METHOD OF DEVELOPING LITHOGRAPHIC PLATES

Abstract: A developing fluid for positive-acting, negative-acting and reversible lithographic printing plates comprises a metasilicate as alkali together with an ethylene oxide/­ propylene oxide block copolymer and, optionally, sodium tetraborate, a phosphate ester, an anti-foaming agent and a water-softening or sequestering agent. The fluid has a high alkali content for reducing the need for frequent replenishment whilst at the same time minimising damage to the image particularly in negative and reversed plates and avoiding unacceptable foaming and turbidity.

Description: The invention relates to a method of developing lithographic printing slates using aqueous alkaline developing fluids. Whilst the method of the invention has application in the development of positive plates, such as those having a light-sensitive coating comprising an orthoquinone diazide, and negative plates, it may also be used in the preparation of negative images on reversible plates such as are described in British Patent No 2082339. In the post-exposure development of lithographic printing plates, developing fluid becomes exhausted by chemical reaction as developments proceeds, and the fluid needs to be replaced or replenished from time to time. Because this step involves an interruption of the operation, and because this interruption is particularly irksome when development is carried out in an automatic plate processing machine, it is desirable that the fluid shall have as high an initial alkali content as practicable, so that it will not exhaust so quickly by chemical reaction. However, it has been found that merely increasing the alkali content, that is to say without at the same time introducing other changes to the fluid, results in damage to substrates and images thereon, and particularly to the negative image produced on reversed plates, and it is an object of the present invention to mitigate these problems. According to the invention there is provided a method of developing exposed lithographic plates, using a fluid which comprises between 5 and 30% by weight of a dissolved salt which confers alkaline characteristics on the fluid, and between 0.01 and 10% by weight of an ethylene oxide/propylene oxide block copolymer. The dissolved salt which confers alkaline characteristics on the fluid is preferably a silicate and particularly sodium metasilicate, and the copolymer is preferably one of a group comprising: (a) a copolymer formed between ethylene oxide, propylene oxide and ethylene diamine; (b) a copolymer wherein the terminal groups are ethylene oxide chains of the same or different length; and (c) a copolymer wherein the terminal groups are propylene oxide chains of the same or different length. The fluid may also comprise any one or more of disodium tetraborate, a phosphate ester, an amine ethoxylate, a water softening or sequestering agent and a dyestuff. The water softening agent or sequestering agent may be one of a group comprising (a) an organophosphonate and (b) an aminocarboxylic acid. Preferably the organophosphonate is sodium pentaethylenehexamine octakis (methylene phosphonate) or the aminocarboxylic acid is 1,3-diamino-2-propanol-N,N,N',N'-tetra acetic acid. The invention will be exemplified by reference to two Examples of fluids which have been found to be particularly effective. Example 1 An aqueous developer fluid was compounded of the following ingredients, expressed in percentage by weight: Sodium metasilicate 15% Borax 2% Triton H66 (T.M. of Rohm & Haas) 2.5% Synperonic T/304 (T.M. of I.C.I.) 0.2% Triton CF-32 (T.M. of Rohm & Haas) 0.02% Cibacron Golden Yellow 2R 0.0036% Cibacron Red 6B 0.0004% Briquest 8106/25S (T.M. of Albright and Wilson) 1.0% Where Triton H66 is a 50% aqueous solution of a phosphate ester, Synperonic T/304 is 100% liquid block copolymer, Triton CF-32 is a 95% aqueous solution of an amine ethoxylate, and Briquest 8106/25S is a 25% aqueous solution of Sodium pentaethylene hexamine octakis (methylene phosphonate). The composition described achieves a balance, not to be expected from the known properties of the individual components, which provides a beneficial required increase in development capacity without unacceptably damaging the image and whilst avoiding other problems such as foaming. When non-preferred alkalis, commonly used in developers, were substituted for the silicate, either the substrate tended to be attacked or the plate coating was not completely developed. The efficacy of the new developer fluid was demonstrated by laboratory tests in which exposed positive lithographic plates were passed through developer until the fluid was exhausted. The table below shows the plate area developed per litre of fluid; the first four fluids being positive developers currently marketed, and the fifth a fluid according to the above exemplified formula. Developer Square Metres per Litre Horsell Hand Developer Type C1290 9.08 Horsell Machine Developer Type C1291 9.08 Horsell Developer Type C1331 10.9 Howson-Algraphy Posidev E 11.74 Improved Positive Developer 18.16 The range of acceptable proportions of the individual components of the fluid is particularly narrow; wide diversion from the values indicated leads to undesirable effects, such as an unacceptable level of attack on the image, the production of excess foam, the formation of a precipitate in hard water or the development of turbidity on storage. Example 2 The formula was identical to that of Example 1 with the replacement of the Briquest by 0.29% by weight of Rexpronal Acid (T.M. of W.R. Grace) which is a solid comprising 86% of 1,3-diamino-2-propanol-N,N,N',N'-tetra acetic acid. A developer according to this example was found to be effective with positive, negative and reversible plates.

Claims: A method of developing exposed lithographic plates, using a fluid which comprises between 5 and 30% by weight of a dissolved salt which confers alkaline characteristics on the fluid, and between 0.01 and 10% by weight of an ethylene oxide/propylene oxide block copolymer. A method as claimed in claim 1, in which the salt comprises a silicate constituting between 12.5 and 17.5% of the total weight, and the copolymer constitutes between 0.1 and 4.0 of the total weight. A method as claimed in claim 1 or claim 2, in which the salt is a metasilicate. A method as claimed in any one of the preceding claims, in which the salt is sodium metasilicate. A method as claimed in any one of the preceding claims, in which the copolymer is one of a group consisting of: (a) a copolymer formed between ethylene oxide, propylene oxide and ethylene diamine; (b) a copolymer in which the terminal groups are ethylene oxide chains of the same or different length; and (c) a copolymer in which the terminal groups are propylene oxide chains of the same or different length. A method as claimed in any one of the preceding claims, in which the fluid includes disodium tetraborate. A method as claimed in claim 6, in which the disodium tetraborate is present in the range 0.5 to 4% by weight. A method as claimed in any one of the preceding claims, in which the fluid includes a phosphate ester. A method as claimed in claim 8, in which the phosphate ester is present as a potassium salt. A method as claimed in any one of the preceding claims, in which the fluid includes an anti-foaming agent. A method as claimed in claim 10, in which the anti-foaming agent comprises an amine ethoxylate. A method as claimed in claim 11, in which the amine ethoxylate is present in the range 0.01 to 0.05% by weight. A method as claimed in any one of the preceding claims, in which the fluid includes a water softening agent or a sequestering agent. A method as claimed in claim 13, in which the water softening agent of the sequestering agent is one of a group consisting of (a) an organophosphonate, and (b) an aminocarboxylic acid. A method as claimed in claim 14, in which the organophosphonate is sodium pentaethylenehexamine octakis (methylene phosphonate). A method as claimed in claim 14, in which the aminocarboxylic acid is 1,3-diamino-2-propanol-N,N,N',N'-tetraacetic acid. A method as claimed in any one of the preceding claims, in which the fluid includes a dyestuff.

Inventors: RILEY, DAVID S., TURNER, GREGORY P.
Applicants: HORSELL GRAPHIC INDUSTRIES LIMITED
Application year: 1987
Main classification: G03F7
IPC codes: G03F7