Fount solutions for printing processes

Abstract: A fount solution which enables high quality prints free from background contamination to be produced, and which shows no tendency to corrode metal parts of printing presses, comprises (a) water; (b) an alkanoic acid; (c) an alkanolamine; (d) an optionally substituted derivative of benzoic acid; and (e) a corrosion inhibiting surfactant, which is typically an alkoxy, aryloxy or alkaryloxy carboxylic acid.

Description: The present invention relates to printing processes and in particular to planographic printing processes. Planographic printing plates, such as lithographic plates, comprise image regions and non-image regions which are essentially co-planar. The image regions are formed from a hydrophobic, oleophilic material to which greasy printing inks are attracted. The non-image regions are formed from a hydrophilic, oleophobic material from which the greasy printing ink is repelled. Thus, on application of the greasy printing ink to the printing plate, the ink is attracted to, and retained on, the image areas and repelled from the non-image areas. The ink can thus be transferred from the printing plate to the printing substrate to produce an image on the printing substrate corresponding to the image areas of the printing plate. The printing plate may, for example, be provided by photomechanical means wherein a hydrophilic substrate (such as a metal sheet) is coated with a layer of radiation sensitive material. The coating of radiation sensitive material is such that, on imagewise exposure of the coating to radiation, a solubility differential is provided between the exposed and non-exposed areas. Treatment of the coating with a suitable developer solution causes the more soluble areas to be removed to reveal the underlying substrate whilst the less soluble areas of the coating are retained on the substrate. Thus, the substrate forms the non-image areas and the coating which remains after exposure and development forms the image areas. A fount solution is conventionally used to assist in maintaining the hydrophilic properties of the non-image areas and to prevent scumming of the ink into the non-image areas. It is usual to use a polar liquid for this purpose, and water itself may perform satisfactorily as a fount solution for a short time. An aqueous solution including various performance enhancing additives is more commonly used as a fount solution. The performance of the fount solution is optimised to ensure that the solution is repelled by the image areas and is retained on and wets the non-image areas of the plate. Additives may also be used to control the interaction of the fount solution with the ink and the substrate. Known additives include aqueous electrolytes, surfactants and water-soluble polymers. Various methods have been employed to apply the fount solution to the printing plate. In many conventional lithographic presses, the means used to apply the fount solution (the dampening system) is entirely separate from the means used to apply the printing ink. In an example of such a method, the fount solution is transferred from a reservoir by a first roller, which is partially immersed in the fount solution, to a second ductor roller. The ductor roller transfers the fount solution (directly or indirectly) to the form rollers which contact the printing plate. The ductor roller oscillates between the first roller and the form rollers (or their precursors) so that contact with each is intermittent, whereby the amount of fount solution which is applied to the plate can be controlled. In alternative methods which attempt to avoid transfer of ink from the printing plate to the dampening system, the fount solution is transferred from the first roller to a brush roller. When rotating, the brush roller flicks droplets of the fount solution onto the form rollers or directly onto the printing plate. Similarly, nozzles can be used to spray a fine mist of fount solution onto the plate or the form rollers. In a further alternative method, the printing plate is contacted only by the inked form rollers. The fount solution must then be transferred from the dampening system to the printing plate via one or more inked rollers. Each of these alternative methods of application does, of course, involve the prolonged direct contact of parts of the lithographic press, such as rollers, nozzles and bearings, with the fount solution. In general, such parts are comprised of metal, especially steel and nickel plated steel, and it has been found that the fount solutions commonly in use are prone to attack these areas of the press, giving rise to corrosion of the various parts. In particular, areas of the press which comprise electroplated nickel are especially vulnerable to attack. The incidence of such corrosion is clearly undesirable, requiring regular and costly replacement of the various parts and, on occasions, resulting in termination of printing runs due to movement of the printing plates on the press, caused by the presence of ill-fitting corroded parts. The corrosion may be associated with any of a number of individual components of the fount solution. The present invention seeks to provide a fount solution which is free from such disadvantages and which may be employed in printing runs on lithographic printing presses comprising metal parts, in particular parts produced from steel and electroplated nickel, without giving rise to corrosion of these parts. The prior art describes a wide variety of corrosion inhibitors which are useful for a range of applications. Typical examples would be derivatives of benzimidazole, together with various phosphonates and phosphates. Many corrosion inhibitors are commercially available under trade names such as Preventol, Cortec, Korantin and Nalco. DE-A-2712900 refers to the use of alkanolamines, fatty acids and/or aromatic carboxylic acids as organic anti-corrosive components, and goes on to disclose formulations comprising aqueous solutions of alkali metal salts, ammonium salts and alkanolamine salts of aliphatic or aromatic monocarboxylic acids in combination with non-ionic surface active compounds and quaternary ammonium surface active compounds, said formulations being of value for cleaning metal surfaces, in particular metal parts in industrial finishing processes, by spray-cleaning methods. Said aliphatic or aromatic monocarboxylic acids include those acids having 6-12 carbon atoms, such as (alkyl)benzoic acids, (phenyl)alkanoic acids and phenylalkenoic acids. However, it has previously been found that many such commercially available corrosion inhibitors are unsuitable or ineffective when used in fount solutions which are employed on lithographic printing presses. The mode of action of the corrosion inhibitors is such that a protective film is formed around the material to be protected. Inevitably, such a film, as well as forming around the metallic parts of the printing press, will also surround the printing plate. In the latter case, the presence of this film, causes a reduction in the differential between hydrophobic image areas and hydrophilic non-image areas which provides the basis of the lithographic process. Thus, fount solutions of this type tend to suffer from either uniform excessive ink acceptance, in which case scumming of the background areas becomes apparent, or uniform inadequate ink acceptance, in which case image areas suffer from "blinding", resulting in poor image quality. In either case, the use of such fount solutions leads to totally unsatisfactory results during printing. The present invention seeks to provide a fount solution which, in addition to showing no tendency to corrode metal parts of printing presses, also enables high quality prints free from background contamination to be produced. Previous attempts by the manufacturers of commercial fount solutions to achieve these objectives involved the use of corrosion inhibitors which are less prone to promote such unsatisfactory printing behaviour and, to this end, solutions are available which incorporate triazole derivatives for this purpose, most notably benzotriazole and, preferably, tolyltriazole. Whilst the printing results observed with such founts are in some respects satisfactory, however, the degree of corrosion protection afforded by the inhibitors falls well short of the levels that would be desired; corrosion of vulnerable parts of the press comprising nickel plated steel, though reduced, is still significant and no protection is afforded to the non-plated steel press parts. The present inventors have now found that it is possible to achieve the dual objectives of a high degree of corrosion protection combined with high quality press performance by the formulation of a fount solution concentrate comprising a specific combination of components which interact synergistically to effectively inhibit corrosion. A working strength fount solution may then be obtained by dilution of this fount solution concentrate with water. According to one aspect of the present invention, there is provided a fount solution concentrate for a lithographic process comprising in admixture (a) water; (b) an alkanoic acid; (c) an alkanolamine; (d) an optionally substituted derivative of benzoic acid; and (e) a corrosion inhibiting surfactant comprising an (optionally substituted) alkoxy, aryl oxy or alkaryl oxy carboxylic acid or a mixture thereof. In preferred embodiments of the invention, the alkanoic acid is a C1-12 carboxylic acid, preferably a C6-10 carboxylic acid, and most preferably it is octanoic acid; the alkanolamine is a di or tri-alkanolamine, preferably a tri-lower alkanolamine, and, most preferably, it is triethanolamine; the optionally substituted derivative of benzoic acid is benzoic acid or a p-alkyl substituted derivative of benzoic acid, preferably a p-C1-8 alkyl substituted derivative of benzoic acid and, most preferably, it is p-tert-butyl benzoic acid; and the corrosion inhibiting surfactant comprises an alkoxy carboxylic acid having the general formula R(OC 2 H 4 ) n OCH 2 CO 2 H where R is an alkyl group and n is an integer between 1 and 30. In the most preferred cases R is a C6-8 alkyl group and n = 7 or, more especially, R is a C4-8 alkyl group and n = 5; these products are commercially available as Akypo LF4 and Akypo LF6, respectively (Akypo is a registered trademark of Chemische Fabrik Chem-Y GmbH). It is, of course, essential that the combination of components is soluble in aqueous media and, in this regard, the corrosion inhibiting surfactant, in addition to its function as a corrosion inhibitor, also acts as a hydrotrope and increases the aqueous solubility of the other components in the composition. In the above fount solution concentrates, the alkanoic acid is present in an amount of from 0.01% to 60%, preferably from 0.1% to 10% volume/volume, the alkanolamine is present in an amount of from 0.01% to 60%, preferably from 0.1% to 15% volume/volume, the optionally substituted derivative of benzoic acid is present in an amount of from 0.01% to 60%, preferably from 0.02% to 8% weight/volume, and the corrosion inhibiting surfactant is present in an amount of from 0.01% to 60%, preferably from 0.1% to 20% volume/volume. Further aspects of the present invention comprehend methods of lithographic printing employing a fount solution concentrate in accordance with the first aspect of the invention as a dampening agent. Still further aspects of the present invention relate to the use of a combination of an alkanoic acid, an alkanolamine, an optionally substituted derivative of benzoic acid and a corrosion inhibiting surfactant in fount solution concentrates for application in lithographic printing. By the incorporation of various additives to the concentrate the efficiency of the fount solution, in terms of maintaining the hydrophilic properties of non-image areas of printing plates and preventing scumming in these areas during printing operations on a press, can be greatly enhanced, and the corrosion inhibiting properties of the fount can be optimised. Thus, in addition to water (which is preferably demineralised), an alkanoic acid, an alkanolamine, an optionally substituted derivative of benzoic acid and a corrosion inhibiting surfactant the fount solution concentrate will preferably include: i) A wetting agent; this additive acts to prevent ink receptivity in the non-image areas and comprises an alkyl sulphate or alkanol containing between 6 and 20 carbon atoms, typically between 8 and 12 carbon atoms, preferred materials being n-ethyl hexyl sulphate, which is available commercially under the trade name Rewopol NEHS, and ethyl n-hexanol. The wetting agent is present in an amount of from 0.001% to 10%, preferably from 0.01% to 8% volume/volume. ii) A buffer system; the inclusion of a buffer system is particularly desirable and the system will preferably comprise a salt and an acid. A pH of about 5.5-6.0 is preferred and the salt and the acid are preferably each present in an amount of from 0.5% to 10% weight/volume. Careful control of the precise buffer formulation can, in fact, assist in achieving reduced corrosion. Suitable salts include trisodium citrate, disodium succinate, sodium glycolate, sodium acetate, sodium tartrate, sodium lactate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, sodium hydroxide and tripotassium citrate. Suitable acids include citric acid, succinic acid, phosphoric acid, lactic acid, tartaric acid, acetic acid, nitric acid and glycolic acid. Particularly preferred buffer systems comprise combinations of citric acid with either trisodium citrate or disodium succinate, and combinations of succinic acid with either trisodium citrate or disodium succinate. As an alternative to a combination of an acid and a salt, it is possible to utilise a buffer system comprising an alkane sulphonic acid containing an optionally substituted nitrogen-containing heterocyclyl ring and having the general formula where A represents the ring atoms necessary to complete a 5- or 6-membered heterocyclic ring which may optionally be substituted, and n is an integer between 1 and 20, preferably between 1 and 5. The compound is preferably present in an amount between 1% and 20%, most preferably between 5% and 15% weight/volume. Particularly preferred examples of such compounds are 2-N-morpholinoethane sulphonic acid and 2-N-morpholino-n-propane sulphonic acid. iii) A desensitising agent; this additive acts to promote water receptivity and prevent or reduce ink receptivity in the non-image areas and is preferably present in an amount of from 0.5% to 20% weight/volume. A preferred desensitising agent is dextrin, but other suitable agents include sodium carboxymethylcellulose, gum arabic, mesquite gum, guar gum, karaya gum and gum tragacanth. iv) A film former; suitable film formers include glycerine and sorbitol. The film former is preferably present in an amount of up to 20% volume/volume. v) A sequestrant; this additive acts to sequester any salts which might otherwise build up. Suitable sequestrants include borax, sodium hexametaphosphate (Calgon R) and salts of EDTA (ethylenediamine tetraacetic acid). Preferably, the sequestrant is present in an amount of up to 10% weight/volume. vi) A biocide; this additive serves to prevent biological spoilage in the solution. The choice is not particularly limited and an amount of up to 10% volume/volume may be included. Preferred biocides include isothiazolinone derivatives sold under trade names such as Bacteron B6, Kathon 886 and Bactrachem BF1 or BF2. General preservatives such as dimethoxane, phenol, sodium salicylate and the like can also be used. Other additives which may usefully be incorporated into these fount solutions include further surfactants acting as wetting agents, anti-foaming or defoaming agents and dyes, such as are generally known in the art. Typical examples include commercially available modified polyethoxylated alcohol non-ionic surfactants such as Triton DF12 and defoaming agents including Airex 900. The fount solutions of the present invention may be used on a wide variety of lithographic printing apparatus. Particular examples included those sold under the trade names Dahlgren, Roland, Miehlematic, Harris Duotron, Komorimatic, Alcolor and Millermatic. The formulations of the present invention are, as is customary in the art, supplied as fount solution concentrates which are diluted before use. All quantities in this specification refer to the fount solution concentrates and are based on the total compositions of the concentrates. The concentrates are diluted with water to form working strength solutions containing from 0.1% to 60% weight/volume of concentrate. The following formulations are illustrative of the fount solution concentrates of the present invention: Example 1 A formulation was prepared which comprised the following: Glycerine 5.0% v/v Disodium succinate 4.0% w/v Succinic acid 4.0% w/v Borax 2.0% w/v Nonanoic acid 3.0% v/v Triethanolamine (90% in demineralised water) 4.0% v/v p-tert-Butyl benzoic acid 1.0% w/v Rewopol NEHS 5.0% v/v Akypo LF4 5.0% v/v Gum Arabic 7.0% w/v Bacteron B6 3.0% v/v Triton DF12 0.2% v/v Airex 900 0.04 v/v the remainder being demineralised water and incidental impurities. One part by weight of the concentrate was diluted with 50 parts by volume of water to form a working strength fount solution. Example 2 A formulation was prepared which comprised the following: Glycerine 6.0% v/v 2-N-Morpholinoethane sulphonic acid 15.0% w/v Ethylenediaminetetraacetic acid 4.0% w/v Octanoic acid 0.5% v/v Triethanolamine (90% in demineralised water) 0.5% v/v p-tert-Butyl benzoic acid 0.3% w/v 2,2-Dimethylhexanol 0.4% v/v Akypo LF4 2.0% v/v Dextrine 7.0% w/v Bactrachem BF2 3.0% v/v Triton DF12 0.1% v/v Airex 900 0.1% v/v the remainder being demineralised water and incidental impurities. One part by weight of the concentrate was diluted with 50 parts by volume of water to form a working strength fount solution. Example 3 A formulation was prepared which comprised the following: Glycerine 5.0% v/v 2-N-Morpholinoethane sulphonic acid 11.0% w/v Borax 4.0% w/v Octanoic acid 0.55% v/v Triethanolamine (90% in demineralised water) 1.0% v/v p-tert-Butyl benzoic acid 0.3% w/v Rewopol NEHS 2.0% v/v Akypo LF4 2.5% v/v Starch 7.0% w/v Bactrachem BF2 3.0% v/v Triton DF12 0.025% v/v the remainder being demineralised water and incidental impurities. One part by weight of the concentrate was diluted with 50 parts by volume of water to form a working strength fount solution. Example 4 A formulation was prepared which comprised the following: Glycerine 5.0% v/v Trisodium citrate 0.035% w/v Citric acid 0.085% w/v Borax 4.0% w/v Octanoic acid 1.7% v/v Triethanolamine (90% in demineralised water) 3.0% v/v p-tert-Butyl benzoic acid 0.08% w/v Rewopol NEHS 6.0% v/v Akypo LF4 7.5% v/v Starch 7.0% w/v Bactrachem BF2 3.0% v/v Triton DF12 0.025% v/v the remainder being demineralised water and incidental impurities. One part by weight of the concentrate was diluted with 50 parts by volume of water to form a working strength fount solution. Example 5 A formulation was prepared which comprised the following: Glycerine 5.0% v/v Trisodium citrate 4.0% w/v Citric acid 2.0% w/v Calgon R 4.0% w/v Nonanoic acid 3.0% v/v Triethanolamine (90% in demineralised water) 4.0% v/v Benzoic acid 1.0% w/v Rewopol NEHS 2.0% v/v Akypo LF6 5.0% v/v Gum Arabic 5.0% w/v Bacteron B6 3.0% v/v Triton DF12 0.1% v/v the remainder being demineralised water and incidental impurities. One part by weight of the concentrate was diluted with 50 parts by volume of water to form a working strength fount solution.

Claims: A fount solution concentrate for a lithographic process comprising in admixture (a) water; (b) an alkanoic acid; (c) an alkanolamine; (d) an optionally substituted derivative of benzoic acid; and (e) a corrosion inhibiting surfactant comprising an (optionally substituted) alkoxy, aryloxy or alkaryloxy carboxylic acid or a mixture thereof. A fount solution concentrate as claimed in claim 1 wherein the corrosion inhibiting surfactant comprises an alkoxy carboxylic acid having the general formula R(OC 2 H 4 ) n OCH 2 CO 2 H where R is an alkyl group and n is an integer between 1 and 30. A fount solution concentrate as claimed in claim 2 wherein R is a C4-8 alkyl group and n = 5. A fount solution concentrate as claimed in claim 2 wherein R is a C6-8 alkyl group and n = 7. A fount solution concentrate as claimed in any of claims 1-4 wherein the alkanoic acid is a C1-12 carboxylic acid, the alkanolamine is a di-or tri-alkanolamine, and the optionally substituted derivative of benzoic acid is benzoic acid or a p-alkyl substituted derivative of benzoic acid. A fount solution concentrate as claimed in claim 5 wherein the alkanoic acid is a C6-10 carboxylic acid, the alkanolamine is a tri-lower alkanolamine and the optionally substituted derivative of benzoic acid is a p-C1-8 alkyl substituted derivative of benzoic acid. A fount solution concentrate as claimed in claim 6 wherein the alkanoic acid is octanoic acid, the alkanolamine is triethanolamine and the optionally substituted derivative of benzoic acid is p-tert-butyl benzoic acid. A fount solution concentrate as claimed in any of claims 1-7 wherein the alkanoic acid is present in an amount of from 0.1% to 10% volume/volume, the alkanolamine is present in an amount of from 0.1% to 15% volume/volume, the optionally substituted derivative of benzoic acid is present in an amount of from 0.02% to 8% weight/volume, and the corrosion inhibiting surfactant is present in an amount of from 0.1% to 20% volume/volume. A fount solution concentrate as claimed in any of claims 1-8 which additionally includes one or more of a wetting agent, a buffer system, a desensitising agent, a film former, a sequestrant and a biocide. A fount solution obtainable by dilution with water of a fount solution concentrate as claimed in any of claims 1-9. A lithographic printing process including the step of applying to a printing plate a fount solution as claimed in claim 10.

Inventors: ARCHER MARGARET, ARCHER, MARGARET
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT N.V.
Application year: 1995
Main classification: B41N3
IPC codes: B41N3