Patent Application
20080038650
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 or charge 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 or charge transport layer. In embodiments, the charge generating layer and hole or charge transport layer can be combined into a single active layer that performs both charge generating and hole transport functions.
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.
1 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 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 or charge transport functions as is known in the art or it may comprise multiple layers such as a charge generator layer and charge 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 be 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 photogenerating 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 charge transport layer comprises a charge 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 charge transporting small molecule dispersed in the polymer, the small molecules being dispersed in the polymer on a molecular scale. Any suitable charge transporting or electrically active small molecule may be employed in the charge transport layer. The expression charge 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 charge 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 charge transporting compounds are dissolved or molecularly dispersed in electrically inactive polymeric film forming materials. Small molecule charge transporting compounds that permit injection of holes from the pigment into the charge generating layer with high efficiency and transport them across the charge 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 charge transport material in the charge transport layer may comprise a polymeric charge transport material or a combination of a small molecule charge transport material and a polymeric charge transport material.
The charge transport layer further comprises, either in addition to or in place of the above-described charge transport materials, carbon nanotube materials dissolved or molecularly dispersed in the film forming binder. In an embodiment, the charge transport layer comprises the carbon nanotube materials, and is free or essentially free of other charge transport materials. In embodiments, the carbon nanotube material comprises carbon nanotubes, carbon nanofibers, or variants thereof, which are chemically functionalized such as with soluble polymeric groups. As the carbon nanotube material, any of the currently known or after-developed carbon nanotube materials and variants can be used. Thus, for example, the carbon nanotubes can be on the order of from about 0.1 to about 50 nanometers in diameter, such as about 1 to about 10 nanometers in diameter, and up to hundreds of micrometers or more in length, such as from about 0.01 or about 10 or about 50 to about 100 or about 200 or about 500 micrometers in length. The carbon nanotubes can be in multi-walled or single-walled forms, or a mixture thereof. In some embodiments, the carbon nanotube materials are particularly of the single-walled form. The carbon nanotubes can be either conducting or semi-conducting, with conducting nanotubes being particularly useful in embodiments. Variants of carbon nanotubes include, for example, nanofibers, and are encompassed by the term “carbon nanotube materials” unless otherwise stated.
In addition, the carbon nanotubes of the present disclosure can include only carbon atoms, or they can include other atoms such as boron and/or nitrogen, such as equal amounts of born and nitrogen. Examples of carbon nanotube material variants thus include boron nitride, bismuth and metal chalcogenides. Combinations of these materials can also be used, and are encompassed by the term “carbon nanotube materials” herein. In embodiments, the carbon nanotube material is desirably free, or essentially free, of any catalyst material used to prepare the carbon nanotubes. For example, iron catalysts or other heavy metal catalysts are typically used for carbon nanotube production. However, it is desired in embodiments that the carbon nanotube material not include any residual iron or heavy metal catalyst material.
Because carbon nanotube materials are generally not soluble in the solvents and film-forming binder used in forming charge transport layers, it is desirable to chemically functionalize the carbon nanotube materials. The chemical functionalization is suitable, for example, for attaching soluble polymeric groups to side walls of the carbon nanotube materials to improve the solubility of the carbon nanotube materials in the charge transport layer components. It is known that carbon centered radicals will react at the surface of a carbon nanotube thereby allowing the carbon centered radical to become covalently bound to the carbon nanotube. One exemplary practical way of performing this transformation is to have a chemical functionality that is stable at room temperature and that becomes labile (or reactive) at elevated temperatures. One such chemical system, known in the art, is polymers prepared by a process commonly referred to as stable free radical polymerization (SFRP) also referred to as nitroxide mediated radical polymerization (NMRP). See, for example, U.S. Pat. Nos. 5,449,724; 5,728,747; and 6,156,858, the entire disclosures of which are incorporated herein by reference. Polymers prepared by this method contain carbon-nitrogen-oxygen residues (carbon capped with nitroxide) at a chain terminus. Heating of these polymers at temperatures of between, for example, 100° C. and 120° C. produces a carbon centered radical at the chain terminus while liberating the nitroxide. If this process is done in the presence of a carbon nanotube, the carbon centered radical will react with the surface of the carbon nanotube and thereby covantely bind the polymer to the carbon nanotube, thereby imparting the desirable characteristics of typical polymers to the carbon nanotube/polymer composite. In the case of application for a photoreceptive device, in embodiments it is desirable to incorporate polymers of relatively low polarity and not containing local dipoles. One example of such suitable polymers is polystyrene.
In embodiments, the carbon nanotube materials can be incorporated into the charge transport layer in any desirable and effective amount. For example, a suitable loading amount can range from about 0.5 or from about 1 weight percent, to as high as about 50 or about 60 weight percent or more. However, loading amounts of from about 1 or from about 5 to about 20 or about 30 weight percent may be desired in some embodiments. Thus, for example, the charge transport layer in embodiments could comprise about 50 to about 60 percent by weight polymer binder, about 30 to about 40 percent by weight hole transport small molecule, and about 5 to about 20 percent by weight carbon nanotube material, although amounts outside these ranges could be used.
A benefit of the use of chemically functionalized carbon nanotube materials in charge transport layers is that charge transport or conduction by the carbon nanotube materials is predominantly electrons. The small size of the carbon nanotube materials also means that the carbon nanotube materials provide low scattering efficiency and high compatibility with the polymer binder and optional small molecule charge transport materials in the layer. Although not limited by theory, it is believed that the electron conduction mechanism through the resultant charge transport layer is by charge transport through the carbon nanotubes themselves, and/or by charge hopping channels between carbon nanotubes formed by closely contacted nanotubes.
Further, the carbon nanotube materials exhibit very high charge transport mobility. Accordingly, the use of chemically functionalized carbon nanotube materials in a charge transport layer can provide charge transport speeds that are orders of magnitude higher than charge transport speeds provided by conventional charge transport materials. For example, the charge transport mobility in a charge transport layer comprising carbon nanotube materials can be 1, 2, 3, 4, 5, 6, 7, or more, such as about 1 to about 4, orders of magnitude higher as compared to a comparable charge transport layer that includes a similar amount of conventional pyrazoline, diamine, hydrazones, oxadiazole, or stilbene charge transport small molecules. This resultant dramatic increase in charge mobility can result in significant corresponding improvements in the printing process and apparatus, such as extreme printing speeds, increased print quality, and increased photoreceptor reliability.
Additional details regarding carbon nanotubes and their charge transport mobilities can be found, for example, in T. Durkop et al., “Extraordinary Mobility in Semiconducting Carbon Nanotubes,” Nano. Lett., Vol. 4, No. 1, 35-39 (2004), the entire disclosure of which is incorporated herein by reference.
Any suitable electrically inactive resin binder insoluble in the alcohol solvent used to apply an optional overcoat layer may be employed in the charge 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 charge transporting polymer may also be utilized in the charge transporting layer. The charge 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 charge 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 charge 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 charge transport layer is between about 10 and about 50 micrometers, but thicknesses outside this range can also be used. The charge transport layer should be an insulator to the extent that the electrostatic charge placed on the charge 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 charge 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 charge 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.
To improve photoreceptor wear resistance, a protective overcoat layer can be provided over the photogenerating 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.
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.
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.
Commonly assigned U.S. patent application Ser. No. ______, filed concurrently herewith (Attorney Docket No. 127968), describes an electrophotographic imaging member comprising: a substrate, an optional intermediate layer, a photogenerating layer, and an optional overcoating layer, wherein the photogenerating layer comprises a carbon nanotube material. Commonly assigned U.S. patent application Ser. No. ______, filed concurrently herewith (Attorney Docket No. 127970), describes an electrophotographic imaging member comprising: a substrate, a photogenerating layer, and an optional overcoating layer wherein the photogenerating layer comprises a multi-block polymeric charge transport material at least partially embedded within a carbon nanotube material. Commonly assigned U.S. patent application Ser. No. ______, filed concurrently herewith (Attorney Docket No. 127971), describes an electrophotographic imaging member comprising: a substrate, a photogenerating layer, and an optional overcoating layer wherein the photogenerating layer comprises a self-assembled carbon nanotube material having pendant charge transport materials. The appropriate components and process aspects of each of the foregoing, such as the photoreceptor materials and processes, may be selected for the present disclosure in embodiments thereof. The entire disclosures of the above-mentioned applications are totally incorporated herein by reference.
