Charge generating elements having modified spectral sensitivity

CROSS REFERENCE TO RELATED APPLICATION
Reference is made to and priority claimed from U.S. Provisional application Ser. No. 60/007,248, filed 6 Nov. 1995, entitled CHARGE GENERATING ELEMENTS HAVING MODIFIED SPECTRAL SENSITIVITY.
1. Field of the Invention
The invention relates to charge generating elements, and more particularly relates to overcoated electrophotographic charge generating elements having modified spectral sensitivity.
2. Background of the Invention
In charge generating elements, incident light induces a charge separation across various layers of a multiple layer device. In an electrophotographic charge generating element, also referred to herein as an electrophotographic element, an electron-hole pair produced within a charge generating layer separate and move in opposite directions to develop a charge between an electrically conductive layer and an opposite surface of the element. The charge forms a pattern of electrostatic potential (also referred to as an electrostatic latent image). The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
The requirements of the process of generating and separating charge place severe limitations on the characteristics of the layers in which charge is generated and holes and/or electrons are transported. For example, many such layers are very soft and subject to abrasion. This places severe constraints upon the design of charge generating elements. Some configurations cannot provide a reasonable length of service unless an abrasion resistant overcoat layer is provided over the other layers of the element. This presents its own problems, since charge must be able to pass through the overcoat.
The resistivity of an overcoat has major consequences in an electrophotographic system. If the overcoat has high resistivity, the time constant for voltage decay will be excessively long relative to the processing time for the electrophotographic element and the overcoat will retain a residual potential after photodischarge of the underlying photoreceptor. The magnitude of the residual potential depends upon the initial potential, the dielectric constants of the various layers, and the thicknesses of each layer. A solution has been to reduce the thickness of the overcoat layer. Another solution is to provide a conductive overcoat. The overcoat must, however, not be too conductive. The electrophotographic element must be sufficiently electrically insulating in the dark that the element neither discharges excessively nor allows an excessive migration of charge along the surface of the element. An excessive discharge ("dark decay") would prevent the formation and development of the electrostatic latent image. Excessive migration causes a loss of resolution of the electrostatic image and the subsequent developed image. This loss of resolution is referred to as "lateral image spread". The extent of image degradation will depend upon processing time for the electrophotographic element and the thicknesses and dielectric constants of the layers. It is thus desirable to provide an overcoat that is neither too insulating nor too conductive.
Silsesquioxanes are siloxane polymers, sometimes represented by the formula (RSiO.sub.1.5).sub.x, that are commonly prepared by the hydrolysis and condensation of trialkoxysilanes. U.S. Pat. No. 4,027,073 to Clark teaches the use of silsesquioxanes as abrasion resistant coatings on organic polymers. Typical applications include scratch resistant coatings on acrylic lenses and transparent glazing materials. This patent teaches that a preferred thickness for good scratch resistance is from 2 to 10 micrometers. U.S. Pat. No. 4,439,509 to Schank teaches photoconducing elements for electrophotography that have silsesquioxane coatings. The silsesquioxane overcoats have a thickness of from 0.5 to 2.0 micrometers. The patent indicates that this thickness optimizes electrical, transfer, cleaning and scratch resistance properties. This contrasts with U.S. Pat. No. 4,027,073, which teaches that a preferred thickness of a silsesquioxane layer, for good scratch resistance, is from 2 to 10 micrometers. U.S. Pat. No. 4,923,775 to Shank teaches that methylsilsesquioxane is preferred since it produces the hardest material in comparison to other alkylsilanes.
U.S. Pat. No. 4,595,602 to Schank teaches a conductive overcoat of cross-linked "siloxanol-colloidal silica hybrid" having a preferred thickness of from 0.3 to 5.0 micrometers. Cross-linkable siloxanol-colloidal silica hybrid was reacted with hydrolyzed ammonium salt of an alkoxy silane. The patent states:
"the ionic moiety of the ammonium salt of an alkoxy silane is both uniformly distributed throughout the overcoating and permanently anchored in place thereby providing sufficient and stable electrical conductivity characteristics to the overcoating under a wide range of temperature and humidity conditions." (col. 6, lines 45-51)
The patent contrasts this with a overcoat layer having migratable ionic species:
"By reacting these ammonium salts of alkoxy silanes with a cross-linkable siloxanol-colloidal silica hybrid material, the moisture sensitivity of the resulting films can be modified so that satisfactory control of the electrical properties of these overcoats can be achieved over an extended relative humidity range of about 10 percent to about 90 percent. Moreover, the overcoatings of this invention permit thicker protective coatings to be used thereby extending the useful life of the photoreceptor. It is hypothesized that when migratable ionic components such as conventional stabilizing acids and alkali metal catalysts are present in a cured cross-linked siloxanol-colloidal silica hybrid material overcoating, the photoreceptor may initially perform well under ordinary ambient conditions. However, upon extended xerographic cycling even under ordinary ambient conditions, repeated exposure to the applied electric field causes the migratable ionic components to migrate to the interface between the overcoating and the photoreceptor thereby forming a concentrated region or layer of ionic components which becomes progressively more electrically conductive. This electrically conductive interface region is believed to be the principal cause of print deletion, particularly at elevated temperatures and high humidity." (col. 6, lines 18-43)
Solid electrolytes, also referred to as solid ionic conductors, are solid materials in which electrical conductivity is provided by the motion of ions not electrons. A variety of solid electrolytes are inorganic crystals. Others are complexes of an organic polymer and a salt, such as complexes of poly(ethylene oxide) and alkali metal salt. "Electrolytes Dissolved in Polymers", J. M. G. Cowrie et al, Annu. Rev. Phys. Chem., Vol. 40, (1989) pp. 85-113 teaches various solid electrolytes. "Solid Ionic Conductors", D. F. Shriver et al, Chemical and Engineering News, Vol. 63, (1985) pp. 42-57; teaches a number of solid electrolytes including a salt-polyphosphazene complex. "Polymer Electrolytes", J. S. Tonge et al, Chapter 5, Polymers for Electronic Applications, ed. J. H. Lai, CRC Press, Boca Raton, Fla., 1989, pp. 157-210, at 162; teaches solid electrolytes having highly flexible, low T.sub.g siloxane backbones. "Fast Ion Conduction in Comb Shaped Polymers", J. M. G. Cowrie, Integration of Fundamental Polymer Science and Technology, Vol. 2, Elsevier Publ., New York, (1988), pp. 54-62; also teaches a solid electrolyte having a siloxane backbone. Electrical conductivities for polymeric and inorganic solid ion conductors are in the range of about 1.times.10.sup.-8 to 10 (ohms/sq).sup.-1. (Surface conductivity is equal to conductivity divided by thickness and is expressed as (ohms/square).sup.-1. Surface resistivity is equal to resistivity divided by thickness and is expressed as ohms/square. For example, a resistivity of 1.times.10.sup.14 ohms-cm, for a layer having a thickness of 5 microns, equates to a surface resistivity of 2.times.10.sup.17.) Solid electrolytes are used for applications including rechargeable lithium batteries, electrochemical sensors, and display devices. Polymeric solid electrolytes tend to be soft materials with little mechanical integrity.
A problem seen in siloxane and silane coatings is a tendency to crack with stress and aging. U.S. Pat. No. 4,227,287 to Frye teaches silicone polycondensates including polysiloxane polyether copolymers having a general structure that can be written: ##STR1## The patent teaches that the addition of about 4 weight percent of these copolymers to the total solids for a polysiloxane produces an aesthetically better coating that is less subject to stress cracking.
A problem of some charge generating elements is spectral responses that do not meet specific needs and cannot be easily modified. One example of this is charge generating elements that have a spectral response that provides a low resistance to photofatigue.
It is therefore desirable to provide charge generating elements which have modified spectral responses and can also have both good resistance to abrasion and good charge transport properties.
SUMMARY OF THE INVENTION
The invention, in its broader aspects, provides a charge generating element having an electrically conductive layer, a charge generating layer overlying the electrically conductive layer, and an overcoat overlying the charge generating layer. The overcoat includes dye and a complex of inorganic oxide polymer and a charge carrier. The overcoat has a surface resistivity of from about 1.times.10.sup.10 to about 1.times.10.sup.17 ohms/sq.
It is an advantageous effect of at least some of the embodiments of the invention that charge generating elements are provided which have modified spectral responses and can also have both good resistance to abrasion and good charge transport properties.
DESCRIPTION OF PARTICULAR EMBODIMENTS
The charge generating elements of the invention have an electrically conductive layer, a charge generating layer, and a layer including dye and/or organic pigment (for brevity, referred to hereafter as "dye") and a complex of inorganic oxide polymer and charge carrier. This layer is also referred to herein as the "overcoat". In different embodiments, these layers are varied and/or used in combination with other layers to provide a variety of devices, such as photovoltaic elements, display devices, sensors and the like. Currently preferred charge generating elements of the invention are configured as electrophotographic elements. These elements are capable of charging positively or negatively and can take a wide variety of forms, as discussed in greater detail below.
The dye in the overcoat modifies the spectral response. Because the overcoat is conductive, the overcoat can be relatively thick, for example, a suitable range is from about 0.5 to about 10 micrometers thick. These overcoat thicknesses allow high concentrations of dye per unit area. The inorganic oxide solid electrolytes disclosed herein also have good abrasion resistance and relatively high glass transition temperatures.
In the charge generating elements of the invention, the charge generating layer overlies the electrically conductive layer. The overcoat overlies the charge generating layer. The charge generating element is described herein as if the element were in the shape of a horizontally disposed flat plate. It is to be understood, however, that the element is not limited to any particular shape and that directional terms refer only to relative positions, not an absolute orientation relative to the environment. The use of the term "overcoat" should not be understood as limiting the scope of the charge generating element, nor even necessarily implying that the overcoat is uppermost, although this is highly preferred.
The overcoat includes one or more dyes or organic pigments which results in a modification of the spectral sensitivity of the charge generating element relative to an element having the same configuration, but lacking dye. The dyes and organic pigments, by virtue of their absorption characteristics, modify the spectral sensitivity of the underlying charge generating element. For example, an electrophotographic element that has too high a sensitivity for use in a particular application, with a particular exposing system, can be less made sensitive such that the sensitivity is perfectly matched to the wavelength and light intensity in that application. This is applicable for both reflection exposures, in an optical copier, and for LED or laser exposure in a printer. Imparting photofatigue resistance is accomplished by incorporating one or more materials into the overcoat which strongly absorb the ultraviolet and short blue wavelengths which interact photochemically with the underlying photoreceptor. In this way these deleterious wavelengths are removed and the overcoated photoreceptor is rendered stable to photofatigue exposure (such as cool white fluorescent light) from the front, that is, from the overcoated side. Photofatigue exposures from the rear are unaffected.
In addition to the dye, the overcoat includes an inorganic oxide polymer complex. Suitable inorganic oxide polymers include silica oxide polymers, such as silsesquioxanes; alumoxanes; titanium oxide polymers; zirconium oxide polymers; and tin oxide polymers.
Silsesquioxanes are currently preferred. The prefix "sesqui-" refers to a one and one-half stoichiometry of oxygen and the "siloxane" indicates a silicon based material. Silsesquioxane can thus be represented by the general structure: (RSiO.sub.1.5).sub.n where R is an organic group and n represents the number of repeating units. This formula, which is sometimes written {Si(O.sub.1/2).sub.3 R}.sub.n is a useful shorthand for silsesquioxanes; but, except as to fully cured silsesquioxane, does not fully characterize the material. This is important, since silsesquioxanes can be utilized in an incompletely cured state. An additional nomenclature, derived from one described in R. H. Glaser, G. L. Wilkes, C. E. Bronnimann; Journal of Non-Crystalline Solids, 113 (1989) 73-87; uses the initials M, D, T, and Q to designate silicon atoms bonded to 1, 2, 3, or 4 oxygen atoms, respectively. The designation T is subdivided as follows, to identify the number of bonds to other silicon atoms:
______________________________________Structure Designation______________________________________ ##STR2## T.sup.0 ##STR3## T.sup.1 ##STR4## T.sup.2 ##STR5## T.sup.3______________________________________
For simplicity, OH groups are shown. The same designations apply to equivalent structures in which hydrolyzeable groups replace one or more hydroxyls.
In fully cured silsesquioxane, substantially all silicons are T.sup.3. In partially cured silsesquioxanes, substantially all silicons are T.sup.2 or T.sup.3. This means that the extent of curing of the silsesquioxane can be quantified as the ratio of T.sup.2 to T.sup.3. This ratio is designated herein: "T.sup.2 -silicon/T.sup.3 -silicon ratio" or "T.sup.2 /T.sup.3 ". The value of T.sup.2 /T.sup.3 decreases with an increase in cure and vice versa.
The dye is dispersed in the complex of inorganic oxide polymer and charge carrier; in other words, the overcoat is a polymeric solid electrolyte that includes dye. The inorganic oxide solid electrolyte is a complex of one or more of the inorganic oxide polymers previously disclosed and a charge carrier.
In a preferred embodiment of the invention, the silsesquioxane of the inorganic oxide solid electrolyte has the general structure: ##STR6## HYDROLYZABLE represents --OH or a "hydrolyzable moiety". The term "hydrolyzeable moiety" is used herein to refer to moieties that readily hydrolyze under the conditions employed during preparation of the polymeric electrolyte. The hydrolyzeable moieties in the polymeric electrolyte represent individual groups that were not hydrolyzed during preparation by reason of steric constraints or the like. Thus, in the inorganic oxide solid electrolyte, all but a small minority of hydrolyzable groups are OH. The following are examples of hydrolyzeable moieties: H; I; Br; Cl; alkoxy having from 1 to about 6 carbons; --O--Ar, where Ar is phenyl or aminophenyl; --(O-ALKYLENE).sub.n --O-ALKYL; where ALKYLENE is an alkylene group having from 2 to about 6 carbons, n is an integer from 1 to about 3, and ALKYL is an alkyl group having from 1 to about 6 carbons; primary and secondary amino having from one to about 6 carbon atoms; --N-(ALKYL).sub.2, where each ALKYL is alkyl having from 1 to about 6 carbons; --NH-(ALKYL), where ALKYL is alkyl having from 1 to about 6 carbons; --O--CO-ALKYL, where ALKYL is an alkyl having from 1 to 6 carbons.
It is preferred that substantially all HYDROLYZABLE moieties be --OH such that the above formula can be rewritten: ##STR7## In these embodiments of the invention, an insubstantial portion of the subunits, about 5 mole percent or less, vary from this general formula. For example, in a small percentage of the subunits an OH group could be replaced by a hydrolyzeable moiety. Similarly, a small percentage of silicon atoms could bear two or three "non-hydrolyzeable" organic groups; or a small percentage of silicons could be replaced by another metal, such as aluminum; or a small percentage of silicons could bear organic groups not within the scope of the definitions of LINK-ACTIVE and INACTIVE.
The silsesquioxane is a relatively large oligomer or a polymer. The value of m, that is, the number of subunits, for the silsesquioxane is greater than 10. As the value of m is increased, the silsesquioxane becomes, in effect, a very large single molecule. Like highly cross-linked polymers, there is theoretically no upper limit on the number of subunits and the value of m can be a very large number.
The value of j corresponds to the mole percentage of T.sup.2 silicons in the silsesquioxane relative to the total of T.sup.2 +T.sup.3 silicons. In the inorganic oxide solid electrolyte of the invention, the value of j is less than 0.5 and greater than or equal to 0. This reflects a T.sup.2 /T.sup.3 ratio of from 1:1 to 0:1. A preferred range for the T.sup.2 /T.sup.3 ratio is from about 0.7:1 to about 0:1.
In the formulas for the silsesquioxane, x+y is substantially equal to 1. The values of x and y, that is, the relative molar concentrations of active subunits (silyl groups bearing a -LINK-ACTIVE moiety) and inactive subunits (silyl groups bearing an -INACTIVE moiety), can be varied to provide a desired resistivity. In a particular embodiment of the invention in which the element is an electrophotographic element, active subunits preferably represent less than about 45 mole percent of the subunits of the polymer. In other words, x/(x+y) is less than about 0.45 and, in an embodiment in which x+y=1, x is from about 5 to about 45 mole percent and y is from about 95 to about 55 mole percent.
INACTIVE represents an aromatic or nonaromatic moiety having from 1 to about 12 carbons. INACTIVE moieties are not capable of participation in a siloxane polycondensation reaction and do not transport charge. The following monovalent or divalent moieties are examples of suitable moieties for INACTIVE: alkyl having from 1 to about 12 carbons, fluoroalkyl having from 1 to about 12 carbons, cycloalkyl having a single, 5 or 6 membered ring, and aryl ring system having a single, 5 or 6 membered ring and from 5 to 12 carbons, including carbons of any substituents. Monovalent moieties are bonded to the Si atom of a single subunit of the polysilsesquioxane. Divalent moieties are bonded to the Si atoms of two subunits. INACTIVE moieties can all be the same or can differ. In the claimed invention, the average number of carbons in INACTIVE moieties is greater than 1, for example, INACTIVE moieties are not all methyl, but can be a mixture of methyl and one or more other moieties. Specific examples of monovalent INACTIVE moieties are: methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, n-decyl, perfluorooctyl, cyclohexyl, phenyl, dimethylphenyl, benzyl, napthyl, trimethylsiloxy. A divalent example is ##STR8## This INACTIVE group links two subunits of the silsesquioxane.
LINK represents divalent moieties corresponding to the monovalent moieties described above in relation to INACTIVE. In particular embodiments of the invention, LINK is selected from alkyl having from 1 to about 12 carbons, fluoroalkyl having from 1 to about 12 carbons; cycloalkyl having a single, 5 or 6 membered ring; and aryl having a single, 5 or 6 membered ring. Suitable LINK moieties include: ##STR9##
ACTIVE is a moiety that is complexed with the charge carrier. In preferred embodiments of the invention, ACTIVE is a monovalent organic moiety having O, S, or N and a total of carbons and heteroatoms from about 4 to about 20. Many ACTIVE moieties include one of the following groups: oxy, thio, ester, keto, imino, and amino. Suitable ACTIVE moieties, which complex cations, include neutral rings and chains of ethylene oxides and propylene oxides and tetramethylene oxides and ethylene imines and alkylene sulfides, glycidoxy ethers, epoxides, pyrolidinones, amino alcohols, carboxylic acids and the conjugate salts, sulfonic acids and the conjugate salts. Suitable ACTIVE moieties, which complex anions, include ammonium salts, phosphonium salts, sulfonium salts, and arsonium salts.
In at least some embodiments of the invention, the ACTIVE moiety is a group that is capable of participation in a siloxane polycondensation reaction as a catalyst. Examples of such groups are primary, secondary, tertiary and quaternary amines. The concentration of catalytic active subunits can be varied to provide a convenient reaction rate. In some preferred embodiments of the invention, from about 0.5 to about 30 mole percent of the subunits in the polymer include the active moiety, --(CH.sub.2).sub.3 --NH.sub.2.
The following are specific examples of -LINK-ACTIVE moieties: ##STR10## In the above, d and e are selected such that the total number of carbons in -LINK-ACTIVE is from 4 to about 25.
The following are specific examples of -ACTIVE moieties: ##STR11## wherein e is from 2 to 5, ##STR12## wherein e is from 2 to 5, ##STR13## where x=2-6 and n=2-6, ##STR14## In the above, unless otherwise indicated, R is H, alkyl, or fluoroalkyl having from to about 12 carbons, n is from 1 to about 12, X is Cl, Br, or I, Ar is aryl having a single 5 or 6 membered ring, and the total number of carbons in -LINK-ACTIVE is from 4 to about 25. Specific examples of some -LINK-ACTIVE moieties include: aminopropyl, dimethylaminopentyl, propylethylene diamine, propylethylene triamine, 3-glycidoxypropyl, 2-(3,4-epoxycyclohexyl)ethyl, 3-acryloxypropyl, 3-methacryloxypropyl, and N-(2-(vinylbenzylamino)ethyl)-3-aminopropyl.
Some considerations apply to both active and inactive subunits of the silsesquioxane. The glassy solid electrolyte can include a mixture of different active subunits or a mixture of different inactive subunits or mixtures of both. The moieties: -LINK-ACTIVE and -INACTIVE should not be substantially hydrolyzed in the siloxane polycondensation reaction used to prepare the glassy solid electrolyte, since the organic substituents would be lost and the resulting polymer would exhibit a very high degree of cross-linking. The moieties: -LINK-ACTIVE and -INACTIVE should not be so large as to cause steric problems. For example, a suitable maximum for the number of carbon and heteroatoms in a -LINK-ACTIVE moiety is 25 and for -INACTIVE moiety is 12.
The charge carrier is selected in tandem with the selection of an ACTIVE moiety. The term "charge carrier" is used herein to describe a substance that complexes with the ACTIVE moiety to yield a mobile species or combination of species that carries charge within the inorganic oxide solid electrolyte. The charge carrier can be a salt or mixture of salts. The mobile species is one or both ions of the salt or one or both ions of the various salts of the mixture. The charge carrier can also be or can include a substance that, as an isolated material, is not a salt. An example of the latter charge carrier is the complexation product of molecular iodine. This type of charge carrier provides a mobile species that forms a donor-acceptor or charge-transfer complex with the ACTIVE moiety in which the resulting charge separation has substantial ionic character.
A wide variety of charge carriers can be used. Selection of a suitable charge carrier for a particular use is a matter of relatively simple trial and error. The charge carrier must be capable of forming a complex with the ACTIVE moiety such that the silsesquioxane-charge carrier complex is electrically conducting. In preferred embodiments of the invention, the charge carrier must be capable of forming a complex with the ACTIVE moiety such that the silsesquioxane-charge carrier complex is electrically conducting in the absence of moisture. For salts, this is commonly described as "dissolving in the matrix". An explanation of this "dissolving" can be provided. Using an example in which ACTIVE is a heteroatomic group and the charge carrier is a salt in which both ions are mobile, it is believed that the "dissolving" is due to the heteroatom acting as a Lewis base or Lewis acid to break up the ion pairing of the low lattice energy salt. The unpaired ions of the salt are free to move from one heteroatom to another to form an ionic conductor. The claimed invention is not, however, limited by any explanation or theory.
Complex formation with a particular ACTIVE moiety can be determined by a variety of means. For example, "Conductivity of solid complexes of lithium perchlorate with poly{�.omega.-methyoxyhexa-(oxyethylene)ethoxy!methylsiloxane}", D. Fish et al, Makromol Chem., Rapid Commun. Vol. 7, (1986) pp. 115-120; teaches that complex formation can be tracked by measuring the increase in glass transition temperature (T.sub.g) as the amount of salt or other charge carrier in the polymer is increased. Care must be taken to account for changes in T.sub.g due to curing during the analysis.
Suitable charge carriers can be selected from materials useful in other solid electrolytes. "Electrolytes Dissolved in Polymers", J. M. G. Cowrie, et al, Annu. Rev. Phys. Chem., Vol. 40, (1989), pp. 85-113, at 87; indicates that useful salts tend to have a low lattice energy or a large anion or both such that the salt will dissolve in the polymer matrix. This article provides the following table of suitable salts for polyethylene oxide based inorganic oxide solid electrolytes:
"A comparison of the tendency for miscible PEO-salt mixtures to form and the lattice energies of the salts. Values in parentheses are either estimated or calculated theoretically.
______________________________________Li.sup.+ Na.sup.+ K.sup.+ Rb.sup.+ Cs.sup.+______________________________________F.sup.- No No No No No 1036 923 821 785 740Cl.sup.- CH.sub.3 COO.sup.- NO.sub.3.sup.- NO.sub.2.sup.- Br.sup.-N.sub.3.sup.- BH.sub.4.sup.- I.sup.- ##STR15##SCN.sup.- Yes Yes Yes Yes Yes 807 682 616 619 568ClO.sub.4.sup.- Yes Yes -- -- -- 723 648 602 582 542CF.sub.3 SO.sub.3.sup.- Yes Yes Yes Yes Yes (.ltoreq.725) (.ltoreq.650) (.ltoreq.605) (.ltoreq.585) (.ltoreq.550)BF.sub.4.sup.- Yes Yes -- -- -- (699) 619 631 605 (556)BPh.sub.4.sup.- Yes Yes Yes Yes Yes (.ltoreq.700) (.ltoreq.630) (.ltoreq.630) (.ltoreq.600) (.ltoreq.550)(end of quote)______________________________________
It is expected that this table (referred to herein as "Table 1") can be used to define salts useful in the invention both in terms of the salts specifically listed and in terms of salts having a cation and an anion of an equivalent size and a similar lattice energy. This table is not all inclusive of suitable salts. Salts such as ammonium halides and hydroxide and quaternary ammonium salts are also expected to be suitable candidates as low lattice energy salts.
The charge carrier and ACTIVE moiety are selected to provide a particular electrical conductivity, and its inverse, resistivity under conditions of low ambient relative humidity (except in embodiments where water provides the charge carrier). Particular ranges are desirable for solid electrolyes used for number of different purposes. For example, a inorganic oxide solid electrolyte used as an overcoat of an electrophotographic element has a desirable surface resistivity for the polymer-electrolyte layer of from about 1.times.10.sup.10 ohms/sq to about 1.times.10.sup.17 ohms/sq; or, more desirably, a surface resistivity of from about 1.times.10.sup.14 ohms/sq to about 1.times.10.sup.17 ohms/sq.
The charge carrier and ACTIVE moiety can also be selected so as to provide other characteristics desired in a particular embodiment of the invention. For example, the charge carrier used in a inorganic oxide solid electrolyte overcoat of an electrophotographic element, can be selected to provide particular tribocharging characteristics, both in terms of polarity and placement in a triboelectric series relative to toner and carrier materials.
For another example, the charge carrier and ACTIVE moiety can be selected such that "blooming" is eliminated or reduced. Ammonium salts can be used as charge carriers; however, these salts "bloom", that is, migrate to the surface of a solid electrolyte resulting in an enhanced degree of ammonium activity on the surface or in an upper layer. (Ammonium salts are commonly used to cure silsesquioxanes. Blooming is a recognized shortcoming of that procedure.) In uses such as electrophotography, blooming is undesirable since it may cause variability in electrophotographic properties, leading to problems such as image artifacts. A charge carrier can be selected that is non-blooming or resistant to migration. The "curing" or catalytic function that would otherwise be provided the ammonium salts can be provided by selection of an ACTIVE moiety that is a siloxane polycondensation catalyst. The ACTIVE moiety is not mobile within the solid electrolyte, thus does not bloom.
The charge carrier can be an inorganic or organic alkali salt, one or both ions may be mobile in the complex. Suitable such salts include: LiCl, CH.sub.3 COO.Li, LiNO.sub.3, LiNO.sub.2, LiBr, LiN.sub.3, LiBH.sub.4, LiI, LiSCN, LiCO.sub.4, LiCF.sub.3 SO.sub.3, LiBF.sub.4, LiBPh.sub.4, NaBr, NaN.sub.3, NaBH.sub.4, NaI, NaSCN, NaClO.sub.4, NaCF.sub.3 SO.sub.3, NaBF.sub.4, NaBPh.sub.4, KSCN, KClO.sub.4, KCF.sub.3 SO.sub.3, KBF.sub.4, KBPh.sub.4, RbSCN, RbClO.sub.4, RbCF.sub.3 SO.sub.3, RbBF.sub.4, RbBPh.sub.4, CsSCN, CsClO.sub.4, CsCF.sub.3 SO.sub.3, CsBF.sub.4, CsBPh.sub.4. ("Ph" used herein represents phenyl.) These salts are highly resistant to blooming when used with the silsesquioxanes disclosed in the Examples. Other suitable salts include: quaternary ammonium salts, ammonium hydroxide, and ammonium halides. These salts and the other salts previously listed can be used individually or in combination.
A suitable concentration of charge carrier is from about 0.1 to 10 weight percent relative to the weight of the inorganic oxide polymer.
A currently preferred charge carrier is LiI. A currently preferred concentration is from about 0.5 to 2 weight percent relative to the weight of the inorganic oxide polymer. LiI is readily soluble in alcohols, and does not display the surface activity of ammonium salts. In particular embodiments of the invention, LiI also acts as a catalyst for the ring opening of epoxide groups of glycidoxypropyl substituents in reactants to give a silsesquioxane in which the ACTIVE groups are the corresponding diol. In other currently preferred embodiments of the invention, the charge carrier is a mixture of LiI and I.sub.2. A suitable mixture has an I.sub.2 concentration of less than 1 mole percent relative to the number of moles of silyl units.
The charge carrier can be water. In a particular embodiment of the invention, the mobile species is the hydrogen ion and the ACTIVE group hydolyzes in the presence of water to yield mobile hydrogen ions. The solid electrolyte has useful properties and can be used as an overcoat on an electrophotographic element. This solid electrolyte has the shortcoming, however, of conductivity that varies with ambient humidity. Under low humidity conditions, the charge carrier is absent, such that the material is no longer a solid electrolyte, but simply a layer of inorganic oxide polymer.
In particular embodiments of the charge generating element of the invention, the silsesquioxane polymer has the general formula: ##STR16## In this equation HYDOLYZABLE has the same meaning as above indicated and is preferably OH. j and m have the same values as above-described. R is ##STR17## a, b, c, x', x", y', and y" have values in the ranges above-discussed in relation to -LINK-ACTIVE and -INACTIVE moieties. In some embodiments of the invention, a is from 1 to about 5, b is is from 1 to about 5, c is from 1 to about 6, x' is from about 5 to about 45 mol %, x" is from about 1 to about 45 mol %, x'+x" is from about 5 to 45, y' is from about 0 to about 95 mol %, and y" is from about 0 to about 95 mol %, and y'+y" is from about 95 to about 55 mol %.
It is currently preferred that the solid electrolyte have a C:Si ratio of greater than about 1.1:1 and a T.sup.2 :T.sup.3 ratio of less than about 0.6:1; or, more preferably, a C:Si ratio of greater than about 1.2:1 and a T.sup.2 :T.sup.3 ratio of less than about 0.6:1. The solid electrolytes, so defined, vary in terms of abrasion resistance, brittleness, and resistivity. It has been ascertained that the primary determinants, among various competing factors, are the organic groups of the silsesquioxane and the extent of cure. A decrease in organic content correlates with an increase in abrasion resistance, but also correlates with an increase in brittleness. An increase in methyl content and an accompanying decrease in ACTIVE groups correlates with an increase in intrisic resistivity. An increase in organic content correlates with an increase in resistivity. As a general rule, charge carrier concentration can be increased to compensate for an increase in intrinsic resistivity, but not an increase in resistivity associated with an increase in organic content. An increase in charge carrier concentration can increase the variability of resistance with changes in ambient relative humidity. Curing is increased by increasing the concentration of a charge carrier that catalyzes curing. An increase in cure is associated with an increase in brittleness. Higher brittleness correlates with higher effective stress in a coating. A relatively higher effective stress can be compensated for by decreasing the coating thickness.
In some preferred solid electrolytes suitable for use in charge generating elements, the C:Si ratio is greater than about 2:1 and the T.sup.2 :T.sup.3 ratio is from about 0.5:1 to about 0.3:1. The following formula is an example of a silsesquioxane useful in such embodiments: ##STR18## In this formula, m and R have the same meanings as indicated above, j is from about 0.4 to about 0.5; x' is from about 5 to about 30 mol %; x" is from about 2 to about 10 mol %; y' is from about 40 to about 90 mol %; and y" is from about 0 to about 55 mol %. These solid electrolytes demonstrate good flexibility and resistivities for use as overcoat layers on electrophotographic element. The silsesquioxane is not fully cured, thus useful life may be limited by changes in brittleness and resistivity associated with further curing that occurs as the solid electrolyte ages.
In some preferred solid electrolytes suitable for use in charge generating elements, the C:Si ratio is greater than about 1.2:1 and the T.sup.2 :T.sup.3 ratio is less than about 0.5:1, or more preferably less than about 0.4:1 The following formula is an example of a silsesquioxane useful in such embodiments: ##STR19## In this formula, m and R have the same meanings as indicated above, j is from about 0.4 to about 0.5; x' is from about 5 to about 30 mol %; x" is from about 2 to about 10 mol %; and y" is from about 60 to about 90 mol %. These solid electrolytes demonstrate increased brittleness as the amount of cure increases, but also increased hardness. These solid electrolytes are useful as relatively thin (for example 1 micrometer thick), relatively high resistivity overcoat layers on electrophotographic elements. The silsesquioxane is not fully cured, thus useful life may be limited by changes in brittleness and resistivity associated with further curing that occurs as the solid electrolyte ages.
In some preferred solid electrolytes suitable for use in charge generating elements, the C:Si ratio is greater than about 1.2:1 and the T.sup.2 :T.sup.3 ratio is less than about 0.1:1, or more preferably less than about 0.05:1, or still more preferably, substantially equal to 0:1. The following formula is an example of a silsesquioxane useful in such embodiments: ##STR20## In this formula, m has the same meaning as indicated above; j is from about 0 to about 0.15; x" is from about 10 to about 40 mol %; and y" is from about 0 to about 90 mol %. These solid electrolytes demonstrate good resistivities and acceptable brittleness for use as overcoat layers on electrophotographic element. These solid electrolytes are moderately brittle, but have the advantage that they are fully or nearly fully cured and are thus very stable.
In many of the solid electrolytes disclosed herein, abrasion resistance and brittleness are complementary, such that an increase in one results in a corresponding decrease in the other. In solid electrolytes having alkylamine substituents, this paradigm can be broken by replacing some of the charge carrier with molecular iodine. The result is a solid electrolyte having increased abrasion resistance relative to the same solid electrolyte having a comparable concentration of charge carrier, but lacking molecular iodine. An explanation can be provided for this phenomenon; the claimed invention is not, however, limited by any particular theory or explanation. The oxidation of alkylamine by iodine has been reported. (D. H. Wadsworth et al., J. Org. Chem. (1984) Vol. 49, p. 2676) It is thought that, during the siloxane polycondensation reaction, the molecular iodine cleaves aminoalkylsilane groups so as to free the amine as ammonia. The iodine is simultaneously reduced to iodide, which then acts as a charge carrier. The ammonia is believed to diffuse to the surface and raise the cure level before the ammonia leaves the coating. There is believed to be a differential in reactivity between the surface and the interior, such that the surface becomes more cured and thus harder, while the interior remains comparatively less cured and thus more flexible. This differential is not fully understood; however, it does correlate well with actual observations.
The overcoat of the invention can include a wide variety of addenda such as fillers, like metal oxide particles and beads of organic polymer. Fillers can be added to modify some of the properties of the resulting material. For example, metal oxide particles could be added to increase abrasion resistance. Fluorocarbon polymer beads could be added to reduce frictional loads on the surface. Filler is added in a concentration that is small enough to not cause deleterious changes in the physical properties of the glassy solid electrolyte. Some fillers can be covalently bonded into the overall matrix of the inorganic oxide. These materials can be expected to show a greater degree of physical integrity at high concentrations of filler, than filler that do not covalently bond into the matrix. An example of a material the covalently bonds into the matrix is a colloidal hydrophilic silica, such as basic Ludox.TM. marketed by DuPont.
In particular embodiments of the invention, the glassy solid electrolyte includes what is referred to herein as a "secondary active agent". The secondary active agent is a non-silsesquioxane compound that includes one or more ACTIVE moieties. The ACTIVE moieties are selected from those defined above for the silsesquioxane. In a particular solid electrolyte, the ACTIVE moieties of the secondary active agent can be the same or different than those of the silsesquioxane and a single secondary active agent or a number of different secondary active agents can be present in the solid electrolyte. The secondary active agent may or may not be involved in charge transport. If the secondary active agent is involved, the additional transport provided increases conductivity less than about about 5 or 10 percent. The secondary active agent can provide additional functions. For example, a secondary active agent could also function as a plasticizer.
In particular embodiments of the invention, the glassy solid electrolyte include an alcohol soluble surfactant. Suitable classes of surfactants include siloxane-alkylene oxide copolymers sold by Dow Corning and OSi Specialties (formerly Union Carbide). These materials act as plasticizers and lubricants and are secondary active agents. Also useful are cationic surfactants such as FC-135.TM. by 3M, which contains a tetra-alkylammonium iodide as the cationic moiety. This material provides charge carrier, with iodide ions as the mobile species, and includes tetra-alkyl ammonium ACTIVE moieties. Also useful are anionic surfactants, such as those sold under the trade name Triton.TM., Aerosol.TM. and Alipal.TM.. These surfactants contain sodium salt moieties which can act as charge carriers, that is, the sodium salt moieties can ionize in the solid electrolyte to provide low lattice energy salts as mobile species. Also useful are the Zonyl FSN surfactants from DuPont, which contain ethylene oxide ACTIVE moieties and iodide salts.
In a particular embodiment of the invention, the surfactant is a poly(alkylene oxide)-co-poly(dimethylsiloxane). A specific example of such a surfactant has the general formula: ##STR21##
R.sup.z can be either hydrogen or a lower alkyl radical, according to product literature on the SILWET.TM. Surface Active Copolymers from OSi Specialties, Inc. A specific example of a surfactant suitable for use in the method of the invention is a material marketed as a "lubricant" by OSi Specialties, Inc. of Danbury, Conn., U.S.A. under the designation: Silwet L-7002.
In particular embodiments of the invention, the glassy solid electrolyte includes a plasticizer. Currently preferred are plasticizers that are incorporated into the silsesquioxane matrix. Examples of classes of suitable plasticizers include: alkyl-tri(polysiloxane polyether copolymers)silanes, which are similar in structure to the surfactants above, but are bulkier and tend to stay in the bulk of the silsesquioxane to a greater degree. An example of a suitable alkyl-tri(polysiloxane polyether copolymers)silane is the material identified in the earlier discussion of U.S. Pat. No. 4,227,287. Materials having this formula are available commercially from OSi Specialties, Inc. of Danbury, Conn. under the designation L-540.TM.; and from Dow Corning Corporation of Midland, Mich. under the designation DC-190.TM.. Suitable concentrations are from about 0.5 to 6 parts by weight based on the weight of the silsesquioxane. Another plasticizer or lubricant is trimethylsiloxyl terminated poly(dimethylsiloxane) having a molecular weight of less than about 5,000 and preferably having a molecular weight from about 300 to about 3000.
Other plasticizers that would remain free to migrate within the silsesquioxane polymer that are not currently preferred, but can be added in amounts small enough to not unacceptably degrade the physical and electrical properties of the resulting element are nylons, such as Elvamide 9061.TM. and Elvamide 8064.TM., marketed by E. I. du Pont de Nemours & Co., of Wilmington, Del.
In particular electrophotographic elements of the invention, the solid electrolyte includes a Lewis base which acts as an acid scavenger. As a practical matter, the acid scavenger should be soluble in the solution used to prepare the silsesquioxane. Examples of suitable materials include: amines, including arylamines and substituted arylamines.
The glassy solid electrolyte is prepared in a manner similar to the preparation of a silsesquioxane. Silsesquioxanes are a class of inorganic/organic glasses which can be formed at moderate temperatures by a type of procedure commonly referred to as a "sol-gel" process. In the sol-gel process, silicon alkoxides are hydrolyzed in an appropriate solvent, forming the "sol"; then the solvent is removed resulting in a condensation and the formation of a cross-linked gel. A variety of solvents can be used. Aqueous, aqueous-alcoholic, and alcoholic solutions are generally preferred. Silsesquioxanes are conveniently coated from acidic alcohols, since the silicic acid form RSi(OH).sub.3 can be stable in solution for months at ambient conditions. The charge carrier and dye are added, in appropriate concentrations along with any other addenda, prior to the polycondensation reaction. The extent of condensation is related to the amount of curing a sample receives, with temperature and time being among the two most important variables.
In the preparation of the glassy solid electrolyte of the invention, the silicon alkoxides include -LINK-ACTIVE and -INACTIVE moieties in the proportions desired in the resulting silsesquioxane. For example, the following are some silicon alkoxides that include catalytic -LINK-ACTIVE moieties: 3-aminopropyltrimethoxysilane; 3-aminopropyltriethoxysilane; 3-aminopropylmethyldiethoxysilane; 3-aminopropyldimethylethoxysilane; 3-aminopropyldiisopropylethoxysilane; 3-aminopropyltris(methoxyethoxyethoxy)silane; 3-(1-aminopropoxy)-3,3-dimethyl-1-propenyltrimethoxysilane; N-(6-aminohexyl)aminopropyltrimethoxysilane; N-2-(aminoethyl)-3-aminopropyltris(2-ethylhexoxy)silane; N-(2-aminoethyl)-3-aminopropyltrimethoxysilane; N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane; (aminoethylaninomethyl)phenethyltrimethoxysilane; 4-aminobutyltriethoxysilane; (N,N-dimethyl-3-aminopropyl)trimethoxysilane; N-methylaminopropyltrimethoxysilane; N-�(3-trimethoxysilyl)propyl!ethylenediamine triacetic acid trisodium salt; N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride; N-trimethoxysilylpropyltri-N-butylammonium bromide.
Particular charge generating elements of the invention include primer bonded between the charge generating layer and the layer of glassy solid electrolyte. The primer is selected so as to provide a good mechanical bond between the charge generating layer and the layer of glassy solid electrolyte, but not interfere with charge related properties. The thickness of the primer layer is from about 0.1 micrometer to about 1.0 micrometer and is preferably less than 0.5 micrometers. It is important that neither the primer, nor the solvent the primer is coated from, damage the photoconducting layers. Suitable solvents include lower alcohols. Suitable primers include polymers that are either soluble in these solvents or that form emulsions. Examples of classes of suitable primers include: acrylics, polyurethanes, pyrrolidones, polyamides, polyesters, and inorganic alkoxide including silane coupling agents. A preferred example of a specific primer is a methacylate-methylmethacrylate-methacrylic acid latex, the synthesis of which is described below. Another example of a specific primer is a copolymer of poly(vinylpyrrolidone-methacrylic acid) (95/5)wt. Another example of a specific primer is partially hydrolyzed aminopropyltrimethoxysilane.
Suitable dyes and organic pigments for the inorganic oxide layer are selected from those useful in electrography, photography, and optical recording. A wide variety of dyes and organic pigments can be used, subject to some practical limitations. The dyes and/or pigments should be soluble or dispersible in the solvent system used to deposit the inorganic oxide layer. In currently preferred embodiments of the invention, aqueous alcohols are the preferred coating solvents. The dyes and/or pigments should be stable under conditions of the process used to prepare the inorganic oxide polymer. In preferred embodiments of the invention, the inorganic oxide polymer is prepared in a sol-gel process at a pH between 3 and 8. The dyes and/or pigments must have a sufficient compatiblity with the inorganic oxide polymer so as to prevent large scale phase separation. As a practical measure, the overcoat should depart from transparency only as to those wavelengths absorbed by the dyes and/or pigments. The dyes and/or pigments should have a thermal stability that withstands the processing conditions for curing the oxide polymer. These conditions are generally dictated by the glass transition temperature of the support. For example, where poly(ethylene terephthalate) is used as the substrate, the maximum allowable curing temperature is approximately 80.degree. C. and the dye selected need be stable only to 80.degree. C. The dye should exhibit photochemical stability over the course of the expected life of the charge generating element. For reusable photoconductors, the dye should resist photofatigue over repeated exposure to the light source used to image the photoconductor.
Some of the classes of dyes which are useful for this invention are listed in The Theory of the Photographic Process, T. H. James, edit., The Macmillan Company, New York, 3rd Edition, 1966: cyanines, including simple cyanine, carbocyanine, dicarbocyanine, isocyanines, thiacyanines, oxacyanines, selenacyanines, thiazolocyanines, thiazolinocyanines; hemicyanines; p-dialkylaminostyrl dyes; trinuclear cyanines such as neoocyanine; phthaleins; oxonols and hemioxonols; merocyanines; holopolor cyanines; azacyanines; phosphocyanines; thioindigo (red); indigo (blue); carbindigo; lignones; bisnaphthoqiunones, coerulignone; aminophenoxazone; rubazonic acid (yellow); indamines; indophenols; indooanilines; and azomethines.
Dyes useful in organic photoconductors are listed in Appendix 2 of Organic Photoreceptors for Imaging Systems, P. M. Borsenberger and D. S. Weiss, Marcel Dekker, Inc., New York, 1993 and also thought to be useful in this application are: rhodamines, including Rhodamine B, Rhodamine 6G; 5-(4-diethylaminobenzylidene)rhodanine; thiapyrylium dyes including 4-(4-dimethylaminophenyl-2,6-diphenylthiapyrylium tetrafluoroborate and 4-(4-dimethylaminophenyl-2,6-diphenylthiapyrylium perchlorate; azo dyes such as Chlorodiane Blue; perylenes; phthalocyanines; porphyrins.
Other dyes useful in this application include metallized dyes, also known as a double salt of an organic cation and a transition metal complex anion. As outlined in U.S. Pat. No. 4,626,496, these salts are generally useful in optical information recording media. The organic dye cation can consists of a polymethine dye cation, triaryl methane dye cation, pyrylium dye cation, phenanthrene dye cation, tetrahydrocholine dye cation, or triarylamine dye cation, and the metal complex anion is a transition metal complex anion.
It is important that the dyes used in this metal oxide layer be stable to both the heat used to process the sol-gel and the light used to record the image onto the photosensitive material. Bleachable dyes are not useful in embodiments of this invention requiring repeated light exposure. Such dyes are outlined in Sabongi and Poon in Great Britain patent publication GB 2 136 590 A.
The electrophotographic elements of the invention can be of various types, including both those commonly referred to as single layer or single-active-layer elements and those commonly referred to as multiactive, or multiple-active-layer elements. All of the electrophotographic elements of the invention have multiple layers, since each element has at least an electrically conductive layer and one photogenerating (charge generating) layer, that is, a layer which includes a charge generation material, in addition to a solid electrolyte overcoat layer.
Single-active-layer elements are so named because they contain only one layer, referred to as the photoconductive layer, that is active both to generate and to transport charges in response to exposure to actinic radiation. Such elements have an additional electrically conductive layer in electrical contact with the photoconductive layer. In single-active-layer elements of the invention, the photoconductive layer contains charge-generation material to generate electron/hole pairs in response to actinic radiation and a charge-transport material, which is capable of accepting electrons or holes generated by the charge-generation material and transporting them through the layer to effect discharge of the initially uniform electrostatic potential. The charge-transport agent and charge generation material are dispersed as uniformly as possible in the photoconductive layer. The photoconductive layer also contains an electrically insulative polymeric film-forming binder. The photoconductive layer is electrically insulative except when exposed to actinic radiation.
Multiple-active-layer elements are so named because they contain at least two active layers, at least one of which is capable of generating charge, that is, electron/hole pairs, in response to exposure to actinic radiation and is therefore referred to as a charge-generation layer (CGL), and at least one of which is capable of accepting and transporting charges generated by the charge-generation layer and is therefore referred to as a charge-transport layer (CTL). In the invention, multiple-active-layer elements have an electrically conductive layer, a CGL, a CTL, and an overcoat layer. Either the CGL or the CTL is in electrical contact with both the electrically conductive layer and the remaining CTL or CGL. The CGL contains charge-generation material and a polymeric binder. The CTL contains a charge-transport agent and a polymeric binder.
Single-active-layer and multiactive layer electrophotographic elements and their preparation and use in general, are well known and are described in more detail, for example, in U.S. Pat. Nos. 4,701,396; 4,666,802; 4,578,334; 4,719,163; 4,175,960; 4,514,481 and 3,615,414, the disclosures of which are incorporated herein by reference.
In preparing the electrophotographic elements of the invention, the components of the photogeneration layer, including binder and any desired addenda, are dissolved or dispersed together in a liquid to form an electrophotographic coating composition which is then coated over an appropriate underlayer, for example, a support or electrically conductive layer. The liquid is then allowed or caused to evaporate from the mixture to form the permanent photoconductive layer or CGL.
The polymeric binder used in the preparation of the coating compositions can be any of the many different binders that are useful in the preparation of electrophotographic layers. The polymeric binder is a film-forming polymer having a fairly high dielectric strength. In a preferred embodiment of the invention, the polymeric binder also has good electrically insulating properties. The binder should provide little or no interference with the generation and transport of charges in the layer. The binder can also be selected to provide additional functions. For example, adhering a layer to an adjacent layer; or, as a top layer, providing a smooth, easy to clean, wear-resistant surface. Representative binders are film-forming polymers having a fairly high dielectric strength and good electrically insulating properties. Such binders include, for example, styrene-butadiene copolymers; vinyl toluene-styrene copolymers; styrene-alkyd resins; silicone-alkyd resins; soya-alkyd resins; vinylidene chloride-vinylchloride copolymers; poly(-vinylidene chloride); vinylidene chloride-acrylonitrile copolymers; vinyl acetate-vinyl chloride copolymers; poly(vinyl acetals), such as poly(vinyl butyral); nitrated polystyrene; poly(methylstyrene); isobutylene polymers; polyesters, such as poly{ethylene-coakylenebis(alkyleneoxyaryl)phenylenedicarboxylate}; phenol-formaldehyde resins; ketone resins; polyamides; polycarbonates; polythiocarbonates; poly{ethylen-coisopeopyliden-2,2-bis(ethylenoxyphenylene)-terephthalate}; copolymers of vinyl haloacrylates and vinyl acetate such as poly(vinyl-m-bromobenzoate-covinyl acetate); chlorinated poly(olefins), such as chlorinated poly(ethylene); cellulose derivatives such as cellulose acetate, cellulose acetate butyrate and ethyl cellulose; and polyimides, such as poly{1,1,3-trimethyl-3-(4'-phenyl)-5-indane pyromellitimide}. Examples of binder polymers which are particularly desirable from the viewpoint of minimizing interference with the generation or transport of charges include: bisphenol A polycarbonates and polyesters such as poly�(4,4'-norbomylidene)diphenylene terephthalate-co-azelate!.
Suitable organic solvents for forming the polymeric binder solution can be selected from a wide variety of organic solvents, including, for example, aromatic hydrocarbons such as benzene, toluene, xylene and mesitylene; ketones such as acetone, butanone and 4-methyl-2-pentanone; halogenated hydrocarbons such as dichloromethane, trichloroethane, methylene chloride, chloroform and ethylene chloride; ethers including ethyl ether and cyclic ethers such as dioxane and tetrahydrofuran; other solvents such as acetonitrile and dimethylsulfoxide; and mixtures of such solvents. The amount of solvent used in forming the binder solution is typically in the range of from about 2 to about 100 parts of solvent per part of binder by weight, and preferably in the range of from about 10 to 50 parts of solvent per part of binder by weight.
In the coating compositions for the CGL or photoconductor layer, the optimum ratios of charge generation material or of both charge generation material and charge transport agent, to binder can vary widely, depending on the particular materials employed. In general, useful results are obtained when the total concentration of both charge generation material and charge transport material in a layer is within the range of from about 20 to about 90 weight percent, based on the dry weight of the layer. In a preferred embodiment of a single active layer electrophotographic element of the invention, the coating composition contains from about 10 to about 70 weight percent of a charge-generation material and from 10 to about 90 weight percent of charge transport material. In a preferred embodiment of a multiple active layer electrophotographic element of the invention, the coating composition contains from 20 to 80 weight percent of charge generation material and from 20 to 60 weight percent of charge-transport material.
Polymeric binders and charge transport materials and concentrations useful for the CGL or photoconductor layer are also useful for a CTL. The CTL can be solvent coated in the same manner as the charge generating layer. The coating composition can utilize the same solvents as in the charge generating layer. A similar process, preparing and then coating an appropriate coating composition, can be followed for charge transport layers.
Any charge generation and transport materials can be utilized in elements of the invention. Such materials include inorganic and organic (including monomeric organic, metallo-organic and polymeric organic) materials); for example, zinc oxide, lead oxide, selenium, phthalocyanine, perylene, arylamine, polyarylalkane, and polycarbazole materials, among many others.
CGL's and CTL's in elements of the invention can optionally contain other addenda such as leveling agents, surfactants, plasticizers, sensitizers, contrast control agents, and release agents, as is well known in the art.
Various electrically conductive layers or supports can be employed in electrophotographic elements of the invention, for example, paper (at a relative humidity above 20 percent) aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, and the like; metal plates such as aluminum, copper, zinc, brass and galvanized plates; vapor deposited metal layers such as silver, chromium, vanadium, gold, nickel, aluminum and the like; and semiconductive layers such as cuprous iodide and indium tin oxide. The metal or semiconductive layers can be coated on paper or conventional photographic film bases such as poly(ethylene terephthalate), cellulose acetate, polystyrene, etc. Such conducting materials as chromium, nickel, etc. can be vacuum-deposited on transparent film supports in sufficiently thin layers to allow electrophotographic elements so prepared to be exposed from either side.
Electrophotographic elements of the invention can include various additional layers known to be useful in electrophotographic elements in general, for example, subbing layers, barrier layers, and screening layers.

The following Examples and Comparative Examples are presented to further illustrate some preferred modes of practice of the invention. Unless otherwise indicated, all starting materials were commercially obtained.
Red and near infrared photosensitivity of electrophotographic elements was evaluated by electrostatically corona-charging the element to an initial potential of -700 volts and exposing the element to 150 microsecond flash of a xenon lamp mounted with a 775 nm narrow band pass filter (approximately 10 nm band, peak intensity output at 775 nm), in an amount sufficient to photoconductively discharge the initial potential down to a level of -350 volts (50% photodischarge). Photosensitivity was measured in terms of the amount of incident actinic radiant energy (expressed in ergs/cm.sup.2) needed to discharge the initial voltage down to the desired level. The lower the amount of radiation needed to achieve the desired degree of discharge, the higher is the photosensitivity of the element. Dark decay was determined by letting an unexposed area of the charged element spontanously discharge in the the dark for seven seconds. The dark decay was calculated by dividing the amount of dark discharge (after seven seconds) by seven.
The surface resistance (ohms/sq), was determined by measuring the time dependent change in shape of an electrostatic image and fitting to Equation 1 with surface resistance as the only adjustable parameter. The elements were affixed to a grounded vacuum platen. The position and velocity of the platen was computer controlled. The film sample was corona charged to a surface potential of about 500 volts in the dark and positioned at a slit opening of 0.25 cm, for a near-contact exposure. Exposure was effected with a shuttered xenon lamp and monochromator. The electrostatic latent image was detected with a Trek Model 344 Electrostatic Voltmeter with a high resolution probe and the analog signal recorded with a Gould TA240 Easy Graf Recorder. Equation 1 describes the time dependent change in shape of a "square well" latent image profile with the image centered about x=0 and a width of 2a. V is the surface potential, V.sub.o is the initial surface potential, R.sub.sq the surface resistance, C the capacitance per unit area, and .DELTA.V.sub.o =V.sub.o -V.sub.exp where V.sub.exp is the surface potential in the exposed area. ##EQU1##
COMPARATIVE EXAMPLE 1
Synthesis of methyl acrylate/methyl methacrylate/methacrylic acid (MaMmE) 70/25/5 wt % latex primer
To a 2 liter three-neck round bottom flask fitted with a mechanical stiffer, condenser and a nitrogen inlet was added 400 mL of deionized water, 20 mL of a 10 % wt/vol solution of sodium dodecylsulfate, 1.0 gram of sodium persulfate and 0.5 grams of sodium bisulfite while the reaction flask was stirred in a 72.degree. C. water bath. An addition funnel containing 70 grams of methyl acrylate, 25 grams of methyl methacrylate and 5 grams of methacrylic acid was placed on the stirred flask and the monomers were added over a 2 hour period. The aqueous phase and the organic phase were purged previous to the monomer addition with nitrogen. The reaction mixture was initially a pale blue color and then became a translucent whitish-blue color. The reaction was allowed to stir overnight, the addition funnel was removed to vent unreacted monomers under a positive nitrogen flow for 50 minutes, and the reaction flask was removed from the water bath and cooled with tap water. The reaction mixture was purified by dialysis against water for 3 days. The polymer had a T.sub.g of 35.degree. C. (midpoint), a number average molecular weight of 22,600, and a weight average molecular weight 177,000. The resulting solution was then diluted to 2 wt % solids and 0.1 wt % of Triton-100.TM. surfactant (added as a 10% wt/vol water solution) was added as a coating aid to provide a "priming solution".
Preparation of 80 wt % propylsilane/20 wt. % glycidoxysilane sol-gel
A sol-gel formulation was prepared as follows. Glacial acetic acid (108.0 grams, 1.80 mol) was added dropwise to a previously prepared, stirred mixture of propyltrimethoxysilane (489.6 grams, 2.97 mol) and 3-glycidoxypropyltrimethoxysilane (122.4 grams, 0.518 mol), followed by the dropwise addition of 3-aminopropyltrimethoxysilane (49.6 grams, 0.277 mol). The acidified silanes were then hydrolyzed by the dropwise addition of excess water (312.0 grams, 17.3 mol). The following day, the clear solution was diluted to approximately 20 wt % solids by the dropwise addition of ethanol (1046 grams) and allowed to stir in a covered vessel for 1 week. DC-190 (16 grams) was subsequently added as a plasticizer, followed by the addition of lithium iodide (9.43 grams, 0.0704 mol) to provide a "sol-gel solution".
Preparation of electrophotographic element.
The above described priming solution was coated onto the upper surface of the image loop (electrophotographic element) of a Kodak 1575 Copier-Duplicator marketed by Eastman Kodak Company of Rochester, N.Y. The image loop had a support of poly(ethylene terephthalate). Overlaying the support was an nickel layer, a charge transport layer, and a charge generation layer.
The image loop was overcoated in the form of a continuous web; that is, prior to being cut to size and spliced into a loop. The priming solution was coated onto the charge generation layer (CGL) using a web coating machine operated at a web speed of 20 ft/min and dryer temperature of 80.degree. F. The resulting coated web, having a primer layer about 0.1-0.5 micrometers thick, was wound on a spool. This web was then coated with the above sol-gel solution at a web speed of 10 ft/min and heating to 200.degree. F., with ramped heating and cooling, and wound on a spool. The web was subsequently cured face down at 180.degree. F. for 24 hours. The cured film was evaluated as follows. Results are presented in Tables 6-8. One piece of overcoated film was evaluated in a Kodak 1575 copier.
Brittleness evaluation
Brittleness was tested by testing samples of the electrophotographic element in accordance with American National Standards Institute Test Standard PH 1.31 Brittleness of Photographic Film, Method B, "WEDGE BRITTLENESS TEST". The following is a description of the procedure.
All samples were tested at about 70.degree. C. and 15 percent relative humidity. The sample size was 15 mm.times.305 mm. The wedge angle was 9.degree.. The wedge Length was 6 inches. The large wedge opening was 1 inch. The small wedge opening was 0.06 inch.
Samples were cut using a 15 mm Thwing-Albert parallel blade cutter. The samples were allowed to condition for at least 24 hours in the specified environment. The wedge was equipped with a clamp mechanism to hold one end of the loop stationary as the other end is pulled (snapped) through the wedge. The samples were placed in the wedge with the side of interest toward the outside when forming a loop. A reference mark was put on the sample at the wedge opening. This mark was considered the "zero" point for the data collection. The sample was then pulled through the wedge as fast as physically possible using a snap motion with the arm. This process was repeated for a total of 6 samples for each example.
Inspection of the samples required piped transmitted light and or surface reflected light to verify the crack location. The two techniques allow for quick observation with the transmitted light but the reflected light is used to verify samples in question. This results because the image belt has two coatings that respond to the test. Both layer's brittle behavior is observed with transmitted light while only the top surface characteristics can be observed in the reflected mode, allowing separation of the two layers when necessary.
The samples were read using the reference mark placed on the sample previous to testing and locating the crack farthest from that reference mark. The farthest crack is the first crack to occur and represents the largest diameter in the loop at failure. The scale accompanying the wedge provides the diameter of the loop at first failure and has units of inches. The larger the number, the more brittle is the specimen. Six specimens were tested and results were averaged and the standard deviation was determined. Results state the diameter of the loop, in inches, at which the first crack was observed.
Solid State Silicon-29 Nuclear Magnetic Resonance.
The extent of cure of the overcoat was measured by determining the silicon-29 solid state NMR spectra. Resonances were observed in the cross-polarized spectra at -60 PPM, corresponding to T.sup.2 silicon atoms, and at -70 PPM, corresponding to T.sup.3 silicon atoms. Results are presented as the ratio of T.sup.2 -silicon atoms to T.sup.3 silicon atoms (designated T.sup.2 /T.sup.3).
Electrical properties under low intensity continuous excitation
One measure of an overcoat's ability to carry charge is to compare film voltage vs. exposure sensitometry using continuous exposure to low intensity light (also referred to as "low intensity continuous exposure" or "LICE"). The overcoated electrophotographic element was evaluated by measuring the exposure necessary at 2 ergs/cm.sup.2 sec and a wavelength of 680 nm (approximately the maximum spectral sensitivity of the charge generation layer) to discharge the element from +500 volts to +100 volts (referred to herein as "Speed (100 V (erg/cm.sup.2)"). The residual voltage or "toe" (referred to herein as "V.sub.toe (LICE)") was measured after 45 seconds discharge.
Electrical properties under high intensity flash and erase cycles
In this procedure a belt of the film was exercised for 5000 of the following cycles. The film was charged to an initial voltage, initially set at +600 volts, and exposed with a xenon flash through a Wratten 92 filter (cut off with 10 % transmission at 630 nm). The film was then erased by a front exposure using green LED's at an exposure of ten times the exposure necessary to discharge the film from +500 volts to +200 volts. This value was measured after 1 cycle. After the 5000 cycles, during which the relative humidity was maintained at 50% and the temperature at 70.degree. F., the voltage was measured immediately after charging ("V.sub.zero (50% RH)") and after erase ("V.sub.erase (50% RH)"). The voltage after erase following 1 cycle was subtracted from V.sub.erase (50% RH) to provide a value of the difference in erase voltages resulting from the exercising (".DELTA.V.sub.erase (50% RH)"). Measurements were taken, in the same manner, after exercising for 5000 cycles at 30% relative humidity and 80.degree. F. (referred to as "V.sub.zero (30% RH)", "V.sub.erase (30% RH)", and ".DELTA.V.sub.erase (30% RH)").
COMPARATIVE EXAMPLE A
Comparative Example A was prepared in substantially the same manner as in Comparative Example 1, with the exception that starting materials were changed as indicated in Tables 2-3. The resulting overcoat was so insulating that it could not be run on a Kodak 1575 copier. This is also reflected in the failure of the overcoated film to discharge in the offline electrical test. Results of evaluations, performed as described above for Example 1, are presented in Tables 6-8.
COMPARATIVE EXAMPLE B-D
According to company literature, Optical Technologies Ultrashield.TM. coating transfers electrical charge and is particularly useful in extending the life of photoconductor drums. The coating has the appearance of a glassy inorganic-organic material. Three coatings were made on the photoconductor used in Comparative Example 1. Results of evaluations, performed as described above for Comparative Example 1, are presented in Tables 6-8.
COMPARATIVE EXAMPLES 2-24
Comparative Examples 2-24 were prepared in substantially the same manner as in Comparative Example 1, with changes in starting materials as indicated in Tables 2-3. Results of evaluations, performed as described above for Comparative Example 1, are presented in Tables 6-8.
Comparative Examples 1-24 illustrate electrophotographic elements having various charge carriers and silsesquioxanes. Comparative Examples 13-17 illustrate a series of elements having a 60/20/20 silsesquioxane containing 5 wt % of Ludox AS with overcoat thickness increasing from 1-5 micrometers. Neither the amount of cure (T.sup.2 /T.sup.3) nor the brittleness show dramatic changes over the series. Comparative Examples 18-22 illustrate a series of elements having a 0/90/10 silsesquioxane with overcoat thickness increasing from 1-5 micrometers. Unlike the series of Examples 13-17, the brittleness of these highly cured overcoats (T.sup.2 /T.sup.3 approximately 0.25) increased as the thickness increased. These elements also showed a decreased ability to carry charge with increasing film thickness.
COMPARATIVE EXAMPLE 25
Comparative Example 25 was prepared and evaluated in substantially the same manner as in Comparative Example 1, with the changes in starting materials indicated in Tables 4-5. Results of evaluations, performed as described above for Comparative Example 1, are presented in Tables 6-8.
COMPARATIVE EXAMPLE 26
Electrophotographic elements were prepared in the same manner as in Comparative Example 1 with the exception that the priming solution was about 50 percent vol./vol. methanol:water. Results were comparable to those in Comparative Example 1, with the exception that an increased residual potential was observed.
COMPARATIVE EXAMPLES 27-31
Comparative Examples 27-31 were prepared and evaluated in substantially the same manner as in Comparative Example 1, with the changes in starting materials indicated in Tables 9-10. Results of evaluations, performed as described above for Comparative Example 1, at relative humidities of about 30-70% relative humidity, are presented in Tables 11-13.
COMPARATIVE EXAMPLES 32-34
Electrophotographic elements were prepared as described in Comparative Example 1, except that silane reactants were varied as indicated in Table 14. Tribocharging properties during electrophotographic development were estimated by use of a linear breadboard incorporating a toner development station as follows. A 5".times.8" piece of each electrophotographic element was striped on an edge with conducting paint and attached to an electrically grounded vacuum platen. The film was initially passed over a positive, DC corona and charged to 300 volts, to remove any negative charge that might be present on the photoconductor. The film voltage was then measured using an electrometer. The electrophotographic element was then passed, at a speed of 1 inch/sec, over a grounded development station having a 20 magnet development brush with a strength of approximately 1200 gauss. The station had a core rotating at 1500 rpm and a shell counterrotating at 50 rpm. The separation between the electrophotographic element and the shell was 0.75 mm. The development station contained 12 g of electrophotographic developer marketed by Eastman Kodak Company of Rochester, N.Y. as Olympus C developer (The toner in this developer charges positively.) The station did not contain any sump. Next, the film was transported over an air knife, where 80 psi air blew a 2 inch wide strip of the photoconductor clean of any toner. The clean area of the photoconductor was then passed over a second electrometer, which recorded the potential on the bare film. These procedures were all performed in the dark. Since the air knife cleaned only a strip of the electrophotographic element clear of toner, an adjacent toned strip was available for transmission densitometry measurements of background density. Background measurements were made using an X-Rite transmission densitometer and are reported in dimensionless units equal to the log of the ratio of intensity of output light divided by the intensity of input light. The background density of the electrophotographic elements after development was zero. In all of these comparative examples, there was a good correlation between the quantity of toner deposited and film voltage. Results for film voltages appear in Table 14.
Results on the linear breadboard were compared to results on a Kodak Ektaprint 1575 electrophotographic copier and a good correlation was found. It was determined that background observed on the copier was also acceptable using these electrophotographic elements.
COMPARATIVE EXAMPLES E-F
The procedures of Comparative Examples 32-34 were repeated using overcoats prepared as described in Comparative Example 1, except that silane reactants were varied as indicated in Table 14. There were good correlations between the quantities of toner deposited and film voltages. Results for film voltages appear in Table 14.
COMPARATIVE EXAMPLES G-H
The procedures of Comparative Examples 32-34 were repeated using an electrophotographic element prepared as described in Comparative Example B. There was a good correlation between the quantity of toner deposited and film voltage. Results for film voltages appear in Table 14. A measurement of the background in Comparative Example H gave a background density of 0.70. This background density level is unacceptably high. The use of the electrophotographic elements on an Ektaprint 1575 copier confirmed the acceptably high background.
COMPARATIVE EXAMPLES 35-36
The electrophotographic elements prepared in Comparative Examples 11-12 were evaluated in an electrophotographic copier. Each element was placed in a Kodak 1575 Copier-Duplicator marketed by Eastman Kodak Company of Rochester, N.Y. and 10,000 copies were produced under both high and low relative humidity conditions. No obvious signs of wear or fatigue were noted for either electrophotographic element.
COMPARATIVE EXAMPLE 37
The electrophotographic element prepared in Comparative Example 24 was evaluated in a Kodak 1575 Copier-Duplicator. Multiple copies were prepared and good image quality was produced on all copies.
TABLE 2______________________________________ pr/me/gly Propyl- Methyl- Glycidoxy- (parts silane silane silane Amino-silaneC. Ex. by weight) (mol) (mol) (mol) (mol)______________________________________C. Ex. 1 80/0/20 2.97 0 0.518 0.277C. Ex. A 100/0/0 3.73 0 0 0.069C. Ex. 2 80/0/20 2.97 0 0.518 0.277C. Ex. 3 75/5/20 2.79 0.225 0.518 0.277C. Ex. 4 70/10/20 2.61 0.449 0.518 0.277C. Ex. 5 60/20/20 2.24 0.899 0.518 0.277C. Ex. 6 60/20/20 2.24 0.899 0.518 0.277C. Ex. 7 60/20/20 2.24 0.899 0.518 0.277C. Ex. 8 60/20/20 2.24 0.899 0.518 0.277C. Ex. 9 60/20/20 2.24 0.899 0.518 0.277C. Ex. 10 60/20/20 2.24 0.899 0.518 0.277C. Ex. 11 60/20/20 2.24 0.899 0.518 0.277C. Ex. 12 60/20/20 2.24 0.899 0.518 0.277C. Ex. 13 60/20/20 2.24 0.899 0.518 0.277C. Ex. 14 60/20/20 2.24 0.899 0.518 0.277C. Ex. 15 60/20/20 2.24 0.899 0.518 0.277C. Ex. 16 60/20/20 2.24 0.899 0.518 0.277C. Ex. 17 60/20/20 2.24 0.899 0.518 0.277C. Ex. 18 0/90/10 0 4.04 0.259 0.277C. Ex. 19 0/90/10 0 4.04 0.259 0.277C. Ex. 20 0/90/10 0 4.04 0.259 0.277C. Ex. 21 0/90/10 0 4.04 0.259 0.277C. Ex. 22 0/90/10 0 4.04 0.259 0.277C. Ex. 23 20/65/15 0.743 2.92 0.389 0.277C. Ex. 24 0/90/10 0 4.04 0.259 0.277______________________________________
TABLE 3______________________________________ DC-190 (wt. % ofC. Ex. Li salt (moles) I.sub.2 (moles) solids) Other addenda______________________________________C. Ex. 1 0.0704 LiI 0 4 noneC. Ex. A 0 0 0 noneC. Ex. 2 0.0352 LiI 0 2 noneC. Ex. 3 0.0352 LiI 0.0175 2 noneC. Ex. 4 0.0352 LiI 0.0175 2 noneC. Ex. 5 0.0352 LiI 0.0175 2 noneC. Ex. 6 0.0165 LiI 0 2 noneC. Ex. 7 0.0165 LiI 0.0008 2 noneC. Ex. 8 0.0165 LiI 0.0016 2 noneC. Ex. 9 0.0473 LiBF.sub.4 0 2 noneC. Ex. 10 0 0.0174 2 noneC. Ex. 11 0.0224 LiI 0 0.5 5% Ludox LSC. Ex. 12 0.0224 LiI 0 0.5 10% Ludox LSC. Ex. 13 0.0224 LiI 0 0.4 5% Ludox ASC. Ex. 14 0.0224 LiI 0 0.4 5% Ludox ASC. Ex. 15 0.0224 LiI 0 0.4 5% Ludox ASC. Ex. 16 0.0224 LiI 0 0.4 5% Ludox ASC. Ex. 17 0.0224 LiI 0 0.4 5% Ludox ASC. Ex. 18 0.0223 LiI 0 0.1 noneC. Ex. 19 0.0223 LiI 0 0.1 noneC. Ex. 20 0.0223 LiI 0 0.1 noneC. Ex. 21 0.0223 LiI 0 0.1 noneC. Ex. 22 0.0223 LiI 0 0.1 noneC. Ex. 23 0.0302 LiI 0 0.1 Silwet 7602C. Ex. 24 0.0299 LiI 0 0.1 none______________________________________
TABLE 4______________________________________ Ethylene- pr/me/gly Methyl- Glycidoxy- diamine (parts Propyl- silane silane silaneC. Ex. by weight) silane (mol) (mol) (mol) (moles)______________________________________C. Ex. 25 100/0/0 3.74 0 0 0.50______________________________________
TABLE 5______________________________________ DC-190 (wt. % ofC. Ex. Li salt (moles) I.sub.2 (moles) solids) Other addenda______________________________________C. Ex. 25 0.022 LiI 0 0.1 none______________________________________
TABLE 6______________________________________ brittleness standard deviation ofC. Ex. T.sup.2 /T.sup.3 number. brittleness number______________________________________C. Ex. 1 0.43 0.25 0.016C. Ex. A 0.64 0.14 0.008C. Ex. B -- 0.48 0.026C. Ex. C -- 0.28 0.062C. Ex. D -- -- --C. Ex. 2 -- 0.25 0.014C. Ex. 3 0.43 -- --C. Ex. 4 0.42 -- --C. Ex. 5 0.39 -- --C. Ex. 6 -- 0.34 0.012C. Ex. 7 -- 0.34 0.017C. Ex. 8 -- 0.31 0.026C. Ex. 9 -- -- --C. Ex. 10 -- -- --C. Ex. 11 -- -- --C. Ex. 12 -- -- --C. Ex. 13 0.38 0.49 0.010C. Ex. 14 -- 0.45 0.008C. Ex. 15 0.35 0.46 0.010C. Ex. 16 -- 0.45 0.010C. Ex. 17 0.30 0.48 0.018C. Ex. 18 0.026 0.50 0.008C. Ex. 19 -- 0.53 0.014C. Ex. 20 0.025 0.57 0.040C. Ex. 21 -- 0.65 0.070C. Ex. 22 0.024 0.81 0.073C. Ex. 23 -- 0.83 0.075C. Ex. 24C. Ex. 25______________________________________
TABLE 7______________________________________ V.sub.zero V.sub.erase DV.sub.erase V.sub.zero V.sub.erase DV.sub.erase (50% (50% (50% (30% (30% (30%C. Ex. RH) RH) RH) RH) RH) RH)______________________________________C. Ex. 1 570 120 90 575 130 80C. Ex. A 650 435 185 -- -- --C. Ex. B 550 165 105 -- -- --C. Ex. C 575 100 50 585 115 50C. Ex. D -- -- -- 525 70 15C. Ex. 2 515 120 70 565 145 100C. Ex. 3 -- -- -- 520 85 60C. Ex. 4 -- -- -- 515 85 60C. Ex. 5 -- -- -- 515 90 60C. Ex. 6 535 90 55 565 105 70C. Ex. 7 535 85 50 565 105 70C. Ex. 8 535 90 55 555 100 65C. Ex. 9 560 115 65 550 150 95C. Ex. 10 595 190 120 575 195 110C. Ex. 11 -- -- -- -- -- --C. Ex. 12 -- -- -- -- -- --C. Ex. 13 -- -- -- 525 85 50C. Ex. 14 -- -- -- 520 85 50C. Ex. 15 -- -- -- 520 90 60C. Ex. 16 -- -- -- 515 80 50C. Ex. 17 -- -- -- 510 80 50C. Ex. 18 -- -- -- 520 100 45C. Ex. 19 -- -- -- 530 125 45C. Ex. 20 -- -- -- 560 160 60C. Ex. 21 -- -- -- 570 175 55C. Ex. 22 -- -- -- 570 200 55C. Ex. 23 -- -- -- 530 75 50C. Ex. 24 -- -- -- 590 165 65C. Ex. 25 -- -- -- -- -- --______________________________________
TABLE 8______________________________________ Speed (100 V) Overcoat thicknessC. Ex. (erg/cm.sup.2) V.sub.toe (LICE) (microns)______________________________________C. Ex. 1 5.52 20 5C. Ex. A -- -- 5C. Ex. B -- -- 5C. Ex. C -- -- 5C. Ex. D 3.52 19 5C. Ex. 2 6.76 40 5C. Ex. 3 3.28 9 5C. Ex. 4 3.34 11 5C. Ex. 5 3.33 13 5C. Ex. 6 3.92 16 5C. Ex. 7 3.77 15 5C. Ex. 8 3.80 16 5C. Ex. 9 5.91 32 5C. Ex. 10 7.96 41 5C. Ex. 11 3.52 8 5C. Ex. 12 3.52 13 5C. Ex. 13 3.76 13 1C. Ex. 14 3.80 9 2C. Ex. 15 3.82 10 3C. Ex. 16 4.08 14 4C. Ex. 17 4.11 13 5C. Ex. 18 4.05 19 1C. Ex. 19 4.07 27 2C. Ex. 20 4.39 36 3C. Ex. 21 4.85 45 4C. Ex. 22 5.07 53 5C. Ex. 23 3.80 16 5C. Ex. 24 4.55 34 3C. Ex. 25 3.68 6 2______________________________________
TABLE 9______________________________________ pr/me/gly Propyl- Methyl- Glycidoxy- (parts silane silane silane Amino-silaneC. Ex. by weight) (mol) (mol) (mol) (mol)______________________________________C. Ex. 27 80/0/20 2.97 0 0.518 0.277C. Ex. 28 75/0/25 2.79 0 0.647 0.069C. Ex. 29 60/20/20 2.24 0.899 0.518 0.277C. Ex. 30 50/25/25 1.86 1.12 0.647 0.069C. Ex. 31 0/50/50 0 2.25 1.29 0.069______________________________________
TABLE 10______________________________________C. Ex. LiI (moles) I.sub.2 (moles) DC-190 (wt. % of solids)______________________________________C. Ex. 27 0 0 4C. Ex. 28 0 0 5C. Ex. 29 0 0 2C. Ex. 30 0 0 5C. Ex. 31 0 0 5______________________________________
TABLE 11______________________________________ brittleness standard deviation ofC. Ex. T.sup.2 /T.sup.3 number. brittleness number______________________________________C. Ex. 27 -- 0.18 0.005C. Ex. 28 0.49 0.19 0.004C. Ex. 29 -- -- --C. Ex. 30 0.4 0.27 0.012C. Ex. 31 0.19 0.49 0.012______________________________________
TABLE 12______________________________________ V.sub.zero V.sub.erase .DELTA.V.sub.erase V.sub.zero V.sub.erase .DELTA.V.sub.erase (50% (50% (50% (30% (30% (30%C. Ex. RH) RH) RH) RH) RH) RH)______________________________________C. Ex. 27 625 295 145 560 450 225C. Ex. 28 540 170 120 -- -- --C. Ex. 29 -- -- -- -- -- --C. Ex. 30 545 200 140 -- -- --C. Ex. 31 535 145 95 -- -- --______________________________________
TABLE 13______________________________________ Speed (100 V) Overcoat thicknessC. Ex. (erg/cm.sup.2) V.sub.toe (LICE) (microns)______________________________________C. Ex. 27 9.15 33 Volts 5C. Ex. 28 -- -- 5C. Ex. 29 -- -- 5C. Ex. 30 -- -- 5C. Ex. 31 -- -- 5______________________________________
TABLE 14______________________________________C. Ex. pr/me/gly (parts by weight) Tribovoltage .+-. 15 volts______________________________________C. Ex. E 100/0/0 +120C. Ex. F 0/100/0 +30C. Ex. G -- -50C. Ex. H -- -80C. Ex. 33 75/0/25 +130C. Ex. 34 0/50/50 +90C. Ex. 35 0/50/50 +45______________________________________
COMPARATIVE EXAMPLES 38-39
Electrophotographic elements were prepared having the overcoat of Comparative Example A, with the exception that the dye diphenoquinone (DPQ) was included in the overcoat in concentrations of 5 (Comparative Example 38) and 10 (Comparative Example 39) weight percent relative to the weight of the overcoat, and the sol-gel solution was applied over the photoconductor by hand using a 3 mil doctor blade. DPQ has the structural formula: ##STR22## and was prepared by the procedure described in Yamaguchi, Y., et al, Chem. Mater., Vol. 3, (1991) PP. 709-714. The electrophotographic elements were evaluated by comparison of electrical properties. One measure of an overcoat's ability to carry charge is to compare film voltage vs. exposure sensitometry using continuous exposure to low intensity light. The overcoated electrophotographic elements were evaluated by measuring the exposure necessary, at the wavelengths given in Table 15, to discharge from +500 volts to +250 volts at an intensity of 1 erg/cm.sup.2 sec. Speed (E or change in log(Exposure)) was not measured to full discharge, because the overcoat is too insulating to permit full discharge.
Optical density (OD) values were obtained using a spectrophotometer to measure the absorbtion of hand coatings of the solid electrolyte prepared on poly(ethylene terephthalate (PET). All coatings were approximately 5 microns.
The electrophotographic element of Comparative Example 39 was evaluated for photofatigue by exposure to cool-white fluorescent light (1500 lux, 15 min) and then photodischarge characteristics at 680 nm were determined again. The element was only slightly photofatigued.
Results are presented in Tables 15-16.
COMPARATIVE EXAMPLE H
An electrophotographic element identical in composition to the element of Comparative Example A, was prepared and evaluated as in Comparative Examples 38-39 with the exception that no dye was added. Results are presented in Tables 15-16. The element was severely fatigued and required greater than 45 erg/cm.sup.2 for discharge, after exposure to fluorescent light in the same manner as described in Comparative Example 39.
COMPARATIVE EXAMPLE I
An unovercoated photoconductor was evaluated as in Comparative Examples 38-39. Results are presented in Tables 15-16.
COMPARATIVE EXAMPLE 40
An electrophotographic element was prepared and evaluated as in Comparative Examples 38-39, with the exception that DPQ was replaced by a dye (referred to herein as "Ni-dye") having the structural formula: ##STR23## at a concentration of 5 weight percent relative to the weight of the overcoat. The Ni-dye was prepared as disclosed in a published European Patent application, EP 649880 A1, filed Oct. 20, 1994. Results are presented in Tables 15-16.
COMPARATIVE EXAMPLE J
An electrophotographic element was prepared and evaluated as in Comparative Examples 38-39, except that the sol-gel solution was prepared as follows.
Preparation of 50 wt % methylsilane/50 wt. % glycidoxysilane sol-gel
Glacial acetic acid (2.7 grams, 0.045 mol) was added dropwise to a previously prepared, stirred mixture of methyltrimethoxysilane (122 grams, 0.746 mol) and glycidoxypropyltrimethoxysilane (122 grams, 0.518 mol), followed by the dropwise addition of aminopropyltrimethoxysilane (0.124 grams, 0.7 mmol). The acidified silanes were then hydrolyzed by the dropwise addition of excess water (31.2 grams, 1.73 mol). The following day, the clear solution was diluted to approximately 20 wt % solids by the dropwise addition of ethanol (104.6 grams) and allowed to stir in a covered vessel for 1 week. Silwet L-7002 (0.1 grams) was subsequently added as a plasticizer. The electrophotographic element was maintained at a relative humidity of 50%, so that the resulting inorganic oxide polymer is complexed with water as the charge carrier.
Results are presented in Tables 15-16.
EXAMPLE 1
An electrophotographic element was prepared as in Comparative Example J except Ni-dye was added to the sol-gel solution in a concentration of 5 weight percent relative to the weight of the overcoat. Results are presented in Tables 15-16.
COMPARATIVE EXAMPLE K
An electrophotographic element was prepared and evaluated as in Comparative Examples 38-39, except that the sol-gel solution was prepared as follows. An alumoxane sol-gel solution was prepared as psuedo-boehmite following the procedure of Yoldas. (B. E. Yoldas, Joournal of Materials Science 10 (1975) 1856) A 3 L, 4 neck Morton type flask fitted with a mechanical stirrer, thermometer, and condensor was charged with isopropanol (676 mL) and water (725 mL) and stirred (250 rpm) at 77.degree. C. Aluminum isopropoxide (306 g, 1.5 mol) was added over 20 min, the reaction stirred at reflux for 24 hours, then approximatly 1 L of solvent was distilled. Acetic acid (9 mL) was added dropwise over a 15 min period, and the stirred reaction mixture was maintained at reflux for an additional 72 hours. The reaction was allowed to cool overnight and filtered to yield a 13 wt % alumoxane solution (1018 g). The wt % was determined by drying the sample in vacuum at 60.degree. C. overnight. A 1 mil doctor blade was used to prepare the overcoat. The electrophotographic element was maintained at a relative humidity of 50%, so that the resulting inorganic oxide polymer is complexed with water as the charge carrier. The overcoat displayed no appreciable optical absorption between 300-850 nm. Results are presented in Tables 15-16.
EXAMPLE 2
An electrophotographic element was prepared as in Comparative Example K except carminic acid (Natural Red 4) was added to the sol-gel solution in a concentration of 37 weight percent relative to the weight of the overcoat as described below. Carminic Acid has the structural formula: ##STR24## Carminic Acid (0.76 g) was dissolved in 1.2 g water, 8.3 g of the above 13 wt % alumoxane solution, and 1 g of glacial acetic acid to give a clear solution. 10 g of a previously prepared 2 wt % aqueous solution of poly(vinylpyrrolidone-methacrylic acid) was added to the alumoxane-dye solution and the clear red solution was coated on the photoconducting film with a 1 mil doctor blade. The poly(vinylpyrrolidone-methacrylic acid) had previously been used to coat the film in order to provide good adhesion between the alumoxane-dye layer and the photoconductor. The electrophotographic element was maintained at a relative humidity of 50%, so that the resulting inorganic oxide polymer is complexed with water as the charge carrier. The element exhibited a strong absorption at 530 nm and a corresponding loss of speed at that wavelength. Results are presented in Tables 15-16.
Photoreceptors with sol-gel overcoats containing DPQ showed no change in speed relative to the control, Comparative Example A, except at 420 nm which is the maximum in the DPQ absorption. Electrophotographic elements of the invention would be prepared in the same manner as Comparative Examples 38-39 except the silane reactants used would be those in Comparative Examples 1-37. In those elements the inorganic oxide includes ACTIVE moieties and is complexed with charge carrier, such as a low lattice energy salt. The electrophotographic elements of the invention thus have application in the prevention of photofatigue by absorbing the ultraviolet and blue wavelengths which cause residual potentials and/or increase dark decay.
Modification of the photsensitivity at specific wavelengths was accomplished with the silsesquioxane overcoats containing the Ni-dye. This dye has a different absorption maximum (and optical density) in each sol-gel (620 nm in Comparative Example 40 and 600 nm in Example 1. The effect on photosensitivity, change in log(Exposure) at a partciular wavelength due to absorption by the dye, is related to the increase in the optical density at that wavelength. Thus, with the Ni-dye in Comparative Example 40, a speed decrease of 0.21 log(Exposure)was observed at 620 nm, relative to Comparative Example I. With the Ni-dye in Example 1, a speed decrease of 0.20 log(Exposure) was observed at 600 nm, as compared to Comparative Example J.
TABLE 15______________________________________PHOTOSENSITIVITY (E) (erg/cm.sup.2) E at 680 nm (photo- fatigued:E at 680 E at 620 E at 600 E at E at 420 1500 lux fornm nm nm 530 nm nm 15 min.)______________________________________C Ex 1.8 2.4 -- -- 3.0 >45C. Ex. 1.6 1.9 2.0 3.0 2.5IC. Ex. 1.9 2.2 -- -- no38 dischargeC. Ex. 1.9 2.1 -- -- no 2.3739 dischargeC. Ex 2.4 3.1 -- -- 4.040C Ex -- -- 1.9 -- --JEx. 1 -- -- 3.0 -- --C. Ex 1.8 -- -- 3.0 --KEx. 2 1.9 -- -- 6.7 --______________________________________
TABLE 16______________________________________Optical Density of OvercoatOptical Optical Optical Optical Opticaldensity at density at density at density at density at680 nm 620 nm 600 nm 530 nm 420 nm______________________________________C Ex H 0 0 -- -- <.05C. Ex. I -- -- -- -- --C. Ex. 38 0 0 -- -- 1.7C. Ex. 39 0 0 -- -- >3C. Ex 40 .09 .10 -- -- .14C Ex J -- -- 0 -- --Ex. 1 -- -- .22 -- --C. Ex K 0 -- -- 0 --Ex. 2 0 -- -- .35 --______________________________________
While specific embodiments of the invention have been shown and described herein for purposes of illustration, the protection afforded by any patent which may issue upon this application is not strictly limited to a disclosed embodiment; but rather extends to all modifications and arrangements which fall fairly within the scope of the claims which are appended hereto:

Number	Name	Date
4027073	Clark	May 1977
4439509	Schank	Mar 1984
4595602	Schank	Jun 1986
4923775	Schank	May 1990
5204201	Schank et al.	Apr 1993

Charge generating elements having modified spectral sensitivity

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