EP0002238B1

EP0002238B1 - Electrophotographic elements and method for their preparation

Info

Publication number: EP0002238B1
Application number: EP19780101449
Authority: EP
Inventors: Jerome Howard Perlstein; George Arthur Reynolds; James Albert Vanallan; Suzanne Patricia Clark
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1977-11-28
Filing date: 1978-11-24
Publication date: 1984-06-13
Also published as: JPS5483837A; JPS6248214B2; CA1129426A; DE2862416D1; EP0002238A1

Description

This invention relates to electrophotography and particularly to an electrophotographic element and a method of enhancing the spectral sensitivity and image resolution of an electrophotographic element.
One type of electrophotographic element particularly useful in electrophotography employs a layer containing a photoconductive material and an electrically insulating, film-forming, resinous binder.
In many uses, it is desirable for an electrophotographic element to exhibit high speed (as measured by an electrophotographic speed or characteristic curve), a low residual potential after exposure, and resistance to electrical fatigue. Sometimes it is also desirable that the electrophotographic element be capable of accepting a high surface potential and have a low rate of dark decay.
High speed electrophotographic elements which exhibit many of the desirable qualities mentioned above have been developed. Such high speed elements are referred to as "aggregate" or "heterogeneous" elements, and are described in U.S. Patent 3,615,414 issued October 26, 1971 to Light, and U.S. Patent 3,732,180 issued May 8, 1973 to Gramza et al. The "aggregate" electrophotographic elements of these patents comprise one or more photoconductive layers which contain a continuous polymer phase having dispersed therein co-crystalline particles of a pyrylium or thiapyrylium salt and a polymer.
The use of thiapyrylium dye salts in photoconductive layers is also disclosed in U.S. 3,973,962, issued August 10, 1976, to Contois et al, and U.S. 3,250,615 issued May 10, 1966 to Van Allen et al. U.S. 3,938,994 issued February 17, 1976 to Reynolds et al discloses the use of 2,4 - diphenyl - 6 - (2,6 - diphenyl - 4H - pyran - 4 - ylidenemethyl) - pyrylium salts; 2,6 - diphenyl - 4 - (2,6 - diphenyl - 4H - pyran - 4 - ylidenemethyl) - pyrylium salts; 4 - (benzoyl - 2,6 - diphenyl - 4H - pyran - 4 - ylidenemethyl) - 2,6 - diphenylpyrylium salts and 2,6 - bis(4 - amyloxyphenyl) - 4 - [2 - (4 - amyloxyphenyl) - 6 - phenyl - 4H - pyran - 4 - ylidenemethyl] - pyrylium salts as sensitizers in preparing electrophotographic elements. For preparing such elements a dope consisting of a photoconductor and a sensitizer initially dissolved in dichloromethane as well as a binder is used. The dope is then coated on a support and dried. The coated product is suitable for use in a standard xerographic process.
The present invention provides an electrophotographic element and method for enhancing both spectral sensitivity and image resolution. The electrophotographic element comprises, on a conductive support, a layer containing an electrically insulating binder, an organic photoconductor and a dye wherein said dye is in the form of a dye-dye complex, said dye having the formula:
wherein

Xε is an anion. The method comprises (1) coating said layer from an organic solvent mixture which contains a solvent for the dye which persists in the coating during drying or (2) subjecting said element to the vapor of an organic solvent for said dye until a color change occurs and thereafter drying said element.

It is believed that the surprising enhancement of the spectral sensitivity and the image resolution results from a dye-dye complex.
The term "dye-dye complex" is used herein to refer to the condition of the dye in the electrophotographic element prepared according to the method of the invention. Although the nature of the combination of the dye molecules with each other is not certain, the observed facts indicate that the dye molecules are present in a close molecular relationship with each other which differs distinctly from co-crystalline dye-polymer "aggregate" of the prior art. Hence the name "dye-dye complex" is used herein to identify the condition of the dye in our novel element.
For some as yet unexplained reason, electrophotographic elements which contain one or more of the dyes described herein surprisingly exhibit enhanced speed, as compared to an element which is otherwise identical, but which has not been treated to produce the "dye-dye complex" condition. The electrophotographic elements of this invention also exhibit a better relationship of speed and image resolution (referred to herein as "speed-resolution product"), as compared with many electrophotographic elements of the prior art. Such improved properties of the dye in our electrophotographic elements are believed due to its molecular relationship and not to the particular method by which such relationship is achieved.
Evidence of the change in the crystalline condition of the dye in our photoconductive layers as described above, is set out in the drawing. The drawing shows the absorption spectrum of a photoconductive layer containing a binder and one of the dyes which we have discovered can be transformed to the "dye-dye complex" condition. In the drawing are set out absorption spectra of the photoconductive layer before and after the dye has been transformed by treatment with solvent vapor.
In the following description "transformed photoconductive layer" is intended to mean a photoconductive layer of an electrophotographic element prepared according to the method of this invention which contains dye in the "dye-dye complex" conditions as described herein.
The electrophotographic elements prepared according to the method of this invention are readily distinguishable from the so-called "aggregate" photoconductive elements of the prior art. Thus, the photoconductive layers of the electrophotographic elements prepared according to this invention have dye in the "dye-dye complex" condition, whereas "aggregate" photoconductive layers contain dye in a "dye-polymer interaction" condition. Dyes useful in "aggregate" photoconductive layers react with a polymer in the layer, and are polymer-dependent (i.e. only certain polymers can be used to produce the "dye-polymer interaction" product). The photoconductive layers of this invention contain dyes which are not dependent upon a particular type of poymer for their valuable electrophotographic properties, and are therefore "polymer-independent".
The absorption spectra of the pyrylium dye salts used to form the aforementioned "aggregate" photoconductive layers also change when a binderless coating of such dye salts is treated with solvent vapors. However, the absorption spectra of vapor-treated layers comprising an electrically insulating binder polymer and the aforementioned pyrylium dyes are different from that of a vapor-treated binderless coating of the pyrylium dye.
Useful dyes within the scope of general Formula I include the dyes shown in Table I.
As stated above, the photoconductive layers of the electrophotographic elements as prepared according to the method of the present invention are transformed photoconductive layers comprising a dye as previously described and an electrically insulating binder. The transformation is the result of solvent action on the dye. The transformation can be carried out as follows: A solution containing the selected dye, the electrically insulating polymer and an organic photoconductor is coated onto a suitable support. The solvent is then evaporated. Transformation of the dye is then achieved by contact of the resulting layer with the vapors of solvent until a color change in the layer occurs. Similarly, coating such a layer from a solvent mixture which also contains solvent which persists in the coating during drying results in the desired transformation.
In general, the photoconductive layers of the examples have been prepared by mixing together separate solutions of the selected dye and the electrically insulating polymer and then adding an organic photoconductor. The resulting coating solution is then coated on a conductive support, such as a nickel-coated poly(ethylene terephthalate) film support, and dried in air or under vacuum at about 60°C for about one hour. The coated layer is then treated with a solvent vapor for a few minutes and then redried under vacuum for about one hour at about 60°C.
The organic solvents useful for preparing coating solution can be selected from a variety of solvents. Useful solvents include organic hydrocarbon solvents, with preferred solvents being halogenated hydrocarbon solvents. The requisite properties of the solvent are that it be capable of dissolving the selected dye and be capable of dissolving or at least highly swelling or solubilizing the polymer in the layer. In addition, it is helpful if the solvent is volatile, preferably having a boiling point of less than 200°C. Particularly useful solvents include halogenated lower alkanes having from 1 to 3 carbon atoms.
The solvents useful in achieving the desired transformation of dye include, among others, dichloromethane, toluene, tetrahydrofuran, p-dioxane, chloroform and 1,1,1-trichloroethane. Such solvents may be used alone or in combination with other volatile organic liquids.
After treatment, the desired transformation is indicated by a change in the absorption spectrum of the photoconductive layer.
The amount of the selected dye incorporated into photoconductive layers and elements of the present invention can be varied over a relatively wide range. When such layers do not include organic photoconductors, the dye may be present in an amount of 0.01 to 50.0 percent by weight of the coated layer on a dry basis. When the photoconductive layer includes an organic photoconductor, useful results are obtained by using the dye in amounts of 0.1 to 30 percent by weight of the photoconductive layer. The upper limit of the amount of dye is a matter of choice and the amount of any dye used will vary widely depending on the particular dye selected, the electrophotographic response desired, the proposed structure of the photoconductive element and the mechanical properties desired in the element.
Conventional electrically insulating film-forming polymers are useful in the present invention. Such polymers include polystyrene, polyvinylethers, polyolefins, polythiocarbonates, polycarbonates, and phenolic resins such as those disclosed in U.S. Patent 3,615,414. Mixtures of such polymers are also useful.
Particularly useful polymers have recurring units as shown in Table II.
Useful organic photoconductors are generally electron acceptors or electron donors for the dyes. They include the organic photoconductors described in the patent literature such as those disclosed in U.S. Patent 3,615,414; U.S. Patent 3,873,311; U.S. Patent 3,873,312 and Research Disclosure 10938, Volume 109, May 1973. Aromatic amines such as tri-p-tolylamine and (di-p-tolylamino- phenyl)cyclohexane are particularly useful.
In general, organic photoconductors, when used, are present in our photoconductive layers in an amount equal to at least 1 weight percent of the combined dry weight of dye, binder, and organic photoconductor in the layer(s). The organic photoconductor can be present in the layer up to the limit of its solubility in the polymeric binder. A polymeric organic photoconductor may also be employed either as the binder or with another polymeric binder. A preferred weight range for the organic photoconductor in the photoconductive layer is from 10 to 40 weight percent.
A wide variety of electrically conducting supports can be used in the practice of this invention, for example, paper (at a relative humidity above 20 percent); aluminum-paper laminates; metal foils such as aluminum foil, zinc foil, etc.; metal plates such as aluminum, copper, zinc, brass and galvanized plates; vapor-deposited metal layers such as silver, chromium, nickel, aluminum, cermet materials and the like coated on paper. Conventional photographic film bases such as cellulose acetate, poly(ethyleneterephthalate) or polystyrene can also be used. Conducting materials such as nickel can be vacuum deposited on transparent film supports in sufficiently thin layers to allow electrophotographic elements prepared therewith to be exposed from either side of such elements. An especially useful conducting support can be prepared by coating a film of poly(ethylene terephthalate) with a conducting layer containing a semiconductor dispersed in a resin.
The photoconductive layers can be coated, if desired, directly on a conducting substrate. In some cases, it may be desirable to use one or more intermediate subbing layers between the photoconductive layer and the conducting substrate to improve adhesion to the conducting substrate and/or to act as an electrical barrier layer between the photoconductive layer and the conducting substrate. Such subbing layers, if used, typically have a dry thickness in the range of 0.1 to 5 micrometers.
Optional overcoat layers may be used in the present invention. For example, to improve surface hardness and resistance to abrasion, the surface layer of the element of the invention may be coated with one or more electrically insulating, organic polymer coatings or electrically insulating, inorganic coatings. A number of such coatings are well known in the art. Typical useful overcoats are disclosed, for example, in Research Disclosure "Electrophotographic Elements, Materials, and Processes", Volume 109, page 63, Paragraph V, May, 1973.
Coating thickness of the phtoconductive layer on the support can vary widely. Normally, a coating in the range of about 0.5 µm to about 300 µm before drying is useful for the practice of this invention. The preferred range of coating thickness is found to be in the range from about 1.0 µm to about 150 µm before drying, although useful results can be obtained outside of this range. The resultant dry thickness of the coating is preferably between 2 pm and 50 pm, although useful results can be obtained with a dry coating thickness between 1 and 200 µm. The symbol ^@ stands for registered Trade Mark.
The elements of the present invention can be employed in any of the well-known electrophotographic processes which require photoconductive layers. One such process is the xerographic process. The following examples are included for a further understanding of the invention.

Example 1

Preparation and testing of photoconductive layer containing dye 1, (Table I).

12.8 mg of dye 1, (Table I) were dissolved in a mixture of 1 ml of dichloromethane, 0.1 ml of HFIP and 5 ml dichloromethane containing Lexan 145^@ (0.1/ml). Lexan 145⁰ is a polycarbonate polymer supplied by General Electric Co., having structure 7 in Table II. The resulting mixture was stirred and heated for 5 minutes. Then 327 mg of tri-p-tolylamine were dissolved in it. The final solution was coated on a nickel coated poly(ethylene terephthalate) support and air-dried at 55°C for 5 minutes. The film was then treated for one minute with methylene chloride vapor and dried in a vacuum oven at 60°C for one hour. Dry film thickness was 6.0µm.
The untreated film appeared blue-green by transmitted light. Upon solvent treatment for one minute with the vapors of methylene chloride, the film turned blue. The optical absorption spectrum for this film before and after vapor treatment is shown in the drawing. The absorption spectrum was determined in a conventional manner using a Cary 14 spectrophotometer. The absorption spectrum 1 for the untreated film had a peak at about 650 nm and a shoulder at 600 nm. The spectrum 2 for the methylene chloride treated film is shifted with narrow peaks at 635 nm and 560 nm. The untreated film did not have a peak at 560 nm.
The photosensitivity of each sample was determined as follows: the surface of the layer away from the support was electrostatically charged negatively under a corona source until the surface potential as measured by a capacitively-coupled probe attached to an electrometer attained an initial dark value, V_o of -500 volts. The rear surface of the charged element was then exposed to monochromatic visible radiation at a wavelength of 650 nm. The exposure caused reduction of the surface potential of the element from -500 volts to -100 volts. The photosensitivity of the element can be considered equivalent to the exposure in ergs/cm² necessary to discharge the element from -500 to -100 volts, after correction for light absorption and reflection by the film support.
The photosensitivities (at 640 nm) of control (untreated) and treated layers of the above example are listed in the following table.

Example 2

A photoconductive layer containing dye 2 (Table I) was tested as in Example I. Upon vapor treatment the layer changed from blue-green to blue and exhibited the same absorption and speed characteristics as dye 1 (Table I).

Example 3

This example shows the combination of high speed and good resolution possessed by electrophotographic elements of the present invention, as compared with the speed and resolution of typical prior art electrgraphic element such as those described in U.S. Patent 3,542,547 and typical "aggregate" electrographic elements such as those described in U.S. Patent 3,615,414 and U.S. Patent 3,873,311.
Three electrophotographic elements were prepared. Element A contained a homogenous photoconductive layer of the type described in U.S. Patent 3,542,547. Element B contained an "aggregate" photoconductive layer of the type described in U.S. Patent 3,873,311. Elements C and D are of this invention and contain dye 1 in their photoconductive layers (Table I). These layers contained the following materials.
Each coating solution was made 24 hours prior to the coating step by dissolving the components in the order listed and allowing sufficient time between additions for complete solvation. Each solution was coated on a transparent nickel or cuprous iodide conductive support. Layer A was made at a coverage of 7.5 gms/m2. Layer B was made at a coverage of 11.3 gms/m². Layers C and D were made at a coverage of 7.5 gms/m2. (Coverages are in terms of dry basis.) The coatings were then dried.
Photosensitivity and resolution data are presented in Table III. Photosensitivity was determined as in Example I for negative charging at a wavelength where the optical density of the film equals 1.0. Discharge was from -600V to -1 OOV. The data in this table shows that electrophotographic elements of the present invention have a higher speed-resolution product than the photoconductive elements of A and B which are representative of the prior art.

Example 3

Examples 4-9

Six different polymers having the recurring units 1, 2, 3, 4, 5 and 6 shown in Table II were used to make six photoconductive layers, each containing a different polymer. Each layer contained dye 1 (Table I). The layers were prepared substantially in accordance with Example I. Each layer was found to have greater photosensitivity after vapor treatment than before such treatment. Each treated layer also had a spectral peak at about 560 nm which did not appear in the film before it was treated. These examples also show that the change in absorption spectrum and enhanced speed is independent of the polymer material and that this transformation probably results from dye-dye complex instead of from dye-polymer co-crystallization.

Claims

1. Method of enhancing the spectral sensitivity and image resolution of an electrophotographic element which comprises, on a conductive support, a layer containing an electrically insulating binder, an organic photoconductor and a dye wherein said dye is in the form of a dye-dye complex, said dye having the formula:

wherein

X^e is an anion, which method comprises (1) coating said layer from an organic solvent mixture which contains a solvent for the dye which persists in the coating during drying or (2) subjecting said element to the vapor of an organic solvent for said dye until a color change occurs and thereafter drying said element.

2. Method according to claim 1 wherein the organic solvent mixture includes hydrocarbons, haiogenated hydrocarbons, p-dioxane and halogenated aliphatic alcohols.

3. Method according to claim 2 wherein solvents contained in said solvent mixture have boiling points of less than 200°C.

4. Method according to claim 1 wherein said dye is 4 - [(2,6 - diphenyl - 4H - thiapyran - 4 - ylidene) - methyl] - 2,6 - diphenylthiapyrylium perchlorate or 4-[(2,6 - diphenyl - 4H - thiapyran - 4 - ylidene)methyl] - 2,6 - diphenylthiapyrylium fluoroborate.

5. An electrophotographic element comprising a conductive support, a layer containing an electrically insulating binder, an organic photoconductor and a dye wherein said dye is in the form of a dye-dye complex, said dye having the formula:

wherein X^⊖ represents an anion.