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(11) | EP 0 002 238 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Electrophotographic elements and method for their preparation |
| (57) Electrophotographic elements which comprise a photoconductive layer on conducting
support, wherein the photoconductive layer contains a electrically insulating binder,
a dye and, optionally, an organic photoconductor. The dye is in a "dye-dye interaction"
condition which results from contacting the layer with vapors of a solvent for the
dye after the layer has been formed. Enhanced photoconductive sensitivity and resolution
result from the transformation of the dye into the "dye-dye interaction" condition.
Dyes found useful in the formation have the structure
wherein Z and Z' independently represent 0, Se, or S and X⊖ represents an anion. The difference between the absorption spectra of the ;pyrylium salts before and after vapor treatment proves the ;existance of a "dye-dye-interaction" condition (curve 2) different from the "dye-polymer-interaction" condition ,(curve 1) of the prior art. |
[0001] This invention relates to electrophotography and particularly to light sensitive materials for phctocondue- tive compositions.
[0002] One- type of photoconductive element particularly useful in electrophotography employs a layer containing a photoconductive material and an electrically insulating, film-forming, resinous binder.
[0003] In many uses, it is desirable for a photoconductive element to exhibit high speed (as measured by an electrical speed or characteristic curve), a low residual potential after exposure, and resistance to electrical fatigue. Sometimes it is also desirable that the photoccn- ductive element be capable of accepting a high surface potential and have a low rate of dark decay.
[0004] High speed photoconductive 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" photoconductive 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 thiopy- rlium salt and a polymer.
[0005] The use of thiopyrylium 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. Certain monomethine thiopyrylium dye salts are also disclosed as sensitizers for photoconductors in U.S. 3,938,994 issued February 17, 1976 to Reynolds et al.
[0006] The present invention provides an electrophotographic element comprising a photoconductive layer on a conductive support. The photoconductive layer contains an electrically insulating binder and a dye. The dye in this layer is in the form of a "dye-dye interaction" which can be (a) formed by treatment of the layer with a solvent for the dye and is (b) polymer-independent, as explained hereinafter. The dyes which are useful in this invention have structures according to Formula I:
wherein Z and Z may be the same or different and represent 0, Se and S, and X⊖ represents an anion such as perchlorate or fluoroborate.
[0007] The present invention also comprises a method of making the electrophotographic element of the present invention, which method includes treating the layer containing the binder, the dye and, optionally, an organic photoconductor with solvent vapor.
[0008] The term "dye-dye interaction" is used herein to refer to the condition of the dye in the electrophotographic element of our 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 the co-crystalline dye-polymer "aggregate" of the prior art. Hence the name "dye-dye interaction" is used herein to identify the condition of the dye in our novel element.
[0009] For some as yet unexplained reason, electrophotographic elements which contain one or more of the dyes described herein in the "dye-dye interaction" condition surprisingly exhibit enhanced speed, as compared to an element which is otherwise identical, but which has not been treated to produce the "dye-dye interaction" condition. The electrophotographic elements of this invention also exhibit a better relationship of speed ana image resolution (referred to herein as "speed-resolution p,,dduct"), 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.
[0010] In order to determine whether a particular electrophotographic element contains dye in the "dye-dye interaction" condition, (regardless of its method of preparation), a simple test is available. The test consists of recording the absorption spectrum in the 450-700 nm region of the element in question (call this spectrum "A"). Next, the absorption spectrum of a layer of the dye absent a polymeric binder is recorded (spectrum "B"). Finally, the absorption spectrum of the binderless layer of dye is recorded (spectrum "C") while such layer is being exposed to vapors of a solvent for the dye. If spectrum "C" differs in absorption maxima from spectrum "B", and spectrum "C" has absorption maxima or shoulders (if any) at the same wavelengths as those for spectrum "A", then the electrophotographic element in question contains dye in the "dye-dye interaction" condition, in accordance with the present invention.
[0011] 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 interaction" 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.
[0012] In the following description "transformed photoconductive layer" is intended to mean a photoconductive layer of this invention which contains dye in the "dye-dye interaction" condition as described herein.
[0013] The electrophotographic elements of this invention are readily distinguishable from the so-called "aggregate" photoconductive elements of the prior art. Thus, our photoconductive layers have dye in'the "dye-dye interaction" 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). Our layers contain dyes which are not dependent upon a particular type of polymer for their valuable electrophotographic properties, and are therefore "polymer-independent".
[0014] 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.
[0015] According to another preferred embodiment of the present invention a photoconductive layer is provided, as described, which also contains an organic photoconductor.
[0017] The symmetrical pyrylium and thiopyrylium monomethine dyes of Formula I may be prepared according to the procedure described in U.S. Patent 3,938,994. The preparation of the sulfur-oxygen unsymmetrical monomethine pyrylium dyes is taught by G. A. Reynolds and J. A. Van Allan, J. Heterocylic Chem., 9, 1105 (1972). The preparation of symmetrical monomethine selenopyrylium, as well as thiopyrylium dyes, is taught by A. J. Tolmachev and M. A. Kudinova, Khimiya Geterotsiklicheskikh Soedinenii, 49 (1974).
[0018] The unsymmetrical selenopyrylium dyes, which are new compositions of matter, can be prepared as follows. In structures III and IV, Z represents 0 or S.
A mixture of 0.31 g of (II) and 0.35 g of (III) in 10 ml of acetic anhydride was heated under reflux for 30 minutes and cooled to room temperature during which time glistening needles of the desired material formed.
[0019] As stated above, the photoconductive layers 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 in several ways. For example, a solution containing the selected dye, the electrically insulating polymer and, if desired, an organic photoconductor can be 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. Also transformation can be achieved by inhibiting solvent removal in an otherwise normal coating operation of a solvent dope containing the dye and polymer. Similarly, coating such a layer from a solvent mixture which also contains solvent which persists in the coating during drying is among the methods for achieving the desired transformation.
[0020] 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.
[0021] 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.
[0022] The solvents useful in achieving the desired transformation of dye include, among others, dichloromethane, toluene, tetrahydrofuran, p-dioxane, chloroform and l,l,l-trichloroethane. Such solvents may be used alone or in combination with other volatile organic liquids.
[0023] After treatment according to one of the above procedures, the desired transformation is indicated by a change in the absorption spectrum of the photoconductive layer.
[0024] 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.
[0025] Conventional electrically insulating film-forming polymers are useful in the present invention. Such polymers include polystyrene, polyvinylethers, polyolefins, poly- thiocarbonates, polycarbonates, and phenolic resins such as those disclosed in U.S. Patent 3,615,414. Mixtures of such polymers are also useful.
[0027] 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-toly- laminophenyl)cyclohexane are particularly useful.
[0028] 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.
[0029] 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 aluminumcop- per, 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 resin.
[0030] 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 microns.
[0031] 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.
[0032] Coating thickness of the photoconductive layer of the support can vary widely. Normally, a coating in the range of about 0.5 microns to about 300 microns 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 microns to about 150 microns before drying) although useful results can be obtained outside cf this range. The resultant dry thickness of the coating is preferably between 2 microns and 50 microns, although useful results can be obtained with a dry coating thickness between 1 and 200 microns.
[0033] 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 - Effect of fuming with solvent
[0034] 24.2 mg of dye 5 (Table I) were dissolved in a mixture of 4 ml of dichloromethane and 0.1 ml 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). This solution was coated on poly(ethylene terephthalate) at 500C and dried. The visible spectrum of the resulting layer has absorption maxima at 630 nm, at 600 nm and at 415 nm.
[0035] Transformation of the dye to the "dye-dye interaction" condition was observed as a change in color when the layer was fumed with tetrahydrofuran vapor. The layer was fumed by suspending it in a Dewar flask which was saturated with the vapor. The flask, fitted for optical access, was placed into a Cary 14 spectrophotometer and the film's optical spectrum was recorded. The amount of time required to form the enhanced photoconductive state of the dye appeared to be dependent on the concentration of solvent fumes in contact with the surface of the layer.
[0036] The absorption spectrum recorded during the fuming of this binderless coating of dye had an absorption maxima at 615 nm and at 415 nm, and a shoulder at 550 nm. The absorption spectrum of the dye in its enhanced photoconductive state in a polymer matrix was substantially the same as for the vapor-treated binderless coating. Example 2 - Preparation and testing of photoconductive
[0037] 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 g/ml). Lexan 145® 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-fumed for one minute with methylene chloride vapor and dried in a vacuum oven at 60°C for one hour. Dry film thickness was 6.0µ.
[0038] The unfumed film appeared blue-given 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 fumed film is shifted with narrow peaks at 635 nm and 560 nm. The untreated film did not have a peak at 560 nm.
[0039] 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, Vo of -500 volts. The rear surface of the charged element was then exposed to monochromatic visible radiation at a wavelength of 640 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.
[0040] The photosensitivities (at 640 nm) of control (unfumed) and fumed layers of the above example are listed in the following table.
Example 3
[0041] A photoconductive layer containing dye 3 (Table I) was tested as in Example 2. Upon vapor treatment the layer changed from blue-green to blue and exhibited the same absorption and speed characteristics as dye 1 (Table 1).
Example
[0042] 15.5 mg of dye 2 (Table I) was dissolved in a mixture of 2 ml CH2C12 with 0.2 ml HFIP. To this was added 5 ml of a solution of poly[4,4'-(2-norbornylidene)diphenylene carbonate] (polymer 5, Table II) (0.075 g of polymer/ ml CH2Cl2) and 297.8 mg of tri-p-tolylamine. The resulting solution was warmed, coated on nickel-coated polyethylene terephthalate) at 250C, and allowed to dry in air for 2-3 minutes at 50°C. The dried layer was then treated for 3 minutes with vapor of toluene and then dried in an oven at 55°C for 1-1/2 hours. The control was a layer of the same material which was not fumed with toluene. The control had maximum absorption at 660 nm and a shoulder at 620 nm. The transformed layer (i.e., vapor treated to form the enhanced photoconductive state) had maximum absorption at 655 nm and a smnaller peak at 580 nm.
[0043] The photosensitivity values shown in Table III of the control layer and the solvent fume layer were determined as in Example 2 for negative charging, front and rear exposures.
Example 5
[0044] 16.1 mg of dye 2 (Table I) were dissolved in a mixture of 2 ml dichloromethane with 0.2 ml HFIP. To this were added 5 ml of a solution of Lexan 145 polycarbonate (0.1 g Lexan/ml CH2C12) and 299.0 mg tri-p-tolylamine were dissolved in it. The resulting solution was heated, coated on Ni/Estar polyester (as in Example 4) at 25°C, and allowed to air dry at 50°C. The resulting photoconductive layer was fumed with tetrahydrofuran vapor for 2 minutes and then oven dried at 550C for 1-1/2 hours.
[0045] The photosensitivity of the treated layer and untreated control layer was determined as in Example 2 for negative charging, front and rear exposure. Results are shown in Table IV.
Example 6
[0046] 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 electrographic elements 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.
[0047] 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.
In Element A
In Element B
In Element C
In Element D
[0052] 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 complet. solvation. Each solution was coated on a transparent
[0053] nickel or cuprous iodide conductive support. Layer A was made at a coverage of 7.5 gms/m. Layer B was made at a coverage of 11.3 gms/m. 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. In this instance, it was not necessary to carry out a separate solvent fuming step, since the coating solutions for layers C and D contained toluene in addition to methylene chloride. The presence of toluene was sufficient to result in the formation of the "dye-dye interaction" condition before all of the toluene had evaporated from the layer.
[0054] Photosensitivity and resolution data are presented in Table V. Photosensitivity was determined as in Example 2 for negative charging at a wavelength where the optical density of the film equals 1.0. Discharge was from -600V to -100V. The data in this table shows that electrophotographic elements of the present invention have a higher speed-resolution product than the photoconductive elements A and B which are representative of the prior art
Examples 7-12
[0055] 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 2. Each layer was found to have greater photosensitivity after vapor treatment than before such treatment. Each fumed layer also had a spectral peak at about 560 nm which did not appear in the film before it was fumed. This example also shows that the change in absorption spectrum and enhanced speed is independent of the polymer material and that this transformation probably results from dye-dye interaction instead of from dye-polymer co-crystallization.
Example 13
[0056] To 12.8 mg of dye 4 (Table I) was added a mixture of 1 ml of dichloromethane, 0.1 ml of hexafluoroisopropanol and a solution of 5 ml dichloromethane containing Lexan 145 polycarbonate (0.1 g/ml). The resulting mixture was stirred and heated for 5 minutes and then 327 mg of tri-p-tolylamine was dissolved in it. The resulting solution was coated on an unsubbed nickel-coated poly-(ethylen terephthalate) support and air-dried at 55°C for 5 minutes. Transformation of the dye occurred upon tact of the resulting photoconductive layer with the vapor of warmed dioxane. The fumed element was dried in a vacuum oven at 60°C for one hour. The absorption spectrum of this element after treatment with solvent vapor had a shoulder at 578 nm and a peak at 605 nm. The spectrum of the unfumed element was different from that of the fumed element.
1. An electrophotographic element which comprises a photoconductive layer on a conductive
support, said photoconductive layer containing a polymeric, electrically insulating
binder and a dye, characterized in that said dye is in the dye-dye interaction condition,
and has the formula
wherein Z and Z' independently represent 0, Se, or S and Xθ represents an anion.
wherein Z and Z' independently represent 0, Se, or S and Xθ represents an anion.
2. An electrophotographic element as in claim 1, wherein said layer also contains
an organic photoconductor.
3. An electrophotographic element as in claim 2, wherein said organic photoconductor
is an aromatic amine photoconductor.
4. An electrophotographic element as in claim 2, wherein said dye is present in said
photoconductive layer in an amount within the range of 0.1 to 30 weight percent of
said layer.
5. An electrophotographic element as in claim 4, wherein said dye is selected from
the group consisting of 4-[(2,6-diphenyl-4H-thiopyran-4-ylidene)-methyl]-2,6-diphenylthiopyrylium
perchlorate; 4-[2,6-diphenyl-4H-thio- pyran-4-ylidene)methyl]-2,6-diphenylseleno-pyrylium
perchlorate; 4-[(2,6-diphenyl-4H-thiopyran-4-ylidene)methyl]-2,6-diphenylthiopyrylium
fluoroborate, 4-[(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenyl-thiopyrylium
perchlorate; 4-[(2,6-diphenyl-4H-pyran-4-ylidene)metriyl]-2,6-diphenyl-selenopyrylium
perchlorate and 4-f(2,6-diphenyl-4H-pyran-4-ylidene)methyl]-2,6-diphenylpyrylium perchlorate;:said
composition being characterized by an absorption spectrum which is substantially similar
to the absorption spectrum of a solvent treated binderless coating of said dye material.
6. An electrophotographic element as in claim 5, wherein said polymeric binder is
selected from the group consisting of poly[4,4'-(hexahydro-4,7-methanoindan-5-ylidene)diphenylene
terephthalate]; poly[4,4'-(isopropylidene) diphenylene 4,4'-oxydibenzoate]; poly[4,4'-(2-norbornylidene(-bis-(2,6-dichlorophenylene)
carbonate]; poly[4,4'-(hexahydro-4,7-methanoindan-5-ylidene)diphenylene carbonate];
poly[4,4'-(2-norbornylidene)diphenylene carbonate]; polystyrene, and poly(4,4'-isopropylidene-
diphenylene carbonate).
7. A method for manufacturing the electrophotographic element of claim 1, which method
comprises the steps of coating on a conductive support a layer containing a film-forming
electrically insulating polymeric binder and said dye, characterized in that after
said layer is coated on said support, said layer is contacted with vapors of a solvent
for said dye.