GB2095620A - A lithographic etch fix - Google Patents

Abstract

The invention relates to a composition for rendering oleophilic a silver image e.g. one which has been produced by the photographic silver salt diffusion transfer process at a water-accepting surface. UK patent specification 1406030 describes and claims inter alia aqueous lithographic fix compositions comprising an acid, iodide ions, an organic cationic compound containing a lipophilic group and either an organic cationic compound having an aromatic group but no nitro group, or, when the silver image requires more oxidation as a step in the process of rendering the silver oleophilic, a heterocyclic organic compound having a nitro group bonded to an aromatic nucleus. It has now been found desirable to use alternatives to aromatic nitro compounds. The present invention provides lithographic fix compositions which contain an acid, iodide ions, one or more organic cationic compounds containing a lipophilic group, one or more heterocyclic onium salts and one or more stable, silver-oxidising compounds having an oxidation potential which is more positive than the potential of a silver iodide/silver electrode which is given by the expression -150 mV -59 log 10[I<->].¦.

Description

SPECIFICATION A lithographic etch fix This invention relates to compositions-useful in lithography particularly for rendering silver images oleophilic. More particularly, this invention relates to a method of rendering oleophilic a silver image which has been produced by the photographic silver salt diffusion transfer process at a water-accepting surface, such as the surface of a hydrophilic colloid layer or the surface of a layer of a hydrophilic pigment in a binder, so that the silver image accepts greasy printing ink.

In lithograp#y, a greasy ink is employed and the printing plate usually comprises a layer which, when wet with water, is oleophobic and carries a line or dot image which is oleophilic.

Accordingly, a lithographic printing plate blank comprises a layer, the nature of whose surface is such that when wetted with water it will not accept greasy ink although, if the ink is applied when the surface is dry, the surface will accept the ink.

The practice of making a lithographic printing plate is to produce an oleophilic image at the surface of the hydrophilic layer which image is sufficiently strongly oleophilic to accept ink in the presence of water.

Where only hundreds of copies are required or even a few thousand copies, it is common to use a paper printing plate or polyester printing plate, but where many thousands of copies are required, it is necessary to use a more robust printing plate and the ones commonly used are made of aluminium sheet or zinc sheet. From these metal sheets, it is a common practice in the art to take as many as fifty thousand copies. Aluminium sheet employed in the art is provided with an inert hydrophilic surface. This is often done by treating the aluminium to produce thereon an aluminium compound which is hydrophilic, for instance, aluminium oxide, as in the case of anodized aluminium, aluminium silicate or chromatized aluminium.

Various methods of forming a silver image on a lithographic printing plate blank can be used. A preferred process involves the photographic silver salt diffusion transfer process in which diffusing silver halide complex contacts silver precipitating nuclei on the surface of the blank to deposit a silver image.

This silver image is then treated with a fix solution containing an oleophilic compound which is adsorbed to the silver image to improve its affinity for a greasy printing ink.

U.K. Specification 1,406,030 describes and claims an aqueous lithographic fix composition containing an acid, iodide ions and components (a) and (b) or (a) and (c), component (a) being an organic cationic compound containing a lipophilic group, component (b) being a heterocyclic compound which has a nitro group bonded to an aromatic nucleus and component (c) being an organic cationic compound having an aromatic group but no nitro group, component (c) being optionally the same as component (a) when the latter is cetyl pyridinium chloride or bromide, and also describes and claims a method of rendering oleophilic a silver image on a lithographic printing plate which comprises contacting the image with the fix composition.Preferably the lithographic printing plate comprises a layer having a water-accepting surface at which surface the silver image has been formed by the photographic salt diffusion transfer process.

According to Specification 1,406,030 a heterocyclic compound having a nitro group bonded to an aromatic nucleus is preferably employed when the silver image requires more oxidation as a step in the process of rendering the silver image oleophilic. Further according to Specification 1,406,030 whether or not more oxidation is required depends in part on the structure of the silver.

To avoid the use of nitro aromatic compounds some of which have hazards associated with their use, it is now proposed to use an auxiliary oxidising agent in combination with compounds (a) and (c) to get the necessary oxidation of silver.

An object of the present invention is to provide a fix composition which contains an oxidising agent for a silver image which is relatively safer to prepare and handle and in which the action of the oxidising agent is restrained so that the silver image is not damaged.

U.K. Specification 1,507,457 indicates that the "lithographic fixer" described in U.K. Specification 1,241,661 which contains cyanoferrate Ill ions, has the disadvantage that these ions should be kept carefully out of contact of acids for acids set free toxic hydrogen cyanide, e.g. in the drain, and an object of the invention described in Specification 1,507,457 is to provide a method for the preparation of a planographic printing plate with less pollution risk by using a new lithographic fixer solution.

The lithographic fixer solution of Specification 1,507,457 is claimed in Claim 18 as follows: 18. Ari aqueous lithographic fixerhaving a pH in the range of 1.0 to 6.5 and containing: (1) an iron (Ill) salt of a polyaminopolycarboxylic acid, (2) a compound yielding in dissolved state iodide ions, and (3) an organic onium compound, which is such that the fixer confers a stronger ink-receptivity on silver image areas for a lithographic ink as hereafter defined that it would if such onium compound were not present in the fixer, the type and concentration of said organic onium compound in said fixer being such that said fixer is capable of making a silver image formed by a silver complex diffusion transfer process on an image-receiving layer as hereafter specified, by treatment of such image for 8 seconds at 220C, sufficiently ink-receptive for said lithographic ink to form by transfer to lithographic printing paper from said silver image in a lithographic press an ink deposit of an optical density at least 0.5 higher than would be obtained under the same circumstances but using a fixer which is free from such onium compound but is otherwise identical; said image-receiving layer being one formed by the application of a composition of: water 890 ml 12.5% by weight aqueous solution of saponin 10 ml aqueous dispersion of colloidal nickel sulphide comprising 0.2 g of nickel sulphide and 10 g of gelatin 100 ml in an amount of 20 g per square metre onto a supported gelatino-silver chlorobromide (1.5 mole % Br) emulsion layer having a silver halide content equivalent to 1 g of silver nitrate per square metre and a ratio by weight of gelatin to silver halide (expressed as silver nitrate) of 10 to 3; and said lithographic ink having the following composition: parts by weight Lake Red C (Cl. 15,585) 80 styrenated linseed-tung oil alkyd of 60 per cent oil length and 10 per cent styrene content 100 aliphatic petroleum having a boiling range of 260~290 C 50 lead naphthenate 1 cobalt naphthenate 0.12 and its use is claimed in Claim 1.

According to Claim 1 the above-defined lithographic fixer is used in a method for the preparation of a planographic printing plate wherein a sheet material comprising an outer hardenable hydrophilic colloid layer on whose surface is concentrated a visible silver image that has been formed thereon from zarnplexed silver halide by a silver complex diffusion transfer process, is treated with said fixer.

We have now devised aqueous lithographic fix compositions which contain an acid, iodide ions, one or more organic cationic compounds containing a lipophilic group, one or more compounds having an oxidation potential as herein defined and one or more heterocyclic onium salts.

The oxidation potential referred to above must be more positive than the potential of a silver iodide/silver electrode which is given by the expression~150 mV -59 log10 [ l#j. The oxidation potential of the compound can be determined by measuring the potential difference between a platinum wire and a normal hydrogen electrode (or other convenient reference electrode) in a dilute equimolar solution of the oxidised and reduced forms of the compound. Apart from having the necessary oxidation potential, the compound must have other characteristics in order to be of practical use. These characteristics are that it must be stable in the presence of iodide ions and that it must oxidise the silver image at a sufficiently high rate. Where the compound is a complex of an oxidising metal ion an organic complexing agent, the latter should form a stronger complex with the oxidising metal ion than does the onium compound. Further such complex must, of course, remain in solution in the fix.

Examples of compounds having the required oxidation potential are as follows:- ferric ion compiexed with one of the following in the molar ratio as indicated.~ ethylenediamine tetracetic acid (1:1); 'Tiron' (disodium salt of 1 ,2-dihydroxybenzene-3,5- disulphonic acid) (1:1); ethylacetoacetate propan-2,4-dione (1:3); sodium metaphosphate (1:1); methyl imino diacetic acid (1:3); nitrilotriacetic acid (1:1); salicyclic acid (1:1) and the disodium salt of 4,5-dihydroxy-2,7-nap hthalene disulphonic acid (1::1) Other 1,3-diketones, 1,3-diacid esters, imino diacetic acids and 1-hydroxy-2-carboxyl benzene derivatives are also agents which when complexed with ferric ion are expected to be useful in this invention.

Examples of complexes with other metal ions having the required oxidation potential are those formed by reacting:~ cupric ion with nitrilotriacetic acid (1:1); cupric ion with disodium salt of ethylene diaminetetracetic acid (1:1); cupric ion with hexametaphosphate (1:1); ceric ion with propan-2,4-dione (1:3); ceric ion with disodium salt of 1 ,2-dihydroxybenzene 3,5-disulphonic acid (1:2); molybdenum Vl ion with propan-2,4-dione (1:3); molybdenum Vl ion with the disodium salt of 1 ,2-dihydroxybenzene 3,5-disulphonic acid (1:1); molybdenum Vl ion with the disodium sait of ethylenediamine tetracetic acid (2::1); cobalt Ill ion with propan-2,4-dione (1:3); cobalt Ill ion with ethylenediamine tetracetic acid (1:1); cobalt Ill ion with the disodium salt of 1,2-dihydroxybenzene- 3,5-disulphonic acid (1:2); manganese Ill ion with propan-2,4-dione (1:3); and manganese ill ion with the disodium salt of ethylenediamine tetracetic acid (1:1).

Preferred onium salts for use in the composition of the invention are N onium salts, more preferably derived from heterocyclic bases such as pyridinium or quinolinium salts and preferably + free from steric hindrance around the N atom. These compounds as well as having lipophilic characteristics also have a capacity to limit the extent of the oxidation of the silver image by interacting with the oxidised surface to prevent further oxidation of the silver image.

A fix composition of the present invention renders the silver image sufficiently oleophilic for printing; the silver image thus does not require further treatment to increase its oleophilicity.

The fix composition of the present invention also does not need to contain an organic solvent.

Some pyridinium salts useful in the present invention are represented by formula I:

wherein R' is an alkyl, substituted alkyl, aryl or substituted aryl group and Xe is an anion such that the salt is soluble in the composition. Preferably R' is benzyl. Preferably X9 is Cle, le, Brs or pT5e.

Some quinolinium salts useful in the present invention are represented by formulae II and Ill:

wherein R1 and X# are as defined for R1 in formula I. Preferably R1 is methyl, ethyl, hydroxyethyl.

Preferably X# is l#, Br#, CI# or pTS#.

Examples of other useful pyridinium and quinolinium salts are described in Specification 1,406,030 in Examples 8 to 13 viz, (8) -ethyl-4-cyanoquinolinium iodide (9) -ethyl-4-ethoxycarbonyl quinolinium iodide (10) 2-methyl-isoquinolinium iodide (11) cetyl pyridinium bromide (12) l-ethyi-quinolinium iodide (13) 1 -ethyl-2-methyl-quinolinium iodide Oxonium and sulphonium salts are examples of other onium compounds that can also be used to restrain oxidation.

The ferric EDTA salt-containing compositions useful in this invention include those in which the EDTA is present in at least equimolar proportion to the ferric salt preferably not exceeding the ferric salt by more than 15% in molar concentration. Other ferric EDTA salts are those which can be used in bleaching silver such as are described in U.S. Patent No. 3,61 5,508.

The cationic surfactant used in the present invention is preferably water-soluble. The surfactant may be a long chain amine, such as n-dodecylamine which is soluble and cationic in acidic composition.

Amines with longer chains are more lipophilic but are less soluble. They can be solubilised, for example by reaction with an alkyl epoxide such as glycidol. The following is a list of other surfactants useful in this composition.

ETHOXYLATED AMINES

n = x+y

*Compound n R Ethomeen C/15 5 012-14 Ethomeen C/25 15 C12-14 Catafor 06 6 alkyl Ethomeen 18/20 10 C", Ethomeen 18/25 15 C18 Ethomeen 18/60 50 C18 Ethomeen 0/12 2 C18U Ethomeen 0/15 5 C18U Ethomeen S/25 \ 15 Ciauu each U means an unsaturated bond in the chain.

ETHOXYLATED DIAMINES

n = x+y+z *Compound n R 18-18 Ethoduomeen T/1 3 10 C1#8 Ethoduomeen T/20 \ 15 \ C16-18 \ Ethoduomeen T/25 CCH#H t ETHOXYLATED QUATEI XL l 2 x+v Ethoquad 18/25 15 C18 1 #HYDROXYETHYL-2-ALKYL IMIDAZOLINES

*Compound R Crodazoline CY C8 Crodazoline C C,O-,8 Crodazoline O C18 QUATERNIZED DERIVATIVES Loraquat M5040 Loramine lM-BT alkyl Bitran TC MISCELLANEOUS *Compound

Other useful cationic compounds are described in Specification No. 1,406,030 on pages 2 and 3 under the designation "compound (a)".

The pH of the fix composition of the invention may be from about 1.5 to about 7 and is preferably between about 1.7 and about 6. The solution may be buffered at the desired pH and an acid preferably an organic acid such as, for example, citric acid, lactic acid or acetic acid, may be used for this purpose.

Amounts of acid between 5 and 150 grams per litre are useful. In a preferred embodiment, about 5 to about 40 grams per litre of an organic acid are used.

The iodide ions used in the present invention are preferably supplied from alkali metal salts; such as, for example, potassium or sodium iodide, preferably potassium iodide; however, other iodide salts can be used as well as any compounds which provide iodide ions including organic materials which dissassociate in aqueous solution.

The following Examples 1 and 4 to 10 are examples of the present invention.

EXAMPLE 1 A silver diffusion transfer image was prepared on a polyethylene coated paper receiver coated with a hydrophilic lithographic layer containing silver precipitating nuclei in a manner analogous to that of Example 1 in U.K. Specification No. 1,406,030. This receiver was then treated with the following etch composition of the invention, which was made by dissolving the components in order in the water, and was applied evenly with a cotton wool pad.

Na Fe"'EDTA 40 g Citric Acid 80 g Ethyl quinolinium iodide 20 g goArmeen 12D 2g Demineralised water 1000 ml pH 2 (eArmeen 1 2D is a product of Akzo Chemie and is a distilled grade of n-dodecylamine).

For long chain amines (such as Armeen 12D) and solubilized amines it is essential that the etch composition be acidic so that these surfactants become positively charged. The citric acid buffer maintains the etch acidic even in the presence of reasonable quantities of alkaline solution used in the preparation of imaged diffusion transfer receivers.

The printing plate thus formed was then put on to a lithographic printing press and moistened with water. The cylinder carrying the printing plate was then rotated 5 times in contact with the moist inking rollers, and then a further 5 times while also contacting the rubber offset blanket. After this paper was fed through the printing press in the normal way, and clean sharp copies were immediately obtained.

EXAMPLE 2 This Example provides a comparison for the result obtained in Example 1. Example 1 was repeated excepting that the Armeen 12D was omitted from the etch formulation. The resulting printing plate was used on the lithographic printing press of Example I, in an analogous manner, and very poor copies resulted, with negligible in king of the image areas. This unacceptable result demonstrates that a secondary oleophile such as Armeen 12D is essential to the invention.

EXAMPLE 3 This Example provides a comparison for the result in Example 1. Example 1 was repeated excepting that the ethyl quinolinium iodide was omitted from the formulation and replaced by potassium iodide (12 g) so as to maintain the iodide concentration in the solution. In this case the silver was rapidly oxidised until all of the silver had been converted to the iodide. When tested on the printing press as in Example 1 negligible inking of the image occurred and very poor copies were produced. The silver iodide image also rapidly wore away. Although the over-etching could be controlled by reducing the concentration of iodide ion in the etch, the rate of etching then became unacceptably slow. Overetching could be prevented by limiting the quantity of etch applied to the plate. However this was not a practical solution to the problem.It is therefore apparent that a compound similar to ethyl quinolinium iodide is essential to the invention for two reasons: Firstly it prevented over-etching of the silver image on the plate; Secondly in combination with a secondary oleophile such as Armeen 12D it allowed satisfactory inking of the printing plate and enabled a large number of good copies to be printed.

EXAMPLES 4 to 8 Example 1 was repeated, but with the ethyl quinolinium iodide replaced in turn by the quinolinium iodide derivatives listed in Table 1. The printed results were obtained in a slightly different way to those of Example 1. After moistening the plate with water the moist inking rollers were made to contact the plate and paper was immediately fed through the printing press. In this way the rate of inking of the image and the quality of the inked image could be subjectively assessed. All of the quinolinium derivatives listed in Table I gave satisfactory results, whereas Example 4 (no added derivatives) did not.

Quinolinium Rate of Degree of Example Derivatives Etching Etching 4 None Very fast Overetched 5 Methyl Fast Slightly overetched 6 Ethyl Satisfactory Optimal 7 Hydroxyethyl Fast Optimal 8 8 Pentyl Satisfactory Optimal Other important parameters were found to be the rate of etching and the degree of etching: that is, whether the etching was satisfactorily limited or continued until the whole of the silver image had been oxidised. The process of the etching can be followed by the colour of the image which progresses from grey through brown to pale yellow.

EXAMPLE 9 Example 1 was repeated but with the ethyl quinolinium iodide replaced by an equimolar quantity of benzyl pyridinium chloride. The iodide concentration in the solution was maintained by adding potassium iodide (12 g). Clean, sharp printed copies were obtained from the printing plate thus obtained.

EXAMPLE 10 Hexadecylamine was made more soluble in aqueous solution by reacting it in a manner described in Research Disclosure 19156 March 1980.

Two grammes of the resulting oil were used, without purification, in place of Armeen 12D in Example 1. In this manner good clean sharp copies were obtained.

Claims (10)

1. An aqueous lithographic fix composition which contains an acid, iodide ions, one or more organic cationic compounds containing a lipophilic group, one or more heterocyclic onium salts and one or more stable, silver-oxidising compounds having an oxidation potential which is more positive than the potential of a silver iodide/silver electrode which is given by the expression~150 mV -59 log io [ I# ] .

2. A fix according to claim 1 wherein said silver oxidising compound is a sodium salt of a complex of ferric iron with ethylenediamine tetracetic acid.

3. A fix according to claim 2 wherein said acid is present in a proportion from equimolar with respect to the ferric iron to 15% more than equimolar.

4. A fix according to claim 1,2 or 3 wherein said onium compound is a pyridinium or quinolinium + salt in which the N onium atom is free from steric hindrance.

5. A fix according to any of claims 1 to 4 wherein said cationic compound is a salt of a long chain amine.

6. A fix according to claim 5 in which said amine is n-dodecylamine.

7. A fix according to any of claims 1 to 4 in which said cationic compound is an amine which has been rendered soluble in the fix by reaction with an alkyl epoxide.

8. A fix according to any of claims 1 to 7 which has a pH of from 1.7 to 6.

9. A fix according to claim 1 substantially as described in any Example selected from Example 1 and Examples 4 to 10.

10. A fix according to claim 1 substantially as herein described.