IMPROVEMENTS IN OR RELATING TO LITHOGRAPHIC PRINTING PLATES

Abstract: A lithographic printing plate having metallic silver image areas is treated with a proteolytic enzyme and an oleophilis­ ing compound to improve ink receptivity.

Description: This invention relates to the treatment of lithographic printing plates. As is well known a lithographic printing plate has ink-receptive (oleophilic) image areas and water-receptive (hydrophilic) non-image areas lying in substantially the same plane. The type of lithographic printing plate to which the method of this invention relates is that in which the image areas are formed from metallic silver. Such plates may be produced by the diffusion transfer process or the physical development process. In the diffusion transfer process, a silver halide emulsion layer is image-wise exposed to form a latent negative image which is developed in contact with a receiving layer containing development nuclei so that a positive metallic silver image is formed. Two systems are known, i.e. the two sheet system in which the silver halide layer and the receiving layer are on separate substrates and the silver halide diffuses from the negative layer to the positive layer when the two are placed in contact in the presence of a developer and the single-sheet system where both the negative silver halide layer and the positive receiving layer are coated onto the same substrate. In the physical development process, a silver halide emulsion layer is image-wise exposed to form latent image nuclei upon which silver is deposited to form the metallic silver image. The substrate used for the single sheet diffusion transfer process, the physical development process and the receiving layer of the two-sheet diffusion transfer process forms the non-image areas of the final printing plate. Thus the substrate may be suitably treated paper or plastics material, or a metal such as aluminium which may be grained and anodised to render its surface more lithographically suitable. The resultant plate needs to be treated with an oleophilising compound to increase the oleophilic properties of the silver image areas. Various treatments are known and are detailed in pages 105 and 106 of "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde (The Focal Press). Also, suitable oleophilising compounds are described in US-A-3 099 209 and FR-A-2 250 444. Even after such treatment, however, it has been found that if gelatin has been used as the binder in the silver-halide emulsion, the surface of the image areas is reluctant to take ink. It is an object of the present invention to improve the ink receptivity of lithographic printing plates having image areas of metallic silver derived from silver halide emulsions. In accordance with the present invention, this is achieved by applying to the plate a proteolytic enzyme and subsequently or simultaneously applying an oleophilising compound. It is known, from FR-A-2 016 906 to treat a lithographic printing plate with an enzyme in order to improve its lithographic properties by preventing scumming. However the image areas of the plate are constituted by hardened gelatin and not by shiny metallic silver as in accordance with the present invention. Also, there is no suggestion to carry out the enzyme treatment in combination with the use of a compound capable of oleophilising silver image areas. US-A-4 285 276 describes a fountain solution for use when lithographic printing. The solution contains an enzyme but there is no suggestion to use the fountain solution when printing from plates having metallic silver image areas or to use the fountain solution in combination with a compound capable of oleophilising silver image areas. Accordingly the present invention provides a method of treating a lithographic printing plate having image areas of metallic silver which method comprises applying a compound capable of rendering the image areas oleophilic characterised in that prior to or simultaneously with the application of said compound said image areas are treated with a proteolytic enzyme. In the case where the proteolytic enzyme and the compound capable of rendering the silver image areas oleophilic are applied simultaneously as a composition, the composition will generally be an essentially aqueous solution containing effective amounts of the enzyme and the oleophilising compound. Typically, the enzyme constitutes from 0.1% to 10.0% by weight of the total composition and the oleophilising compound constitutes from 0.05% to 5.0% by weight of the total composition. The enzyme is chosen from that class of enzymes which specifically catalyse the breakdown of proteins. For example, trypsin, pepsin, ficin, papain or bacterial proteinases such as "Gelatase C" (ABM Chemicals Ltd.) may be used although trypsin and papain are preferred. Enzyme activity is very pH dependent and enzymes in general will not withstand extremes of pH or temperature or the presence of strong oxidising or reducing agents. Hence the composition should be formulated and used in a manner such that these are avoided. The oleophilising compound may be any of those disclosed on pages 105 and 106 of "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde (hereby incorporated by reference) but mercapto compounds and cationic surfactants such as quaternary ammonium compounds are preferred oleophilising compounds. In accordance with a preferred embodiment, a desensitising treatment for the non-image areas is carried out either simultaneously with or subsequently to the application of said enzyme and the oleophilising compound. In the former case, the composition additionally comprises an effective amount of a lithographic desensitising compound. Typically, the desensitising compound constitutes from about 1.0 to 10.0% by weight of the total composition. Carbohydrates such as gum arabic and dextrin may be used as desensitising compounds without affecting the enzyme. The run-length of the treated plates may be increased by applying a lacquer (based, for example on phenol or cresol novolak resins) to the image areas. It is believed that treatment of the plate in accordance with the invention provides better adhesion of the lacquer to the image areas and thus a longer run-length. The following Examples illustrate the invention. Example 1 Single Sheet DTR System Electrochemically grained and anodised aluminium substrates were coated, in order, with a nucleation layer comprising silver sol in gelatin binder and a silver chlorobromide-gelatin emulsion. These coated substrates were then exposed and developed in a diffusion transfer type developer to form lithographic printing plates. The composition of the developer was as follows:- Phenidone is a proprietory form of 1-phenyl-3-pyrazolidone. The plates were then washed with water to remove the chemically developed silver and the silver image areas remaining on the plates were subjected to the treatments detailed below. In all cases the plates were buff-dried after the treatment and then put on a FAG Offset Press 104, which is a proofing press, and the ink-receptivity determined from the number of impressions required before good quality prints were obtained. In general, the ink-receptivity was considered to be very good if less than 5 impressions were required, good if from 5 to 10 impressions were required, moderate if from 10 to 15 impressions were required. If greater than 20 impressions did not yield a satisfactory print, the image was considered to have no oleophilicity for practical purposes. In each case, the treatment solution was applied to a plate at an amount of 50 mls/m² for 20 seconds at room temperature. Treatment 1 The following solution was used: (Triton X-100 is a proprietory form of octyl phenoxy polyethoxy ethanol). Treatment 2 The following buffered trypsin solution was used Treatment 3 This treatment was identical to Treatment 2 except that the excess solution was wiped-off and the plate was then treated as in Treatment 1. Treatment 4 The solution of Treatment 2 was used to which had been added 100mls of a 2% solution of 2-mercapto benzthiazole in polyethylene glycol 200. Treatment 5 Treatment 6 The solution of Treatment 2 was used to which had been added 100 mls of a 2% solution of phenylmercaptotetrazole in PEG 200. Treatment 7 The following buffered solution of Ficin was used: Treatment 8 The following solution of Pepsin was used: Treatment 9 The following solution of papain was used: Treatment 10 A solution was used which was similar to the solution of Treatment 9 but containing 1 gm/litre papain Treatment 11 The solution of Treatment 2 was used to which had been added 10gms/litre cetyltrimethylammonium bromide Treatment 12 The solution of Treatment 2 was used to which had been added 10 gms/litre cetylpyridinium chloride. Treatment 13 The solution of Treatment 2 was used to which had been added 1gm/litre trypsin. Treatment 14 The solution of Treatment 2 was used to which had been added 0.1gm/litre trypsin. Treatment 15 The following solution of a bacterial enzyme was used: Proteinase 200L is bacterial neutral proteinase obtained from ABM Chemicals. The following results were obtained. Example 2 Two sheet DTR System This system comprised an electrochemically grained aluminium substrate coated with a silver sol. of average particle diameter 50Å and a paper base coated with a silver chlorobromide-gelatin emulsion. The paper was exposed and the assembly processed in the conventional manner in an ADT 026 processing machine (Develop GmbH) using the developer of Example 1. After the sheets were peeled apart the silver image remaining on the aluminium receptor sheet was treated with fixing solution prior to testing the resultant lithographic printing plate on the proof press. Mottle blinding is often associated with plates produced in this way. The extent of it depends on several factors including, in particular, the amount of gelatin or other binder, in the donor sheet and the degree to which it has been hardened. A plate showing this effect when treated in the conventional manner was treated as in treatment 6 of Example 1. This treatment removed the blinding and resulted in a plate with excellent ink receptivity over the whole of the silver image. Example 3 An electrochemically grained and anodised aluminium sheet was coated with a silver iodobromide-gelatin emulsion to about 0.1 gm Ag m⁻². This was exposed and developed for 20 seconds in a physical developer containing 10 gm/litre silver nitrate and an Fe²⁺ /Fe³⁺ redox couple to produce a lithographic printing plate having a metallic silver image of about 5 gm Ag m⁻². This plate when treated as in treatment 6 showed much better ink receptivity than one treated with an oleophilising solution not containing the enzyme. Example 4 The following finishing composition was prepared: The pH of the composition was 7.8. The buffer solution had the following ingredients: A similar finishing composition was prepared except that the trypsin was omitted from the buffer solution. A single-sheet diffusion transfer plate consisting of an electrolytically grained and anodised aluminium substrate coated with a positive receiving layer consisting of silver particles in a gelatin binder and a silver chlorobromide/gelatin emulsion, was exposed and developed in the manner of Example 1. The resultant lithographic printing plate was washed with water, and the two finishing compositions were applied respectively to separate areas thereof. The plate was rubbed with normal lithographic inking-in ink. The area treated with the enzyme-containing finishing composition accepted the ink immediately whereas considerable rubbing was necessary before the other area reluctantly accepted the ink. Example 5 Example 4 was repeated except that papain was the enzyme used and the composition had a pH of 6. Similar results were obtained. Example 6 Examples 4 and 5 were repeated except that phenylmercaptotetrazole was used as the oleophilising compound. Similar results were obtained. Example 7 The following finishing composition was prepared: The pH was adjusted to 8.0 with 1N sodium hydroxide. An equivalent composition without the trypsin was also prepared and the two finishing compositions were applied to separate areas of a developed lithographic printing plate produced as in Example 4. Results similar to those of Example 4 were obtained. Example 8 Examples 4 to 7 were repeated using a lithographic printing plate produced by the two-sheet diffusion transfer system. Similar results were again obtained. The two-sheet system consisted of an electrolytically grained aluminium substrate coated with a silver particle/gelatin positive receiving layer and a paper substrate coated with a silver halide/gelatin emulsion. Example 9 The finishing compositions of Examples 4 to 7 were applied to lithographic printing plates produced by a physical development process and similar results were obtained. The plates used consisted of an electrolytically grained aluminium substrate coated with a silver halide/gelatin emulsion. After exposure, the plates were placed in a physical developer containing a silver salt and a reducing agent.

Claims: A method of treating a lithographic printing plate having image areas of metallic silver which method comprises applying a compound capable of rendering the image areas oleophilic characterised in that prior to or simultaneously with the application of said compound said image areas are treated with a proteolytic enzyme. A method according to claim 1 wherein the enzyme is trypsin, pepsin, ficin, papain or a bacterial proteinase. A method according to claim 1 or 2 wherein said compound is a mercapto compound. A method according to claim 3 wherein said compound is 2-mercaptobenzthiazole or phenylmercaptotetrazole. A method according to claim 1 or 2 wherein said compound is a cationic surfactant. A method according to claim 5 wherein said compound is cetyltrimethyl ammonium bromide or cetylpyridinium chloride. A method according to any one of the preceding claims, comprising the step of applying a lithographic desensitising compound to said image areas either simultaneously with or subsequently to the application of said compound. A method according to claim 7, wherein said lithographic desensitising compound is gum arabic or dextrin.

Inventors: WATKISS, PHILIP JOHN
Applicants: E.I. DU PONT DE NEMOURS AND COMPANY (A DELAWARE CORPORATION)
Application year: 1984
Main classification: G03F7
IPC codes: G03F7