Table 3. Electrophotographic speed (E 50 ) results are also shown in Table 3. Particle Size Distribution in Micrometers Dispersion 0.00-0.049 0.05-0.15 0.16-0.20 0.21-0.30 0.31-0.40 0.41-0.5 0.51-0.60 0.6-0.7 0.7-2.0 total % E 50 ergs/ cm2 Comparative Example 3 % distribution 0.00 0.00 3.42 25.46 48.73 12.06 4.38 1.42 4.55 100 1.60 Comparative Example 4 % distribution 0.00 13.63 16.95 30.46 23.23 4.95 2.45 1.32 7.01 100 2.10 Comparative Example 5 % distribution 6.87 30.10 17.28 25.31 16.55 2.81 1.13 0.00 0.00 100 4.00 It can be seen again that the speed is dependent upon the particle size distribution in the charge generation layer. The dispersion containing the larger number of small particle size has the lower speed. In particular the dispersion with 6.9 % of its particle with size below 0.05 is about sixty percent slower than the dispersion having less than 4% of its particle above 0.20 micron. Particle Size Distribution in Micrometers Comments 0.00-0.049 0.05-0.15 0.16-0.20 0.21-0.30 0.31-0.40 0.41-0.5 0.51-0.60 0.6-0.7 0.7-2.0 total % E 50 ergs/cm 2 Example 2 0.00 25.78 25.04 31.44 15.37 1.88 0.49 0 0 100 1.10 Example 3 0.00 1.60 14.29 46.18 33.30 3.78 0.85 0 0 100 1.10 Example 4 0.00 0.00 9.05 39.54 42.55 6.87 1.99 0 0 100 1.00 Example 5 0.00 20.96 30.92 33.52 12.57 1.49 0.54 0 0 100 1.00 Example 2 A 2.5 gallon attritor (1S series) made by Union Process Company, was loaded at 53% with stainless steel media 3 mm (1/8") spheres media, 192 g of the cocrystal of preparation 4, 48 g of poly[4,4-xylylene-co-2,2'-oxydietylene (46/54) isophthalate-co-5-sodiosufoisophthalate 95/5], 1800 g of dichloromethane, and 1200 g of 1,1,2-trichloroethane. The media height was leveled with the liquid. The mixture was milled at 125 rpm for 3 hours. Then the mill was lowered to 100 rpm. A premixed solution consisting of 144 g of poly[4,4-xylylene-co-2,2'-oxydietylene (46/54) isophthalate-co-5-sodiosufoisophthalate 95/5], 11,369 g of dichloromethane, and 3,444 g of 1,1,2-trichloroethane was added to the attritor. The mixture was milled at 100 rpm for 15 minutes. Then a mixture of 259.28 g of dichloromethane, and 111.12 g of 1,1,2-trichloroethane was added. Mixing is continued for another five minutes, before the dispersion is discharged through a screen, and diluted to 2% solid. The particle size distribution for the sample is shown in table 4. Electrophotographic speed (E 50 ) results are also shown in table 4. Example 3 This dispersion was prepared in the same conditions as in example 2, except that the binder was the polyvinyl butyral SLEC-BMS from Sekui Chemical Company. Particle size distribution and electrophotographic speed (E 50 ) data are shown in table 4. Example 4 This dispersion was prepared similar to example 1 using a Sweco mill, except that the pigment was the cocrystalline mixture of preparation 4, and 400 g of media were used particle size distribution and electrophotographic speed (E 50 ) data are shown in Table 4. Example 5 This dispersion was prepared using a 1.5 gallon attritor, 3300 mL of 3 mm stainless steel media, 23,68 g of the cocrystalline mixture of Preparation 4, 5.9 g of SLEC-BMS polyvinyl butyral binder, 222.24 g of dichloromethane, and 148.16 g of 1,1,2, trichloroethane. The letdown solution uses17.8 g of binder, 591.6 g of dichloromethane, and 200.62 g of 1,1,2 trichloroethane. The final solution was diluted to 3% solid. Particle size and electrophotographic data are shown in Table 4. As can be seen from Table 4 all the examples show very good speed (the cocrystalline mixture is 100% faster than the plain dichloromethane treated titanyl fluorophthalocyanine). Also all samples have less than 2% of its particles bigger than 0.5 micron, and less than 30 % of its particles smaller than 0.15 micron. No particles are smaller than 0.05 micron. The dispersion of example 2 was coated on 30 mm diameter (254 mm long) Laserjet 4 like drum substrate over a series of amilan CM8000 barrier layer thickness. All drums were coated with charge transport layer for a 28 micrometers total thickness. Evaluation of breakdown spot was conducted for image quality samples generated using a Laserjet 4 printer. The results are shown in table 5. the drum coated with no barrier layer has a very high H. C. Kan breakdown index . The breakdown index is decreased as the barrier layer thickness is increased. This experiment proves that not only the particle size distribution of the dispersion has to be in a certain area, but the presence of a barrier layer is required to minimize breakdown spot to an acceptable level. Sample Barrier g/ft 2 (g/0.092903m 2 ) CGL g/ft 2 (g/0.092903m 2 ) Breakdown Index 1 0 0.023 0.452 2 0.0155 0.0407 0.239 3 0.0371 0.0407 0.089 4 0.0538 0.0407 0.048 The dispersion of example 3 was used to coat a series of drum on 30 mm Laserjet 4 like substrate. The barrier layer thickness was optimized and kept constant, but the thickness of the charge generation layer was varied. The results of table 6 show no significant variation in the H. C. Kan breakdown index. They all are below 0.10. Sample CGL g/ft 2 (g/0.092903 m 2 ) Breakdown Index 1 0.002 0.025 2 0.0041 0.067 3 0.011 0.078 4 0.0151 0.02 5 0.0282 0.098 6 0.0306 0.058 7 0.0448 0.042

Claims: An electrophotographic element comprising an electrically conductive support, a barrier layer and a photoconductive layer containing a polymeric binder having dispersed therein charge generating pigments characterized in that the pigments are (a) selected from the group consisting of titanyl fluorophthalocyanines and cocrystalline mixtures of titanyl fluorophthalocyanine and unsubstituted titanyl phthalocyanine; wherein the cocrystalline mixtures have a distinct crystallogram exhibiting major peaks of the Bragg angle 2-theta with respect to X-rays of Cu ka at a wavelength of 1.541Å at 7.5, 10.2, 12.7, 13.2, 15.1, 16.1, 17.2, 18.5, 22.4, 24.2, 25.3, 28.7, all+/-0.2, for a wide range of weight ratio of the starting phthalocyanines and (b) have a particle size from 0.05 to 0.7 µm, characterized in that up to 30% of the particles of the pigments are smaller than 0.15 µm and less than 8% of such particles are larger than 0.6 µm. An electrophotographic element according to claim 1 wherein the photoconductive layer comprises a charge generation layer and a charge transport layer and the the charge generation layer contains a polymeric binder having dispersed therein the charge generating pigments. The element of claim 1 or 2 wherein the average particle size of the pigments is between 0.20-0.30 µm. The element of claim 1, 2 or 3 wherein the titanyl fluorophthalocyanines have the general structure: where each of k, l, m, and n is independently an integer from 0 to 4 and at least one of k, l, m, and n is an integer from 1 to 4. The element of claim 4 wherein the titanyl fluorophthalocyanines are titanyl tetrafluorophthalocyanine. The element of claim 5 wherein the charge generation layer comprises poly[4,4-xylylene-co-2,2'-oxydietylene (46/54) isophthalate-co-5-sodiosufoisophthalate 95/5] as a binder. The element of claim 6 wherein the mole ratio of titanyl phthalocyanine to titanyl fluorophthalocyanine is 75:25. The element of claim 7 wherein the barrier layer is a polyamide.

Inventors: CORBIN DOUGLAS DEAN, MOLAIRE MICHEL F, CORBIN, DOUGLAS DEAN, MOLAIRE, MICHEL, F.
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1997
Main classification: G03G5
IPC codes: G03G5