Background of the Invention

Xerography is a well-known process for the formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means. The process involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the imaging surface and then exposing the charged surface selectively to light. The electrostatic charge is selectively dissipated in irradiated areas leaving a member having selected areas of charged surface. Thereafter, the image is rendered visible by development with a finely divided colored electroscopic material known in the art as a "toner" which is principally attracted to the charges areas of the surface.

A number of materials, hereinafter referred to as "persistent photoconductors", have been noted which are characterized by exhibiting a substantial lag in returning to their original state of dark conductivity after illumination. This permits the imaging sequence to be reversed from that of xerography, namely an uncharged imaging layer is initially selectively irradiated to render areas persistently conductive in an imagewise fashion and thereafter a charge is imposed on the entire surface, but is only selectively processed by the persistently conductive areas. Persistent photoconductors and their use are described in a number of references, including i.e., Persistent Conductivity. Electrophotography. R.M. Schaffert ed. (Foral Press 1965) pp. 70-77; Nisho and Inoue, Photo-Induced Memory Effect of Organic Photoconductor. Photographic Science & Engineering 22:194 (1978), 22:35 (1981) and 26:24 (1982); and Hanna and Inoue, The Design of An Organic Photoreceptor With a Charge-Acceptance Memory. Photographic Science & Engineering 25:209 (1981), 26:69 (1982), and 27:51 (1983).

A problem which has characterized persistent photoconductivity compositions heretofore is that the image quality is both generally insufficient and the quality has a tendency to degenerate as the speed of exposure is increased. Also, there is a low toner image density. A number of additives have been described which improve the persistent conductivity characteristics, that is, permit extremely short exposure times to produce relatively long persistent conductivity, but such additives have not been found to improve image quality. In the experimentation which led to the present invention, it was also found that some additives improved image quality but they caused a decrease in the persistent conductivity characteristics of the persistent photoconductive composition.

In simple xerography, after the toner has been fused into the charged area of the surface upon which it is present, any toner in contact with the uncharged area is removed resulting in a relatively sharp division between coated and uncoated areas. In a system in which the decoating step is effected after fusing of the toner, the division between the coated and uncoated areas of the surface is not sharp. Moreover, the fusing step is energy intensive but even increasing the energy input does not substantially improve the poor contrast.

Diazo sensitizers have previously been used in an electrostatic coating system where the toner image is obtained by use of a liquid developer. The toner adhered to the charged areas and acted as an opaque mask to prevent ultraviolet light from activating the sensitizer and causing a conversion from alkali insolubility to solubility. However, it was required that the OPC layer and the diazo sensitizer layer be maintained separate from one another because when they were combined, the electrical properties required for the OPC coating degenerated.

SUMMARY OF THE INVENTION

This invention relates to an improved persistent conductivity composition, plate and the use thereof. More particularly,

the invention relates to an improved composition which contains a persistent photoconductor and a diazo sensitizer dispersed in an inert coherent matrix, a plate comprising such composition on a conductive substrate, and the use of the same for forming an image. In a preferred embodiment, the composition also contains a photoinitiator and a phenolic hydroxyl or acetate containing resin. The diazo sensitizer can be carried by the resin. In a particularly preferred procedure, the plate is exposed to UV light after toning.

DESCRIPTION OF THE INVENTION

In accordance with the present invention, a conventional persistent photoconductive composition which comprises a persistent photoconductor dispersed in an inert coherent matrix is improved by incorporating a diazo sensitizer therein. This composition can be further improved by incorporating a photoinitiator and a condensation product of a polyhydroxylated aromatic hydrocarbon and a ketone therein which allows extremely short exposure times to be employed.

The basic persistent photoconductivity composition is composed of organic photoconductive electron donor materials, including what has been termed a "small molecule" photoconductor, dispersed in an inert coherent matrix. These so-called "small molecule" photoconductive materials include the following: oxadiazoles; e.g. , 2,5-bis[4'-diethylaminophenyl]-1,3,4-oxadiazole, 2,5-bis[4'-n-propylamino] -2-chlorophenyl-(l')] -1,3,4-oxadiazole,

2,5-bis[4'-N-ethyl-N-n-propyl-aminophenyl-(l')]-1,3,4- oxadiazole; triazoles, e.g., l-methyl-2,5-bis- [4'-dimethylaminophenyl] -1,3,4-triazole imidazoles, e.g., 2-(4'-dimethylaminophenyl) -6-methoxy-benzimidazole; oxazoles, e.g., 2-(4'-chlorophenyl)- phenanthreno-(9'-10':4,5)-oxazole; thiazoles, e.g. ,

2-(4'-dimethylaminophenyl)-benzathiazole; thiophenes, e.g., 2,3,5-triphenylthiophene; triazines, e.g. 3-(4'-aminophenyl) -5,6-dipyridyl-(2')-l,2,4-triazine, 3-(4'-dimethylamino¬ phenyl-5,6-di(4'-phenoxyphenyl)-1,2,4-triazine; hydrazones, e.g., 4-dimethylaminobenzaldehyde isonicotinic acid hydrazone; styryl compounds, e.g., 2-(4'-dimethylaminostyryl)-6-methyl-4-pyridone,

2-(4'-dimethylaminostyryl)-5-(or 6)-amino-benzimidazole, bis(4-dimethylaminostyryl) ketone; azo ethines, e.g., 4-dimethylaminobenzylidene-j_y- naphthylamine; acylhydrazones, 4-dimethylaminobenzylidene-4-hydroxybenzoic hydrazide, 4-dimethylaminobenzylidene-2-aminobenzoic hydrazide,

4-dimethylaminobenzylidene-4-methoxybenzoic hydrazide, 4-dimethylaminobenzylidene-iso-nicotinic hydrazide, 4-dimethylaminobenzylidene-2-methylbenzoic hydrazide; pyrazolines, e.g., 1,3,5-triphenylpyrazoline, l,3-diphenyl-5-[4'-methoxy-phenyljpyrazoline, l,3-diphenyl-5-[4'-dimethylaminophenyl]pyrazoline; 1,5-dipheny1-3-styrylpyrazoline; l-phenyl-3-[4'-dimethylamino¬ styryl]-5-[4'-dimethylaminophenyl]-pyrazoline; imidazolones, e.g. , 4-[4'-dimethylaminophenyl]-5-phenylimidazolone, 4-furfuryl-5-phenylimidazolone; i idazolethiones, e.g., 4-[4'-dimethylaminophenyl]-5-phenylimidazolethione, 3,4,5-tetraphenylimidazolethione, l,3,5-triphenyl-4-[4'- dimethylaminophenyl]imidazolethione, 1,3,4-triphenyl- 5-furfuryl-imidazolethione; benzimidazoles, e.g., 2-[4'-dimethylamino-phenyl]-benzimidazole, 1-methyl -2-[4'-dimethylaminophenyl]-benzimidazole, 1-phenyl -2-[ '-dimethylaminophenyl]-benzimidazole; benzoxazoles, e.g., 2-[4'-dimethylaminophenyl]-benzoxazole; and benzothiazoles, e.g., 2-[4'-dimethylamino-phenyl]-benzothiazole.

Materials which can be effectively used to provide the inert cohesive matrix for dispersion of the above "small molecule" photoconductors are polymers having fairly high dielectric strength and which are good electrically insulating film forming vehicles. Typical of such inert polymer matrices are: styrene butadiene copolymers; silicone resins, styrene-alkyd resins; soya-alkyd resins; polyvinyl chloride; styrene-maleic anhydride; copolymers; polyvinyl acetate; vinyl acetate-vinyl chloride copolymers: polyvinyl acetals, such as polyvinyl formal; polyacrylic and methacrylic esters, such as polymethyl methacrylate, poly-n-butyl methacrylate, polyisobutyl methacrylate; polystyrene; nitrated polystyrene; polymethylstyrene; isobutylenepolyvinylidene chloride; vinylidene chloride-aer lonitrile polymers; polyesters, such as

polyethylenealkaryloxyalkylene terephthalate; phenol formaldehyde resins; ketone resins; polyamides; and polycarbonates.

The phenolic hydroxyl or acetate containing resin used in the present invention includes commercially available materials such as cellulose acetate phthalate, polyvinyl acetate, phenol-formaldehyde resins or can be formed by reacting a polyhydroxylated aromatic hydrocarbon with an aldehyde ketone, preferably in the presence of a catalyst. The formation of this product generally parallels the formation of bisphenol-A from acetone and phenol. The polyhydroxylated aromatic hydrocarbon may be in a single-ring compound based on benzene, i.e. have a phenyl nucleus, or may be a multiple fused-ring nucleus such as naphthyl. Examples include catechol, resorcinol, phonoglucinol and pyrogallol among other aromatics. The aldehyde ketones preferably contain one to about eight carbon atoms such as formaldehyde, acetone, methyl ethyl ketone, pentanone, hexanone, methyl isobutyl ketone and the like. The preferred condensation product is the polyhydroxyphenyl formed from the reaction of acetone and pyrogallol at about room temperature in the presence of a catalyst, for example, phosphorus oxychloride. This product can be dissolved in ketones, alcohols and the like, and has a molecular weight of about 1,000 to 1,800.

The photoinitiators used in the composition of the present invention can be any photoinitiator which has been used heretofore for the preparation of photosensitive compositions. Such materials include such diverse materials as benzophenone, anthraquinone, penanthrenequinone, Micheler's ketone, dihydroxybenzophenone, chlorobenzophenone, tribromoimidazole, trichlorpyrimidine and the like. The preferred photoinitiators are a biimidazolidine, triazine or ketone type initiators, preferably a per(halophenylated) biimidazolidine, 2,6-di(trichloromethyl) -4-substituted-l,3,5 triazines, or a benzophenone. The most preferred photoinitiators are N- [l-(2' ,3' ,5-tri-o-chlorophenyl) imidazolidinyl]-2,4,5-tri-o-chlorophenyl-imidazolidine, 4-(p-methoxyphenyl)-2,6-di(trichloromethyl)-1,3,5-triazine, 4-(2-naphthyloxy)-2,6-di(trichloromethyl)-1,3,5-triazine,

4-phenylethynyl-2,6-di(trichloromethyl)-1,3,5-triazine, and chlorobenzophenone, tribromoimidazole, trichlorpyrimidine and

the benzophenonetetracarboxyl acid diarihydride.

The solvent soluble, negative-acting or positive-acting, light sensitive diazonium sensitizers used in the present invention result in improved toner acceptance and can be any of the commonly used lithographic diazo compounds or reaction or condensation products of such diazo compounds with agents therefore that do not materially impair the light sensitivity of the diazo. Negatively-acting diazo compounds are broadly diazo-aromatics and more particularly are diazo-arylamines that can be substituted on the aromatic nucleus or on the amino nitrogen, preferably p-diazo-diphenylamine and derivatives thereof, for example, condensation products thereof with organic condensing agents containing reactive carbonyl groups such as aldehydes and acetyls, particularly condensates with compounds such as formaldehyde and paraformaldehyde. The positive-active diazo resins include several possible types of light sensitive materials. One type is esters of a 1,2-diazoquinone-sulfonic acid chloride or a 1,2-diazo -naphthoquinone-sulfonic acid chloride with a phenolic hydroxyl containing resin such as that described above. Another type are esters of sulfonic acids of ortho-diazophenyls, particularly quinone-(l,2)-diazides and naphthoquinone-(l,2)-diazides with phenol formaldehyde resins. Another suitable diazo-type resin are phosphotungstate diazonium salts, such as for example a reaction product of phosphotungstic acid with an acid salt of hexafluorophosphoric acid, and the condensation product of para-diazodiphenylamine with formaldehyde.

The coating composition of the present invention uses a persistent photoconductivity effective amount of the persistent photoconductor, preferably about 10 to 40 percent based on the weight of the total composition, and an effective resolution enhancing amount of the diazo sensitizer, preferably about 1 to 40 percent based on the weight of the total composition and preferably about 2 to 10 percent by weight. When present, the photoinitiator can comprise about 0.5 to 8 percent, preferably about 1 to 4 percent and the phenolic hydroxyl or acetate resin can comprise about 0.5 to 5 weight percent, and preferably about 1 to 3.5 weight

percent. Any one or more of each of the photoconductors, diazo sensitizers, initiators, resins or matrix can be used as a mixture of materials if so desired. Further, if so desired, the diazo sensitizer and the phenolic hydroxyl or acetate resin can be present in the form of a reaction product thereof. It has been found that this reaction does not interfere with the resolution enhancing effect of the diazo sensitizer and the effect of the resin on the exposure time when used in conjunction with the photoinitiator. At the longer exposure times, the resin acts as if it were part of the inert matrix, even though it has been reacted with the diazo sensitizer.

The persistent photoconductive coating compositions of the present invention are prepared by dispersing the constituents in an appropriate dispersion medium and applying the dispersion to a suitable substrate which is preferably self-supporting and conductive to form a film thereon. The medium, which can be for instance, benzene, toluene, acetone, butanone, chlorinated hydrocarbons such as methylene chloride, ethylene ethers such as tetrahydrofuran, or a mixture thereof, is therefor evaporated. Any standard coating technique can be used and film thickness can be controlled either by adjustment of the viscosity of the dispersion or by mechanical means or a combination of both. The films thus produced form a substantially uniform, continuous and adhering coating on the substrate and ordinarily have an average film thickness of about 1 to 50 microns, preferably about 3 to 5 microns.

Examples of suitable substrates include conductive paper, metals such as copper, aluminum, zinc, iron, tin and lead, polyethyleneteraphthalate having a thin overcoating of aluminum and copper and NESA glass.

In an especially preferred procedure, the persistent photoconductive plate is processed in generally the same manner as in the prior art combining aspects of standard persistent photoconductivity processing and presensitized lithographic plate processing. The plate thus is photoconductivity processed by

first exposing the film surface selectively to light to render areas persistently conductive in an image-wise fashion followed by imposing a charge on the entire area which is only selectively processed by the persistent photoconductive areas. A conventional liquid toner is applied and the thus treated persistent conductivity plate is then processed as if it was a standard presensitized lithographic plate. In other words, the plate is exposed to an appropriate light source such as a high intensity ultraviolet light to activate the diazo sensitizer to create distinct alkali solubility differences on different areas of the plate. A standard alkali developing media is then used to remove the alkali soluble portions of the coating. The amount of radiation applied to the composition during the initial processing steps is regulated so that the diazo sensitizer remains substantially unaffected during the initial irradiation so that it can be activated by further irradiation after application of the toner.

The conventional processing of a persistent photoconductive plate requires fusing of the toner using for instance a heat energy input on the order of 100 to 200 watts per linear inch. Exposure to ultraviolet light is effected at ambient temperature using energy on the order of about one watt per linear inch effectively eliminating the fusing step. As a result, a highly energy intensive and hence costly procedure can be omitted. In the absence of the diazo sensitizer, a strong developer is required whose action cannot be substantially confined to the area desired to be removed and which undercuts the desired retained areas. This can lead to shorter press runs and loss of detail and small elements such as periods, and the like. Use of the diazo sensitizer and ultraviolet light permits the much weaker and standard alkali developers to be used and the greater solubility differences together with such developers permits the plate to substantially resist undercutting.

In order to further describe the present invention, various representative examples are set forth below. The first several examples describe the preparation of base persistent

photoconductive coating compositions and their use. Throughout these examples, as well as throughout the balance of this specification and claims, all temperatures are in degrees Centigrade and all parts and percentages are by weight unless otherwise specified.

Example 1

44.4 parts of 2,5-bis[4'-diethylaminophenyl] -1,3,4-oxadiazole and 55.6 parts of a styrene maleic anhydride resin were dissolved in a 1:1 mixture of methyl Cellosolve and methyl ethyl ketone which was stirred for about 30 minutes and then coated on an aluminum substrate. The mixture was allowed to dry until the mixed solvent constituted less than 2 percent of the coating. The resulting plate, composed of the conductive aluminum carrying a film of about 5 microns in thickness, was exposed to a low power UV light at 1.5 mj/cm exposure and then charged with a high voltage corona. An oppositely charged toner was applied to the plate and fused in the image areas followed by washing the non-toned areas from the plate. The persistent photoconductivity characteristics of the coating was rated as good, but the quality of the image formed was rated as bad.

Example 2

Example 1 was repeated except that 2.1 parts of the resin matrix was replaced with N- [l-(2' ,3' ,5-tri-o-chlorophenyl)- imidazolidinyl] -2,4,5, -tri-o-chlorophenyl-imidazolidine and 2.78 parts of the resin matrix was replaced with a condensation product of pyrogallol and acetone. The latter was prepared by dissolving 50 grams of pyrogallol in 350 grams of acetone to which was added 5 grams of phosphorus oxychloride as a condensation catalyst and after allowing the solution to stand overnight at room temperature, the solution was added dropwise into water with stirring to form a tarry resin. The resin was redissolved in acetone and reprecipitated in water. The precipitate solids were recovered by filtration and dried and found to have a melting point of 200-215°C.

The basic formulation containing the photoinitiator and the condensation product showed persistent photoconductivity characteristics which were very good coupled with an image quality which was also very good. The % PEM was 85.2%

Examples 3 - 6

Example 2 was repeated with equivalent results replacing the photoinitiator with the following photoinitiators:

Example No. Photoinitiator % PEM

3 4-(p-hydroxyphenyl)-2,6- 74.2 di(trichloromethyl-l,3,5-triazine

4 4-(naphthyloxy)-2,6- 71.4 di(trichloromethyl)-1,3,5-triazine

5 4-(phenylethynyl)-2,6,- 78.6 di(trichloromethyl)-1,3,5-triazine 6 benzophenone etracarboxy acid 71.6 dianhydride

Example 7

Example 2 was repeated except that the condensation product was a resorcinol-acetone resin which had been prepared in the presence of concentrated hydrochloric acid as a catalyst. The % PEM was greater than 90% and the image quality was comparable to that achieved in Example 2.

Example 8

The image density (net of background) for Example 1 was determined and is set forth in the following table in which the image density for the same composition but without the phenolic hydroxyl or acetate containing resin is also set forth for comparison.

Net Image Resins Density

None 0.78

Pyrogallol-acetone condensation product 1.46

Examples 9 - 12

A base formulation containing 47.39 parts of a styrene maleic anhydride resin 42.1 parts of 2,5-bis[4'-diethylamino- phenyl]-1,3,4-oxadiazole and 5.3 parts of succinic anhydride dissolved in a 1:1 mixture of methyl Cellosolve and methylethyl ketone was prepared. Four aliquots were taken and 5.3 parts of a different diazo sensitizer were added to three of the aliquots. The sensitizers were 1,2-naphthoquinone-diazide -5-sulfonyl ester of 2-(4-hydroxy-3-methoxystyryl) -4,6-bis(trichloromethyl)-5-triazine; ester of pyrogallol- acetone condensation product and 1,2-naphthoquinone -diazide-4-sulfonyl chloride and 1,2-naphthoquinone-diazide -5-sulfonyl chloride. Each aliquot was stirred for about 30 minutes and then coated on an aluminum substrate. The coating was dried in an oven maintained at lOO'C. until the mixed solids constituted less than 2% of the coating.

The resulting plate, composed of the aluminum carrying a photoconductive film of about 3.5 microns in thickness was exposed to a low power ultraviolet light at 1.5 j/cm exposure and then charged with a high voltage corona. An oppositely charged liquid toner was applied to the plate which was then exposed to a high power ultraviolet light. On the three plates prepared from a formulation containing the sensitizer, the light activated (decomposed) the diazo sensitizer in the non-charged areas. The decomposition products of the sensitizer made those areas more alkali soluble than the area which had been charged. When washed with a commercially available aqueous alkali developer, the composition in the non-charged areas was much more alkali soluble and dissolved in the washing liquid at a much faster rate. As a result, a relatively sharp demarcation between the charged and toned areas and the non-charged and non-toned areas were obtained. The plate prepared from the formulation without any sensitizer did not have any significant amount of any area removed when treated in an identical manner with the same developer.

Examples 13 - 15

Example 10 is repeated replacing the diazo sensitizer with the same number of parts of the following sensitizers:

Example No. Diazo Sensitizer

13 p-diazo-diphenylamine

14 ester of 1,2-naphthoquinone-diazide-4- sulfonyl chloride and condensation product of resorcinol and acetone

15 ester of 1,2-diazonaphthoquinone-sulfonyl chloride with condensation product of pyrogallol and acetone

Example 16

A persistent photoconductivity coating composition was prepared by dissolving 44.4 parts of 2,5-bis[4'4'-diethylamino phenyl]-1,3,4-oxadiazole, 2.1 parts of N-[l-(2'3' ,5-tri-o- chlorophenyl)imidazolidinyl]-2,4,5-tri-o-chlorophenyl- imidazolidine, 2.78 parts of the reaction product of the condensation product described in Example 2 with 1,2-diazo-naphthoquinone non-sulfonic acid chloride and 50.72 parts of a styrene maleic anhydride resin were dissolved in 1:1 mixture of methyl Cellosolve and methyl ethyl ketone. After stirring for about 30 minutes, the coating composition was coated on an aluminum substrate and dried in an oven at 100°C. until the mixed solids constituted less than 2% of the coating. The resulting plate, composed of the aluminum carrying a photoconductive film of about 3 microns in thickness was then processed as described in Examples 9 - 12.

Various changes and modifications can be made in the products and processes of the present invention without departing from the spirit and scope thereof. The various embodiments which were disclosed herein were for the purpose of further illustrating the invention, but were not intended to limit it.