Patent Application
20080020306
Electrophotographic imaging members are known in the art. Electrophotographic imaging members may be prepared by any suitable technique. Typically, a flexible or rig,id 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 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 charge 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 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 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 polymer 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 phthaliocyanine, chloroaluminuim 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.
However, in embodiments, the photogenerating material incorporated into the charge generating layer is a titanium phthalocyanine, denoted TiOPc, such as a high sensitivity titanyl phthalocyanine. In this respect, the sensitivity of photogenerator materials is commonly ranked according to the material's half exposure energy, namely E1/2, which represents the amount of exposure energy required to achieve 50% of photodischarge in xerographic measurement. Therefore, in comparative measurement, a “high” sensitivity pigment has lower E1/2 values than a “low” sensitivity material. Accordingly, as used herein, high sensitivity generator materials typically have E1/2 values of less than about 1 erg/cm2 for a 30 micron thick imaging member photodischarged from 800 volts. Any suitable titanium phthalocyanine pigment can be used in the charge generating layer, including known or developed titanium phthalocyanine denoted as Type I, II, III, X, IV, Y, and X. For example, Type I titanium phthalocyanines can be prepared by various known methods, including those described in U.S. Pat. Nos. 5,153,094; 5,166,339; 5,189,155; and 5,189,156, the entire disclosures of which are incorporated herein by reference.
Some embodiments in particular use a Type V titanium phthaliocyanine such as those disclosed in U.S. patent application Ser. No. 10/992,500, filed Nov. 18, 2004, the entire disclosure of which is incorporated herein by reference. A Type V titanyl phthalocyanine is distinguishable from, for example, Type IV titanyl phthalocyanines, in that a Type V titanyl phthlalocyanine exhibits an x-ray powder diffraction spectrum having four characteristic peaks at 9.0°, 9.6°, 24.0°, and 27.2°, while Type IV titanyl phthalocyanines typically exhibit only three characteristic peaks at 9.6°, 24.0°, and 27.2°.
The average particle size of the titanyl phthalocyanine can be suitably selected to provide desired results. For example, in embodiments, the average particle size may be selected to be from about 10 nm to about 500 nm, although sizes outside this range can be used.
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, 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.
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.
However, in embodiments, an additional component, a glycol compound, is used in preparing the photogenerating layer coating mixture, in order to stabilize the photogenerating material such as titanyl phthalocyanine against polymorphic (crystal structure) change in the polymer binder/solvent mixture in which the pigment is otherwise mixed. The additional component, in embodiments, is a particular glycol compound, which can be used either in addition to the above film-forming polymer, or in place of some of the above film-forming polymer. In particular embodiments, the glycol is used to first stabilize and disperse the photogenerating material, and the resultant dispersion of photogenerating material and glycol is then added to a further solution of the film-forming polymer and solvent. Thus, for example, while the glycol compound can be used both to stabilize the photogenerating material, it is desired in embodiments that the final composition include both a glycol compound and a film-forming polymer binder that in embodiments does not include hydroxyl groups.
Thus, for example, the photogenerating layer coating mixture can be prepared by first preparing a mixture of the photogenerating material and the glycol compound in an amount of the film-forming polymer binder, optionally in a solvent. Once the mixture is formed, whereby the photogenerating material is stabilized against polymorphic (crystal structure) change by the glycol compound, the mixture can be added to the remaining bulk film-forming polymer and solvent to form the final photogenerating layer coating mixture. The solvents used in forming the first mixture and in forming the final mixture can be the same or different.
Any suitable glycol compound can be used to stabilize the photogenerating material. Examples of suitable glycol compounds include alkyldiol compounds having, for example from about 3 to about 20 carbon atoms, such as from about 3 to about 12 or to about 15 carbon atoms, or from about 3 to about 8 carbon atoms. In addition, in embodiments, the glycol compound desirably has two hydroxyl groups bonded to adjoining carbon atoms in the carbon chain, as such glycol compounds have been found to provide improved results even over similar compounds of similar carbon chain length but with two hydroxyl groups bonded to different carbon atoms in non-adjacent positions. Thus, for example, where the glycol compound is a hexanediol, the hexanediol is, for example, 1,2-hexanediol, 2,3-hexanediol, or 3,4-hexanediol, rather than, for example, 1,3-hexanediol, 1,4-hexanediol, 1,5-hexanediol, 1,6-hexanediol, 2,4-hexanediol, or 2,5-hexanediol.
Examples of specific suitable glycol compounds include propanediols such as 1,3-propanediol; butanediols such as 1,2-butanediol and 2,3-butanediol; pentanediols such as 1,2-pentanediol and 2,3-pentanediol; hexanediols such as 1,2-hexanediol, 2,3-hexanediol, and 3,4-hexanediol; heptanediols such as 1,2-heptanediol, 2,3-heptanediol, and 3,4-heptanediol; octanediols such as 1,2-octanediol, 2,3-octanediol, 3,4-octanediol, and 4,5-octanediol; nonanediols; decanediols; undecanediols, dodecanediols; and the like. In addition, the glycols can be unsubstituted, or substituted by groups such as alkyl groups, and the like. For example, alkyl substituted glycol compounds include 3-methyl-1,2-pentanediol, 3,3-dimethyl-1,2-pentanediol, 3-methyl-3-ethyl-1,2-pentanediol, 2-methyl-2,3-pentanediol, 3,3-dimethyl-1,2-propanediol, and the like.
The glycol compound can be combined with the photogenerating material in any desired or suitable amount, although in embodiments the glycol compound is combined with the photogenerating material in a smaller amount relative to the photogenerating material. For example, the glycol compound can be present in an amount of from about 0.1 to about 10 percent by weight, compared to the photogenerating material that can be present in an amount of from about 30 to about 70 percent by weight. A solvent can also be added in any desired amount. As a result, the final dried charge generating layer can include the photogenerating composition, polymeric film-forming polymer binder, and glycol compound in any desired amounts to provide desired functional effect. However, in embodiments, the materials are generally present in amounts of from about 10 to about 90 parts by weight such as about 30 to about 70 parts by weight photogenerating pigment, from about 90 to about 10 parts by weight such as about 70 to about 30 parts by weight polymeric film-forming polymer binder, and from about 0.01 to about 9 parts by weight such as about 0.03 to about 7 parts by weight glycol compound, although not limited to these ranges.
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″-diethlylamino 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. A small molecule charge transporting compound that permits injection of holes from the pigment into the charge generating layer with high efficiency and transports them across the charge transport layer with very short transit times is N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-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.
Any suitable electrically inactive resin binder insoluble in the alcohol solvent used to apply the 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′-cyclohexylinediphenylene 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 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 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 charge 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 a high sensitivity photogenerating material, such as a high sensitivity titanium phthalocyanine photogenerating pigment which is protected against polymorphic (crystal structure) change in the coating solution. As a result, the desired sensitivity of the photogenerating material is not lost to crystal structure change during formation of the charge generating layer. In addition to maintaining high photosensitivity of the photogenerating material, the glycol compounds may help prevent the sedimentation of pigment in the coating dispersion as the hydroxyl groups of the glycols tend to adsorb on active sites on the pigment surface, keeping pigment particles from interacting with each other to form large agglomerates.
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 charge generating layer coating dispersion was prepared as follows: 0.60 gram of TiOPc pigment was mixed with 0.003 g of 1,2-butanediol, 0.113 gram of Iupilon 200 (PC-Z 200) polymer available from Mitsubishi Gas Chemical Corp. and 11.2 grams of tetrahydrofuran in a 30 mL glass bottle containing 70 grams of approximately ⅛ inch stainless steel balls. Four different compositions were made, where each composition was rolled in the roll mill for 2, 4, 6, or 8 hours, respectively. 4 gram of milled pigment dispersion from each bottle was transferred to another bottle and further diluted with a solution of 3 g tetrahydrofuran and 0.19 g of PC-Z200 to for m a final coating dispersion to be used for making charge generator layer.
Charge generating layer coating dispersions are prepared as in Example 1, except that 1,2-butanediol was not included. Four different compositions are made, where each composition is rolled in the roll mill for 2, 4, 6, or 8 hours, respectively.
Charge generating layer coating dispersions are prepared as in Example 1, except that 0.0009 g 1,4-butanediol (Comparative Example 2) or 0.003 g 1,4-butanediol (Comparative Example 3) is substituted for the 0.003 g 1,2-butanediol. Four different compositions are made for each example, where each composition is rolled in the roll mill for 2, 4, 6, or 8 hours, respectively.
Imaging member sheets are formed using the charge generating layer coating compositions of Example 1 and Comparative Examples 1-3. Each imaging member sheet is formed as follows: Layered photoconductive imaging members were prepared according to the following procedure. The pigment coating dispersion was coated using a Bird's bar (0.00025 inch gap) onto a titanized MYLAR® substrate of 75 microns in thickness, which had a gamma-aminopropyltriethoxy silane layer, 0.1 micron in thickness, thereover, and E.I. DuPont 49,000 polyester adhesive thereon in a thickness of 0.1 micron was used as the base conductive film. Thereafter, the photogenerator layer formed was dried in a forced air oven at 120° C. for 1 minute.
A transport layer solution was prepared by mixing 6.34 grams of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenyl-4,4′-diamine, 6.34 grams of polycarbonate resin (available as MAKROLON® 5705 from Bayer A.G.), and 72 grams of methylene chloride. The transport solution was coated onto the above photogenerating layer using a Bird's bar of 5 mil gap. The resulting members were dried at 120° C. in a forced air oven for 1 minute. The final dried thickness of the transport layer was about 29 microns.
The xerographic electrical properties of prepared photoconductive imaging members can be determined by known means, including electrostatically charging the surfaces thereof with a corona discharge source until the surface potentials, as measured by a capacitively coupled probes attached to an electrometer, attained an initial value V0 of about −800 volts. After resting for 0.5 second in the dark, the charged members attained a surface potential of Vddp, dark development potential. Each member was then exposed to light from a filtered Xenon lamp thereby inducing a photodischarge that resulted in a reduction of surface potential to a Vbg value, background potential. The percent of photodischarge was calculated as 100×(Vddp−Vbg)/Vddp. The desired wavelength and energy of the exposed light was determined by the type of filters placed in front of the lamp. The monochromatic light photosensitivity was determined using a narrow band-pass further. The photosensitivity of the imaging member is usually provided in terms of the amount of exposure energy in ergs/cm2, designated as E1/2, required to achieve 50 percent photodischarge from Vddp to half of its initial value. i.e. from 800 to 400 volts. The higher the photosensitivity, the smaller is the E1/2 value. The device was finally exposed to an erase lamp of appropriate light intensity (200-250 erg/cm2) and any residual potential (Vresidual) was measured. The imaging members were tested with an exposure monochromatic light at a wavelength of 780 nanometers and an erase light with the wavelength of 600 to 850 nanometers.
The Table below includes the electrical performance data -for the various imaging members.
From the results, it can be seen that dispersions made with glycol compounds demonstrate superior stability as compared to conventional binders. However, it is also seen that dispersions made with glycol compounds having two hydroxyl groups bonded to adjoining carbon atoms in the carbon chain demonstrate still better stability as compared to dispersions made with glycol compounds of similar carbon chain length but with two hydroxyl groups bonded to different carbon atoms in non-adjacent positions.
Although not limited by theory, it is speculated that the hydroxyl groups may be chelating or adsorbing to certain sites of TiOPc pigment surface that would help to stabilize the pigment crystal against polymorphic change. Example 1 members had E1/2 values around 1 erg/cm2 for all milling times. Comparative Example 1 members showed increased E1/2 values with milling time. A three fold increase in E1/2 value for 6 hour milling meant sensitivity had decreased to ⅓ of photosensitivity observed for the 2 hour milling. Comparative Example 2 and 3 members had reasonably stable E1/2 values up to 6 hours of milling but at 8 hours, the E1/2 increased by about 2 to 2.4 times relative to 2 hour milling.
Optical measurement also provides additional evidence that the polymorphic stability of high sensitivity TiOPc is greatly enhanced by glycol compounds bonded to adjoining carbon atoms in the carbon chain as compared to the conventional PCZ binders or to glycol compounds of similar carbon chain length but with two hydroxyl groups bonded to different carbon atoms in non-adjacent positions.
Optical absorption spectra of TiOPc imaging members were obtained using Shimadzu Model UV-160 spectrophotometer in the wavelength region from 400 to 1000 nm. The variation of optical absorption spectrum with milling time provided some qualitative indication of polymorphic stability of TiOPc dispersion. For example, the shift of absorption peak position, or the change in the absorbance ratio of peak (800 nm)/tail (1000 nm), would indicate a polymorphic change. Example 1 members showed stable optical absorption for all milling times. The absorption peak stayed at 800 nm and the absorbance ratio remained at about 5 in all cases. Comparative Example 1 members prepared from 6 and 8 hour milling both exhibited absorbance ratio of 1.6, which was different from the initial ratio of 5 obtained at 2 hour milling.
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.
