Process For Preparing A Planographic Printing Form

Abstract

A planographic printing plate prepared by electrostatic exposure and development and carrying a hydrophobic deposit from such development in the image areas thereof is hydrophilized by treatment with an aqueous solution containing a polymeric compound having in the recurring units thereof an amino salt and/or quaternary salt group the polymer compound being free of long chain hydrocarbon groups. Preferred hydrophilizing compounds are polyamines having aliphatic linkages between the amino groups and polyvinyl compounds having hydrogen-containing heterocyclic side groups.

Description

This invention relates to a process for preparing planographic printing plate and to the printing plate obtained therewith.

Planographic printing is based on the physical property of repellence of greasy materials for water. The printing surface, which is substantially flat, contains the pattern of the image to be printed in terms of a differentiation in water repellency. In the ordinary lithographic or planographic printing processes a printing plate is prepared by affixing to a water-attractive, hydrophilic surface, a water repellent, hydrophobic image, usually greasy, resinous or waxy in nature.

There are several methods known for preparing a lithographic printing plate. According to one of them use is made of a photoconductive insulating material such as an electrophotographic recording layer containing photoconductive zinc oxide dispersed in an electrically insulating binder. To prepare the printing surface said layer is electrostatically charged and image-wise exposed.

The electrostatic image according to the electrical sign of the electrostatically attractable developer particles in respect of the applied charge may be developed as a negative or a positive of the original to be reproduced. According to a common technique the electrostatic image is rendered visible by applying a developer powder, which is held electrostatically to the charged areas of the sheet. The powder image preferably composed of hydrophobic fusible powder particles may be fixed by heating. Normally the difference in hydrophobicity between the developed image parts and the noncovered areas of the recording layer is not high enough for high quality planographic printing and has to be increased. Therefor it has been proposed to treat the recording layer chemically in the undeveloped areas in order to make said areas highly water-receptive.

It has been found now that a photoconductive insulating recording layer comprising an electrically insulating binder having suspended therein an inorganic photoconductive substance that can provide zinc ions, such as photoconductive zinc oxide, can be provided with improved hydrophility by treating it with a polyamino compound comprising at least two nonaromatic amino groups, e.g. aliphatic, cycloaliphatic, cyclo- or heterocyclic amino groups without aromatic character, at least one of these amino groups being present in salt form or quaternized form.

The aliphatic or cycloaliphatic chains of the nonaromatic amine groups may consist entirely of chains of carbon atoms, or the chains may contain carbon atoms interspaced with oxygen, sulfur nitrogen or phosphorus atoms or combinations thereof. Thus the chains may contain ester, amide, ether, imidazole, or urethane linkages, or their thio-equivalents.

Use is preferably made of a polymeric compound containing one or more aliphatic amino groups in the recurring units e.g. a polyalkyleneimine compound wherein part of the amino groups or all of them are transformed into a salt form or a quaternized form.

Particular representatives of such polyamino compounds are listed in the following table.

TABLE

1. h.sub.2 n-(ch.sub.2 -ch.sub.2 -nh).sub.n -CH.sub.2 -CH.sub.2 -NH.sub.2 (acid neutralized) wherein:

n is an integer from 2 to 50.

wherein:

R is a C.sub.1 -C.sub.5 alkyl group;

x is an integer from 2 to 50, and

compounds of this type are sold under the trade name

Solidogen by Cassella Farbwerke Mainkur A.G. Frankfurt

(Main). A preferred product is "SOLIDOGEN FFL."

wherein:

m is an integer from 2 to 50.

(partly quaternized polyethyleneimine).

wherein:

X.sup.- is an anion, e.g. a chloride ion and

n is an integer from 2 to 50. ##SPC1## ##SPC2## Poly(N-Vinyl-2-methylimidazolium methylsulphate) sold under the trade name ANTI-STATIN LF by Badische Anilin- & Soda-Fabrik A.G. Ludwigshafen, West Germany.

Further polymeric amino compounds suitable for use according to the present invention are described in the published Dutch Pat. applications Nos. 6,614,230, 6,614,291 and 6,711,152.

The polyamino compounds for use according to the present invention are preferably dissolved in an aqueous liquid in a concentration of 0.1 to 20 percent by weight calculated on the total weight of the liquid.

The polyamino compounds comprising amino group(s) in neutralized or quaternized form are preferably used in combination with water-soluble ferricyanides and/or water-soluble ferrocyanides since the sparingly water-soluble hydrophilic zinc salts formed therewith have a relatively high adsorption power for cationic compounds and therefore have a mordanting effect on said neutralized or quaternized polyamino compounds.

A printing plate containing in the hydrophilized portions a coprecipitate of zinc ferri- and/or zinc ferrocyanide and a neutralized or quaternized polyamine has a high resistance to wear, which is important for long run printing. That resistance to wear can still be increased with hydrophilic colloids which may be applied simultaneously with or after the treatment with said polyamines. Suitable hydrophilic colloids for that purpose are e.g. gum arabic, alginic acid, water-soluble alginates e.g. ammonium alginate, caseinates, gelatin, polyacrylic acid esters, polystyrene sulfonic acid, polyvinyl alcohol, and carboxymethylcellulose. A treatment with hydrophilic precipitating or hardening agents for these colloids has proved to be advantageous. So, use can be made of formaldehyde as protein hardener and heavy metal salts forming a precipitate e.g. with alginates.

In order to set free zinc ions from the photoconductive layer containing a photoconductive zinc compound, the pH of the hydrophilizing liquid is preferably below 7 and more preferably within the range of 3 to 6. For acidifying the developing liquid water-soluble mineral and/or organic acids can be used e.g. orthophosphoric acid, nitric acid, sulfuric acid, hydrochloric acid, boric acid, acetic acid, citric acid, tartaric acid, tannic acid and lactic acid.

The hydrophilizing action of the hydrophilizing agent may be improved by a pretreatment of the developed and fixed recording layer with a softening or swelling agent for the binder occasionally in combination with a wetting agent improving the penetrating power of the hydrophilizing agent in the recording layer. In this way much more zinc oxide grains are reached by the hydrophilizing agent and hydrophilized.

As swelling agents preferably water-miscible compounds such as acetone and methyl ethyl ketone are used.

According to a preferred embodiment the electrophotographic recording layer is rendered water-receptive at the areas to be hydrophilized after the printing master has been mounted on the press, thus obviating any separate immersion treatment. The hydrophilizing treatment of said layer may be carried out by means of an absorbent pad impregnated with an aqueous solution containing the partly or wholly neutralized or quaternized polyamine.

Electrophotographic recording materials, which are especially suited to be used in the preparation of a planographic printing plate, are described e.g. in the published Dutch Pat. applications 6,608,814 and 6,608,815.

Any known process for forming the electrostatic latent image and hydrophobic image may be applied.

According to a common technique the hydrophobic image is formed by the consecutive steps of producing an electro-static image on a photoconductive zinc oxide/hydrophobic binder layer by integrally electrostatically charging that layer, subsequently image-wise exposing and developing the latter with a hydrophobic developer powder, which is fixed to the recording layer e.g. by heating.

The powder image can be formed by the known dry "carrier-toner development" or by a liquid development based on electrophoresis wherein charged hydrophobic particles are attracted from an electrically insulating liquid to be charged areas of the recording layer. Such development technique is described e.g. in the U.K. Pat. specification 755,486.

According to a developing technique in the U.K. Pat. specifications 987,766, 1,010,505, 1,020,503, 1,033,419 and 1,033,420 an electrostatic latent image can also be developed by a selective deposit of an aqueous developing composition. That composition can e.g. contain a dispersed hydrophobic polymer, which on drying the liquid image is image-wise left and forms a hydrophobic pattern.

It is self-explanatory that the polyamines applied according to the present invention can be used in combination with other hydrophilizing compounds.

The following examples illustrate the present invention without, however, limiting it thereto.

EXAMPLE 1

A layer of photoconductive material consisting of photoconductive zinc oxide dispersed in an insulating binder (3 parts by weight of zinc oxide to 1 part by weight or resin binder) was coated on a sheet of aluminum foil laminated to a paper support. The resin binder contained on a weight basis 80 % of styrene-butadiene copolymer (80/20), 20 % of silicone resin (Silicone Resin SR-82 supplied by General Electric Silicone Products Department Waterford, N.Y., U.S.A.). The recording layer was charged with a negative corona with a tension of - 6000 v. on the corona wires and exposed through a graphic original. Development was carried out by using the magnetic brush technique with iron particles as carrier and gilsonite as fusible toner. The powder image was fixed to the layer by heating for 90 seconds at 140.degree. C.

A cotton pad was impregnated with a solution containing the following ingredients:

50 percent aqueous solution of a high molecular weight (15,000) polyethyleneimine quaternized with dimethylsulphate 3 cc.

water 97 cc.

The plate was rubbed softly with the cotton pad while on the press. The quality of prints was very good.

On using 1 percent by weight of the said hydrophilizing compound together with phytic acid for maintaining the pH to 5 in the liquid of the damping system of a common offset printing device up to 5000 of excellent sharp copies could be obtained.

EXAMPLE 2

Example 1 was repeated with the proviso, however, that the hydrophilizing liquid used contained 3 percent by weight of compound 1 (n= 3) neutralized with 3 cc. of a 1 percent aqueous tannic acid solution. The quality of the plate was only a little inferior to that of example 1.

EXAMPLE 3

Example 1 was repeated with the proviso, however, that the hydrophilizing liquid used contained 3 percent by weight of compound 3 (m= 10). The quality of the plate was equivalent to that of example 1.

EXAMPLE 4

Example 1 was repeated, with the proviso, however, that the hydrophilizing liquid used contained 3 percent by weight of 4 (X.sup.-= C1.sup.- and n= 30). The quality of the plate was equivalent to that of example 1.

EXAMPLE 5

Example 1 was repeated, with the proviso, however, that the hydrophilizing liquid used contained 6 percent by weight of polyethyleneimine (n= 30-50). 50 percent of the -NH- groups being quaternized with dimethyl sulfate

EXAMPLE 6

A layer of photoconductive material consisting of photoconductive zinc oxide dispersed in an insulating binder (6 parts by weight of zinc oxide to 1 part by weight of binder) was coated on a flexible aluminum foil.

The binder was prepared from a mixture of EPOK X-1772 (ammoniacal alkyd resin salt marketed as a 66 to 68 percent (by weight) aqueous solution by British Resin Products Ltd.) and EPOK W-9801 (a 72 to 75 percent aqueous solution of a melamine-formaldehyde resin marketed by British Resin Products). The coated layer was dried for 24 hours at room temperature and cured for 1 min. at 100.degree. C.

The cured recording layer was charged with a negative corona with a tension of - 6000 v. on the corona-wires and exposed to a line-copy.

The electrostatic image was electrophoretically developed by means of an electrophoretic developer obtained by diluting the hereinafter described concentrated developer composition in a volume ratio of 15/1000 with the hydrocarbon solvent SHELLSOL T (trade name)

carbon black (average particle size 20 .mu. ) 30 g.

zinc monotridecyl phosphate 1.5 g.

Shellsol t (trade name) 750 cc.

resin solution prepared as described

hereinafter 150 g.

The resin binder solution was prepared by heating 500 g. of ALKYDAL L 67 [ trade name by Farbenfabriken Bayer AG, Leverkusen, West Germany, for a linseed oil (67 percent by weight) modified alkyd resin] and 500 cc. of white spirit containing 11 percent by weight of aromatic compounds at 60.degree. C. till a clear solution was obtained, and subsequent cooling.

Hydrophilization of the developed plate was effected by softly rubbing with a cotton pad soaked in an aqueous solution containing the following ingredients:

Solidogen ffl (trade name) 5 cc.

sodium ferrocyanide 1 g.

sodium sulfite (antioxidant) 2.5 g.

water 100 cc.

The quality of the plate was better in respect of wear than that of the plate obtained in example 1.

EXAMPLE 7

Example 6 was repeated, with the proviso, however, that the hydrophilizing liquid was acidified till pH 3 by means of tartaric acid, phosphoric acid, citric acid, phytic acid, or oxalic acid.

Hydrophilization was more effective than was the case according to the procedure of example 6.

EXAMPLE 8

Example 1 was repeated, with the proviso, however, that the hydrophilizing liquid used contained 5 percent by weight of the compound 14. The plate quality was practically the same as that obtained according to the procedure of example 1.

EXAMPLE 8-11

Example 1 was repeated with the proviso, however, that the hydrophilizing liquid was acidified till pH 6 and that it contained 5 percent by weight of the quaternized polyamino compound No. 6 of the table. The hydrophilization obtained was equivalent to that according to the procedure of example 1. Results of slightly inferior quality were obtained by using the compounds Nos. 7, 8 and 9 in the same concentration at pH values of 6, 2 and 6 respectively.

EXAMPLES 12-15

Example 1 was repeated with the proviso, however, that aqueous hydrophilizing liquid was acidified with hydrochloric acid till pH 2 and that it contained in a concentration of 5 percent by weight the compounds Nos. 10, 11, 12 or 13 respectively.

Claims

What we claim is:

1. In a process for preparing a planographic printing plate comprising the steps of forming a latent electrostatic image on a photoconductive insulating recording layer containing an inorganic photoconductive substance, which can provide zinc ions, and developing said image with a toner forming an image-wise hydrophobic deposit on said recording layer, the improvement of treating the portions of the recording layer which are not covered with said hydrophobic deposit with an aqueous hydrophilizing solution containing a a water-soluble polyamino compound comprising at least two nonaromatic amino groups with each adjacent pair of such amino groups being connected by an aliphatic linkage including a hydrocarbon chain of at least two but not more than six carbon atoms, said compounds containing not more than six carbon atoms in a continuous carbon chain, at least part of the amino groups in said compound being in amine salt or quaternized salt form.

2. Process for preparing a planographic printing plate according to claim 1, wherein the polymeric compound is applied in the aqueous liquid of a damping system of a planographic printing device.

3. Process for preparing a planographic printing plate according to claim 1, wherein the polymeric compound is applied in the aqueous phase of a planographic printing ink composed of a lipophilic phase dispersed in a hydrophilic phase.

4. Process for preparing a planographic printing plate according to claim 1, wherein the polymeric compound is used in dissolved form in an aqueous liquid in a concentration of 0.01 to 20 percent by weight calculated on the total weight of the liquid.

5. Process for preparing a planographic printing plate according to claim 1, wherein polymeric compound is used in an aqueous liquid having a pH lower than 7.

6. Process for preparing a planographic printing plate according to claim 1, wherein the polymeric compound is used in combination with ferricyanide and/or ferrocyanide ions.

7. Process for preparing a planographic printing plate according to claim 1, wherein the polymeric compound is used in combination with a hydrophilic colloid.

8. ##SPC3##

9. Process for preparing a planographic printing plate according to claim 10, wherein the polyamine compound is polyalkylene-polyimine compound having at least part of the amino groups thereof transformed into a salt form or a quaternized form.