US3653886A - Preparation of printing forms by the ionic polymerization of photoconductors - Google Patents

Abstract

This invention relates to an electrophotographic reproduction process for the preparation of printing plates from electrophotographic reproduction material in which a latent electrostatic image is produced on a layer containing at least one polymerizable organic photoconductor having olefinic double bonds therein, or capable of forming such bonds during heating, the image is made visible by means of a developer which comprises at least one fnely-divided substance which acts as a catalyst for the ionic polymerization of olefins, the reproduction material with the developed image thereon is heated to a temperature between about 50* and 300* C., and the organic photoconductor layer then is removed in a conventional manner from the non-image areas by dissolving it away.

Description

United States Patent Lind et al. Apr. 4, 1972 [54] PREPARATION OF PRINTING FORMS 3,317,315 5/1967 Nicoll et al. ..96/1.1 Y THE IONIC POLYMERIZATION 01? 3,418,116 12/1968 Inami et al. ..96/ 1.5

PHOTOCONDUCTORS Primary Exammer-George F. Lesmes [72] Inventors: Erwin Lind, Auringen Uber Wiesbaden; A i mm E i M B, Wittenberg Wolfgang Wiedemann, Niederwal- An j 5 B a luf, Rhineland, both ofGer many [73] Assignee: Kalle Aktiengesellschaft, Wiesbaden- {57] ABSTRACT This invention relates to an electrophotographic reproduction [22] Filed; Apt 0 9 3 process for the preparation of printing plates from electrophotographic reproduction material in which a latent elec- [21] Appl' 720383 trostatic image is produced on a layer containing at least one polymerizable organic photoconductor having olefinic double [30] Foreign Application Priority Dat bonds therein, or capable of forming such bonds during heating, the image is made visible by means of a developer which Apr. 13, 1967 Germany ..I( 62004 comprises at least one fnely divided substance which acts as a catalyst for the ionic polymerization of olefins, the reproduc- [52] U.S. Cl. ..96/l R, 96/363, 110015145662; on material with the developed image thereon is heated to a 51 In: c1 ..G03g G030 5/00 temperature between and and the garlic 58 Field of Se arch: ..96/1 363- 161/456 463 Photoconductor layer is Iemmd a conventional manner from the non-image areas by dissolving it away.

[56] References Cited 6 Claims No Drawings UNITED STATES PATENTS 3,231,374 1/1966 Sciambi ..96/1

PREPARATION OF PRINTING FORMS BY THE IONIC POLYMERIZATION OF PHOTOCONDUCTORS The present invention relates to the preparation of printing forms by electrophotographic means. Further, it relates to an electrophotographic developer. By the process of the invention and with the use of the developer of the invention, planographic printing forms are preferably prepared, but intaglio printing forms also may be prepared.

It is known to use photoconductive organic compounds in the layers of electrophotographic reproduction materials. Furthermore, it is known to process electrophotographic reproduction materials containing organic photoconductors into printing forms. In this case, they must have a support which is suitable for printing purposes. In the unprocessed state, they are also called electrophotographic printing plates. Thus, an electrophotographic printing plate is known the photoconductive layer of which consists of a monomolecular photoconductor and high molecular weight alkali-soluble substances. This layer is electrically charged, exposed and developed with a colored resin powder in the normal manner. The resin powder is fixed by buming-in, and the layer is then treated with an alkaline solution, the non-image areas being dissolved away and a planographic printing form resulting therefrom. This process has the disadvantage that by the treatment with the alkaline solution, it may occur that relatively small components of the image are underwashed or even washed away.

Further, an electrophotographic printing plate is known the layer of which contains, as the photoconductor, finely distributed zinc oxide in a binder capable of cross-linking, and also a method for processing such printing plates, in which for developing the latent electrostatic image a substance which acts as a catalyst for the cross-linking of the binder is employed as a finely divided solid. When the developer is burned in, the image areas become less soluble or even insoluble in certain solvents in which the non-image areas, which are not cross-linked, are readily soluble. For the preparation of the printing form, the photoconductive layer is thus dissolved away in the non-image areas. Here, too, the already mentioned effect of underwashing of the image areas occurs, because the layer is not evenly cross-linked down to the support due to the zinc oxide particles embedded therein. To eliminate this drawback, in a variation of this process, electrophotographic printing plates are used which, between the support and the photoconductive layer, contain a metal chromate or phosphate intermediate layer which increases adhesion. By this means, the underwashing effect of the image areas is substantially prevented, but the process still has the disadvantage that organic solvents must be used for removal of the non-image areas.

It also has been suggested to prepare printing plates by a method in which an electrophotographic layer containing photoconductors with olefinic double bonds is electrically charged in the normal manner, then image-wise exposed, and the latent electrostatic image thus produced is then developed by means of a radical-forming solid substance. During the ensuing heating step, radicals are formed in the image areas so that the layer is thoroughly polymerized in these areas. The unpolymerized areas then may be removed from the support in known manner. This method has the disadvantage that the radical-forming substances are rather expensive and can be dispersed in hydrocarbons only with difficulty, so that the otherwise preferable liquid developers are of unsatisfactory serviceability when prepared with these substances.

The present invention is based on the known process for the preparation of printing forms from electrophotographic reproduction material comprising a support suitable for printing purposes and a photoconductive layer thereon, by electrically charging and image-wise exposure to light of the layer, development of the latent electrostatic image in the layer with a finely divided toner, with subsequent heating, and removal of the photoconductive layer in the non-image areas. The latent electrostatic image is produced on a layer which contains at least one known polymerizable organic photoconductor containing olefinic double bonds or capable of forming such bonds during heating, the image is made visible by means of a developer which at least partially consists of one or more finely divided substances which act as catalysts for the ionic polymerization of olefins, the reproduction material with the developed image thereon is heated to a temperature between about 50 and 300 C., preferably to a temperature between and 200 C., and the organic photoconductor layer is removed in a conventional manner from the non-image areas by dissolving it away.

By the above-described process, a planographic printing form is obtained. When a suitable support is used, this planographic printing form may be converted into a relief printing form or, if desired, also into an intaglio printing form.

The novel feature of the present invention is that a monomeric polymerizable photoconductor is image-wise polymerized by a ionic mechanism, the image-wise distribution of the polymerization catalyst having the ionic action being achieved by using it as a developer substance.

The electrophotographic printing plate serving as the starting material for this process is known in principle. The use of photoconductive organic substances having olefinic double bonds in electrophotographic layers is also known. The use thereof in printing plates has not been specifically described. It is, however, known from German Pat. No. 1,1 17,391, that they, in principle, may be used for this purpose.

In the process of the invention, photoconductor substances having vinyl or vinylidene groups may be used, and also those in which three or even all four ethylene hydrogens are substituted. Suitable polymerizable compounds also may be found among the photoconductive stilbene and styrene derivatives.

Compounds containing vinyl groups are preferably employed. The following have proved to be particularly suitable, e.g.: vinyl carbazole, substituted vinyl carbazole, substituted vinyl oxazoles, substituted vinyl triazoles, and vinyl dibenzofuran. Suitable compounds which do not form the vinyl group before heating are the substituted hydroxyethyl-diphenyltriazoles. They split off water during heating, so that a vinyl group is formed.

A very suitable representative of substances containing the vinylidene group is acenaphthylene.

When the polymerizable organic photoconductors are sufficiently film-forming themselves, they may be applied directly to a suitable support and processed according to the process of the invention. Since this is not normally the case to a satisfactory degree, it has proved to be of advantage to employ a printing plate the layer of which, in accordance with German Pat. Specification No. 1,117,391, in addition to the organic photoconductor substance, contains resins which are soluble in aqueous alkali. High molecular weight substances carrying groups which render them alkali-soluble are suitable as such resins. Groups of this type are, e.g.: acid anhydride groups, carboxylic groups, sulfonic groups, sulfonamide groups, or sulfonimide groups. Resins having a high acid number are preferred, because they are particularly readily soluble in aqueous alkaline solutions. Copolymers containing anhydride groups may be used with particularly good results, because due to the absence of free acid groups the conductivity in the dark of the electrophotographic layer is low in spite of its good alkali-solubility. Copolymers of styrene and maleic anhydride have proved particularly suitable.

It is, therefore, a preferred embodiment of the process -of the invention to prepare the latent electrostatic image on a layer which contains at least one polymerizable organic photoconductor substance having a vinyl group and at least one alkali-soluble binder and, after development and heating, remove the non-image areas of the photoconductor layer in known manner by means of an aqueous alkaline liquid, which may contain an organic solvent, if desired. Suitable alkaline liquids are described in the above-mentioned German Pat. Specification No. 1,1 17,391.

The layer of the electrophotographic printing plate serving as starting material also may contain conventional sensitizers. They are added to the reproduction layer in small quantities, i.e. about 0.00,00l to about 0.1 per cent by weight of the photoconductor component. Suitable sensitizers, primarily dyestuffs, are described e.g., in Belgian Pat. Specification No. 558,078, and US. Pat. No. 3,189,447.

The light-sensitivity of the layer may be increased in known manner by the addition of so-called activator substances. These are electron acceptors or electron donors which form molecular complexes with the photoconductor substances. More detailed information is found in German Pat. Specification No. 1,127,218, and U.S. Pat. Nos. 3,287,1l3-3,287,l23.

All materials known to be suitable for the purpose may be used as supports for the electrophotographic printing plate employed as starting material, e.g. aluminum, zinc, magnesium, or copper plates, and also cellulose products, such as special papers, or cellulose hydrate, cellulose acetate, or cellulose butyrate films, the latter especially when they are in a partially saponified form. ln some cases, plastic materials, such as polyamides in the form of films, also may be used as supports.

For performing the process of the present invention, an electrophotographic printing plate, as described above, is charged in the usual manner by means of a corona discharge. As desired, a positive or a negative charge may be sprayed thereon. A latent electrostatic charge image is then produced on the charged layer by image-wise exposure to light. Contact exposure or diascopic or episcopic exposure is possible. Alternatively, the image may be screened by means of a contact screen or an engraved glass screen. The latent image is then made visible by means of a developer which consists of or contains substances which, when heated, initiate ionic polymerization of the photoconductor substance.

All compounds which are used for the ionic polymerization of olefins and are in the form of solids may be used as developers. In the present case, substances having a cationic action as well as those having an anionic action may be used. The substances may be of inorganic or organic nature. The following are mentioned as examples: metals, such as magnesium, aluminum, zinc or copper in a finely divided form; metal oxides, such as copper-l-oxide, copper-ll-oxide, calcium oxide, zinc oxide, magnesium oxide, mercury oxide, boric oxide, aluminum oxide, titanium dioxide, silicon dioxide, lead oxide, lead dioxide, tin-lV-oxide, manganese-(ll)-oxide, antimony (Ill) oxide, bismuth trioxide, molybdenum trioxide, chromium (Ill) oxide, iron (lll) oxide, and mixed oxides, such as those of copper and chromium; metal sulfides, such as antimony trisulfide and antimony pentasulfide, metal halides, metal carbonates, metal nitrates and metal silicates, e.g. zinc chloride, bismuth oxychloride, lithium carbonate, sodium bicarbonate, potassium bicarbonate, barium carbonate, zinc carbonate, silver carbonate, basic copper carbonate, nickel carbonate, cadmium carbonate, lithium nitrate and aluminum silicates; metal salts of organic acids, such as copper acetate, copper (ll) oxalate, manganese (I1) oxalate, iron (ll) oxalate, tin'(ll) oxalate, calcium malonate, as well as formiates and tartrates; metal chelates, such as aluminum, copper or zinc oxinate, further acetyl acetonate, bissalicyl aldoximes and glycinates; and carbonium compounds, e.g. triphenylmethane dyestuffs, such as malachite green.

For development, these substances may be used either in the dry state or in dispersion, in the form of a liquid developer.

The present invention also provides a dry developer, in the form of a finely divided powder, for performing the process of the invention, which consists of or comprises one or more substances which act as catalysts for the ionic polymerization of olefins.

Normally, the pure substance initiating ionic polymerization is employed as the developer. However, if the image is to be made clearly visible, a colored component may be added. The proportion of catalyst in the dry developer should not, however, fall below 20 per cent by weight. Furthermore, it has proved to be of advantage to pulverize the substances in question to a grain size of less than 10;! In most cases, the developer substance is applied to the layer to be developed with the aid of a carrier. Mixing with iron filings or using the developer powderin the known magnetic brush or magnetic drum also lead to favorable results.

Also in the case of the present invention, liquid development has proved to be of particular advantage because it enables a clean working and a high degree of resolution. The present invention also provides a liquid developer comprising a highly electrically resistant liquid phase and a finely divided solid phase dispersed therein, in which the solid phase consists entirely or partially of one or more substances which act as catalysts for the ionic polymerization of olefins. Here, too, the pure compound is preferred as the solid substance. in some cases, however, it is of advantage to add a dyestuff to the solid phase if this is not itself colored. The particle size likewise should be below 10 4. Normally, the solid substance is pulverized in a ball mill.

As dispersing agents, those liquids are used which do not dissolve the solid phase. Some halogenated hydrocarbons are suitable, e.g. particularly, however, those aliphatic hydrocarbons of relatively high molecular weight which are liquids, e.g. a product known under the trade name Shellsol T (a product of Deutsche Shell Chemie GmbH, Frankfurt/Main, (Germany). The polarity of the charge of the dispersed solid phase depends upon the properties of the catalyst itself and also upon the nature of the solvent. When dispersed in Shellsol, the above-indicated substances initiating ionic polymerization have the following polarities: antimony trisulfide, antimony pentasulfide, bismuth oxychloride and the oxinates of aluminum, copper, and zinc are negatively charged, all other substances are positively charged.

it is, however, possible to change or stabilize the polarity of the charge in the conventional manner by means of additives. Suitable additives are naphthenates, for example. They are particularly advantageously employed together with a small quantity of binding agent, such as linseed oil. Their technical effect is known from the German Auslegeschrift No. 1,047,616.

Application of the liquid developer may be performed in a conventional manner, e.g. by immersion or roller application. Preference is given, however, to roller application because of greater uniformity of application.

After development of the latent electrostatic image with one of the above-described developers of the invention, the substance initiating ionic polymerization loosely adheres image-wise to the polymerizable layer. Polymerization is then initiated by heating. Polymerization is preferably effected, as already indicated, in the temperature range of to 200 C. Heating is advantageously performed in an oven already heated to the required temperature. Infrared radiators of sufficient intensity also are suitable. However, in this case it is advisable to use a colored developer in order to increase the energy reception of the developed image areas. The duration of the heat-treatment depends upon the photoconductor to be polymerized, the substance which initiates ionic polymerization, the temperature employed, and the thickness of the layer to be polymerized. Sometimes, heating for 30 seconds is sufficient to thoroughly polymerize the layer. Sometimes, however, several minutes are required. A duration of 5 minutes can be regarded as the upper limit of the heating time. The image areas polymerized by heating become insoluble in suitable solvents, i.e. in the preferred embodiment of the invention they are insoluble in aqueous alkaline solutions.

The solutions are coated on the layer, for example by means of a cotton pad. Alternatively, the plates may be directly immersed into the liquid. Suitably constructed devices, for example with application rollers for the liquid, may be used for removing the non-image areas. Thus, a differentiation into hydrophilic areas and oleophilic areas is achieved, which is desired for offset printing, the polymerized organic photoconductor layer forming the oleophilic areas, and the surface of the support constituting the hydrophilic areas.

After treatment with the alkaline liquid, the printing plate is advantageously rinsed with water and, if desired, the hydrophilic properties are further increased by wiping it over with dilute phosphoric acid solution. After inking up with greasy ink, prints can be made therefrom in a conventional manner in planographic printing machines.

Alternatively, it is possible to produce printing forms for relief, or, if desired, also for intaglio printing by a subsequent partial removal of the bared support. The removal may be performed in one-step or multi-step etching machines known for this purpose.

The printing forms prepared in accordance with the present invention, with the aid of one of the described developers, are capable of long runs. The photoconductive layer, which generally has a thickness of only a few microns, is completely polymerized through to the support and is firmly anchored thereto. Underwashing during removal of the non-image areas does not occur. Since the organic photoconductor layers are homogeneous, grainless layers, the finest screen dots easily can be faithfully reproduced. The process of the invention is therefore preferably employed in the preparation of printing forms of very fine screen half-tone originals. This field has hitherto been substantially inaccessible to electrophotographic printing plates.

The invention will be further illustrated by the following examples:

EXAMPLE 1 A superficially roughened aluminum foil of about 100/J. thickness is coated with a solution of 2 g. of N-dimethylvinyl carbazole, 2 g. of a polystyrene resin (Lytron 820) and 4 mg. of Rhodamine B (Color Index No. 45,170) in 40 g. of glycol monomethylether, and dried. The coated material is negatively charged to a potential of about 400 volts by means of a corona discharge. It is then exposed under a positive transparent half-tone screen in a vacuum printing frame. When an incandescent lamp of 200 watts is used at a distance of 75 cm., the exposure time is 2 seconds. The charge pattern thus produced on the photo-semiconductor layer is then developed in a developer bath containing 4 g. offinely divided copper powder per 100 g. of a high boiling point liquid hydrocarbon (Shellsol T). After evaporation of the excess dispersion agent adhering to the layer, the metallic copper image formed is heated for 3 minutes to 200 C., so that the copper initiates the ionic polymerization of the N-dimethyl vinylcarbazole photo-semiconductor layer in the image areas. The layer is removed from the non-image areas by bathing the foil for 2 minutes in a solution of 5 g. of sodium metasilicate in a mixture of 100 m1. of water, 40 ml. of methanol, 35 ml. of glycol and 20 ml. of glycerol, and then wiping it with a sponge. A printing form is thus obtained from which a very long run, of several tens of thousands of prints, may be printed by the offset process.

EXAMPLE 2 The N-dimethyl-vinylcarbazole solution used in Example 1 is replaced by a solution of the corresponding quantity of 2- vinyl-4-(o-chloro-phenyl)-5-(p-diethylaminophenyl)-oxazole for coating an aluminum foil. The layer is negatively charged to a potential of 400 volts by means of a corona device. Exposure is effected under a contact screen and a positive transparency. The exposure time depends on the light-source used and the enlargement scale. Generally, it is of the order ofa few seconds. The exposed plate is developed with a liquid developer, obtained by diluting with 1,000 g. of Shellsol T," 3 g. of a concentrate prepared from 90 g. of Fe203 (iron oxide black), g. of a pentaerythritol resin ester (Pentalyn H), 30 g. of linseed stand oil, 0.5 g. of cobaltic naphthenate, and 100 g. of a high boiling point hydrocarbon (Shellsol T) and by finely grinding in a ball mill overnight. The subsequent procedure is as described in Example 1. A half-tone printing form is thus produced.

EXAMPLE 3 A zinc plate suitable for etching is coated with a solution of 10.5 g. of 2-vinyl-4-(o-chlorophenyl)-5-(p-diethylaminophenyl)-oxazole, 25 g. of a spirit-soluble novolak resin (Alnovol 429 K), 20 g. of polyvinyl acetate (Mowilith CT 5) and 120 mg. of Rhodamine B (Color Index No. 45,170) in 300 g. of glycol monoethylether, and then dried. The thickness of the dry photoconductor layer should be in the range of IO to l2 ;1.. The layer is then negatively charged to a charge potential of 400 volts by means of a corona device. Exposure may be effected with the aid of a contact copying frame as described in EXAMPLE 1. The latent charge pattern produced by exposure is developed with a dispersion of 3 g. of basic copper carbonate in 1,000 g. of a high boiling point hydrocarbon (Shellsol T). To facilitate dispersion, 0.5 g. of a pentaerythritol resin ester (Pentalyn H) and 1.0 g. of stand oil are added to the Shellsol. The plate is then heated to 200 C. and decoated. The zinc surface bared in the image-free areas by decoating then may be deep-etched with commercial etching solutions. In this manner, a relief printing form is obtained.

EXAMPLE 4 A superficially roughened aluminum foil is coated with a solution of 5 g. of l-hydroxyethyl-2,5-bis-( pdiethylaminophenyl)-l,3,4-triazole, 5 g. of a polystyrene resin (Lytron 820), and 10 mg. of Rhodamine B (Color index No. 45,170) in g. of glycol monoethylether The layer is positively charged to a potential of 300 volts and then exposed in a reproduction camera equipped with eight 500 watt incandescent lamps. A half-tone photograph serves as the original. Exposure is under an engraved glass screen. After an exposure of 60 seconds, the plate is developed, in the manner already described, with a dispersion of 5 g. of antimony pentasulfide in 100 g. of a high boiling point hydrocarbon (shellsol l') to which 10 g. of linseed stand oil are added to facilitate dispersion. Heating requires about 5 minutes, i.e. somewhat longer than in the case of vinyl compounds, because water must first be split off from the hydroxyethyl group. After decoating of the non-image areas, a printing form for offset printing is obtained.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

l. A process for the preparation of a printing plate from electrophotographic reproduction material, the latter comprising a supported layer of a polymerizable organic photoconductor containing an olefinic double bond, which process comprises electrostatically charging the layer in the absence of light, exposing the charged layer to light under a master, developing the resulting latent electrostatic image with a finely divided developer material which acts as a catalyst for the ionic polymerization of olefins, heating the reproduction material with the developed image thereon to a temperature in the range of about 100 to 200 C., and decoating the photoconductive layer in the non-image areas.

2. A process according to claim 1 in which the olefinic double bond is formed in the photoconductor during heating.

3. A process according to claim 1 in which the photoconductor contains a vinyl group and is in admixture with at least one alkali-soluble binding agent.

4. A process according to claim 3 in which, after development and heating, the non-image areas are decoated by means of an alkaline aqueous liquid.

5. A process according to claim 1 in which the finely divided developer material is contacted with the latent electrostatic image in the form of a dry powder.

6. A process according to claim 1 in which the finely divided developer material is contacted with the latent electrostatic image in the form of a liquid developer.

Claims (5)

1. 2. A process according to claim 1 in which the olefinic double bond is formed in the photoconductor during heating.
2. 3. A process according to claim 1 in which the photoconductor contains a vinyl group and is in admixture with at least one alkali-soluble binding agent.
3. 4. A process according to claim 3 in which, after development and heating, the non-image areas are decoated by means of an alkaline aqueous liquid.
4. 5. A process according to claim 1 in which the finely divided developer material is contacted with the latent electrostatic image in the form of a dry powder.
5. 6. A process according to claim 1 in which the finely divided developer material is contacted with the latent electrostatic image in the form of a liquid developer.