Process for preparing a multilayer electrophotographic imaging member

Abstract: An electrophotographic imaging member having a perylene-containing charge generating layer is prepared by forming a dispersion of a perylene charge generating material in an acetate solvent and applying the dispersion to an electrophotographic imaging member layer by solution coating. An imaging member is prepared by forming an interphase region between a charge generating layer and a charge transport layer.

Description: This invention relates in general to electrophotography and, in particular, to a process for preparing electrophotoconductive imaging members having multiple layers. In electrophotography, an electrophotographic plate, drum, belt or the like (imaging member) containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image on the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly to a support, such as paper. The imaging process may be repeated many times with reusable imaging members. An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. A layered photoreceptor having separate photogenerating and charge transport layers is disclosed in US-A-4,265,990. The photogenerating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer. Degradation of image quality is encountered during extended cycling with more advanced, higher speed electrophotographic copiers, duplicators and printers. Complex, highly sophisticated higher speed duplicating and printing systems place stringent requirements on photoreceptors. These requirements impose narrow operating limits. The numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multilayered photoreceptor that has been employed as a belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl backing layer and an overcoating layer. Suitable and economical coating methods used for applying layers in multi-layer electrophotographic imaging members include dip coating, roll coating, Meyer bar coating, bead coating, curtain flow coating and vacuum deposition. Solution coating is a preferred approach because it is more economic than vacuum coating and can be used to deposit a seamless layer. US-A-4,082,551 discloses a process of coating multiple layers onto an insulating, polyester substrate by applying solutions having dissolved coating substance and drying each applied layer before coating a subsequent layer. The coated elements, when tested, indicate no chemical interaction between the photogenerating and conducting layers and essentially no change in electrical resistivity of the conducting layer. US-A-4,571,371 discloses an electrophotographic photosensitive member having a charge generating layer and a charge transport layer. A dispersion of charge generating material dissolved in solvent is applied to a cured polyamide resin layer by soaking and drying at 100°C for 10 minutes to form a charge generating layer. Subsequently, a solution containing a charge transfer material is applied to the dried charge generating layer followed by drying at 100°C for 60 minutes. US-A-5,395,722 describes a process for making a photoreceptor having a charge generating layer, wherein said layer is formed using a dispersed coating fluid containing perylene pigment in a solvent. Conventional electrophotographic imaging members, having at least a charge generating layer and a charge transport layer suffer numerous disadvantages. For example, electrophotographic imaging members can suffer from poor charge acceptance and can have limited photosensitivity due to limited injection of charge generated by absorbed photons into the charge transport layer. In addition, charge transport materials may diffuse and come in contact with the conductive layer, adversely affecting the electrophotographic imaging member. Notably, devices manufactured using conventional processes have limited photoresponse. Photoreceptors with perylene charge generating pigments, particularly benzimidazole perylene, show superior performance with extended life. The perylene-containing charge generating layers can be applied by a vacuum coating process. Vacuum coated charge generating layers containing perylenes show a high photosensitivity. However, vacuum coating is expensive. Solution coating is a more economical and convenient method of applying charge generating layers. However, perylene pigments are difficult to disperse and unstable dispersions are encountered with coating perylene pigment charge generating layers from solution. Unstable dispersions cause pigment flocculating and settling that leads to coating quality problems. Unstable dispersions are difficult to process, especially in a dip coating process. Dip coated perylene-containing charge generating layers show the substantial depreciation in sensitivity described above. The invention is directed to a process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer, comprising: a) dispersing a perylene charge generating material in an acetate solvent to form a dispersion; b) providing a charge transport material; and c1) applying said dispersion to form a perylene-containing charge generating layer as a wet underlying layer and subsequently overcoating said charge transport material on the underlying layer prior to drying to form a charge transport layer and an interphase region comprising a mixture of perylene charge generating material and charge transport material between said charge generating layer and said charge transport layer, or c2) simultaneously applying said dispersion and said charge transport material to form a perylene-containing charge generating layer overcoated with a charge transport layer and an interphase region comprising a mixture of perylene charge generating material and charge transport material between said charge generating layer and said charge transport layer. Specific embodiments of the invention are set forth in the attached claims. The invention is directed to a process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer. In one embodiment, the process comprises forming a dispersion of a perylene pigment in an acetate solvent, applying the dispersion to an electrophotographic imaging member layer to form a wet underlying layer and overcoating a charge transport layer on the underlying charge generating layer prior to drying the charge generating layer to allow charge transport material in the charge transport layer to diffuse into the wet underlying charge generating layer to form an interphase region comprising a mixture of perylene charge generating material and charge transport material. The present invention provides electrophotographic imaging members that enhance the injection of photogenerated charge into the charge transport layer. The interphase region comprises perylene charge generating material and charge transport material. The present invention relates to a method of solvent coating charge generating layers containing perylene pigments to produce a photoreceptor with improved sensitivity. The present invention provides a process for preparing a multi-layer electrophotographic imaging member having a perylene-containing charge generating layer that can be applied by solution coating from stable solutions and that results in perylene-containing charge generating layers of improved sensitivity. The present invention relates to a process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer comprising dispersing a perylene charge generating material in an acetate solvent to form a dispersion and applying the dispersion to form the charge generating layer. Preferred acetate solvents include n-butylacetate, ethylacetate, isopropylacetate and methylacetate. Unexpectedly, it has been found that perylenes form stable dispersions in acetate solvent for the purposes of application by solvent coatings such as dip coating. Further, it has been found that photoreceptors that include charge generating layers containing perylene charge generating materials applied from dispersions in acetate solvent display an increased sensitivity. For example, a 30% increase in sensitivity is obtained when a benzimidazole perylene dispersion in n-butylacetate is dip coated onto a photoreceptor to form a charge generating layer as compared to a photoreceptor having a charge generating layer prepared from a BZP dispersion in cyclohexanone. A representative electrophotographic imaging member may include a supporting substrate, optional adhesive layer(s), a conductive layer, a blocking layer, a perylene-containing charge generating layer, an interphase region and a charge transport layer. Other combinations of layers suitable for use in electrophotographic imaging members are also within the scope of the invention. For example, an anti-curl backing layer and/or a protective overcoat layer may also be included, and/or the substrate and conductive layer may be combined. Additionally, a ground strip may be provided adjacent the charge transport layer at an outer edge of the imaging member. The ground strip is coated adjacent to the charge transport layer so as to provide grounding contact with a grounding device. The substrate, conductive layer, blocking layer and adhesive layer(s), if incorporated into an electrophotographic imaging member, may be prepared and applied using conventional materials and methods. An electrophotographic imaging member prepared according to the present invention comprises a perylene-containing charge generating layer, a charge transport layer and an interphase region between the charge generating layer and the charge transport layer. The interphase region contains a mixture of charge transport matenal and charge generating material. In one embodiment, the interphase region is formed by applying a charge transport material to an underlying layer of perylene charge generating material, prior to drying or curing the underlying layer. Application of charge transport material before the underlying layer has completely dried or cured can produce the interphase region comprising a mixture of the charge generating material and the charge transport material. This method permits the charge transport material and/or the charge generating material to migrate across the charge transport layer/charge generating layer interface to form the interphase region, thereby increasing the photosensitivity of the resulting imaging member. Such an interphase region can have the charge generating material and the charge transport material mixed on a molecular level. The interphase region, preferably having the perylene-containing charge transport material in an increasing gradient layer on a molecular level in a direction approaching the charge transport layer, may enhance the injection of photogenerated charge from the charge generating material into the charge transport layer to enhance the charge transport efficiency throughout the charge generating layer. A gradual mixing of the perylene charge generating material and the charge transport material in the interphase region between the charge generating layer and the charge transport layer can be achieved by diffusion of the charge transport material into solvent-rich, undried charge generating layer during the coating process. The gradient transition between the charge generating layer and the charge transport layer significantly enhances the photoresponse of the electrophotographic imaging member and provides remarkably improved performance over imaging members produced using conventional means. The mixture in the interphase region is preferably characterized by a decreasing gradient of charge generating material and an increasing gradient of charge transport material in the direction of the charge transport layer of the electrophotographic imaging member. In another related embodiment, the charge transport layer can contain a minor amount (relative to the charge transport material) of a charge generating material, and/or the charge generating layer can contain a minor amount (relative to the charge generating material) of a charge transport material. The composition of the interphase region may be directly controlled by the specific type of process used to apply the underlying charge generating layer and the charge transport layer. For example, a method for simultaneously applying the charge generating material and the charge transport material controls the concentration of the charge generating material and the charge transport material at various depths in the interphase region. Specifically, a spraying apparatus fed by two reservoirs respectively containing charge generating material and charge transporting material may be passed over a suitable substrate several times. The amount of charge generating material may be decreased and the amount of charge transport material increased so that, with each successive pass, a gradual transition from charge generating material to charge transporting material is achieved, thus producing the interphase region gradient. Generally, the cumulative thickness of the layers in a multi-layered electrophotographic imaging member does not exceed 30 µm. Therefore, preferred interphase region thicknesses range from 0.1 µm to 10 µm. Any suitable perylene charge generating material may be applied to the substrate or other layer. The charge generating materials for use in the present invention are compositions comprising a perylene pigment. The perylene pigment is dissolved in an acetate solvent for application of the charge generating layer. Preferably, the perylene pigment is dispersed in a film forming binder and the resulting dispersion is dissolved in the acetate solvent. Examples of photogenerating pigments include the perylene pigments disclosed in US-A-4,587,189. Benzimidazole perylene is a preferred pigment. The benzimidazole perylenes include the following structures: and Any suitable polymeric film-forming binder material may be employed as a matrix in the charge generating layer. The binder polymer preferably 1) adheres well to the substrate or other underlying layer; and 2) dissolves in the acetate solvent. Examples of materials useful as the film-forming binder include polyvinylcarbazole, phenoxy resin, polycarbonate, polyvinylbutyral, polystyrene, polystyrenebutadiene and polyester. Polyvinylbutyral is the preferred binder polymer. The acetate solvent is a lower alkylacetate. Preferrably the alkyl has 1 to 4 carbon atoms. Examples of acetate solvents include methylacetate, ethylacetate, isopropylacetate, n-propylacetate, n-butylacetate, sec-butylacetate and tert-butylacetate. The preferred acetate solvent is n-butylacetate. Generally, from 5 percent by volume to 95 percent by volume of the perylene pigment is dispersed in no more than 95 percent by volume of the film-forming binder. In one embodiment, a volume ratio of the photogenerating pigment and film-forming binder is about 1:12, corresponding to about 8 percent by volume of the photogenerating pigment dispersed in about 92 percent by volume of the film-forming binder. In another embodiment, the volume ratio of the film-forming binder and photogenerating pigment is about 1:9, corresponding to 90 percent of the photogenerating pigment dispersed in 10 percent binder. Exemplary charge generating layer thicknesses include thicknesses ranging from 0.1 µm to 5.0 µm, and preferably from 0.3 µm to 3 µm. Charge generating layer thickness generally depends on film-forming binder content. Higher binder content generally results in thicker layers for photogeneration. Thicknesses outside the above exemplary ranges are also within the scope of the invention. The charge transport layer comprises any suitable organic polymer or non-polymeric material capable of transporting charge to selectively discharge the surface charge. It may not only serve to transport charge, but may also protect the imaging member from abrasion, chemical attack and similar destructive elements, thus extending the operating life of the electrophotographic imaging member. Alternatively or in addition, a protective overcoat layer may provide these protective functions. The charge transport layer should exhibit negligible, if any, discharge when exposed to a wavelength of light useful in xerography, e.g. 400 to 900 nm . Therefore, the charge transport layer is substantially transparent to radiation in a region in which the photoreceptor operates. Charge transport materials for use in the invention are preferably compositions comprising a hole transporting material dispersed in a resin binder and dissolved in a solvent for application. Hole transporting materials for use in compositions used according to the present invention include a mixture of one or more transporting aromatic amines or hydrozons. Exemplary aromatic amines include triaryl amines such as triphenyl amines, poly triaryl amines, bisarylamine ethers and bisalkylaryl amines. Preferred bisarylamine ethers include bis (4-diethylamine-2-methylphenyl) phenylmethane and 4'-4"-bis (diethyl-amino)-2'2"dimethyltriphenylmethane. Preferred bisalkylaryl amines include N,N'-bis (alkylphenyl)(1,1'-biphenyl)-4,4'-diamine wherein the alkyl is, for example, methyl, ethyl, propyl or n-butyl. Meta-tolyl-bis-diphenylamino benzadine and N,N'-diphenylN,N'-bis (3"-methylphenyl)-(1,1'biphenyl)-4,4'-diamine are preferred transporting aromatic amines. Exemplary resin binders used in charge transport compositions used according to the present invention include polycarbonate, polyvinylcarbazole, polyester, polyarylate, polyacrylate, polyether and polysulfone. Molecular weights of the resin binders can vary from 20,000 to 1,500,000. Preferred resin materials are polycarbonate resins having molecular weights from 20,000 to 120,000, more preferably from 50,000 to 100,000. Highly preferred resin materials are 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 General Electric Company; polycarbonate resin having a molecular weight of from 50,000 to 100,000, available as Makrolon from Farben Fabricken Bayer A.G.; polycarbonate resin having a molecular weight of from 20,000 to 50,000 available as Merlon from Mobay Chemical Company; polyether carbonates; and 4,4'-cyclohexylidene diphenyl polycarbonate. Solvents useful to form charge transport layers include monochlorobenzene, tetrahydrofuran, cyclohexanone, methylene chloride, 1,1,1-trichloroethane, 1,1,2-trichloroethane, dichloroethylene and toluene. Monochlorobenzene is a desirable component of the charge transport layer coating mixture for adequate dissolving of all the components and for dip coating applications. An especially preferred charge transport layer material for multi-layer photoconductors comprises from 25 percent to 75 percent by weight of at least one charge transporting aromatic amine, and 75 percent to 25 percent by weight of a polymeric fiimforming resin in which the aromatic amine is soluble. As discussed above, an exemplary mechanism for mixing charge generating material and charge transport material to form an interphase region comprises molecular mixing in which charge transport material migrates across the charge generating material/charge transport material interface to achieve a gradient of charge transport material in the interphase region, and combinations of this and other mechanisms. Combinations of charge generating material and charge transport material in an electrophotographic imaging member preferably include materials which are capable of molecular mixing. In a process of the invention for producing the electrophotographic imaging member having an interphase region, a perylene-containing charge generating layer is applied from an organic acetate solution to form an underlying layer; the underlying layer is overcoated, prior to drying, with a charge transport material to form a charge transport layer; the charge transport material is allowed to diffuse into the undried underlying layer; and the underlying layer and charge transport layer are dried or cured to fix the interphase region having a mixture of a charge generating material and a charge transport material. Another exemplary process according to the invention, which permits control of the concentration of the charge generating material and charge transport material in the interphase region, includes simultaneously applying the charge generating material and charge transport material and decreasing the amount of the charge generating material while increasing an amount of the charge transport material. Any suitable technique, which has been appropriately selected and/or modified in accordance with the process herein described, may be utilized to mix and thereafter apply any of the charge generating layer composition, the charge transport layer composition or simultaneously applied charge generating material and charge transport material layer to the substrate or other underlying layer. Typical application techniques include spray coating, dip coating, roll coating, Meyer bar coating, bead coating and curtain flow coating. Drying of the deposited coating can be carried out by any suitable conventional technique to remove solvent from an applied layer or interphase region. Examples of drying techniques include oven drying, infrared radiation drying and air drying. When the coating is dried, it may be dried at room temperature or elevated temperature. In the embodiment in which the charge transport layer is applied to the charge generating layer prior to drying, the charge transport layer can be applied immediately after application of the charge generating layer or can be applied to a partially or nearly completely solidified charge generating layer. In the embodiment wherein the charge generating material and charge transport material are simultaneously applied, the materials may be applied to a dried charge generating layer or to a partially or completely dried charge generating layer. Correspondingly, the applied interphase region may be completely or only partially dried prior to application of the charge transporting layer. Each layer can be applied to a previously applied layer in the wet state or in any state including a dry or nearly solidified state. A previously applied layer may be dried for a period of 0 to 20 minutes or longer before application of the next layer. In various embodiments of the invention, a previously applied layer may be dried for a period of 0 to 20 minutes, 5 to 15 minutes or 10 to 12 minutes. In some embodiments, the previously applied layer can be dried for a period of 0 to 5 minutes or 0 to 10 minutes or 18 to 20 minutes or 15 to 18 minutes. The period of drying will depend upon the conditions of drying. Additionally, the period of drying will depend upon the physical state of the previously applied layer necessary to carry out the objectives of the process of the invention. The invention will further be illustrated in the following examples. EXAMPLE 1 A nylon charge blocking layer is fabricated from an 8 weight% solution of nylon in butanol, methanol and water mixture. The butanol, methanol and water mixture percentages are 55, 36 and 9 weight% respectively. A charge generating layer is prepared from a 3% by weight solids solution of benzimidazole perylene and polyvinylbutyral B79 (Mansanto Chem. Co.) (68/32 weight %) in n-butylacetate. The dispersion is prepared by roll milling the pigment/B79/n-butylacetate solution for 5 days in a bottle charged with 3.2mm dia. stainless steel shot. A charge transport layer is prepared from a 20% by weights solids solution of N,N'diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-biphenyl)-4,4' diamine and poly(4,4'-diphenyl-1,1'-cyclohexane carbonate) (35/65 weight %) in monochlorobenzene. The charge blocking layer is dip coated onto an aluminum substrate and is dried at a temperature of about 105°C for about 5 minutes. The dried nylon containing blocking layer has a thickness of about 1.5 µm. The charge generating layer is then coated onto the charge blocking layer and allowed to air dry for 5 minutes. The layer thickness is about 0.5 µm. The charge transport layer is dip coated onto the charge generating layer and is dried at about 130°C for about 60 minutes. The dried charge transport layer has a thickness of about 20 µm. For comparison, a sister photoreceptor sample is prepared by the same method as above, except that the charge generating layer is dried at 110°C for 10 minutes. The two samples are tested in a cyclic scanner at ambient conditions, i.e., about 25°C, for photosensitivity. The device is first charged with a scorotron to 600V, then is exposed to a light of 670 nm wavelength 0.47 s after charging. Light intensity is varied to monitor the surface voltage change amount. Photosensitivity is calculated by dividing the amount of surface voltage change by the exposed light intensity. Device dV/dX(V.cm 2 /ergs) measured att = 0 measured 2 months later Dried charge generating layer coated from cyclohexanone 65 65 Dried charge generating layer coated from n-butylacetate 110 114 Dried charge generating layer coated from n-butylacetate 112 105 No dry charge generating layer coated from n-butylacetate 135 138 No dry charge generating layer coated from n-butylacetate 149 141 A 30% increase in sensitivity is achieved when dip coating the benzimidazole perylene dispersion (in polyvinylbutyral binder) from n-butylacetate instead of cyclohexanone. A further increase in sensitivity is achieved by eliminating the drying step between the charge generating layer and charge transport layer and coatings. The n-butylacetate dispersion is stable (newtonian with no yield point) over a long period of time with no particle size or rheological property change over a three month monitoring period. No streak or other coating defects known to be associated with charge generating layer dispersion qualities are observed. The dispersion can be manufacturable in large quantities by a dynomilled process. Consistent sensitivity values are observed with photoreceptor devices coated periodically over a three month time frame. The devices are remeasured 3 months after fabrication and the sensitivity remains unchanged. The 140 V.cm2/ergs sensitivity for a 20 µm thick device is satisfactory for a commercial product. Eliminating of the charge generating layer drying step reduces cycle time and reduces photoreceptor cost.

Claims: A process for preparing an electrophotographic imaging member having a perylene-containing charge generating layer, comprising: a) dispersing a perylene charge generating material in an acetate solvent to form a dispersion; b) providing a charge transport material; and c1) applying said dispersion to form a perylene-containing charge generating layer as a wet underlying layer and subsequently overcoating said charge transport material on the underlying layer prior to drying to form a charge transport layer and an interphase region comprising a mixture of perylene charge generating material and charge transport material between said charge generating layer and said charge transport layer, or c2) simultaneously applying said dispersion and said charge transport material to form a perylene-containing charge generating layer overcoated with a charge transport layer and an interphase region comprising a mixture of perylene charge generating material and charge transport material between said charge generating layer and said charge transport layer. The process according to claim 1, wherein the amount of the perylene charge generating material is decreased and the amount of the charge transport material is increased as the materials are applied in said forming step. The process according to claim 1, comprising: (a) applying the perylene charge generating material; (b) forming said interphase region by applying a decreasing amount of perylene charge generating material and an increasing amount of charge transport material to form a gradient mixture of charge generating material and charge transport material; and (c) applying the charge transport material. The process according to any of the preceding claims, wherein the perylene charge generating material and/or the charge transport material are applied by spray coating, dip coating, roll coating, Meyer bar coating, bead coating or curtain flow coating. The process according to any of the preceding claims, wherein said perylene charge generating material is dispersed with a polyvinylbutyral binder in said acetate solvent. The process according to claim 5, wherein said acetate solvent is selected from the group consisting of methylacetate, ethylacetate, isopropylacetate, n-propylacetate, n-butylacetate, sec-butylacetate and tert-butylacetate. The process according to any of the preceding claims, wherein said perylene-containing charge generating layer comprises benzimidazole perylene.

Inventors: CHAMBERS JOHN S, YUH HUOY-JEN, CHAMBERS, JOHN S., YUH, HUOY-JEN
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1996
Main classification: G03G5
IPC codes: G03G5