Charge generation layer containing mixture of terpolymer and copolymer

BACKGROUND OF THE INVENTION
This invention relates in general to electrophotographic imaging members and more specifically, to an electrophotographic imaging member having an improved charge generation layer.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable electrophotographic imaging members.
The electrophotographic imaging members may be in the form of plates, drums or flexible belts. These electrophotographic members are usually multilayered photoreceptors that comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer.
A conventional technique for coating cylindrical or drum shaped photoreceptor substrates involves dipping the substrates in coating baths. The bath used for preparing photoconducting layers is prepared by dispersing photoconductive pigment particles in a solvent solution of a film forming binder. Unfortunately, some organic photoconductive pigment particles cannot be applied by dip coating to form high quality photoconductive coatings. For example, organic photoconductive pigment particles such as hydroxygallium phthalocyanine and titanyl phthalocyanine pigment particles tend to settle when attempts are made to disperse the pigments in a solvent solution of a film forming binder. The tendency of the particles to settle requires constant stirring which can lead to entrapment of air bubbles that are carried over into the final photoconductive coating deposited on a photoreceptor substrate. These bubbles cause defects in final prints xerographically formed with the photoreceptor. The defects are caused by differences in discharge of the electrically charged photoreceptor between the region where the bubbles are present and where the bubbles are not present. Thus, for example, the final print will show dark areas over the bubbles during discharged area development or white spots when utilizing charged area development. Moreover, many pigment particles tend to agglomerate when attempts are made to disperse the pigments in solvent solutions of film forming binders. The pigment agglomerates lead to non-uniform photoconductive coatings which in turn lead to other print defects in the final xerographic prints due to non-uniform discharge. The film forming binder selected for photoconductive pigment particles in a charge generating layer can adversely affect the particle dispersion uniformity, coating composition rheology, residual voltage after erase and electrophotographic sensitivity. Some binders can lead to unstable pigment particle dispersions which are unsuitable for coating photoreceptors. Thus, for example, when a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate such as VYHH terpolymer from Union Carbide is utilized to disperse hydroxygallium phthalocyanine photoconductive particles, an unstable dispersion is obtained. Moreover, a charge generating layer containing this copolymer has poor light sensitivity and gives high residual voltage after erase. Combinations of some polymers can result in unacceptable coating or electrical properties. For example, some polymers are incompatible with each other and cannot form coatings in which the polymers or particles are distributed uniformly throughout the final coating.
Photoconductive compositions are also difficult to modify for electrophotographic copiers, duplicators and printers characterized by different sensitivity requirements. Thus, custom photogenerating layer compositions must be prepared for each type of machine having its own different specific sensitivity requirement. The addition of a relatively insensitive pigment to a highly sensitive photoconductive pigment can alter the overall sensitivity of a photoreceptor. However, uniform electrical characteristics from one batch to the next batch is difficult to achieve because of uneven pigment distribution of the two different pigment particles in the final dried charge generation layer. Variations in distribution might be due to property differences of the different pigment materials employed such as size, shape, wetting characteristics, density, triboelectric charge, and the like. For example, some dispersions behave in a non-uniform manner when deposited as a coating on a photoreceptor substrate to form discontinuous coatings during dip coating or roll coating operations. It is believed that these discontinuous coatings are caused by the coating material flowing in some regions of the areas being coated and not in other regions.
INFORMATION DISCLOSURE STATEMENT
U.S. Pat. No. 5,296,323 to T. Kobata et al., issued Mar. 22, 1994--A laminated organic photosensitive material is disclosed which provides a copy image having substantially no interference fringe-like unevenness in darkness thereon. The laminated organic photosensitive material comprises an electroconductive e support, an undercoat, a charge producing layer and a charge transporting layer in sequence wherein each of the undercoat and the charge producing layer has a thickness of d (NM) which satisfied a specified formula. Various specific polymers for the charge producing layer such as certain vinyl polymers and mixtures of polymers are also disclosed.
U.S. Pat. No. 5;079,120 to Kobata et al., issued Jan. 7, 1992--A laminated organic photosensitive material is disclosed which comprises an electroconductive support, a charge producing layer and a charge transporting layer formed thereon wherein the charge producing layer contains X-type non-metal phthalocyanine as a charge producing substance and a halogen-containing resin as a binder resin for the layer, and the charge transporting layer contains a hydrazone compound having a specified formula, a charge transporting substance and a halogen-containing resin as a binder resin for the charge producing layer.
U.S. Pat. No. 5,521,306 to Richard Burt et al., issued May 28, 1996--A process for preparation of Type V hydroxygallium phthalocyanine is disclosed comprising the in situ formation of an alkoxy-bridged gallium phthalocyanine dimer, hydrolyzing the dimer to hydroxygallium phthalocyanine and subsequently converting the hydroxygallium phthalocyanine product obtained to Type V hydroxygallium phthalocyanine.
U.S. Pat. No. 5,420,268 to Denis Desilets et al., issued on May 30, 1995--A layered photoconductive imaging member is disclosed comprising a supporting substrate, a photogenerating layer and a transport layer, and wherein the photogenerating layer contains Type IV oxytitanium phthalocyanine obtained by hydrolysis of a dihalotitanium phthalocyanine in a strong acid.
U.S. Pat. No. 5,322,755 to Ah-Mee Hor et al, issued Jun. 21, 1994--An electrophotographic recording element is disclosed. In addition, some dispersions react non-uniformly when deposited as a coating on a photoreceptor substrate to form discontinuous coatings during dip coating or roll coating operations. It is believed that these discontinuous coatings are caused by the coating material flowing in some regions of the coating and not in other regions.
U.S. Pat. No. 5,418,107. to Richard Nealey et al., issued May 23, 1995--A process is disclosed for fabricating an electrophotographic imaging member including providing a substrate to be coated, forming a coating comprising photoconductive pigment particles having an average particle size of less than about 0.6 micrometer dispersed in a solution of a solvent comprising n-alkyl acetate having from 3 to 5 carbon atoms in the alkyl group and a film forming polymer consisting essentially of a film forming polymer having a polyvinyl butyral content between about 50 and about 75 mol percent, a polyvinyl alcohol content between about 12 and about 50 mol percent, and a polyvinyl acetate content is between about 0 to 15 mol percent, the photoconductive pigment particles including a mixture of at least two different phthalocyanine pigment particles free of vanadyl phthalocyanine pigment particles, drying the coating to remove substantially all of the alkyl acetate solvent to form a dried charge generation layer comprising between about 50 percent and about 90 percent by weight of the pigment particles based on the total weight of the dried charge generation layer, and forming a charge transport layer.
US-A to et al, issued May 19, 1992--An electrophotographic photoreceptor is disclosed having a light-sensitive layer on an electroconductive base. The light-sensitive layer is formed from a dispersion in which a titanyl phthalocyanine having at least two predominant peaks at Bragg angle 2.sub.-- at 9.6.degree..+-.0.2.degree. and 27.2.+-.2.degree. in a diffraction spectrum obtained with characteristic x-rays of Cu K at a wavelength of 1.54 Angstrom is dispersed in a dispersion medium that contains at least one of branched acetate ester and alcohol solvents as a chief component. Charge generation particle sizes having an average particle size of 2 micrometer or below, preferably 1 micrometer or below are also disclosed.
U.S. Pat. No. 4,728,592 to Ohaku et al., issued Mar. 1, 1988--An electrophotoconductor is disclosed having a light sensitive layer comprising a titanyl phthalocyanine dispersed in a binder, the titanyl phthalocyanine having a certain specified structure. The titanyl phthalocyanine may be employed in combination with a binder such as butyral resin.
U.S. Pat. No. 4,898,799 to Fujimaki et al., issued Feb. 6, 1990--A photoreceptor for electrophotography is disclosed containing a titanyl phthalocyanine compound which has certain specified major peaks in terms of Bragg's 2.theta. angles. The binders used to form the carrier generator layer may include polyvinyl butyral.
U.S. Pat. No. 4,265,990 to Stolka et al., issued May 5, 1981--A photosensitive member is disclosed having at least two electrically operative layers. The first layer comprises a photoconductive layer and the second layer comprises a charge transport layer. The charge transport layer comprises a polycarbonate resin and a diamine having a certain specified structure. Also, metal phthalocyanines are disclosed as useful as charge generators. A photoconductor particle size of about 0.01 to 5.0 micrometers is mentioned.
U.S. Pat. No. 3,121,006 to Middleton et al., issued Feb. 11, 1964--A xerographic process is disclosed which utilizes a xerographically sensitive member comprising an insulating organic binder having dispersed therein finely-divided particles of an inorganic photoconductive insulating metallic-ions containing crystalline compound. Various specific insulating organic binders are disclosed.
CROSS REFERENCE TO COPENDING PATENT APPLICATIONS
U.S. application Ser. No. 08/786,009, filed concurrently herewith in the names of R. Nealey et al., entitled "CHARGE GENERATION LAYER CONTAINING HYDROXYALKYL ACRYLATE REACTION PRODUCT" and identified by the Docket number D/96631--An electrophotographic imaging member is disclosed comprising a substrate, a charge generating layer and a charge transport layer, the charge generating layer comprising photoconductive hydroxygallium phthalocyanine particles dispersed in a polymer matrix, the matrix comprising a polymeric film forming reaction product of at least vinyl chloride, vinyl acetate and hydroxyalkyl acrylate.
As described above, there is a continuing need for versatile high quality photoreceptors.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an improved photoreceptor which overcomes the above-noted deficiencies.
It is yet another object of the present invention to provide an improved photoreceptor have high quality photoconductive coatings.
It is still another object of the present invention to provide an improved photoreceptor that have uniform continuous photoconductive coatings.
It is another object of the present invention to provide an improved photoreceptor that exhibit improved electrical properties.
It is yet another object of the present invention to provide an improved more versatile tunable photoreceptor.
These and other objects of the present invention are accomplished by providing an electrophotographic imaging member comprising a substrate, a charge generating layer and a charge transport layer, the charge generating layer comprising photoconductive particles selected from the group consisting of hydroxygallium phthalocyanine particles and titanyl phthalocyanine particles dispersed in a polymer matrix, the matrix comprising a uniform mixture of a film forming terpolymer reaction product of vinyl chloride, vinyl acetate and maleic acid and a film forming copolymer reaction product of vinyl chloride and vinyl acetate.
Electrophotographic imaging members, i.e. photoreceptors, are well known in the art. Typically, a substrate is provided having an electrically conductive surface. At least one photoconductive layer is then applied to the electrically conductive surface. A charge blocking layer may be applied to the electrically conductive surface prior to the application of the photoconductive layer. If desired, an adhesive layer may be utilized between the charge blocking layer and the photoconductive layer. For multilayered photoreceptors, a charge generation binder layer is usually applied onto the blocking layer and charge transport layer is formed on the charge generation layer. However, if desired, the charge generation layer may be applied to the charge transport layer.
The substrate may be opaque or substantially transparent and may comprise numerous suitable materials 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 rigid or flexible, such as thin webs.
The thickness of the substrate layer depends on numerous factors, including beam strength and economical considerations, and thus this layer for a flexible belt may be of substantial thickness, for example, about 125 micrometers, or of minimum thickness less than 50 micrometers, provided there are no adverse effects on the final electrostatographic device. In one flexible belt embodiment, the thickness of this layer ranges from about 65 micrometers to about 150 micrometers, and preferably from about 75 micrometers to about 100 micrometers for optimum flexibility and minimum stretch when cycled around small diameter rollers, e.g. 19 millimeter diameter rollers. Substrates in the shape of a drum or cylinder may comprise a metal, plastic or combinations of metal and plastic of any suitable thickness depending upon the degree of rigidity desired.
The conductive layer may vary in thickness over substantially wide ranges depending on the optical transparency and degree of flexibility desired for the electrostatographic member. Accordingly, for a flexible photoresponsive imaging device, the thickness of the conductive layer may be between about 20 angstrom units to about 750 angstrom units, and more preferably from about 100 Angstrom units to about 200 angstrom units for an optimum combination of electrical conductivity, flexibility and light transmission. The flexible conductive layer may be an electrically conductive metal layer formed, for example, on the substrate by any suitable coating technique, such as a vacuum depositing technique. Where the substrate is metallic, such as a metal drum, the outer surface thereof is normally inherently electrically conductive and a separate electrically conductive layer need not be applied.
After formation of an electrically conductive surface, a hole blocking layer may be applied thereto. Generally, electron blocking layers for positively charged photoreceptors allow holes from the imaging surface of the photoreceptor to migrate toward the conductive layer. Any suitable blocking layer capable of forming an electronic barrier to holes between the adjacent photoconductive layer and the underlying Conductive layer may be utilized. Blocking layers are well known and disclosed, for example, in U.S. Pat. Nos. 4,291,110, 4,338,387, 4,286,033 and 4,291,110. The disclosures of U.S. Pat. Nos. 4,338,387, 4,286,033 and 4,291,110 are incorporated herein in their entirety. The blocking layer may comprise an oxidized surface which inherently forms on the outer surface of most metal ground plane surfaces when exposed to air. The blocking layer may be applied as a coating 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. 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. The blocking layer should be continuous and have a thickness of less than about 2 micrometer because greater thicknesses may lead to undesirably high residual voltage.
An optional adhesive layer may applied to the hole blocking layer. Any suitable adhesive layer well known in the art may be utilized. Satisfactory results may be achieved with adhesive layer thickness between about 0.05 micrometer (500 angstroms) and 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.
The photogenerating layer of this invention photoconductive particles selected from the group consisting of hydroxygallium phthalocyanine particles and titanyl phthalocyanine particles dispersed in a polymer matrix, the matrix comprising a uniform mixture of a film forming terpolymer reaction product of vinyl chloride, vinyl acetate and maleic acid and a film forming copolymer reaction product of vinyl chloride and vinyl acetate. Preferably, the photoconductive particle size is also less than the thickness of the dried photoconductive coating in which it is dispersed. Photoconductive hydroxygallium phthalocyanine particles and titanyl phthalocyanine particles are well known in the art. These particles are available in numerous polymorphic forms. Any suitable hydroxygallium phthalocyanine or titanyl phthalocyanine polymorph may be used in the charge generating layer of the photoreceptor of this invention. Hydroxygallium phthalocyanine and titanyl phthalocyanine polymorphs are extensively described in the technical and patent literature. For example, hydroxygallium phthalocyanine Type V and other polymorphs are described in U.S. Pat. No. 5,521,306 and titanyl phthalocyanine Types I, II, IV and other polymorphs are described in U.S. Pat. No. 5,420,268, the entire disclosures of these patents being incorporated herein by reference. Generally, the photoconductive pigment particle size utilized is less than the thickness of the dried charge generating layer and the average particle size is less than about 1 micrometer. Satisfactory results are achieved with an average photoconductive particle size of less than about 0.6 micrometer when the photoconductive coating is applied by dip coating. Preferably, the average photoconductive particle size is less than about 0.4 micrometer. Optimum results are achieved with an average particles size of less than about 0.1 micrometer.
The polymer matrix in the charge generating layer of this invention comprises a uniform mixture of a film forming terpolymer reaction product of vinyl chloride, vinyl acetate and maleic acid and a film forming copolymer reaction product of vinyl chloride and vinyl acetate. These materials are commercial available resins obtainable from Union Carbide Corp under the trade names VMCH, VYHH and VYNS. For utilization in the photoconductive layer of this invention the terpolymer is formed from a reaction mixture comprising between about 81 percent and about 86 percent by weight vinyl chloride, between about 13 percent and about 17 percent by weight vinyl acetate and between about 1 percent and about 2 percent by weight maleic acid, based on the total weight of the reactants for the terpolymer. When the proportion of maleic acid exceeds about 2 weight percent, high dark decay can occur and charging becomes unacceptable. A proportion of maleic acid of less than about 0.5 weight percent adversely affects the quality of the dispersion of pigment particles in the coating composition This effect is most clearly seen in the particle size of the agglomerates as measured by a Model CAPA 700 (available from Horiba Instruments Corp.) where particle size increases when the amount of maleic acid in the mixture is reduced to below 0.5 weight. The terpolymer may be represented by the following formula: ##STR1## wherein x is the proportion of the terpolymer derived from a reaction mixture comprising between about 81 percent and about 86 percent by weight vinyl chloride,
y is the proportion of the terpolymer derived from a reaction mixture comprising between about 13 percent and about 17 percent by weight vinyl acetate, and
z is the proportion of the terpolymer derived from a reaction mixture comprising between about 1 percent and about 2 percent by weight maleic acid, based on the total weight of the reactants for the terpolymer.
These film forming terpolymers are commercially available and include, for example, VMCH resin--a terpolymer reaction product of 86 weight percent vinyl chloride, 13 weight percent vinyl acetate and 1 weight percent maleic acid having a weight average molecular weight of about 27,000 (available from Union Carbide Co.), VMCC resin--a terpolymer reaction product of 83 weight percent vinyl chloride, 16 weight percent vinyl acetate and 1 weight percent maleic acid having a weight average molecular weight of about 19,000 (available from Union Carbide Co.), VMCA resin--a terpolymer reaction product of 81 weight percent vinyl chloride, 17 weight percent vinyl acetate and 2 weight percent maleic acid having a weight average molecular weight of about 15,000 (available from Union Carbide Co.),), and the like. Satisfactory results may be achieved when the terpolymer is a solvent soluble polymer having a weight average molecular weight of at least about 10,000. Preferably, the terpolymer has a weight average molecular weight of between about 10,000 and about 50,000. When the molecular weight is below about 10,000, poor film forming properties and undesirable dispersion characteristics can be encountered. A molecular weight greater than about 50,000 can be acceptable if the terpolymer remains solvent soluble. When a terpolymer reaction product of 86 weight percent vinyl chloride, 13 weight percent vinyl acetate and 1 weight percent maleic acid such as VMCH terpolymer from Union Carbide is utilized as the sole matrix material for dispersing hydroxygallium phthalocyanine photoconductive particles, a stable Newtonian dispersion can be obtained. However, the maleic acid content present can contribute to high dark decay values and/or print defects due to localized dark decay points. It is advantageous to reduce the amount of acid content to a minimum without impacting the quality of the dispersion.
The film forming copolymer is reaction product of vinyl chloride and vinyl acetate prepared by conventional emulsion or suspension techniques and contains a spine of carbon atoms. The chain length can be controlled by varying the reaction temperature and time. For utilization in the photoconductive layer of this invention the copolymer is formed from a reaction mixture comprising between about 80 percent and about 90 percent by weight vinyl chloride and between about 20 percent and about 10 percent by weight vinyl acetate, based on the total weight of the reactants for the copolymer. ##STR2## wherein s is the proportion of the copolymer derived from a reaction mixture comprising between about 80 percent and about 90 percent by weight vinyl chloride and
t is the proportion of the copolymer derived from a reaction mixture comprising between about 20 percent and about 10 percent by weight vinyl acetate, based on the total weight of the reactants for the terpolymer.
These film forming copolymers are commercially available and include, for example, VYNS resin--a copolymer reaction product of 90 weight percent vinyl chloride and 10 weight percent vinyl acetate and having a weight average molecular weight of about 44,000 (available from Union Carbide Co.), VYHH resin--a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate and having a weight average molecular weight of about 27,000 (available from Union Carbide Co.), VYHD having a structure similar to VYHH but having a weight average molecular weight of 22,000 (available from UNION Carbide), and the like. Satisfactory results may be achieved when the copolymer is a solvent soluble polymer having a weight average molecular weight of at least about 22,000. Preferably, the copolymer has a weight average molecular weight of between about 22,000 and about 80,000. When the molecular weight is below about 22,000, poor film forming properties and undesirable dispersion characteristics can be encountered. A molecular weight greater than about 80,000 can be acceptable if the copolymer remains solvent soluble. When a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate (VYHH copolymer from Union Carbide) is utilized as the sole matrix to disperse hydroxygallium phthalocyanine photoconductive particles, a stable Newtonian dispersion cannot be obtained and high erase residual voltage is observed. However, satisfactory dispersions and electrical properties, including reduce erase voltage and good print quality, can be achieved with a combination of the terpolymer and copolymer, particularly when the matrix comprises between about 95 percent and about 50 percent by weight of the terpolymer and between about 5 percent and about 50 percent by weight of the copolymer, based on the total weight of said matrix in the charge generating layer after drying. Preferably, photoreceptor the matrix comprises between about 95 percent and about 50 percent by weight of the terpolymer and between about 5 percent and about 50 percent by weight of the copolymer, based on the total weight of the matrix in said charge generating layer. Optimum photoreceptor performance is achieved when the matrix comprises about 50 percent by weight of the terpolymer and about 50 percent by weight of the copolymer, based on the total weight of the matrix in the charge generating layer. When the proportion of the copolymer in the binder mixture is greater than about 50 percent by weight, the quality of the dispersion of pigment particles in the coating mixture degrades and an increase in the erase residual voltage in the final photoreceptor is observed. When a terpolymer (e.g. VMCH) is used alone a high dark decay is observed and when a copolymer (e.g. VYHH) is used alone a poor quality dispersion is obtained as seen in particle size measurements and high dark decay is also seen. The optimum mixture is a 50/50 blend which retains the good dispersion quality of VMCH yet also shows a significant decrease in dark decay without significantly impacting the photo response of the device.
Any suitable solvent may be employed to dissolve the mixture of two film forming polymers utilized in the charge generating layer matrix of this invention. Typical solvents include, for example, esters, ethers, ketones, mixtures thereof, and the like. Specific solvents include, for example, n-butyl acetate, cyclohexanone, tetrahydrofuran, methylethyl ketone, toluene, mixtures of methylethyl ketone and toluene, mixtures of tetrahydrofuran and toluene, and the like.
Any suitable technique may be utilized to disperse the pigment particles in the solution of the two film forming polymers dissolved in a suitable solvent. Typical dispersion techniques include, for example, ball milling, roll milling, milling in vertical attritors, sand milling, and the like which utilize milling media. The solids content of the mixture being milled does not appear critical and can be selected from a wide range of concentrations. Typical milling times using a ball roll mill is between about 4 and about 6 days. If desired, the photoconductive particles with or without film forming binder may be milled in the absence of a solvent prior to forming the final coating dispersion. Also, a concentrated mixture of photoconductive particles and binder solution may be initially milled and thereafter diluted with additional binder solution for coating mixture preparation purposes. The resulting dispersion may be applied to the adhesive blocking layer, a suitable electrically conductive layer or to a charge transport layer. When used in combination with a charge transport layer, the photoconductive layer may be between the charge transport layer and the substrate or the charge transport layer can be between the photoconductive layer and the substrate.
Any suitable technique may be utilized to apply the coating to substrate to be coated. Typical coating techniques include dip coating, roll coating, spray coating, rotary atomizers, and the like. The coating techniques may use a wide concentration of solids. Preferably, the solids content is between about 2 percent by weight and 8 percent by weight based on the total weight of the dispersion. The expression "solids" refers to the pigment particle and binder components of the coating dispersion. These solids concentrations are useful in dip coating, roll, spray coating, and the like. Generally, a more concentrated coating dispersion is preferred for roll coating. 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.
Satisfactory results are achieved when the dried photoconductive coating comprises between about 50 percent and about 65 percent by weight of the photoconductive hydroxygallium phthalocyanine or titanyl phthalocyanine particles based on the total weight of the dried charge generating layer. When the pigment concentration is less than about 50 percent by weight, the particle to particle contact is lost resulting in deterioration of electrical performance.. Optimum imaging performance is achieved when the charge generating layer comprises about 60 percent by weight of the photoconductive particles based on the total weight of the dried charge generating layer. Since the photoconductor characteristics are affected by the relative amount of pigment per square centimeter coated, a lower pigment loading may be utilized if the dried photoconductive coating layer is thicker. Conversely, higher pigment loadings are desirable where the dried photoconductive layer is to be thinner.
For multilayered photoreceptors comprising a charge generating layer and a charge transport layer, satisfactory results may be achieved with a dried photoconductive layer coating thickness of between about 0.1 micrometer and about 10 micrometers. Preferably, the photoconductive layer thickness is between about 0.2 micrometer and about 1 micrometer. Optimum results are achieved with a generating layer has a thickness of between about 0.3 micrometer and about 0.7 micrometer. However, these thicknesses also depend upon the pigment loading. Thus, higher pigment loadings permit the use of thinner photoconductive coatings. Thicknesses outside these ranges can be selected providing the objectives of the present invention are achieved.
The active charge transport layer may comprise any suitable activating compound useful as an additive dispersed in electrically inactive polymeric materials making these materials electrically active. These compounds may be added to polymeric materials which are incapable of supporting the injection of photogenerated holes from the generation material and incapable of allowing the transport of these holes therethrough. This will convert the electrically inactive polymeric material to a material capable of supporting the injection of photogenerated holes from the generation material and capable of allowing the transport of these holes through the active layer in order to discharge the surface charge on the active layer. An especially preferred transport layer employed in one of the two electrically operative layers in the multilayered photoconductor of this invention comprises from about 25 percent to about 75 percent by weight of at least one charge transporting aromatic amine compound, and about 75 percent to about 25 percent by weight of a polymeric film forming resin in which the aromatic amine is soluble.
The charge transport layer forming mixture preferably comprises an aromatic amine compound of one or more compounds having the general formula: ##STR3## wherein R.sub.1 and R.sub.2 are an aromatic group selected from the group consisting of a substituted or unsubstituted phenyl group, naphthyl group, and polyphenyl group and R.sub.3 is selected from the group consisting of a substituted or unsubstituted aryl group, alkyl group having from 1 to 18 carbon atoms and cycloaliphatic compounds having from 3 to 18 carbon atoms. The substituents should be free form electron withdrawing groups such as NO2 groups, CN groups, and the like.
Examples of charge transporting aromatic amines represented by the structural formulae above 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 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. Molecular weights can vary from about 20,000 to about 150,000.
Any suitable and conventional technique may be utilized to mix and thereafter apply the charge transport layer coating mixture to the coated or uncoated substrate. Typical application techniques include spraying, dip coating, roll coating, wire wound rod coating, and the like. Drying of the deposited coating may be effected by any suitable conventional technique such as oven drying, infra red radiation drying, air drying and the like.
Generally, the thickness of the hole transport layer is between about 10 to 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 layer is preferably maintained from about 2:1 to 200:1 and in some instances as great as 400:1.
The preferred electrically inactive resin materials are polycarbonate resins have a molecular weight from about 20,000 to about 150,000, more preferably from about 50,000 to about 120,000. The materials most preferred as the electrically inactive resin material is poly(4,4'-dipropylidene-diphenylene carbonate) with a molecular weight of from about 35,000 to about 40,000, available as Lexan 145 from General Electric Company; poly(4,4'-isopropylidenediphenylene carbonate) with a molecular weight of from about 40,000 to about 45,000, available as Lexan 141 from the General Electric Company; a polycarbonate resin having a molecular weight of from about 50,000 to about 120,000, available as Makrolon from Farbenfabricken Bayer A. G. and a polycarbonate resin having a molecular weight of from about 20,000 to about 50,000 available as Merlon from Mobay Chemical Company. Methylene chloride solvent is a desirable component of the charge transport layer coating mixture for adequate dissolving of all the components and for its low boiling point.
Examples of photosensitive members having at least two electrically operative layers include the charge generator layer and diamine containing transport layer members disclosed in U.S. Pat. No. 4,265,990, U.S. Pat. No. 4,233,384, U.S. Pat. No. 4,306,008, U.S. Pat. No. 4,299,897 and U.S. Pat. No. 4,439,507. The disclosures of these patents are incorporated herein in their entirety. The photoreceptors may comprise, for example, a charge generator layer sandwiched between a conductive surface and a charge transport layer as described above or a charge transport layer sandwiched between a conductive surface and a charge generator layer.
Optionally, an overcoat layer may also be utilized to improve resistance to abrasion. In some cases an anti-curl back coating may be applied to the side opposite the photoreceptor to provide flatness and/or abrasion resistance where a web configuration photoreceptor is fabricated. These overcoating and anti-curl back coating layers are well known in the art and may comprise thermoplastic organic polymers or inorganic polymers that are electrically insulating or slightly semi-conductive. Overcoatings are continuous and generally have a thickness of less than about 10 micrometers. The thickness of anti-curl backing layers should be sufficient to substantially balance the total forces of the layer or layers on the opposite side of the supporting substrate layer. An example of an anti-curl backing layer is described in U.S. Pat. No. 4,654,284 the entire disclosure of this patent being, incorporated herein by reference. A thickness between about 70 and about 160 micrometers is a satisfactory range for flexible photoreceptors.
A number of examples are set forth hereinbelow and are illustrative of different compositions and conditions that can be utilized in practicing the invention. All proportions are by weight unless otherwise indicated. It will be apparent, however, that the invention 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.

EXAMPLE I
A dispersion was prepared by dissolving a film forming binder composition in CXN solvent and then adding hydroxygallium phthalocyanine pigment. The binder concentration, based on the total weight of binder in the solution was 1 00 percent by weight of a terpolymer reaction product of 86 weight percent vinyl chloride, 13 weight percent vinyl acetate and 1 weight percent maleic acid having a weight average molecular weight of about 27,000 (VMCH, available from Union Carbide Co.). The pigment concentration in the dispersion was 60 percent by weight based on the total solids weight (pigment and binder). The dispersion was milled in a ball mill with 1/8 inch (0.3 cm) diameter stainless steel shot for 4 days. The dispersion was filtered to remove the shot. The average particle size of the milled pigment was about 0.06 micrometer. The dispersion quality of the coasting mixture was examined. Next, the charge generating layer coating mixture was applied by a dip coating process in which a cylindrical 40 mm diameter and 310 mm long aluminum drum coated with a 1.5 micrometers thick nylon coating was immersed into and withdrawn from the charge generating layer coating mixture in a vertical direction along a path parallel to the axis of the drum at a rate of 200 mm/min. The applied charge generation coating was dried by in oven at 106.degree. C. for 10 minutes to form a layer having a thickness of approximately 0.2 micrometers. This coated charge generator layer was then dip coated with a charge transport mixture containing 36 percent N,N'-diphenyl-N,N'-bis(3methylphenyl)-1,1'-biphenyl-4,4'diamine and polycarbonate dissolved in monochlorobenzene solvent. The applied charge transport coating was dried by in a forced air oven at 118.degree. C. for 25 minutes to form a layer having a thickness of 20, micrometers. The electrophotographic imaging members prepared were tested by electrically charging it at a field of 800 volts and discharging it with light having a wavelength of 780 nm. The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYHH Particle Power Yield Vm% % Size Law Viscosity Point (3 erg) Ve______________________________________100 0 0.06 0.969 14 0.004 56 18______________________________________
All particle size determinations were accomplished on a Model CAPA 700 (available from Horiba Instruments Corp.) particle size distribution analyzer in the solvents used for the pigment milling step. The expression "power law" is obtained by plotting the viscosity against the shear rate and measuring the slope of the resulting line. A value that approximates 1 is indicative of a newtonian fluid, i.e exhibits no change in viscosity with increasing shear. The viscosity values are in centipoise units. The expression "yield point" is defined as the resistance to flow until a certain shear value is applied. A value approximating 0 has no yield point and is desirable for dip coating purposes. This yield point value demonstrates that no yield point is observed in this dispersion. Vm(3 erg) is the voltage resulting when a photoreceptor is charged to 800V and then exposed to 3 ergs/sq.cm. of light and is a measure of the photoresponse of the device. Verase is the voltage remaining under the same test conditions but after an exposure of 300 ergs/sq.cm. and is somtimes referred to as the residual voltage, i.e. the device cannot be discharged with light to below this level.
EXAMPLE II
An electrophotographic imaging member was prepared as described in Example I using the same procedures and materials except that instead of using 100 weight percent of VMCH in the binder component, 75 percent VMCH was mixed with 25 weight percent of a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate having a weight average molecular weight of about 27,000 (VYHH copolymer from Union Carbide). The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYHH Particle Power Yield Vm% % Size Law Viscosity Point (3 erg) Ve______________________________________75 25 0.06 0.96 13 0.015 63 20______________________________________
EXAMPLE III
An electrophotographic imaging member was prepared as described in Example I using the same procedures and materials except that instead of using 100 weight percent of VMCH in the binder component, 50 percent VMCH was mixed with 50 weight percent of a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate having a weight average molecular weight of about 27,000 (VYHH copolymer from Union Carbide). The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYHH Particle Power Yield Vm% % Size Law Viscosity Point (3 erg) Ve______________________________________50 50 0.08 0.964 14 0.004 60 23______________________________________
EXAMPLE IV
An electrophotographic imaging member was prepared as described in Example I using the same procedures and materials except that instead of using 100 weight percent of VMCH in the binder component, 25 percent VMCH was mixed with 75 weight percent of a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate having a weight average molecular weight of about 27,000 (VYHH copolymer from Union Carbide). The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYHH Particle Power Yield Vm% % Size Law Viscosity Point (3 erg) Ve______________________________________25 75 0.15 0.93 18 0.023 68 29______________________________________
EXAMPLE V
An electrophotographic imaging member was prepared as described in Example I using the same procedures and materials except that instead of using 100 weight percent of VMCH in the binder component, 100 weight percent of a copolymer reaction product of 86 weight percent vinyl chloride and 14 weight percent vinyl acetate having a weight average molecular weight of about 27,000 (VYHH copolymer from Union Carbide) was employed. The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYHH Particle Power Yield Vm% % Size Law Viscosity Point (3 erg) Ve______________________________________0 100 0.38 0.807 187 0.258 97 51______________________________________
EXAMPLE VI
A dispersion was prepared by dissolving a film forming binder composition in cyclohexanone solvent and then adding hydroxygallium phthalocyanine pigment. The binder concentration, based on the total weight of binder in the solution was 100 percent by weight of a terpolymer reaction product of 86 weight percent vinyl chloride, 13 weight percent vinyl acetate and 1 weight percent maleic acid having a weight average molecular weight of about 27,000 (VMCH, available from Union Carbide Co.). The pigment concentration in the dispersion was 60 percent by weight based on the total solids weight (pigment and binder). The dispersion was milled in a ball mill with 1/8 inch (0.3 cm) diameter stainless steel shot for 4 days. The dispersion was filtered to remove the shot and diluted to 3.4 weight percent solids for coating purposes. The average particle size of the milled pigment was about 0.05 micrometer. The dispersion quality of the coating mixture was examined. Next, the charge generating layer coating mixture was applied by a dip coating process in which a cylindrical 40 mm diameter and 310 mm long aluminum drum coated with a 1.5 micrometers thick nylon coating was immersed into and withdrawn from the charge generating layer coating mixture in a vertical direction along a path parallel to the axis of the drum at a rate of 200 mm/min. The applied charge generation coating was dried by in oven at 106.degree. C. for 10 minutes to form a layer having a thickness of approximately 0.2 micrometers. This coated charge generator layer was then dip coated with a charge transport mixture containing 36 percent N,N'-diphenyl-N,N'-bis(3methylphenyl)-1,1'-biphenyl-4,4'diamine and polycarbonate dissolved in monochlorobenzene solvent. The applied charge transport coating was dried by in a forced air oven at 118.degree. C. for 55 minutes to form a layer having a thickness of 20 micrometers. The electrophotographic imaging members prepared were tested by electrically charging it at a field of 800 volts and discharging it with light having a wavelength of 780 nm. The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYNS Part Power Yield Vm dV/% % Size Law Viscosity Point 7.0 Ve dX______________________________________100 0 0.05 0.909 8 0 92 35 326______________________________________
EXAMPLE VII
An electrophotographic imaging member was prepared as described in Example VI using the same procedures and materials except that instead of using 100 weight percent of VMCH in the binder component, 60 percent VMCH was mixed with 40 weight percent of a copolymer reaction product of 90 weight percent vinyl chloride and 10 weight percent vinyl acetate having a weight average molecular weight of about 44,000 (VYNS copolymer from Union Carbide). The dispersion properties of the coating mixture used to prepare the photoreceptor and electrical properties of the prepared photoreceptor are summarized in the following table:
______________________________________VMCH VYNS Part Power Yield Vm dV/% % Size Law Viscosity Point 7.0 Ve dX______________________________________60 40 0.9 0.927 22 0.023 123 51 277______________________________________
Although the invention has been described with reference to specific preferred embodiments, it is not intended to be limited thereto, rather those having ordinary skill in the art will recognize that variations and modifications may be made therein which are within the spirit of the invention and within the scope of the claims.

Number	Name	Date
3121006	Middleton et al.	Feb 1964
3811904	Zola	May 1974
4265990	Stolka et al.	May 1981
4728592	Ohaku et al.	Mar 1988
4898799	Fujimaki et al.	Feb 1990
5204200	Kobata et al.	Apr 1993
5322755	Allen et al.	Jun 1994
5399452	Takegawa et al.	Mar 1995
5418107	Nealey et al.	May 1995
5420268	Desilits et al.	May 1995
5521306	Burt et al.	May 1996
5633046	Petropoulos et al.	May 1997

Charge generation layer containing mixture of terpolymer and copolymer

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