GB2103818A - Photoconductive compositions and electrophotographic elements coated with them - Google Patents

Abstract

A photoconductive composition is sensitive to aqueous composition and comprises inorganic photoconductor, photoconductive organic pigment and insulating resin that is sensitive to aqueous composition. An electrophotographic element comprises a conductive support carrying a coating of this composition. The support may be of lithographically suitable material and lithographic printing plates may be made using the composition.

Description

SPECIFICATION Photoconductive compositions and elements and processes using them This invention relates to photoconductive compositions and electrophotographic elements and, especially, to compositions and elements suitable for use in lithography.

Electrophotographic reproduction is based on the ability of some normally insulating materials, whose surfaces have been electrically charged, selectively to conduct electric charge upon imagewise irradiation whereby an electrostatic latent image is formed upon such a material. Thus the charge is conducted away from the surface and through those portions of the insulating materials which have been exposed to the radiation while the areas of the surface which have not been irradiated retain their original charges. The electrostatic latent image is an invisible electrostatic charge pattern generated in a usual exposure procedure, e.g. by lens-projected imaging or contact-printing methods, wherein the charge density at the various areas of the surface is a function of the intensity of irradiation at said areas during exposure.

The thus formed latent image may be developed (i.e. made visible) by treatment with a powder (which may or may not be coloured and/or mixed with a binding resin) which is attracted to the non-irradiated surface areas which have retained their charges thereby providing a sharp contrast between the irradiated (non-image) and non-irradiated (image) areas.

The visible image is then fixed by causing it to become permanently attached to a support on which the image is desired.

The support may be the original support to which the composition was applied before exposure or a material to which the image was transferred after development and is chosen in accordance with the desired end use. For instance, if the final product is to be a lithographic printing plate then the supporting material is a lithographically suitable supporting material to which the photoconductive composition is applied and the developed image is fixed directly thereon.

In that case the exposed areas are removed (de-coated) from the plate after the image has been fixed thereon, by means of decoating compositions, i.e. aqueous compositions which attack and either dissolve or disperse the exposed photoconductive composition so that it no longer adheres to the support material. The developing agent must of course be chosen so that after fixation it is resistant to subsequently applied compositions, for instance the aqueous composition required for producing a lithographic element, and prevents attack by the composition on the underlying photoconductive layer.

The electrostatic latent image is formed on the surface of an insulating photoconductive layer carried on a conductive support. For example, the free surface of the photoconductive layer is uniformly charged, in the dark, e.g. by application of a corona discharge, and most of the charge is maintained on said surface, due to the insulating character of the layer, in the absence of irradiation.

Upon imagewise exposure, however, the conductivity of the layer is greatly increased in the irradiated areas in proportion to the intensity of irradiation. Thus, the surface charge in such areas is permitted to "leak" off while the charge in the irradiated areas is not affected. This pattern of charge and uncharge surface areas is the aforementioned electrostatic latent image.

Electrophotographic materials and processes are of great importance in many areas of the graphic arts industry including the preparation of lithographic printing plates, where they have been found to be preferable to other conventional methods which require additional process steps of preparing a mask or transparency from the original image prior to the exposure step. This requirement has, inter alia, the disadvantage of using a film for the transparency which is expensive (especially if the film uses a silver halide coating) and requiring additional equipment for preparing the transparencies. In the electrographic processes the above step is not required as the image is formed on the printing plate directly from the original resulting in considerable savings in money and time.

This increased interest has created a need for suitable materials meeting stringent requirements regarding, e.g. spectral range of use (i.e. range of radiation wavelengths within which material is photoconductive), simplicity of handling, and reliability. In the past such materials have included elemental and inorganic substances such as, selenium, sulphur, zinc oxide, and tellurium and organic substances such as anthracene and anthraquinone. These materials, however, suffer from a number of disadvantages, e.g. in the areas of spectral range of use, ease of handling, light sensitivity and stability.

These disadvantages have been avoided partially by the use of compositions comprising the substituted oxadiazoles described in U.S.

Patent No. 3,189,447 or the substituted oxazoles described in U.S. Patent No. 3,257,203 and which may also comprise water insoluble resins, as well as photosensitisers to increase their sensitivity to the visible areas of the spectrum. However, these compositions suffer the disadvantages of poor resolution and sensitivity to the pre-exposure effect and high material costs. Sensitivity to the pre-exposure effect refers to that property of some insulating photoconductive materials whereby their capacity to be charged, as well as their photoconductivity upon irradiation, is temporarily decreased upon exposure to radiation, prior to the electrostatic charging. These decreased capacities return to their normal values after the elements have been stored in the dark for a period of time.

The need for such dark storage is disadvantageous either with respect to an increase in "down-time" or by requiring an excess of inventory.

Additionally, the elements comprising the above photoconductive compositions can only be decoated using organic solvents. At present, when the environmental impact of all industrial processes and materials is being critically scrutinised, that factor can be an extreme limitation on the use of such compositions and provides an incentive for greater research efforts to produce elements which can be decoated by means of aqueous compositions.

A photoconductive composition according to the invention is sensitive to aqueous compositions and comprises at least one inorganic photoconductor, at least one photoconductive organic pigment and at least one insulating resin that is sensitive to aqueous compositions. By the invention it is possible to provide compositions having good spectral sensitivity and good resolution and which are decoatable by aqueous compositions and are not subject to the pre exposure effect and which may be formulated from relatively low cost ingredients.

Suitable inorganic photoconductors for use in the invention include titanium dioxide and, preferably, zinc oxide.

Suitable photoconductive organic pigments that may be used include anthraquinone class pigments, preferably C.l. Pigment Red 1 68 (e.g.

Monolite Red 2Y, I.C.I. Co.) and C.l. Pigment Orange 43 (e.g. Indofast, Hermon Chemical Co., Hostaperm Orange GR, Hoechst Co.). C.l. indicates "Colour Index". Monolite, Indofast and Hostaperm are trade marks.

The composition and the insulating resin are both defined as being sensitive to aqueous composition and this means that the resin is soluble or dispersible in the aqueous composition with the result that the photoconductive composition can be decoated by application of this aqueous composition. Suitable resins for this purpose include phenol-formaldehyde resins, phenoxy resins, epoxy resins, and homo- and copolymers comprising residues of vinyl alcohol (which exists only in polymers), hydroxyalkyl acrylates or methacrylates, vinyl acetate, acrylic monomers, N-vinylpyrrolidone-2, vinyl-sulphonic acid, styrenesulphonic acid, acrylamide and its derivatives, maleic anhydride, vinyl ethers, and the like. Preferred resins are phenoxy resins (e.g.

Bakelite phenoxy resin PKHH, from Union Carbide Corp.) and poly(vinyl acetate) homo- or copolymers. Bakelite is a trade mark.

The decoating aqueous compositions may be any of those known to the art for decoating aqueous composition-sensitive resins, preferably aqueous alkaline solutions of pH of greater than 10.

The use of the compositions containing the three ingredients defined above may be rather limited due to the tendency of such compositions to have a relatively slow speed. It has been found that the speed of the compositions may be increased by addition of dyestuff photosensitisers.

Photosensitisers which may be used in this invention include triarylmethane dyes, such as Methyl Violet, xanthene dyes such as the rhodamines (e.g. Rhodamine B) and phthaleins (e.g. Rose Bengal); and mixtures thereof. A preferred photosensitiser is Rose Bengal. Thus by the invention it is possible to obtain compositions that have good speed.

A preferred photoconductive composition comprises 3 to 25% wt. of the inorganic photoconductor, amounts of organic photoconductive pigment in the range of 4 to 30% wt. and 20 to 80% wt. of the aqueous composition-sensitive resin, all of the percentages being based on the weight of the dried coating.

Most preferably the composition further comprises up to 5%, preferably 0.5 to 5%, of dyestuff photosensitisers based on the total weight of the dried coating.

In addition, if desired, the photoconductive composition may comprise other additives selected from other resins, i.e. resins which are not aqueous composition-sensitive, reactive diluents which are converted to aqueous compositionsensitive resins or the said other resins upon irradiation, plasticisers, fillers, colourants, thermally and/or photolytically activated polymerisation initiators, thermal polymerisation inhibitors and mixtures thereof.

The resins that may be used as the said other resins may be illustrated by polystyrene, ABS terpolymers, and ethylene-propylene rubbers.

The reactive diluents may be any of those known in the art, such as, vinyl acetate, styrene, alkyl acrylates and methacrylates and maleic anhydride.

An electrophotographic element according to the invention comprises an electrically conductive support having adherent to at least one surface thereof any of the photoconductive compositions of the invention as described above.

The electrically conductive support may comprise any of those supports known in the art including metal sheets or foils, glass plates, paper sheets or webs or plastic sheets or webs especially those made of electrically conductive resins. The supports, dependent upon the intended end use of the elements, may be opaque, translucent or transparent. If an absorbent support, such as paper, is used it is desirable that it be pretreated to prevent adsorption of the coating solution. The pretreatment may be accomplished by e.g. adsorption of methyl cellulose or poly(vinyl alcohol) from aqueous solutions or polyamides from aqueous alcoholic solutions. If the elements are to be used for lithography then the support is preferably a lithographically suitable material, preferably being formed of aluminium or an aluminium alloy, or a plastic sheet coated with metal foil.

The photoconductive compositions may be applied to the support by any means known to the art, such as, spray, meniscus, wire wound rod reverse coating, gravure coating, whirl coating from solutions in appropriate solvents or by melt coating of the undissolved photoconductive compositions.

The conductive compositions and elements are of particular use in lithography and the invention includes a method of preparing lithographic printing plates which comprises the steps of: I. electrostatically charging the free surface of the photoconductive layers of any of the above elements, prepared according to the invention, wherein the conductive support comprises a lithographically suitable material; II. image-wise exposing the charged surface to radiation, whereby the charge is discharged in the exposed areas in proportion to the intensity of the radiation, to provide an electrostatic latent image; III. developing said latent image to provide a visible image; IV. fixing said visible image, and V. removing the exposed photoconductive composition by treatment with a suitable aqueous composition to produce the desired printing plate.

If desired, stages I to IV may be repeated several times, for instance 1 to 6 times, before going to step V, using different masks in order to superimpose several images upon each other in the final plate.

While the coatings are, in themselves, nonradiation-sensitive application of a positive or negative electrostatic charge thereto, e.g. by means of a corona discharge, renders them radiation-sensitive. The electrostatic latent image is formed on the charged element by imagewise irradiation thereof. The radiation may be selected from the whole electromagnetic spectrum including, visible light, UV or IR radiation and electron beam. Preferred radiation is in the UV and visible light areas of the spectrum.

Development of the latent electrostatic image may be accomplished by any means known in the art including treatment of the imagewise exposed element with liquid or dry toners such as, dye filled resins and the carbon black or coloured resin described in U.S. Patent 3,189,447.

The developed (visible) image is then made permanent, (i.e. fixed) e.g. by heating in the presence or absence of solvent vapours, or by treatment with steam and/or pressure.

The thus fixed image element is then converted to the desired lithographic printing plate by treatment with an appropriate aqueous composition, preferably an aqueous alkali solution preferably of a pH of greater of 1 0, to remove the exposed aqueous composition soluble or dispersible photoconductive composition. If desired, the aqueous composition may also comprise organic solvents to aid in wetting the resinous components of the photoconductive composition whereby removal of the exposed composition from the support is faciiitated.

The following examples are illustrative of the invention and methods for its accomplishment.

Example 1 A mixture comprising 10.0 gms of Monolite Red 2Y, 8.0 gms of ZnO, 0.1 gms of Rose Bengal, 50 ml. of methyl Cellosolve (MC) and 100 ml methyl ethyl ketone (MEK) was ground in a ball mill (using Borundum balls) for 1 6 hours. To the above mixture was then added a solution comprising 40.0 gms of Resyn 28-2930 (a carboxylated vinyl acetate terpolymer manufactured by National Starch and Chemical Corp., Bridgewater, N.J.) 50 ml of MC and 100 ml MEK. The resultant mixture was then milled an additional 3 hours. Monolite, Resyn, Cellosolve and Borundum are trade marks.

The above mixture was applied by means of No 18 wire wound rod to a 37 cm x 56 cm, 12 gauge sheet of aluminium which had first been pumice grained and anodised. The resultant element was then dried using a hot air blower followed by heating at 1000 Cfor 1/2 hours to yield the desired electrophotographic element.

The free insulating surface of the above element was then electrostatically charged by means of a corona discharge, image-wise exposed to a Visible/Argon Laser (Muirhead, Inc.) at 80 amps Laser power, toned and fixed by heating.

The resultant plate was then decoated, to remove exposed (non-image) portions of the coating, by means of an aqueous alkaline decoating solution.

The decoated plate was then gummed by conventional means and used on an ATF Chief press to produce 121,000 acceptable impressions. Chief is a trade mark.

Example 2 Example 1 was repeated except that 5.0 gms of Resyn 28-2930 were replaced by 5.0 gms of Bakelite phenoxy resin PKHH, the Monolite Red was replaced by 10.0 gms of Hostaperm Orange GR and only 4.0 gms of ZnO were used. Similar results were obtained.

Claims (14)

1. A photoconductive composition that is sensitive to aqueous composition and that comprises at least one inorganic photoconductor, at least one photoconductive inorganic pigment and at least one insulating resin that is sensitive to aqueous composition.

2. A photoconductive composition according to claim 1 also comprising at least one dyestuff sensitiser.

3. A photoconductive composition according to claim 2 in which the sensitiser is selected from triaryl methane and xanthene dyes.

4. A photoconductive composition according to claim 3 in which the sensitiser is Rose Bengal.

5. A photoconductive composition according to any preceding claim in which the inorganic photoconductor is selected from TiO2 and ZnO.

6. A photoconductive composition according to claim 5 in which the inorganic photoconductor is ZnO.

7. A photoconductive composition according to any preceding claim in which the photoconductive organic pigment is an anthraquinone pigment.

8. A photoconductive composition according to claim 7 in which the pigment is selected from C.i.

Red 168, and C.l. Pigment Orange 43.

9. A photoconductive composition according to any preceding claim in which the resin is sensitive to aqueous composition and is selected from phenol-formaldehyde resins, phenoxy resins, epoxy resins and homo- and co-polymers comprising residues of vinyl alcohol, hydroxy-alkyl acrylates or methacrylates, vinyl acetate, acrylic monomers, N-vinylpyrrolidone-2, vinylsulphonic acid, styrenesulphonic acid, acrylamide and its derivatives, maleic anhydride, and vinyl ethers.

10. A photoconductive composition according to claim 1 comprising ZnO as inorganic photoconductor, C.l. Pigment Red 1 68 or C.l.

Pigment Orange 43 as organic photoconductor, a carboxyl containing polyvinyl acetate copolymer as aqueous composition sensitive insulating resin, and Rose Bengal as dyestuff sensitiser.

11. A photoconductive composition according to any preceding claim containing 3 to 25% inorganic photoconductor, 4 to 30% organic photoconductive pigment, 20 to 80% aqueous composition sensitive resin and 0 to 5% dyestuff photosensitiser, all percentages being by weight on the weight of the dried coating.

12. An electrophotographic element comprising a conductive support material having coated on at least one surface a composition according to any preceding claim.

1 3. An element according to claim 12 in which the conductive support is a lithographically suitable material.

14. An element according to claim 13 in which the support is selected from aluminium, aluminium alloys and plastic sheet coated with metal foil.

1 5. A method of preparing a lithographic plate comprising the steps of I. electrostatically charging the free surface of the photoconductive layer of an element according to claim 13 or claim 14: II. image-wise exposing the charged surface to radiation, whereby the surface charge is discharged in the exposed areas to provide an electrostatic latent image; III. developing the latent image to provide a visible image; IV. fixing the visible image, and V. removing the exposed photoconductive composition by treatment with a suitable aqueous composition to produce the desired printing plate.

1 6. A method according to claim 1 5 wherein steps I to IV are repeated 1 to 6 times wherein the image in each of the steps Il is different.