GB2107889A - Lithographic image deletion fluid - Google Patents

Abstract

A lithographic image deletion fluid comprises a solution in a dipolar aprotic solvent, or a solvent mixture containing such a solvent, of a silver bleaching agent and a thickening agent. The solvent or solvent mixture, which is capable of dissolving oleophilic substances not adequately dissolved by previously proposed deletion compositions, has values of fp and fh of greater than 32 and less than 25, respectively, where fp is the percentage contribution of polar forces, and fh the percentage contribution of hydrogen bonding forces, to the solubility parameter ( delta ). A preferred fluid contains an iodine- iodide silver bleaching agent dissolved in a gamma -butyrolacetone-water mixture.

Description

SPECIFICATION Lithographic image deletion fluid This invention relates to a fluid composition for deleting silver images from lithographic printing plates.

A known photographic method of making lithographic printing plates comprises forming a silver image on a hydrophilic support by a diffusion transfer process and treating that image with an appropriate solution to render it hydrophobic, and so ink-receptive. When the support is of metal, often grained aluminium, the silver is treated with a solution of a compound containing one or more groups with an affinity for silver (such as sulphur-containing or quaternary ammonium groups) and one or more oleophilic groups, such a solution commonly being called a 'fix'. Lithographic fix compositions containing more than one active constituent have also been suggested (see, for example, United Kingdom patent specification 1 ,27 1 225).

When the support consists of a hydrophilic layer coated on, for instance, paper or a polymeric film support, the silver image is produced within the hydrophilic layer, and it has been found desirable to convert part of the image silver to a more bulky compound (for instance, silver iodide), as well as treating the image with at least one compound which renders it ink-receptive. A solution which has these two functions is termed an 'oxidising fix', or an 'etch', and commonly contains a substance which oxidises the silver to a silver salt of low solubility, and also an ink-receptive compound which reacts with, or adsorbs to, that silver salt (see, for example, United Kingdom patent specification 1,406,030).

An ink-receptive image formed as outlined above may be lacquered to improve its ink-receptivity, to protect it during storage and to increase its printing life (see, for instance, United Kingdom patent specification 1,227,205).

From the above brief account of the preparation of lithographic printing plates by a silver-salt diffusion transfer process, it is evident that the printing image contains silver and at least one oleophilic substance combined with or adsorbed to silver, and may also contain an insoluble silver salt, such as silver iodide. Furthermore, the image may be covered with a lacquer and, if it has been used, will be covered with lithographic printing ink. If, therefore, it is desired to correct a printing image by deleting a part of it chemically, it is desirable to have a fluid composition which can be applied locally and which will dissolve all these materials.

Deletion compositions containing a silver bleach and organic solvents, to dissolve oleophilic materials, including ink, have already been proposed (see, for example, United Kingdom patent specification 1,549,774) but these compositions do not adequately dissolve all the components of images produced using certain etch solutions. If any ink-receptive material is left on the hydrophilic support in the image areas to be deleted, then even if the image silver has been completely bleached away from those areas so that they are no longer visible, they will still be inked on the press.

An aqueous lithographic deletion composition for silver images will, of course, bleach any photographic silver image, and is often used for correcting proof images produced by diffusion transfer in gelatin-coated receiver layers.

The present invention provides a lithographic deletion fluid which contains a silver bleaching agent and a thickening agent dissolved in a solvent, or solvent mixture, which satisfies the criteria described below.

The constituents of the deletion fluid will now be described in turn.

a) The solvent or solvent mixture The solvent is, or the solvent mixture contains a dipolar aprotic solvent, that is to say an organic compound which has a high dipole moment (above approximately 2.5 debyes) and which is no more than a very weak hydrogen bond donor, all the hydrogen atoms in the molecule being bound to carbon atoms. A dipolar aprotic solvent suitable for use in this invention is one which has a dipole moment of at least 3.0 debyes and preferably at least 3.4 debyes.

The solvent power of a solvent may conveniently be considered with the aid of the concept of the 'solubility parameter' (S) the square root of the 'cohesive energy density' which latter is the energy required to evaporate 1 cc of the solvent. The solubility parameter is expressed in 'hildebrand' units.

The solubility parameter can be split into components due to dispersion forces (#d), polar forces (Sp) and hydrogen-bonding (and similar associative) forces (h), the values of these components satisfying the equation: a2=(~d)2+(~p)2+(~h)2.

Values of ad, ap and 8h have been tabulated for numerous solvents* The solvent characteristics of a solvent, or solvent mixture, are largely governed by the relative values of bd, bp and# aht and Teas proposed a convenient plotting method in which the three components of the solubility parameter are expressed as percentages of their total (fd, fp and fh, where *Kirk-Othmer 'Encyclopaedia of Chemical Technology' (1 971) pp 892 to 5.

f,=1006d(S,+6,+6,) and so on) and then plotted on a triangular graph. The points for solvents having similar solvent power fall within a limited region in such a graph and it is often possible to draw, for a particular solute, a boundary in the graph defining a 'solubility domain', solvents within the boundary dissolving the solute and those outside not doing so. The 'solubility parameter' concept has been applied to a number of problems in lithography (J. H. Boielle, Proceedings of the Institute of Printing, 16 2-8 (1972)).

Solvents which have been found to be satisfactory for inclusion as the sole solvent in a composition of the invention intended for use with metal plates are listed in Table 1 , which gives their solubility parameters (S), the components adt Sp #and bh of these parameters, the corresponding percentages fd, fp and fh used for triangular plotting, and in most cases the dipole moment.

Table 1 Dipole moment Solvent 8 zd 8p zh fd fp fh (debyes) Acetonitrile 11.9 7.5 8.8 3.0 38.9 45.6 15.5 3.9 Nitromethane 12.3 7.7 9.2 2.5 40 48 12 3.4 Dimethylsulphoxide 12.9 9.0 8.0 5.0 40.5 36 23.5 4.0 Diethylsulphate 11.1 7.7 7.2 3.5 41.8 39.1 19.0 Acrylonitrile 12.2 8.0 8.5 3.5 40 42.5 17.5 3.8 Ethylenecarbonate 14.5 9.5 10.6 3.5 42.0 46.9 11.1 4.9 y-Butyrolactone 12.8 9.3 8.1 3.6 44.3 38.6 17.1 4.1 2-Furaldehyde 11.9 9.1 7.3 2.5 48.1 38.6 13.2 3.6 Nitroethane 11.1 7.8 7.6 2.2 44.3 43.2 12.5 3.6 Butyronitrile 10.0 7.5 6.1 2.5 46.6 37.9 15.5 4.0 Propylenecarbonate 13.3 9.8 8.8 2.0 47.6 42.7 9.7 5.0 N-methyl-2-pyrrolidone 11.2 8.8 6.0 3.5 48 32.5 19.5 4.1 2-Nitropropane 10.0 7.9 5.9 2.0 50 37.3 12.7 3.7 Nitrobenzene 10.6 8.6 6.0 2.0 51.8 36.1 12.0 4.3 Corresponding values for various other solvents some of which are not satisfactory on their own but can be used in admixture with an appropriate dipolar aprotic solvent are given below.

Table 2 Dipole moment Solvent 8 8d Sp 8h fd fD fh (debyes) Water 23.5 6.0 15.3 16.7 15.8 40.3 43.9 1.8 Acetone 9.77 7.58 5.1 3.4 47.5 31.5 22.5 2.9 Cyclohexanone 9.88 8.65 4.1 2.5 56.6 26.5 16.5 2.7 Benzonitrile 9.7 8.5 4.4 1.6 59 30 11 4.4 Dipropylene glycol 15.5 7.8 9.9 9.0 29.2 37.1 33.7 Ethanolamine 15.5 8.4 7.6 10.4 31.8 28.8 39.4 2.3 Diethylenetriamine 12.6 8.2 6.5 7.0 37.8 30.0 32.2 N-N-dimethylacetamide 11.1 8.2 5.6 5.0 43.6 29.8 26.6 3.7 From the data in Table lit is evident that the satisfactory solvents have a solubility parameter (h) of at least 10, component (6p) being at least 5.5 and the hydrogen bonding component (h) being from 2 to 5.The satisfactory solvents all lie in a region of the triangular plot for which fp is greater than 32 and fh is less than 25. The solvents of Table 1 all lie within a triangular area bonded by the lines for fd=39, fp=32 and fh=9.

If a solvent mixture is used, instead of a single dipolar aprotic solvent, then the mixture should have values for the percentage contributions of dispersion, polar and hydrogen-bonding forces which lie within the specified region of the triangular plot. This usually results in the mixture containing at least 50% by weight of aprotic solvent. The values for the mixture can normally be calculated with sufficient accuracy by adding fractions of the solubility parameter components for the individual solvents which correspond to the proportions by weight of those solvents in the mixture and then exposing the mixture 'solubility parameter components' obtained as percentages of their total.

A satisfactory solvent or solvent mixture can be selected by experimental trial using the testing technique described in Example 1 in which a silver image formed by diffusion transfer process in a hydrophilic layer containing polyvinyl alcohol is treated with an etch solution containing a quinoxalinium compound and an organic cationic compound containing a lipophilic group dissolved in an aqueous acidic solution also containing iodide ions. This etch solution produces a hydrophobic image substance which is barely, if at all, soluble in previously proposed organic solvent-containing deletion fluids. The solvent, or solvent mixture, being tested does not have to be embodied in a deletion composition because if it can dissolve the hydrophobic material formed on the silver image, it will render the image hydrophilic, the silver not being sufficiently hydrophobic to accept ink.Thus inking the plate will show whether or not the solvent has been effective. It is necessary in practice for a deletion fluid of the invention to bleach silver so that the user can see what portions of an image have been deleted before the plate is inked.

For images produced near the surface of the hydrophilic layer, as on metal printing plates, any appropriate solvent or solvent mixture which can dissolve the silver bleach and thickener can be employed for the deletion fluid. For silver images produced within a hydrophilic colloid layer, such as a gelatin layer (as in 'Kodak' Instafax 'Proofmaster' paper, 'Kodak' 'Instafax' and 'Proofmaster' are trade marks), a solvent mixture including some water must be used so that the layer is swollen sufficiently to allow the bleach to diffuse to the image silver.

The solvent, or solvent mixture, should not be volatile so that it will remain, and not lose its solvent power, on the plate during use. Each solvent present preferably has a boiling point in the range 90--21 OOC. Ethanol and, especially, acetone are not fully satisfactory in this respect having boiling points of 78.5 and 56.5 C, respectively.

The solvent or solvent mixture preferably dissolves any lacquer, and any ink, which has been applied to the image. Dipolar aprotic solvents tend, in general to be suitable for dissolving lacquers but may not readily dissolve all lithographic inks, especially if these had had time to oxidise. Where the deletion fluid is to be used with images treated with ink which is hard to dissolve, it may be practicable to incorporate in the fluid a dispersion of an excellent ink solvent, such as white spirit, made with the aid of a surface active agent. Alternatively, the plate may be de-inked with an ink-solvent before image deletion is carried out.

A very satisfactory solvent mixture consists of y-butyrolactone (b.pt. 2060C) and water. This will dissolve difficulty-soluble hydrophobic substances produced by fix and etch solutions and will swell hydrophilic layers. It is preferred for the mixture to contain at least 60% by weight of 1 -butyrolactone.

b) The silver bleach Because a composition of the invention contains a dipolar aprotic solvent, possibly in admixture with one or more other solvents, it has to contain a silver bleach which is soluble in the chosen solvent or solvent mixture. The silver bleach normally comprises a silver oxidizing agent and-a silver ion complexing agent. Examples of suitable silver oxidizing agents are: benzoquinone ferric acetyl acetonate iodine phenazine quinoxaline.

The oxidizing agent chosen should not be so powerful as to oxidize the silver ion complexing agent during normal storage of the composition. Examples of suitable silver ion complexing agents are: ammonium and alkali metal bromides, iodides and thiocyanates; urea, certain organic thioethers (see U.K. patent specification 765,053).

Thiosulphates tend to have inadequate solubility and stability. The nature and concentration of the bleach have to be such as to give the required rate of bleaching, a bleaching time of less than one minute being very desirable.

In the preferred j'-butyrolactone-water mixture, an iodine-iodide mixture gives very satisfactory bleaching.

c) The thickener For convenient application, a deletion fluid of the invention has to be viscous enought not to flow into image areas adjacent to those to which it is applied. The compositions should be applicable with, for example, a small brush. It is therefore necessary to include a viscosity increasing agent in the fluid which is stable therein. Suitable thickeners include certain cellulose derivatives, such as hydroxypropyl cellulose, and other water-soluble polymers having good stability in organic solvents such as certain acrylic acid polymers (e.g. 'Carbopol'-trade mark7-resins).

The invention is illustrated by the following Examples.

Example 1 For testing the deletion of images formed on metal foil, images were prepared as follows.

Sheets of sulphuric acid-anodized aluminium foil were treated in phosphoric acid (10% w/v) for 4 minutes, washed in demineralized water, dried and then dipped in a nucleating bath of the composition (see U.K. patent specification 1,561,378): colloidal silver solution (prepared by the Carey Lea method) 654 ml mild silver protein (B.P.C.) 8.45 g distilled water 4320 ml 'Lissapol N' (trade mark-a non-ionic surfactant sold by Rohm and Haas) 10 ml The excess was removed with an air knife, and the sheets were then dried. Images were produced on the sheets by-exposing a layer of a contact-speed developer-incorporated silver chloride negative material and contacting this with the nucleated sheet when wet with a developer solution containing a silver ion complexing agent.This developer solution was (see U.K. patent specification 1,111,684) hydroquinone 15g 1-phenyl-3-pyrazolidone 1 g sodium hydroxide 13 g sodium sulphite 75 g sodium thiosulphate 5H20 8g potassium bromide 0.35 g carboxymethylhydroxyethyl cellulose (grade 37) 5 g water to 1 litre The silver images obtained were rendered hydrophobic by treatment with a fix solution containing 1phenyl-5-mercaptotetrazole (1.8 girl) and cetyltrimethylammonium bromide (4 g/l) in a water-organic solvent mixture thickened with carboxymethyl cellulose.

For testing the deletion of images formed on paper plates, images were prepared as follows.

Polyethylene coated paper subbed with chrome alum-hardened gelatin was provided with a hydrophilic surface by coating with a composition containing (see U.K. patent specification 1,419,511, Examples 1 and 3): fumed silica (average unit diameter 1200 nm 6% dispersion in water) 64 cc mild silver protein (10% solution in water) 2.8 cc polyvinyl alcohol (6.7% solution in water) 30 cc colloidal silica (average diameter 2000 nm, 30% solids) 7 cc tetraethyl orthosilicate hydrolyzed 8 cc Images were formed in the coating using the silver chloride negative material and the developer solution used for Example 1.These images were then rendered hydrophobic by treatment with the following etch solution methyl quinoxalinium p-toluenesulphonate 10 g potassium iodide 10 g 'Ethomeen Tri 5' (trade mark for amine of tallow oil solubilized by reaction with five moles of ethylene oxide per mole ofamine) 2 g glacial acetic acid 20 ml water to 1 litre adjust pH to 4 with sodium acetate.

Both kinds of test image were tested both as prepared and after lacquering with a lacquer comprising a solution of red-pigmented epoxy resin in cyclohexanone dispersed in an aqueous gum arabic solution.

A deletion solution of the invention was tested as described below.

A deletion solution was made of the following composition.

y-butyrolactone 799 g water 125 g carboxypolymethylene ('Carbopol 934', trade mark, from B.F. Goodrich) 12.9 g ammonium iodide 166g iodine 8.3 g This solution deleted, within 30 to 60 seconds, both unlacquered and lacquered images formed on aluminium foil and coated paper prepared as described above. After bleaching the plates were wiped with a pad wetted with water.

The solubility parameter components, expressed as percentages, for this preferred solvent mixture, are as follows: fp 38.0 39.0 23.0 These fall within the specified region of the triangular plot.

It should be possible to vary the concentrations of the constituents in the above deletion solution within the following ranges, provided that the solvent mixture chosen has appropriate solubility parameters:iodine 0.5 to 80 grams/litre; ammonium iodide 20 to 500 grams/litre, depending on the iodine concentration; water, 500 to 1000 grams/litre of butyrolactone, and thickener 0 to 35 grams/litre. If an ink solvent is dispersed in this, or some other, solution of the invention, from 2 to 20% by weight is a suitable concentration, being dispersed with the aid of from 0.25 to 10% of a surfactant (e.g. 'Zonyl A'. trade mark, from DuPont).

Silver bleach solutions prepared as described in U.K. patent specification 1,549,774, Examples 4 and 5 satisfactorily bleached the test images formed on aluminium foil but did not bleach those formed in paper plates.

Example 2 A deletion solution of the invention was prepared from the following: propylene carbonate 425 ml dimethyl sulphoxide 425 ml water 150 ml iodine 10 g potassium iodide 200 g hydroxypropyl cellulose ('Klucel H'-trade mark) from Hercules Inc. 7.5 g This solution satisfactorily bleached images on aluminium foil and coated paper (both unlacquered and lacquered) prepared as described in Example 1 above. Bleaching was complete in less than one minute.

The solubility parameter components, expressed as percentages, for the solvent mixture are as follows.

4 38.1 39.7 22.2 Example 3 A deletion solution was prepared having the following composition: y-butyrolactone 850 ml water 150 ml ferric acetyl acetonate 250 g potassium thiocyanate 250 g acetic acid, glacial 20 ml hydroxypropyl cellulose ('Klucel H'-trade mark) 7.5 g This solution deleted the unlacquered and lacquered test images on aluminium and coated paper prepared as described in Example 1 above, but was less convenient than the solutions of Examples 1 and 2 because it was highly coloured.

Claims (filed 13.10.83,' 1. A lithographic deletion fluid which comprises a solution in a solvent or solvent mixture of a silver bleaching agent and a thickening agent, the solvent being, or the solvent mixture containing, a dipolar aprotic solvent having a dipole moment of at least 3.0 debyes, and the solvent or solvent mixture having a percentage contribution of the polar forces (fp) to the solubility parameter () greater than 32 and a percentage contribution of the hydrogen-bonding forces (fh) to the solubility parameter less than 25, the values of fp and fh being calculated, in the case of a solvent mixture, using solubility parameter component values obtained by adding fractions of the solubility parameter components for the individual solvents which correspond to the proportions by weight of those solvents in the mixture.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. This solution deleted, within 30 to 60 seconds, both unlacquered and lacquered images formed on aluminium foil and coated paper prepared as described above. After bleaching the plates were wiped with a pad wetted with water. The solubility parameter components, expressed as percentages, for this preferred solvent mixture, are as follows: fp 38.0 39.0 23.0 These fall within the specified region of the triangular plot. It should be possible to vary the concentrations of the constituents in the above deletion solution within the following ranges, provided that the solvent mixture chosen has appropriate solubility parameters:iodine 0.5 to 80 grams/litre; ammonium iodide 20 to 500 grams/litre, depending on the iodine concentration; water, 500 to 1000 grams/litre of butyrolactone, and thickener 0 to 35 grams/litre. If an ink solvent is dispersed in this, or some other, solution of the invention, from 2 to 20% by weight is a suitable concentration, being dispersed with the aid of from 0.25 to 10% of a surfactant (e.g. 'Zonyl A'. trade mark, from DuPont). Silver bleach solutions prepared as described in U.K. patent specification 1,549,774, Examples 4 and 5 satisfactorily bleached the test images formed on aluminium foil but did not bleach those formed in paper plates. Example 2 A deletion solution of the invention was prepared from the following: propylene carbonate 425 ml dimethyl sulphoxide 425 ml water 150 ml iodine 10 g potassium iodide 200 g hydroxypropyl cellulose ('Klucel H'-trade mark) from Hercules Inc. 7.5 g This solution satisfactorily bleached images on aluminium foil and coated paper (both unlacquered and lacquered) prepared as described in Example 1 above. Bleaching was complete in less than one minute. The solubility parameter components, expressed as percentages, for the solvent mixture are as follows. 4 38.1 39.7 22.2 Example 3 A deletion solution was prepared having the following composition: y-butyrolactone 850 ml water 150 ml ferric acetyl acetonate 250 g potassium thiocyanate 250 g acetic acid, glacial 20 ml hydroxypropyl cellulose ('Klucel H'-trade mark) 7.5 g This solution deleted the unlacquered and lacquered test images on aluminium and coated paper prepared as described in Example 1 above, but was less convenient than the solutions of Examples 1 and 2 because it was highly coloured. Claims (filed 13.10.83,'

1. A lithographic deletion fluid which comprises a solution in a solvent or solvent mixture of a silver bleaching agent and a thickening agent, the solvent being, or the solvent mixture containing, a dipolar aprotic solvent having a dipole moment of at least 3.0 debyes, and the solvent or solvent mixture having a percentage contribution of the polar forces (fp) to the solubility parameter () greater than 32 and a percentage contribution of the hydrogen-bonding forces (fh) to the solubility parameter less than 25, the values of fp and fh being calculated, in the case of a solvent mixture, using solubility parameter component values obtained by adding fractions of the solubility parameter components for the individual solvents which correspond to the proportions by weight of those solvents in the mixture.

2. A deletion fluid according to claim 1 which comprises a solvent mixture which contains at least

50% by weight of the dipolar aprotic solvent.

3. A deletion fluid according to claim 1 or 2 wherein the solvent, or each solvent in the solvent mixture, has a boiling point in the range 90--21 OOC.

4. A deletion fluid according to claim 3 which comprises a mixture of fl-butyrolactone and water.

5. A deletion fluid according to any of the preceding claims which contains a dispersed ink solvent.

6. A deletion fluid according to any of the preceding claims wherein the silver bleaching agent comprises benzoquinone, ferric acetylacetonate, iodine, phenazine or quinoxaline as a silver oxidising agent and urea, an organic thiourea, or an alkali metal or ammonium bromide, iodide or thiocyanate, as a silver ion complexing agent.

7. A deletion fluid according to claims 4 and 6 wherein the bleaching agent comprises iodine and an alkali metal or ammonium iodide.

8. A lithographic deletion fluid according to claim 1 substantially as described in any of the Examples herein.