Electrophotographic imaging member with a support layer containing polyethylene naphthalate

Abstract: An electrophotographic imaging member including a support substrate layer comprising polyethylene naphthalate substantially free of any oligomers, having a glass transition temperature of between about 100°C and about 140°C, having a thickness between about 75 micrometers (3 mils) and about 125 micrometers (5 mils) and having a Young's modulus of between about 650,000 and about 1,000,000 pounds per square inch, an electrically conductive ground plane layer c including titanium, a hole blocking layer, an optional adhesive layer, a charge generation layer including photoconductive particles dispersed in a film forming binder, and a hole transport layer, the hole transport layer being substantially non-absorbing in the spectral region at which the charge generation layer generates and injects photogenerated holes but being capable of supporting the injection of photogenerated holes from the charge generation layer and transporting the holes through the hole transport layer.

Description: This invention relates in general to electrophotography and more specifically, to an electrophotographic imaging member having an improved support layer. As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated, duplicating and printing systems employed flexible photoreceptor belts, operating at very high speeds, have also placed stringent mechanical requirements and narrow operating limits as well on photoreceptors. For example, the layers of many modern multilayered photoreceptor belt must be highly flexible, adhere well to each other, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. An encouraging advance in electrophotographic imaging which has emerged in recent years is the successful fabrication of a flexible imaging member which exhibits excellent capacitive charging characteristic, outstanding photosensitivity, low electrical potential dark decay, and long term electrical cyclic stability. This imaging member employed in belt form usually comprises a substrate, a conductive layer, a solution coated hole blocking layer, a solution coated adhesive layer, a thin charge generating layer comprising a sublimation deposited perylene or phthalocyanine organic pigment or a dispersion of one of these pigments in a selected binder resin, a solution coated charge transport layer, a solution coated anti-curl layer, and an optional overcoating layer. Multi-layered photoreceptors containing charge generating layers, comprising either vacuum sublimation deposited pure organic pigment or an organic pigment dispersion of perylene or phthalocyanine in a film forming binder exhibit characteristics that are superior to photoreceptor counterparts containing a trigonal selenium dispersion in the charge generating layer. Unfortunately, these multi-layered perylene photoreceptors have also been found to develop a serious charge deficient spots problem, particularly the dispersion of perylene pigment in the matrix of a bisphenol Z type polycarbonate film forming binder. The expression "charge deficient spots" as employed herein is defined as localized areas of dark decay that appear as toner deficient spots when using charged area development, e.g. appearance of small white spots having an average size of between 0.2 and 0.3 millimeter on a black toner background on an imaged hard copy. In discharged area development systems, the charge deficient spots appear in the output copies as small black toner spots on a white background. Moreover, multi-layered benzimidazole perylene photoreceptors have also been observed to curl after coating and drying. A curled photoreceptor cannot be electrostatically charged uniformly because different parts of the photoreceptor surface are at different distances from charging devices such as corotrons and scorotrons. Also a curled photoreceptor adversely affects image development and transfer. Further, an upwardly curled edge of a photoreceptor carrying a ground strip along one edge can short out a charging device in electrophotographic imaging machines. Typically, flexible photoreceptor belts are fabricated by depositing the various layers of photoactive coatings onto long webs which are thereafter cut into sheets. The opposite ends of each photoreceptor sheet are overlapped and ultrasonically welded together to form an imaging belt. When conventional photoreceptor substrates such as polyethylene terephthalate webs are coated and dried, the resulting dried photoreceptor web usually has a pronounced camber which adversely affects the circumference uniformity of welded belts, particularly large welded belts for high volume, high speed electrophotographic duplicators and printers. In the fabricated belt form, the welded seam of the photoreceptor tends to protrude excessively above the rest of the outer surface of the photoreceptor. This protrusion is undesirable because it collides with cleaning blades and other closely spaced subsystems arranged around the web path. Collisions rapidly wear down cleaning blades and can cause vibrations which adversely affect development and registration of toner images, particularly in color imaging machines. In addition, some photoreceptor belts tend to stretch whereas others tend to shrink during image cycling around support rollers, at least one of the rollers being spring loaded to maintain belt tension. The tendency of some photoreceptors to shrink with cycling is aggravated under high operating temperature conditions such as imaging systems that have a rapid first copy out feature where the fuser temperature is increased dramatically to achieve the shorter fusing times needed for a more rapid first copy out. Also, when a photoreceptor belt containing a polyethylene terephthalate substrate is placed under high belt tension to help flatten the belt, the high tension, particularly at high operating temperatures, damages the charge transport layer of the photoreceptor. When the belt stretches or .shrinks, the relative location changes for different sections of the belt such as the seam and regions for imaging. Such relative location changes are difficult to track and require complex, sophisticated and costly detection and timing equipment. Photoconductor belts containing polyethylene terephthalate substrates also tend to absorb water under high humidity operating conditions. Absorption of water causes undesirable alteration of the electrical properties of the photoreceptor and can cause it to swell. Photoconductor belts containing polyethylene terephthalate substrates also exhibit a wavy surface pattern on the exposed surface of the charge transport layer due to stress imbalance in the member being coated. This wavy pattern is undesirable because of uneven charging of the photoreceptor, incomplete transfer of toner images, and the formation of dark and light patterns. Photoconductor belts containing polyethylene terephthalate substrates form low frequency ripples in the belt during cycling. These ripples tend to have peaks and valleys that run longitudinally of the belt and, therefore, are parallel to the edges of the belt. The presence of ripples markedly reduces the quality of charging, exposure and final toner image. Attempts to utilize alternative materials for the substrate layer in a electrophotographic imaging belts have encountered difficulties. For example, substrates comprising polyetheramide or polyvinylidene fluoride (Kynar) cannot be readily welded and therefore are less desirable for photoreceptor substrates. Belt substrates of polyethersulphone (PES) are adversely effected by solvents used in the applied coating layers such as methylene chloride solvents. Thus, there is a continuing need for improved photoreceptors that exhibit freedom from charge deficient spots and are more resistant to curling, stretching, camber formation, conicity variation. US-A 4,026,703 to Hayashi et al., issued May 31, 1997 - An electrophotographic photoreceptor is disclosed for producing an electrostatic latent image on the top layer thereof which comprises from the bottom up: a. a substrate b. a layer of metallic palladium having thickness of from 0.5 to 10 nm (5A to 1000A). c. a layer including vitreous selenium having a thickness of from 0.05 to 3 micrometer (0.05 to 3) microns and d. a top layer including polyvinyl carbazole represented by a specific formula. Numerous specific substrates are disclosed including, for example, polyethylene naphthalate. US-A-5,114,818 relates to an electrostatographic imaging apparatus comprising an organic electrostatographic imaging member having at least one arcuate surface, a heat fuser roll and a thin heat shield comprising a solid polymer substrate having a T g of about 100°C, coated with a thin heat reflective metallic layer interposed between said fuser roll and the adjacent arcuate surface, said metallic layer of said shield being concentric to and facing said fuser roll. Further, a flexible belt photoreceptor comprises one or more photoconductive layers on a flexible supporting substrate such as a polyethylene terephthalate polyester. Polyethylene naphthalate is mentioned in a list of suitable polymers in an embodiment. US-A-5,709,765 discloses an electrophotographic imaging member comprising a support substrate layer having a T g of between 100° and 140°C, an electrically conductive ground plane layer, a hole blocking layer, an optional adhesive layer, a charge generation layer and a hole transport layer. A suitable material for the substrate is polyethylene naphthalate. It was the object of the present invention to provide an improved electrophotographic member exhibiting increased flatness during coating and during image cycling, having greater resistance to stretching or distortion, having flatter welded joints, having less camber, exhibiting more uniform conicity after welding into a belt, exhibiting greater resistance to the formation of charge deficient spots, that having more stable electrical properties under high humidity operating conditions, which is resistant to shrinking, which resists deformation under high temperature drying conditions, and having coatings that are more uniform in thickness. The foregoing objects and others are accomplished in accordance with this invention by providing an electrophotographic imaging member as claimed in claim 1. Preferred embodiments are set forth in the subclaims. This photoreceptor is utilized in an electrophotographic imaging process. The photoreceptor substrate consists of polyethylene naphthalate. The polyethylene naphthalate substrate is transparent to visible light. This substrate also blocks the transmission of ultraviolet radiation having a wavelength of less than about 380 nanometers emanating from erase lamps such as fluorescent erase lamps, thereby preventing damage to the charge transport layer of the photoreceptor and to charge generating layers containing UV sensitive materials such as vanadyl phthalocyanine. The polyethylene naphthalate substrate should also have a thickness of between 75 micrometers (3 mils) and 125 micrometers (5 mils). A thickness of between 87.5 micrometers (3.5 mils) and 112.5 micrometers (4.5 mils) is preferred. Optimum results are achieved with a polyethylene naphthalate substrate layer thickness of about 90 micrometers (3.5 mils). When the thickness is less than 75 micrometers, waviness and ripples become unacceptable because of print and charge nonuniformities. For example, at less than 75 micrometers (3 mils) thickness, a polyethylene naphthalate substrate unexpectedly forms a photoreceptor that has an early end of life at which point the charge transport layer begins to crack. When the thickness is greater than 125 micrometers charge transport layer cracks during image cycling. Thus, it is surprising that some polyethylene naphthalate substrate materials at a critical thickness provide superior properties compared to other polyethylene naphthalate materials at different thicknesses. The polyethylene naphthalate in the substrate should be substantially free of any oligomers. The term "oligomers" as employed herein is defined as monomer units such as, for example, dimers, trimers, tetramers and the like in a polymer. The expression "substantially free" as employed herein is defined as present in an amount of less than 0.5 percent by weight based on the total weight of polyethylene naphthalate in the substrate. Polyethylene terephthalate commonly contains an oligomer content of 1.5 percent by weight based on the total weight of polyethylene terephthalate in the substrate. When the presence of oligomers in the substrate layer become excessive, the oligomers lead to coating defects in the subsequently applied photoreceptor layers. For example, oligomers can accumulate on rollers in coating applicators and cause charge transport layer blotch and charge generator layer backing roll mottle. The oligomers can also accumulate in coater dryers and be dislodged onto freshly coated webs by coater vibration. Excess reactants should not be used to form the polyethylene naphthalate substrate layer. Generally, excess reactants are utilized to form polyethylene terephthalate substrate layers so that unreacted oligomer materials are present in polyethylene terephthalate substrate layers. The polyethylene naphthalate utilized in the photoreceptors of this invention should have a glass transition temperature of between 100 °C and 140°C. In addition, the polyethylene naphthalate should stretch or shrink less than 0.22 percent at 130°C and have an oxygen permeability of less than 12.8 cubic centimeters per square meter per day for a thickness of 25 micrometers (1 mil). Unlike the polyethylene naphthalate substrate of this invention, polyethylene terephthalate has an oxygen permeability of about 52.3 cubic centimeters per square meter per day for a thickness of 25 micrometers (1 mil). The polyethylene naphthalate utilized in the photoreceptors of this invention should also have a Young's modulus of between 4.5x10 9 Pa and 6.9x10 9 Pa. Polyethylene naphthalate having the foregoing properties is commercially available, for example, Kaladex 2000, available from ICI Films or E. I. Du Pont De Nemours & Co. Inc. The substrate may have any suitable shape such as, for example, a flexible web, sheet, belt and the like. Preferably, the final coated substrate support layer is in the form of an endless flexible belt. Attempts to utilize other materials for a substrate layer in a electrophotographic imaging belt have encountered difficulties. For example, substrates comprising polyetheramide or polvinlylidene fluroride (e.g. Kynar) cannot be readily welded and therefore are less desirable for photoreceptor substrates. Belt substrates of polyethersulphome are adversely effected by solvents used in the applied coating layers such as methylene chloride solvents. The titanium and optional zirconium layers may be formed by any suitable coating technique, such as vacuum deposition. Typical vacuum depositing techniques include sputtering, magnetron sputtering, RF sputtering, and the like. Magnetron sputtering of titanium or zirconium onto a substrate can be effected by a conventional type sputtering module under vacuum conditions in an inert atmosphere such as argon, neon, or nitrogen using a high purity titanium or zirconium target. The vacuum conditions are not particularly critical. In general, a continuous titanium or zirconium film can be attained on a suitable substrate, e.g. a polyester web substrate such as Mylar available from E.I. du Pont de Nemours & Co. Inc. with magnetron sputtering. It should be understood that vacuum deposition conditions may all be varied in order to obtain the desired titanium or zirconium thickness. The conductive layer preferably comprises a plurality of metal layers with the outermost metal layer (i.e. the layer closest to the charge blocking layer) comprising at least 50 percent by weight of zirconium. At least 70 percent by weight of zirconium is preferred in the outermost metal layer for even better results. The multiple layers may, for example, all be vacuum deposited or a thin layer can be vacuum deposited over a thick layer prepared by a different techniques such as by casting. Thus, as an illustration, a zirconium metal layer may be formed in a separate apparatus than that used for previously depositing a titanium metal layer or multiple layers can be deposited in the same apparatus with suitable partitions between the chamber utilized for depositing the titanium layer and the chamber utilized for depositing zirconium layer. The titanium layer may be deposited immediately prior to the deposition of the zirconium metal layer. Generally, for rear erase exposure, a conductive layer light transparency of at least 15 percent is desirable. The combined thickness of a two layered conductive layer should be between 10 to 30 nm (100 and 300 angstroms). A typical zirconium/titanium dual conductive layer has a total combined thickness of 20 nm (200 angstroms). Although thicker layers may be utilized, economic and transparency considerations may affect the thickness selected. Regardless of the technique employed to form the titanium or zirconium layer, a thin layer of titanium or zirconium oxide forms on the outer surface of the metal upon exposure to air. Thus, when other layers overlying the zirconium layer are characterized as "contiguous" layers, it is intended that these overlying contiguous layers may, in fact, contact a thin titanium or zirconium oxide layer that has formed on the outer surface of the metal layer. Ground planes comprising zirconium tend to continuously oxidize during xerographic cycling due to anodizing caused by the passage of electric currents, and the presence of this oxide layer tends to decrease the level of charge deficient spots with xerographic cycling. Generally, a zirconium layer thickness of at least 6 nm (60 angstroms) is desirable to maintain optimum resistance to charge deficient spots during xerographic cycling. A typical electrical conductivity for conductive layers for electrophotographic imaging members in slow speed copiers is 10 2 to 10 3 ohms/square. After deposition of at least a titanium metal layer, a hole blocking layer is applied thereto. Generally, electron blocking layers for positively charged photoreceptors allow the photogenerated holes in the charge generating layer at the top of the photoreceptor to migrate toward the charge (hole) transport layer below and reach the bottom conductive layer during the electrophotographic imaging processes. Thus, an electron blocking layer is normally not expected to block holes in positively charged photoreceptors such as photoreceptors coated with charge a generating layer over a charge (hole) transport layer. For negatively charged photoreceptors, any suitable hole blocking layer capable of forming an electronic barrier to holes between the adjacent photoconductive layer and the underlying zirconium or titanium layer may be utilized. A hole blocking layer may comprise any suitable material. Typical hole blocking layers utilized for the negatively charged photoreceptors may include, for example, polyamides such as Luckamide, hydroxy alkyl methacrylates, nylons, gelatin, hydroxyl alkyl cellulose, organopolyphosphazines, organosilanes, organotitanates, organozirconates, silicon oxides, zirconium oxides, and the like. Preferably, the hole blocking layer comprises nitrogen containing siloxanes. Typical nitrogen containing siloxanes are prepared from coating solutions containing a hydrolyzed silane. Typical hydrolyzable silanes include 3-aminopropyl triethoxy silane, (N,N'-dimethyl 3-amino) propyl triethoxysilane, N,N-dimethylamino phenyl triethoxy silane, N-phenyl aminopropyl trimethoxy silane, trimethoxy silylpropyldiethylene triamine and mixtures thereof. During hydrolysis of the amino silanes described above, the alkoxy groups are replaced with hydroxyl group. An especially preferred blocking layer comprises a reaction product between a hydrolyzed silane and the zirconium and/or titanium oxide layer which inherently forms on the surface of the metal layer when exposed to air after deposition. This combination reduces spots at time 0 and provides electrical stability at low RH. The imaging member is prepared by depositing on the zirconium and/or titanium oxide layer of a coating of an aqueous solution of the hydrolyzed silane at a pH between 4 and 10, drying the reaction product layer to form a siloxane film and applying electrically operative layers, such as a photogenerator layer and a hole transport layer, to the siloxane film. The blocking layer may be applied by any suitable conventional technique such as spraying, dip coating, draw bar coating, gravure coating, silk screening, air knife coating, reverse roll coating, vacuum deposition, chemical treatment and the like. For convenience in obtaining thin layers, the blocking layers are preferably applied in the form of a dilute solution, with the solvent being removed after deposition of the coating by conventional techniques such as by vacuum, heating and the like. This siloxane coating is described in US-A 4,464,450 to L. A. Teuscher. After drying, the siloxane reaction product film formed from the hydrolyzed silane contains larger molecules. The reaction product of the hydrolyzed silane may be linear, partially crosslinked, a dimer, a trimer, and the like. The siloxane blocking layer should be continuous and have a thickness of less than 0.5 micrometer because greater thicknesses may lead to undesirably high residual voltage. A blocking layer of between 0.005 micrometer and 0.3 micrometer (50 Angstroms - 3000 Angstroms) is preferred because charge neutralization after the exposure step is facilitated and optimum electrical performance is achieved. A thickness of between 0.03 micrometer and 0.06 micrometer is preferred for zirconium and/or titanium oxide layers for optimum electrical behavior and reduced charge deficient spot occurrence and growth. The adhesive layer is applied to the charge blocking layer. The adhesive layer may comprise any suitable film forming polymer. Typical adhesive layer materials include, for example, copolyester resins, polyarylates, polyurethanes, blends of resins, and like. A preferred copolyester resin is a linear saturated copolyester reaction product of four diacids and ethylene glycol. The molecular structure of this linear saturated copolyester in which the mole ratio of diacid to ethylene glycol in the copolyester is 1:1. The diacids are terephthalic acid, isophthalic acid, adipic acid and azelaic acid. The mole ratio of terephthalic acid to isophthalic acid to adipic acid to azelaic acid is 4:4:1:1. A representative linear saturated copolyester adhesion promoter of this structure is commercially available as Mor-Ester 49,000 (available from Morton International Inc., previously available from duPont de Nemours & Co.). The Mor-Ester 49,000 is a linear saturated copolyester which consists of alternating monomer units of ethylene glycol and four randomly sequenced diacids in the above indicated ratio and has a weight average molecular weight of about 70,000. This linear saturated copolyester has a T g of 32°C. Another preferred representative polyester resin is a copolyester resin derived from a diacid selected from the group consisting of terephthalic acid, isophthalic acid, and mixtures thereof and diol selected from the group consisting of ethylene glycol, 2,2-dimethyl propane and mixtures thereof; the ratio of diacid to diol being 1:1, where the T g of the copolyester resin is between 50°C 80°C. Typical polyester resins are commercially available and include, for example, Vitel PE-100, Vitel PE-200, Vitel PE-200D, and Vitel PE-222, all available from Goodyear Tire and Rubber Co. Another polyester resin is Vitel PE-200 available from Goodyear Tire & Rubber Co. This polyester resin is a linear saturated copolyester of two diacids and two diols where the ratio of diacid to diol in the copolyester is 1:1. The diacids are terephthalic acid and isophthalic acid. The ratio of terephthalic acid to isophthalic acid is 1.2:1. The two diols are ethylene glycol and 2,2-dimethyl propane diol. The ratio of ethylene glycol to dimethyl propane diol is 1.33:1. The Goodyear PE-200 linear saturated copolyester consists of randomly alternating monomer units of the two diacids and the two diols in the above indicated ratio and has a weight average molecular weight of about 45,000 and a Tg of 67°C. Alternatively, the adhesive interface layer may comprise polyarylate (ARDEL D-100, available from Amoco Performance Products, Inc.), polyurethane or a polymer blend of these polymers with a carbazole polymer. Adhesive layers are well known and described, for example in US-A 5,571,649, US-A 5,591,554, US-A 5,576,130, US-A 5,571,648, US-A 5,571,647 and US-A 5,643,702. Any suitable solvent may be used to form an adhesive layer coating solution. Typical solvents include tetrahydrofuran, toluene, hexane, cyclohexane, cyclohexanone, methylene chloride, 1,1,2-trichloroethane, monochlorobenzene, and the like, and mixtures thereof. Any suitable technique may be utilized to apply the adhesive layer coating. Typical coating techniques include extrusion coating, gravure coating, spray coating, wire wound bar coating, and the like. The adhesive layer should be continuous. Satisfactory results are achieved when the adhesive layer has a thickness between 0.03 micrometer and 2 micrometers after drying. Preferably, the dried thickness is between 0.05 micrometer and 1 micrometer. The charge generating layer of the photoreceptor of this invention comprises any suitable photoconductive particle dispersed in a film forming binder. Typical photoconductive particles include, for example, phthalocyanines such as metal free phthalocyanine, copper phthalocyanine, titanyl phthalocyanine, hydroxygallium phthalocyanine, vanadyl phthalocyanine and the like, perylenes such as benzimidazole perylene, trigonal selenium, quinacridones, substituted 2,4-diamino-triazines, polynuclear aromatic quinones, and the like. Especially preferred photoconductive particles include hydroxygallium phthalocyanine, benzimidazole perylene and trigonal selenium. Typical perylene pigments particles include, for example, perylenes represented by the following cis and trans structures and mixtures thereof: wherein X is o-phenylene, pyridimediyl, pyrimidinediyl, phenanthrenediyl, naphthalenediyl, and the corresponding methyl, nitro, chloro, and methoch substituted derivatives. It is preferred that the perylene pigment is benzimidazole perylene which is also referred to as bis(benzimidazole). This pigment exists in the cis and trans forms and mixtures of these forms. The cis form is also called bis-benzimidazo(2,1-a-1',1'-b) anthra (2,1,9-def:6,5,10-d'e'f') disoquinoline-6,11-dione. The trans form is also called bisbenzimidazo (2,1-a1',1'-b) anthra (2,1,9-def:6,5,10-d'e'f') disoquinoline-10,21-dione. The cis form may be represented by the following formula: The transform may be represented by the following formula: The benzimidazole perylene pigment may be prepared by reacting perylene 3,4,9,10-tetracarboxylic acid dianhydride with 1,2-phenylene. Benzimidazole perylene is ground into fine particles having an average particle size of less than 1 micrometer and dispersed in a preferred polycarbonate film forming binder of poly(4,4'-diphenyl-1,1'-cyclohexane carbonate). Optimum results are achieved with a pigment particle size between 0.2 micrometer and 0.3 micrometer. Benzimidazole perylene is described in US-A 5,019,473 and US-A 4,587,189. Any suitable film forming binder material may be employed in the charge generator layer. Typical organic resinous binders include, for example, polyvinyl butyral, polycarbonates, acrylate polymers, vinyl polymers, cellulose polymers, polyesters, polysiloxanes, polyamides, polyurethanes, epoxies, and the like. Many film forming binder are disclosed, for example, in US-A 3,121,006 and US-A 4,439,507. The photogenerating particles are present in the film forming binder composition in various amounts. Preferred film forming polymers include poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) and polystyrene/polyvinylpyridene copolymers. Polystyrene/vinylpyridene copolymers include, for example, AB block copolymers of polystyrene/poly-4-vinylpyridene having a weight average molecular weight of from 7,000 to 80,000, and more preferably from 10,500 to 40,000 and wherein the percentage of vinylpyridene is from 5 to 55 and preferably from 9 to 20. Block copolymers of polystyrene/polyvinylpyridene are known in the art and described, for example in US-A 5,384,223, US-A 5,484,223, and US-A 5,571,649. Electrical life is improved dramatically by the use of poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) film forming binder. Poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) has repeating units represented in the following formula: wherein "S" in the formula represents saturation. Preferably, the film forming polycarbonate binder for the charge generating layer has a weight average molecular weight between 20,000 and 140,000. Satisfactory results may be achieved when the dried charge generating layer contains between 20 percent and 90 percent by volume benzimidazole perylene dispersed in poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) based on the total volume of the dried charge generating layer. Preferably, the perylene pigment is present in an amount between 30 percent and 80 percent by volume. Optimum results are achieved with an amount between 35 percent and 45 percent by volume. Poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) allow a reduction in perylene pigment loading without an extreme loss in photosensitivity. Any suitable solvent may be utilized to dissolve the film forming binder. Typical solvents include, for example, tetrahydrofuran, toluene, methylene chloride, and the like. Tetrahydrofuran is preferred because it has no discernible adverse effects on xerography and has an optimum boiling point to allow adequate drying of the generator layer during a typical slot coating process. Coating dispersions for charge generating layer may be formed by any suitable technique using, for example, attritors, ball mills, Dynomills, paint shakers, homogenizers, microfluidizers, and the like. Any suitable drying technique may be utilized to solidify and dry the deposited coatings. Typical drying techniques include oven drying, forced air drying, infrared radiation drying, and the like. Satisfactory results may be achieved with a dry charge generating layer thickness between 0.3 micrometer and 3 micrometers. Preferably, the charge generating layer has a dried thickness of between 1.1 micrometers and 2 micrometers. Any suitable charge transport layer may be utilized. The active charge transport layer may comprise any suitable transparent organic polymer of non-polymeric material capable of supporting the injection of photo-generated holes and electrons from the charge generating layer and allowing the transport of these holes or electrons through the organic layer to selectively discharge the surface charge. The charge transport layer in conjunction with the generation layer in the instant invention is a material which is an insulator to the extent that an electrostatic charge placed on the transport layer is not conducted in the absence of illumination Thus, the active charge transport layer is a substantially non-photoconductive material which supports the injection of photogenerated holes from the generation layer. An especially preferred transport layer employed in one of the two electrically operative layers in the multilayer photoconductor of this invention comprises from 25 to 75 percent by weight of at least one charge transporting aromatic amine compound, and 75 to 25 percent by weight of a polymeric film forming resin in which the aromatic amine is soluble. A dried charge transport layer containing between 40 percent and 50 percent by weight of the small molecule charge transport molecule based on the total weight of the dried charge transport layer is preferred. The charge transport layer forming mixture preferably comprises an aromatic amine compound. Most preferably, the charge transport layer comprises an arylamine small molecule dissolved or molecularly dispersed in a polycarbonate. Typical aromatic amine compounds include triphenyl amines, bis and poly triarylamines, bis arylamine ethers, bis alkyl-arylamines and the like. Examples of charge transporting aromatic amines for charge transport layers capable of supporting the injection of photogenerated holes of a charge generating layer and transporting the holes through the charge transport layer include, for example, triphenylmethane, bis(4-diethylamine-2-methylphenyl)phenylmethane; 4'-4"-bis(diethylamino)-2',2"-dimethyltriphenylmethane, N,N'-bis(alkylphenyl)-[1,1'-biphenyl]-4,4'-diamine wherein the alkyl is, for example, methyl, ethyl, propyl, n-butyl, etc., N,N'-diphenyl-N,N'-bis(chlorophenyl)-[1,1'-biphenyl]-4,4'-diamine, N,N'-diphenyl-N,N'-bis(3"-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine, and the like dispersed in an inactive resin binder. Any suitable inactive resin binder soluble in methylene chloride or other suitable solvent may be employed in the process of this invention. Typical inactive resin binders soluble in methylene chloride include polycarbonate resin, polyvinylcarbazole, polyester, polyarylate, polyacrylate, polyether, polysulfone, and the like. Weight average molecular weights can vary from 20,000 to 1,500,000. The preferred electrically inactive resin materials are polycarbonate resins have a molecular weight from 20,000 to 120,000, more preferably from 50,000 to 100,000. The materials most preferred as the electrically inactive resin material is poly(4,4'-dipropylidene-diphenylene carbonate) with a molecular weight of from 35,000 to 40,000, available as Lexan 145 from General Electric Company; poly(4,4'-isopropylidene-diphenylene carbonate) with a molecular weight of from 40,000 to 45,000, available as Lexan 141 from the General Electric Company; a polycarbonate resin having a molecular weight of from 50,000 to 100,000, available as Makrolon from Farbenfabricken Bayer A. G.; a polycarbonate resin having a molecular weight of from 20,000 to 50,000 available as Merlon from Mobay Chemical Company; and poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) available as PCZ -200 from Mltsubishi Gas Chemical. Examples of photosensitive members having at least two electrically operative layers including a charge generator layer and diamine containing transport layer are disclosed in US-A 4,265,990, US-A 4,233,384, US-A 4,306,008, US-A 4,299,897 and US-A 4,439,507. 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. Coated photoreceptors containing polyethylene naphthalate substrates may be dried at a higher drying temperature than coated photoreceptors containing polyethylene terephthalate substrates. The use of higher drying temperatures to dry the photoconductive layers, particularly the charge transport layer, reduces stress in the photoreceptor and promotes a flatter photoreceptor that is resistant to curl. Photoreceptor belts containing polyethylene terephthalate substrates tend to deform during high temperature coating drying operations. This affects camber of the coated web and conicity of the final welded photoreceptor belt. However, coated photoreceptor webs containing polyethylene naphthalate substrates can be dried at much higher temperatures than coated photoreceptor webs contain polyethylene terephthalate substrates. Typically, coatings applied to photoreceptor webs contain polyethylene terephthalate substrates are dried at a temperature of about 135°C. Thus, even when higher temperature coating drying conditions are utilized, photoreceptors containing polyethylene naphthalate substrates show no detectable deformation thereby avoiding camber and conicity problems due to shrinkage at one edge of the coated web compared to the operative edge. For example, measurements of the shrinkage of polyethylene naphthalate and polyethylene terephthalate at photoreceptor drying temperatures of 135°C, there was a 4 fold improvement in shrinkage and a 4 fold reduction in camber. Generally, the thickness of the transport layer is between 5 micrometers to 100 micrometers, but thicknesses outside this range can also be used. A dried thickness of between 18 micrometers and 35 micrometers is preferred with optimum results being achieved with a thickness between 24 micrometers and 29 micrometers. Although a charge transport layer formed on a photoreceptor having a polyethylene terephthalate substrate layer has an outer surface resembling a wave pattern, surprisingly, the size of the waves on the outer surface of a charge transport layer applied to a polyethylene naphthalate substrate is much lower than that applied to a polyethylene terephthalate substrate. The peak to valley distance of the waves on the surface of a charge transport layer applied to a charge generating layer on a polyethylene terephthalate substrate having a thickness of 76 micrometers (3 mils) is 300 micrometers to 600 micrometers. The peak to valley distance measured in a direction perpendicular to the surface of the substrate of waves on the surface of a charge transport layer applied to a charge generating layers supported on a polyethylene naphthalate substrate having a thickness of 90 micrometers (3.5 mils) is only 30 to 150 micrometers. The peak to valley distance on the outer surface of a charge transport layer in a photoreceptor containing a polyethylene naphthalate substrate layer having a thickness of 76 micrometers (3 mils) is 100 micrometers to 250 micrometers. Thus, the wavy pattern on the outer surface of a photoreceptor of this invention is substantially less in amplitude. Other layers such as conventional ground strips comprising, for example, conductive particles disposed in a film forming binder may be applied to one edge of the photoreceptor in contact with the zirconium and/or titanium layer, blocking layer, adhesive layer or charge generating layer. Optionally, an overcoat layer may also be utilized to improve resistance to abrasion. In some cases a back coating may be applied to the side opposite the photoreceptor to provide flatness and/or abrasion resistance. These overcoating and backcoating layers may comprise organic polymers or inorganic polymers that are electrically insulating or slightly semi-conductive. Generally, the photoreceptor is fabricated by applying coatings to a web shaped substrate and the resulting coated web is cut into sheets. Opposite ends of the sheet are thereafter joined by any suitable technique such as by ultrasonic welding. If desired, joining may be effected by other methods such as with adhesives, tapes, and the like. These joining techniques are well known in the art. A typical welding process is described in US-A 4,878,985. The welded belt may optionally be treated to release stress in the seam area. Stress release may be accomplished by heating the seam area of the belt to a temperature above the glass transition temperature while the seam region of the belt is bent over a support having an arcuate surface. After heating, the seam is cooled to ambient temperature while it is still bent over the support. Because the initial rectangular shape of the belt of this invention is maintained substantially intact and camber is avoided during the coating, drying and welding operations, the conicity of the belt can be precisely predicted. Thus, electrophotographic imaging belts of this invention can be fabricated with a difference in conicity significantly less than belts fabricated with polyethylene terephthalate substrates. The belt of this invention can be mounted in electrophotographic imaging copiers, printers and duplicators without any major adjustments to compensate for large difference in conicity from one edge of the belt to the other edge. Thus, for example, a polyethylene naphthalate substrate belt of this invention having an 87.5 micrometer (3.5 mill) thickness has been electrophotographically cycled in an electrophotographic imaging system more than 600,000 electrophotographic imaging cycles. Some of the improved mechanical properties achieved include increased flatness, higher elastic modulus which resists stretching or distortion, and flatter welded joints. Thus, for example, the belt lies flatter when supported on backer bars. Further, large segments of a coated web have less camber so that there is more uniform conicity (i.e., more uniform circumference across the width of the web) after the segment is cut and the opposite ends welded together. The combination of layers in the photoreceptor of this invention can achieve a flatness equal to or less than 300 micrometers. The measurement technique for measuring this flatness is described in Example VI below. Also, shrinkage of the photoreceptor of this invention imaging member is less than 0.5 percent in the machine direction when exposed to a temperature of 130°C for 30 minutes. The expression "machine directions" as employed herein is defined as in a direction parallel to the movement of the photoreceptor during xerographic image cycling. It has been found that when a polyethylene naphthalate is employed as a substrate, the photoreceptor belt lies flatter and the thickness of the photoconductive layers on the belt are more uniform. It is believed that the presence of fewer oligomers in the polyethylene naphthalate substrate of this invention results in fewer defects in the applied photoconductive coatings thereby resulting in a reduction of defects. Photoreceptor belts containing polyethylene terephthalate substrates tend to deform during high temperature coating drying operations. This adversely affects camber of the coated web and conicity of the final welded photoreceptor belt. However, coated photoreceptor webs containing polyethylene naphthalate substrates can be dried at much higher temperatures than coated photoreceptor webs contain polyethylene terephthalate substrates. Thus, even when higher temperature coating drying conditions are utilized, photoreceptors containing polyethylene naphthalate substrates show no detectable deformation thereby avoiding camber and conicity problems due to shrinkage at one edge of the coated web compared to the opposite edge. Further with a 75 to 125 micrometer (3 to 5 mils) thick polyethylene naphthalate substrate, mechanical life is improved. In addition, less power is required to achieve a welded seam having acceptable mechanical properties. Further, the polyethylene naphthalate substrate photoreceptors of this invention may be utilized in electrophotographic imaging systems generating a high temperature environment such as imaging systems that have a rapid first copy out feature where the fuser temperature is increased dramatically to achieve the shorter fusing times needed for a more rapid first copy out. Generally, high belt tension is need to help flatten a photoreceptor belt containing polyethylene terephthalate substrates. These high tensions, particularly at high temperatures damage the charge transport layer of the photoreceptor. Since less tension is needed to for photoreceptor containing polyethylene naphthalate substrates of this invention to achieve a flat belt and since such lower tensions reduces the likelihood of stretching, the charge transport layer is more resistant to damage. Also, because belt photoreceptors containing polyethylene naphthalate substrates of this invention are flatter, the coatings thereon are more uniform, the deposit of electrostatic charge is more uniform and the final image is more uniform. Moreover, the coatings on a polyethylene naphthalate substrate of this invention exhibit fewer surface defects. Thus, more complete development of the electrostatic latent image and more complete transfer of the deposited toner image occurs with photoreceptors containing polyethylene naphthalate substrates of this invention. Also unexpected, is that the wavy characteristic of the outer surface of a charge transport layer applied to a polyethylene naphthalate substrate of this invention is much lower than that applied to a polyethylene terephthalate substrate. Unlike polyethylene terephthalate substrate belts, the polyethylene naphthalate substrate belts of this invention resist shrinkage and stretching under high tension and high temperature operating conditions. COMPARATIVE EXAMPLE I A polyethylene terephthalate (Melinex, available from ICI Americas Inc.) substrate having a thickness of 76 micrometers was vacuum coated by sputtering with a titanium layer having a thickness of about 10 nm (100 Angstroms). Without breaking the vacuum, the titanium layer was coated by sputtering a zirconium metal layer having a thickness of about 10 nm (100 Angstroms). The exposed zirconium surface was oxidized by exposure to oxygen in the ambient atmosphere. A siloxane hole blocking layer was prepared by applying a 0.22 percent (0.001 mole) solution of 3-aminopropyl triethoxylsilane to the oxidized surface of the zirconium layer with a gravure applicator. The deposited coating was dried at 135°C in a forced air oven to form a layer having a thickness of 12 nm (120 Angstroms). A coating of polyester resin (duPont 49,000, available from I. I. DuPont de Nemours & Co.) was applied to the siloxane coated base with a gravure applicator. The polyester resin was dried to form a film having a thickness of about 0.05 micrometer. A slurry coating solution of 40 percent by volume benzimidazole perylene and 60 percent by volume poly(4,4'-diphenyl-1,1'-cyclohexane carbonate (PCZ-200, available from Mitsubishi Gas Chem.) dispersed in tetrahydrofuran was extrusion coated onto the polyester coating to form a layer having a wet thickness of about 26 micrometers. The coated member was dried at 135°C in a forced air oven to form a layer having a thickness of about 1 micrometer. A charge transport layer was formed on this charge generator layer by applying a mixture of a 60-40 by weight solution of Makrolon, a polycarbonate resin having a molecular weight from about 50,000 to about 100,000 available from Farbenfabriken Bayer A. G. and N,N'-diphenyl-N,N'bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine dissolved in methylene chloride to give a 15 percent by weight solution. The components were extrusion coated on top of the generator layer and dried at temperature of about 135°C to form a 24 micrometer thick dry layer of hole transporting material. A grounding strip coating and an anti curl backing coating were also applied. This photoreceptor was then cut and welded by conventional ultrasonic welding to form a continuous belt. The belt was 353 millimeters wide and 836 millimeters in circumference. EXAMPLE II A photoconductive imaging member was prepared as described in Comparative Example I, except that the web of titanium and zirconium coated polyethylene terephthalate (Melinex, available from ICI Americas Inc.) substrate having a thickness of 76 micrometers was substituted by a web of titanium and zirconium coated polyethylene naphthalate (Kaladex, available from ICI Films) substrate having a thickness of 76 micrometers. The polyethylene naphthalate substrate was substantially free of any oligomers, had a glass transition temperature of about 110°C and had a Young's Modulus of 6x10 9 Pa (871,000 pounds per square inch). EXAMPLE III A photoconductive imaging member was prepared as described in Comparative Example I, except that the web of titanium and zirconium coated polyethylene terephthalate (Melinex, available from ICI Americas Inc.) substrate having a thickness of 76 micrometers was substituted by a web of titanium and zirconium coated polyethylene naphthalate (Kaladex, available from ICI Films) substrate having a thickness of 90 micrometers. The polyethylene naphthalate substrate was substantially free of any oligomers, had a glass transition temperature of about 110°C and had a Young's Modulus of 6x10 9 Pa (871,000 pounds per square inch). EXAMPLE IV A photoconductive imaging member was prepared as described in Comparative Example I, except that the web of titanium and zirconium coated polyethylene terephthalate (Melinex, available from ICI Americas Inc.) substrate having a thickness of 76 micrometers was substituted by a web of titanium and zirconium coated polyethylene naphthalate (Kaladex, available from ICI Films) substrate having a thickness of 100 micrometers. The polyethylene naphthalate substrate was substantially free of any oligomers, had a glass transition temperature of about 110°C and had a Young's Modulus of 6x10 9 Pa (871,000 pounds per square inch). EXAMPLE V A photoconductive imaging member was prepared as described in Comparative Example I, except that the web of titanium and zirconium coated polyethylene Terephthalate (Melinex, available from ICI Americas Inc.) substrate having a thickness of 76 micrometers was substituted by a web of titanium and zirconium coated polyethylene naphthalate (Kaladex, available from ICI Films) substrate having a thickness of 125 micrometers. The polyethylene naphthalate substrate was substantially free of any oligomers, had a glass transition temperature of about 110°C and had a Young's Modulus of 6x10 9 Pa (871,000 pounds per square inch). EXAMPLE VI The mechanical flatness properties of photoconductive imaging members of Examples I through V were evaluated by mounting the devices in a tri roller mechanical fixture consisting of two twenty five millimeter diameter rollers mounted with centers located 63 millimeters apart, and a third thirty millimeter diameter roller mounted in a spring loaded holder oriented perpendicular to the line connecting the centers of the first two rollers, midway between the rollers. The third roll maintains a belt tension of 192.6 newton per meter. The flatness was evaluated by mechanically scanning a laser triangulation sensor (Keyence LC-2440, available from the Keyence Corporation of America) along a line midway between the two 25 millimeter diameter rollers. The peak to peak variation in surface position for the device described in Example I was about 500 micrometers, the peak to peak variation in surface position for the device described in Example II was about 300 micrometers, the peak to peak variation in surface position for the device described in Example III was about 100 micrometers, and the peak to peak variation in surface position for the devices described in Examples IV and V were less than 100 micrometers. All of the examples fabricated with the polyethylene naphthalate exhibited flatness properties that are superior to those observed in Example I. EXAMPLE VII A photoconductive imaging member was prepared as described in Example I, except that the sample size was 2500 millimeters in circumference and 414 millimeters in width. EXAMPLE VIII A photoconductive imaging member was prepared as described in Example II, except that the sample size was 2500 millimeters in circumference and 414 millimeters in width. EXAMPLE IX A photoconductive imaging member was prepared as described in Example IV, except that the sample size was 2500 millimeters in circumference and 414 millimeters in width. EXAMPLE X A photoconductive imaging member was prepared as described in Example V, except that the sample size was 2500 millimeters in circumference and 414 millimeters in width. EXAMPLE XI The devices fabricated in Examples VII through X were measured on a circumference gauge to determine the actual belt circumference. Each sample was measured at the time of fabrication, 16 days later, and 50 days later to determine the amount of shrinkage. Example VII, the polyethylene terephthalate reference material, shrank 1.2 millimeters in 16 days and shrank 1.8 millimeters in 50 days. Example VIII shrank 0.8 millimeters in 16 days and 0.9 millimeters in 50 days. Example IV shrank 0.8 millimeters in 16 days and 0.9 millimeters in 50 days. Example X shrank 0.6 millimeters in 16 days and 0.7 millimeters in 50 days. The polyethylene naphthalate samples in this test had about one half the amount of shrinkage of the polyethylene terephthalate reference material. EXAMPLE XII The 836 millimeter circumference devices fabricated in Examples I, II, IV, and V were measured on a circumference gauge at the time of fabrication and 44 days later to determine the amount of shrinkage. Example I shrank 0.7 millimeter, Example II shrank 0.4 millimeter, Example IV shrank 0.3 millimeter and Example V shrank 0.25 millimeter.

Claims: An electrophotographic imaging member comprising in this order: a support substrate layer, an electrically conductive ground plane layer comprising titanium, a hole blocking layer, an optional adhesive layer, a charge generation layer comprising photoconductive particles dispersed in a film forming binder, and a hole transport layer, the hole transport layer being substantially non-absorbing in the spectral region at which the charge generation layer generates and injects photogenerated holes but being capable of supporting the injection of photo-generated holes from the charge generation layer and transporting the holes through the hole transport layer, characterized in that said support substrate layer comprises a polyethylene naphthalate having an amount of oligomers of less than 0.5% by weight based on the total weight of polyethylene naphthalate in the substrate, said polyethylene naphthalate substrate has a Young's modulus of between 4.5 x 10 9 and 6.9 x 10 9 Pa (650,000 and 1,000,000 pounds per square inch) and a glass transition temperature of between 100°C and 140°C, and said substrate has a thickness between 75 micrometers (3 mils) and 125 micrometers (5 mils). An electrophotographic imaging member according to claim 1 wherein the hole transport layer has an outer surface, the outer surface having a wavy pattern with a peak to valley distance measured in a direction perpendicular to the surface of the substrate between 30 micrometers and 250 micrometers. An electrophotographic imaging member according to claim 1 wherein the adhesive layer comprises a film forming resin selected from a copolyester, a polyarylate and a polyurethane. An electrophotographic imaging member according to claim 3 wherein the film forming resin in said adhesive layer is a linear saturated copolyester reaction product of ethylene glycol with terephthalic acid, isophthalic acid, adipic acid and azelaic acid. An electrophotographic imaging member according to claim 1 wherein the photoconductive particles are selected from hydroxygallium phthalocyanine particles, benzimidazole perylene particles and trigonal selenium particles. An electrophotographic imaging member according to claim 1 wherein the film forming binder in the charge generation layer is selected from poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) and polystyrene/polyvinylpyridene copolymer. An electrophotographic imaging member according to claim 1 wherein the conductive ground plane has a total thickness of between 10 and 30 nm (100 angstroms and 300 angstroms) and comprises the titanium layer overcoated with a zirconium layer. An electrophotographic imaging member according to claim 7 wherein the zirconium layer has a thickness of at least 6 nm (60 angstroms). An electrophotographic imaging member according to claim 1 wherein the hole blocking layer comprises a siloxane, preferably an amino siloxane. An electrophotographic imaging member according to claim 1 wherein the charge generation layer comprises between 20 percent and 90 percent by volume of the benzimidazole perylene particles, based on the total volume of the charge generation layer.

Inventors: EVANS KENT J, GRABOWSKI EDWARD F, PARIKH SATISH R, ROETKER MICHAEL S, EVANS, KENT J., GRABOWSKI, EDWARD F., PARIKH, SATISH R., ROETKER, MICHAEL S.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1999
Main classification: G03G5
IPC codes: G03G5