Patent Application
20070281226
Electrophotographic imaging members are known in the art. Electrophotographic imaging members may be prepared by any suitable technique. Typically, a flexible or rigid substrate is provided with an electrically conductive surface. A charge generating layer is then applied to the electrically conductive surface. A charge blocking layer may optionally be applied to the electrically conductive surface prior to the application of a charge generating layer. If desired, an adhesive layer may be utilized between the charge blocking layer and the charge generating layer. Usually the charge generation layer is applied onto the blocking layer and a hole transport layer is formed on the charge generation layer, followed by an optional overcoat layer. This structure may have the charge generation layer on top of or below the hole transport layer.
The substrate may be opaque or substantially transparent and may comprise any suitable material having the required mechanical properties. Accordingly, the substrate may comprise a layer of an electrically non-conductive or conductive material such as an inorganic or an organic composition. As electrically non-conducting materials there may be employed various resins known for this purpose including polyesters, polycarbonates, polyamides, polyurethanes, and the like which are flexible as thin webs. An electrically conducting substrate may be any metal, for example, aluminum, nickel, steel, copper, and the like or a polymeric material, as described above, filled with an electrically conducting substance, such as carbon, metallic powder, and the like or an organic electrically conducting material. The electrically insulating or conductive substrate may be in the form of an endless flexible belt, a web, a rigid cylinder, a sheet and the like. The thickness of the substrate layer depends on numerous factors, including strength desired and economical considerations. Thus, for a drum, this layer may be of substantial thickness of, for example, up to many centimeters or of a minimum thickness of less than a millimeter. Similarly, a flexible belt may be of substantial thickness, for example, about 250 micrometers, or of minimum thickness less than 50 micrometers, provided there are no adverse effects on the final electrophotographic device.
In embodiments where the substrate layer is not conductive, the surface thereof may be rendered electrically conductive by an electrically conductive coating. The conductive coating may vary in thickness over substantially wide ranges depending upon the optical transparency, degree of flexibility desired, and economic factors. Accordingly, for a flexible photoresponsive imaging device, the thickness of the conductive coating may be about b 20 angstroms to about 750 angstroms, such as about 100 angstroms to about 200 angstroms for an optimum combination of electrical conductivity, flexibility and light transmission. The flexible conductive coating may be an electrically conductive metal layer formed, for example, on the substrate by any suitable coating technique, such as a vacuum depositing technique or electrodeposition. Typical metals include aluminum, zirconium, niobium, tantalum, vanadium and hafnium, titanium, nickel, stainless steel, chromium, tungsten, molybdenum, and the like.
An optional hole blocking layer may be applied to the substrate. Any suitable and conventional blocking layer capable of forming an electronic barrier to holes between the adjacent photoconductive layer and the underlying conductive surface of a substrate may be utilized.
An optional adhesive layer may be applied to the hole blocking layer. Any suitable adhesive layer known in the art may be utilized. Typical adhesive layer materials include, for example, polyesters, polyurethanes and the like. Satisfactory results may be achieved with adhesive layer thickness of about 0.05 micrometer (500 angstroms) to about 0.3 micrometer (3,000 angstroms). Conventional techniques for applying an adhesive layer coating mixture to the charge blocking layer include spraying, dip coating, roll coating, wire wound rod coating, gravure coating, Bird applicator coating, and the like. Drying of the deposited coating may be effected by any suitable conventional technique such as oven drying, infra red radiation drying, air drying and the like.
At least one electrophotographic imaging layer is formed on the adhesive layer, blocking layer or substrate. The electrophotographic imaging layer may be a single layer that performs both charge generating and hole transport functions as is known in the art or it may comprise multiple layers such as a charge generator layer and hole transport layer. Charge generator layers may comprise amorphous films of selenium and alloys of selenium and arsenic, tellurium, germanium and the like, hydrogenated amorphous silicon and compounds of silicon and germanium, carbon, oxygen, nitrogen and the like fabricated by vacuum evaporation or deposition. The charge generator layers may also comprise inorganic pigments of crystalline selenium and its alloys; Group II-VI compounds; and organic pigments such as quinacridones, polycyclic pigments such as dibromo anthanthrone pigments, perylene and perinone diamines, polynuclear aromatic quinones, azo pigments including bis-, tris- and tetrakis-azos; and the like dispersed in a film forming polymeric binder and fabricated by solvent coating techniques.
Phthalocyanines have been employed as photogenerating materials for use in laser printers utilizing infrared exposure systems. Infrared sensitivity is required for photoreceptors exposed to low cost semiconductor laser diode light exposure devices. The absorption spectrum and photosensitivity of the phthalocyanines depend on the central metal atom of the compound. Many metal phthalocyanines have been reported and include, oxyvanadium phthalocyanine, chloroaluminum phthalocyanine, copper phthalocyanine, oxytitanium phthalocyanine, chlorogallium phthalocyanine, hydroxygallium phthalocyanine magnesium phthalocyanine and metal-free phthalocyanine. The phthalocyanines exist in many crystal forms which have a strong influence on photogeneration.
Any suitable polymeric film forming binder material may be employed as the matrix in the charge generating (photogenerating) binder layer. Typical polymeric film forming materials include those described, for example, in U.S. Pat. No. 3,121,006, the entire disclosure of which is incorporated herein by reference. Thus, typical organic polymeric film forming binders include thermoplastic and thermosetting resins such as polycarbonates, polyesters, polyamides, polyurethanes, polystyrenes, polyarylethers, polyarylsulfones, polybutadienes, polysulfones, polyethersulfones, polyethylenes, polypropylenes, polyimides, polymethylpentenes, polyphenylene sulfides, polyvinyl acetate, polysiloxanes, polyacrylates, polyvinyl acetals, polyamides, polyimides, amino resins, phenylene oxide resins, terephthalic acid resins, phenoxy resins, epoxy resins, phenolic resins, polystyrene and acrylonitrile copolymers, polyvinylchloride, vinylchloride and vinyl acetate copolymers, acrylate copolymers, alkyd resins, cellulosic film formers, poly(amideimide), styrenebutadiene copolymers, vinylidenechloride-vinylchloride copolymers, vinylacetate-vinylidenechloride copolymers, styrene-alkyd resins, polyvinylcarbazole, and the like. These polymers may be block, random or alternating copolymers.
The photogenerating composition or pigment is present in the resinous binder composition in various amounts. Generally, however, from about 5 percent by volume to about 90 percent by volume of the photogenerating pigment is dispersed in about 10 percent by volume to about 95 percent by volume of the resinous binder, such as from about 20 percent by volume to about 30 percent by volume of the photogenerating pigment dispersed in about 70 percent by volume to about 80 percent by volume of the resinous binder composition. In one embodiment about 8 percent by volume of the photogenerating pigment is dispersed in about 92 percent by volume of the resinous binder composition. The photogenerator layers can also fabricated by vacuum sublimation in which case there is no binder.
Any suitable and conventional technique may be utilized to mix and thereafter apply the photo generating layer coating mixture. Typical application techniques include spraying, dip coating, roll coating, wire wound rod coating, vacuum sublimation and the like. For some applications, the generator layer may be fabricated in a dot or line pattern. Removing of the solvent of a solvent coated layer may be effected by any suitable conventional technique such as oven drying, infrared radiation drying, air drying and the like.
The hole transport layer comprises a bole transporting small molecule dissolved or molecularly dispersed in a film forming electrically inert polymer such as a polycarbonate. The term “dissolved” as employed herein is defined herein as forming a solution in which the small molecule is dissolved in the polymer to form, a homogeneous phase. The expression “molecularly dispersed” as used herein is defined as a hole transporting small molecule dispersed in the polymer, the small molecules being dispersed in the polymer on a molecular scale. Any suitable hole transporting or electrically active small molecule may be employed in the hole transport layer. The expression hole transporting “small molecule” is defined herein as a monomer that allows the free charge photogenerated in the transport layer to be transported across the transport layer. Typical hole transporting small molecules include, for example, pyrazolines such as 1-phenyl-3-(4′-diethylamino styryl)-5-(4″-diethylamino phenyl)pyrazoline, diamines such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, hydrazones such as N-phenyl-N-methyl-3-(9-ethyl)carbazyl hydrazone and 4-diethyl amino benzaldehyde-1,2-diphenyl hydrazone, and oxadiazoles such as 2,5-bis(4-N,N′-diethylaminophenyl)-1,2,4-oxadiazole, stilbenes and the like. As indicated above, suitable electrically active small molecule hole transporting compounds are dissolved or molecularly dispersed in electrically inactive polymeric film forming materials. Small molecule hole transporting compounds that permit injection of holes from the pigment into the charge generating layer with high efficiency and transport them across the hole transport layer with very short transit times are N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine, N,N,N′,N′-tetra-p-tolylbiphenyl-4,4′-diamine, and N,N′-Bis(3-methylphenyl)-N,N′-bis[4-(1-butyl)phenyl]-[p-terphenyl]-4,4′-diamine. If desired, the hole transport material in the hole transport layer may comprise a polymeric hole transport material or a combination of a small molecule hole transport material and a polymeric hole transport material.
Any suitable electrically inactive resin binder insoluble in the alcohol solvent used to apply the overcoat layer may be employed in the hole transport layer. Typical inactive resin binders include polycarbonate resin, polyester, polyarylate, polysulfone, and the like. Molecular weights can vary, for example, from about 20,000 to about 150,000. Exemplary binders include polycarbonates such as poly(4,4′-isopropylidene-diphenylene)carbonate (also referred to as bisphenol-A-polycarbonate, poly(4,4′-cyclohexylidinediphenylene) carbonate (referred to as bisphenol-Z polycarbonate), poly(4,4′-isopropylidene-3,3′-dimethyl-diphenyl)carbonate (also referred to as bisphenol-C-polycarbonate) and the like. Any suitable hole transporting polymer may also be utilized in the hole transporting layer. The hole transporting polymer should be insoluble in any solvent employed to apply the subsequent overcoat layer described below, such as an alcohol solvent. These electrically active hole transporting polymeric materials should be capable of supporting the injection of photogenerated holes from the charge generation material and be incapable of allowing the transport of these holes therethrough.
Any suitable and conventional technique may be utilized to mix and thereafter apply the hole transport layer coating mixture to the charge generating layer. Typical application techniques include spraying, dip coating, roll coating, wire wound rod coating, and the like. Drying of the deposited coating may be effected by any suitable conventional technique such as oven drying, infra red radiation drying, air drying and the like.
Generally, the thickness of the hole transport layer is between about 10 and about 50 micrometers, but thicknesses outside this range can also be used. The hole transport layer should be an insulator to the extent that the electrostatic charge placed on the hole transport layer is not conducted in the absence of illumination at a rate sufficient to prevent formation and retention of an electrostatic latent image thereon. In general, the ratio of the thickness of the hole transport layer to the charge generator layers is desirably maintained from about 2:1 to 200:1 and in some instances as great as 400:1. The hole transport layer, is substantially non-absorbing to visible light or radiation in the region of intended use but is electrically “active” in that it allows the injection of photogenerated holes from the photoconductive layer, i.e., charge generation layer, and allows these holes to be transported through itself to selectively discharge a surface charge on the surface of the active layer.
In accordance with embodiments, a strong electron accepting material is also dispersed in at least one layer of the photoreceptor, to provide improved electrical properties to the photoreceptor. In embodiments, the strong electron accepting material can be dispersed in any of the photoreceptor layers, such as an undercoating layer, an overcoating layer, or the photogenerating layer. However, in particular embodiments, the strong electron accepting material is incorporated or dispersed in the hole transport layer and/or in an overcoating layer along with the hole transporting small molecule.
When so incorporated, the strong electron accepting material can be incorporated into the whole layer, such as to make a uniform dispersion of the strong electron accepting material in the layer, or it can be incorporated into the layer in a varying amount to make a concentration gradient of the strong electron accepting material in the layer. In these embodiments, there would thus be a relatively larger amount or concentration of the strong electron accepting material in one portion of the layer, and a relatively smaller amount or concentration of the strong electron accenting material in another portion of the layer, such as in radially inner and outer portions of the layer. In other embodiments, the layer can be provided in two or more distinct sub-layers, where the two or more distinct sub-layers include different amounts or concentrations of the strong electron accepting material. In these embodiments, the thicknesses of the two or more different sub-layers can have any proportions relative to the whole thickness and to each other. For example, the strong electron accepting material in these embodiments can be restricted to only a portion of the overall layer, such as an overall hole transport layer and/or an overcoating layer. In embodiments, the use of sub-layers allows the strong electron accepting material to be restricted to, for example, from about 1 or about 5 to about 50 or about 75% or more of the overall layer thickness. Thus, for example, a thickness ratio of a thickness of the sub-layer containing the strong electron acceptor material to a thickness of a sub-layer not containing the strong electron acceptor material is from about 1:99 to about 99:1, or about 5:95 to about 95:5, or about 25:75 to about 75:25. Restricting the strong electron accepting material to only a portion of the overall layer thickness can be useful, for example, to help provide a photoreceptor with lower residual voltage values (such as from about 0 to about 5 volts or front about 0 to about 3 volts) and/or reduced cycling changes, while still providing minimal undesired increases in dark decay, such as a dark decay of from about 0 to about 30 volts. However, some of these benefits can be provided without restricting the strong electron accepting material to only a portion of the overall layer thickness.
The term “strong electron accepting material” refers, for example, to a material or chemical species that is capable of oxidizing another material that co-exists in the same layer of a photoreceptor device, where that other material is typically a hole transport material, such as, for example a hole transport material in a hole or charge transport layer or an overcoating layer. It is believed that the capacity of the strong electron accepting material to oxidize the hole transport material arises from the strong electron affinity of the strong electron accepting material. In embodiments, the electron affinity is, for example, no less than about 2 electron Volts (eV), and typically no less than about 3 eV, and sometimes no less than about 5 eV. There is generally no upper limit, although the electron affinity is, for example, no more than about 15 electron Volts (eV), and typically no more than about 10 eV. Such strong electron accepting materials are sometimes characterized by having a lowest unoccupied molecular orbital (LUMO) that has an energy, ELUMO, of no less than about 2 eV versus the vacuum energy level EVAC (where energy of EVAC is taken, by convention, as 0 eV), and typically no less than about 3 eV, and sometimes no less than about 5 eV.
Suitable examples of such strong electron accepting materials include, but are not limited to, Tetracyanoquinonedimethane (TCNQ) and its derivatives, such as the fluorinated TCNQ-analog 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinonedimethane (sometimes referred to as F4TCNQ), and organometal complexes of TCNQ, such as M-TCNQ where M represents a metal such as Li, Na, K, Ag, Cut, or Fe. Other suitable examples of such strong electron accepting materials include Lewis acid compounds such as FeCl3, AlCl3, AlBr3, BF3, BF32C6H5OH, BF3[O(C2H5)2]2, TiCl4, SnCl4, AlC2H5Cl2, SbCl5, SbF5, ZrCl4, HfCl4, NbCl5, TaCl5, MoCl5, and WCI6 and the like. Other suitable examples of such strong electron accepting materials include fullerenes, such as, for example, C60 and C70 and their derivatives. Other strong electron acceptor materials include iodine, tris(4-bromophenyl)ammonium hexachloroantimonate (TBAHA), quinones fused with sulfur containing heterocycles, N,N′-dicyanoquinone diimine (DCNQi) analogues, radialenes containing a sulphur, selenium or tellurium atoms and others like those described by Yamashita and Tomura [J. Material Chemistry, Volume 8, pages 1933-1944 (1998)] and others.
In forming the layer containing the strong electron accepting material, the strong electron accepting material can be simply mixed with the other layer components to forms a uniform or substantially uniform dispersion, and thereafter applied to form the layer. For example, where the strong electron accepting material is included in a hole transport layer, the strong electron accepting material can be mixed with the hole transport material and applied to form the hole transport layer. In other embodiments, however, it may be desirable to first form a solution of the strong electron accepting material in a suitable solvent, such as CH2Cl2, and then to mix the resultant solution with the other layer components to form a layer-forming composition. These two-step process can be used to help ensure complete mixing of the strong electron accepting material in the layer-forming composition.
The strong electron accepting material can be included in the respective photoreceptor layer in any desired amount, such as from greater than 0% up to about 10% or up to about 20% by weight oft the final applied layer. However, much smaller amounts of the strong electron accepting material can be used in forming the layers. Thus, for example, in embodiments, the strong electron accepting material can be present in an amount of from greater than 0% up to about 1%, such as up to about 0.5% up to about 0.1% by weight, or up to about 0.025 or up to about 0.05%, by weight of the total solid content of the layer. Of course, other amounts can be used as desired
To improve photoreceptor wear resistance, a protective overcoat layer can be provided over the hole transport layer (or other underlying layer). Various overcoating layers are known in the art, and can be used as long as the functional properties of the photoreceptor are not adversely affected.
Advantages provided by the present disclosure include, in embodiments, photoreceptors having desirable electrical and function properties. For example, photoreceptors in embodiments have one or more of (i) a low residual voltage (Vr) value, such as from about 0 to about 10 volts or from about 0 to about 5 volts or from about 0 to about 3 volts, (ii) reduced cycling changes over at least about 10,000 cycles, such as, for example a cycling-up change (increase) in Vr of no more than 15 Volts when cycled for about 10,000 cycles.
Also, included within the scope of the present disclosure are methods of imaging and printing with the imaging members illustrated herein. These methods generally involve the formation of an electrostatic latent image on the imaging member; followed by developing the image with a toner composition comprised, for example, of thermoplastic resin, colorant, such as pigment, charge additive, and surface additives, reference U.S. Pat. Nos. 4,560,635, 4,298,697 and 4,338,390, the disclosures of which are totally incorporated herein by reference; subsequently transferring the image to a suitable substrate; and permanently affixing the image thereto. In those environments wherein the device is to be used in a printing mode, the imaging method involves the same steps with the exception that the exposure step can be accomplished with a laser device or image bar.
An example is set forth hereinbelow and is illustrative of different compositions and conditions that can be utilized in practicing the disclosure. All proportions are by weight unless otherwise indicated. It will be apparent, however, that the disclosure can be practiced with many types of compositions and can have many different uses in accordance with the disclosure above and as pointed out hereinafter.
A hole transport layer coating solution is prepared by introducing into an amber glass bottle a weight ratio of 1:1 N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4˜4′-diamine, and Makrolon 5707, a polycarbonate resin having a weight average molecular weight of about 120,000 commercially available from Bayer A.G. The resulting mixture is dissolved to give a 15 percent by weight solid in 85 percent by weight methylene chloride.
Four hole transport layer coating solutions are prepared in the same manner as Comparative Example 1, by introducing into an amber glass bottle a weight ratio of 1:1 N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4˜4′-diamine, and Makrolon 5707, a polycarbonate resin having a weight average molecular weight of about 120,000 commercially available from Bayer A.G. The resulting mixture is dissolved to give a 15 percent by weight solid in 85 percent by weight methylene chloride. To the mixture is added tetrafluorotetracyanoquinonedimethane (F4TCNQ) in amounts to form final solutions having 0.017, 0.025, 0.030, and 0.50% by weight F4TCNQ.
Imaging member sheets or belts are formed using various of the hole transport layer coating compositions of Example 1 and Comparative Example 1. Each imaging member sheet or belt is formed as follows: A production machine coated PEN/Mylar/TiZr/Silane/Ardel substrate was provided and a HOGaPc/PCZ-200 photogenerating, layer was production machine coated over the substrate. A hole transport layer was hand coated on the charge generating layer using web coating methods. The hole transport layer coating compositions are applied as two sub-layers of thickness ratios of 10:90, 50:50, or 90:10, where a first sub-layer is denoted CTL1 and a second sub-layer is denoted CTL2. The respective solutions are applied onto the photogenerator layer to form a coating that upon drying has a total hole transport layer thickness of around 29 micrometers. In the various imaging member sheets or belts, the hole transport layer solutions used are as shown in the Table below. The coating was dried in a forced air oven for about 1 minute at about 120° C.
Device performance is evaluated using time zero PIDC measurements and long term electrical cycling over 10,000 cycles in ambient conditions. The imaging members are tested for their electrostatographic sensitivity and cycling stability in a scanner. The scanner is known in the industry and equipped with means to rotate the drum while it is electrically charged and discharged. The charge on the sample is monitored through use of electrostatic probes placed at precise positions around the circumference of the device. The samples in this Example are charged to a negative potential of 700 Volts. As the device rotates, the initial charging potential is measured by voltage probe 1, and then the potential after dark decay is measured by voltage probe 2, and the value of Vdd is calculated. The sample is then exposed to monochromatic radiation of known intensity, and the surface potential measured by voltage probes 3 and 4. Finally, the sample is exposed to all erase lamp of appropriate intensity and wavelength and any residual potential, Vr, is measure by voltage probe 5. The process is repeated under the control of the scanner's computer, and the data is stored in the computer. The PIDC (photo induced discharge curve) is obtained by plotting the potentials at voltage probes 3 and 4 as a function of the light energy.
The Table below includes the dark decay voltage (Vdd) and residual voltage (Vr) values. Only Vdd and Vr are shown here because other electrical characteristics remain essentially unchanged among the various configurations. From the results, it can be seen that the use of F4TCNQ substantially reduces Vr in all device configurations as compared to the reference device configurations (Samples D and I). It can also be seen that limiting the use of F4TCNQ to only a part of the entire hole transport layer (that is in either CTL1 or CTL2 but not both) and to lower concentrations (e.g. 0.025 wt % or lower) leads to desirable Vdd (<30 volts). In that regard, Samples E and K are observed to provide low Vr coupled with low Vdd.
Imaging member sheets or belts are formed in the same manner as in Example 2. A control imaging member sheet is formed using no F4TCNQ, for comparison to an exemplary imaging member sheet formed using 0.050 weight % F4TCNQ in the first hole transport sub-layer (CTL1) and no F4TCNQ in the second hole transport sub-layer (CTL2).
Device performance is evaluated using time zero PIDC measurements and long tern electrical cycling over 10,000 cycles in ambient conditions, as in Example 2. The results show that for the exemplary imaging member sheet, the device containing the strong electron accepting material shows less cycling up; pointing to the increased cycling stability.
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.
