GB1561378A - Anodised aluminium sheets for making lithographic printing plates - Google Patents

Description

(54) ANODISED ALUMINIUM SHEETS FOR MAKING LITHOGRAPHIC PRINTING PLATES (71) We, KODAK LIMITED, a Company registered under the law of England, of Kodak House, Station Road, Hemel Hempstead, Hertfordshire, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to anodised aluminium sheets for making lithographic printing plates.

Commercially available aluminium silver salt diffusion transfer receiving sheets, intended for making lithographic printing plates, having an anodised surface tend either to have fast ink-up rates (less than 10 cylinder impressions) but yield relatively short press runs (less than 20,000 copies if the image is not lacquered) or rather slow ink-up rates (around 50 cylinder impressions) but provide longer press runs (25,000 to 50,000 copies). It is believed that this phenomenon is influenced by the nature of the anodised surface. If it is such that the silver precipitating nuclei are deposited deep in the anodic layer, the silver image will also be deep making inking up difficult or, in the extreme, impossible. On the other hand, if the nuclei are at or near the surface the silver image will be accessible and therefore easy to ink-up. Such an image if too near the surface, however, will be comparatively simple to remove by rubbing and will only give a short press run.

The anodised aluminium required in this and related processes is different from that required in presensitised litho plates comprising an anodised support carrying a layer of a photohardenable polymer. Generally we have observed that anodised supports suitable for such presensitised plates tend not to be suitable for the present usage because their anodic coating is too porous.

We have discovered that if anodised aluminium is treated with a phosphate solution and used to make silver salt diffusion transfer receiving sheets for example, and lithographic printing plates are made therefrom, longer press runs can be obtained without the disadvantage of slow ink-up rates.

Accordingly the present invention provides a method of making an anodised aluminium sheet capable of forming a lithographic printing plate which comprises the steps of treating an anodised aluminium sheet containing an aluminium compound more water-soluble than aluminium phosphate in the anodic layer, with an aqueous solution containing phosphate ions and having a pH less than 4 or greater than 10.5 in such a manner that a lithographic printing plate prepared therefrom has an increased press run, then washing the anodised aluminium with an aqueous solution having a pH between 4 and 10.5, preferably between 5 and 9, and a degree of total hardness of not more than 70 ppm and, before or after the above treatment and wash steps, uniformly depositing image-forming particles on the anodic surface.

The image-forming particles are preferably nuclei for physical development or particles from which such nuclei can be formed in imagewise distribution. When the anodised sheet is a silver salt diffusion transfer receiving sheet it will contain a uniform distribution of silverprecipitating nuclei. Alternatively the image-forming particles may be capable of forming an imagewise distribution of nuclei for physical development after imagewise exposure to radiation, e.g. titanium dioxide, zinc oxide, radiation-sensitive palladium compounds or silver halide particles. Further mechanisms which can be used on the present anodised sheets include imagewise destruction or poisoning of development nuclei and photo-solubilisation of silver halide/dye combinations.

Prior to anodising, the aluminium may be subjected to conventional treatments including cleaning, graining (either mechanical or electrolytic), alkaline and/or acid etching.

The anodic coating is preferably formed by anodising in a sulphuric acid solution but anodisation may be carried out using other electrolytes for example chromic acid, oxalic acid, sodium carbonate or sodium hydroxide or mixtures thereof, for example, a mixture of sulphuric and chromic acids. Such coatings contain hydrated aluminium oxides which are more water-soluble than aluminium phosphate.

After anodising has been completed the anodic surface is preferably washed with water which may be at ambient temperature or preferably at elevated temperature. Such a washing treatment at 70-900C renders the anodic coating substantially non-porous. Sealing treatments of this kind are known in the aluminium treating art wherein the hot water baths may contain additives, e.g. nickel salts to improve the sealing effect.

Preferably the phosphate ion solution has a pH either between 0.5 and 3.5 or between 10.5 and 13.5, preferably from 11 to 13. The solution may comprise phosphoric acid but solutions of alkali metal phosphates including mixtures thereof may equally be used, for example trisodium phosphate. The temperature of the treatment with the phosphate solution may vary over a wide range. The time of treatment is approximately halved on raising the temperature 10 C.

The silver precipitating nuclei required for the silver salt diffusion transfer receiving sheets of the invention may be those known in the art, for example, colloidal silver particles. A Carey Lea colloidal silver dispersion containing no hydrophilic colloid is the preferred source of nuclei and it may also contain silver protein. Nucleation of anodised aluminium supports is described, for example, in British Specifications 934,692 and 934,693 and the use of silver protein is described in Canadian Specification 598,598.

The phosphate treatment of the present invention may be carried out on the anodised aluminium supports before or after the deposition of the image-forming particles. If done after, it may be unnecessary to deposit further image forming particles but this must be determined by experiment.

In use, the present diffusion transfer receiving sheets may be employed in a manner conventional in silver salt diffusion transfer processes and when separated from the negative are treated with a "fix" solution which converts the silver image to an ink-receptive hyd rophobic image, for example, as described in British Specification 934,691 and 934,694.

In general terms the process of the present invention leads to considerably improved lengths of press runs without the usual corresponding slow inking up rates.

The invention is illustrated by the following examples. The words "Lissapol" and "Instafax" are trade marks.

Example 1 Three sheets of anodised aluminium foil prepared by wire brush-graining, sulphuric acid anodising and washing in water at 70-90"C to obtain a substantially non-porous anodic layer supplied by Anocoil Limited of Bletchley, Buckinghamshire having an anodic layer one micron thick were wetted in turn with demineralised water to prevent air pockets affecting subsequent treatment. Each sample of foil was drained for about five seconds, immersed in a solution containing phosphoric acid (10% W/V) at a pH of 1.6 for between two and seven minutes, washed immediately in demineralised water, and then dried. Each sample was then dipped in a nucleating bath of the following composition: Colloidal Silver Solution (prepared by the Carey Lea Method) 654 ml Mild Silver Protein (B.P.C.) 8.45 grams Distilled water 4320 ml Lissapol - N (a non-ionic surfactant sold by Rohm and Haas) 10 ml The excess was removed using an air knife, and the above samples were dried and subsequently imaged by the chemical transfer process and 'Instafax' Offset Fix applied. They were then mounted on an A.B. Dick (model 350) press alongside a fourth control sample (A) of sulphuric acid anodised foil, obtained from Anocoil Limited, which had been previously nucleated with a solution of the same composition. The results, summarised in Table 1, show that as the immersion time in phosphoric acid is increased, the press run increases, but the rate at which the image accepts the ink (No. of copies to fully ink plate) is reduced. However a condition exists (Sample B) where a run length of 40,000 copies is achieved with a reasonably fast ink-up (less than 10 impressions).

Tablel Time of immer- No. of copies sion in I 10% to fully ink Press run length Sample H3PO4 (min) plate (No. of copies) A 0 10 11,000 B 2 10 40,000 C 5 20 92,000 D 7 100 100,000 Example 2 A further three samples of the anodised aluminium foil were prepared as an Example 1 except that instead of phosphoric acid being used in the etching treatment, trisodium phosphate (10% W/V) (pH 12.8) was used with immersion times between twenty seconds and one minute. These samples were nucleated, imaged and fixed as in Example 1. They were then mounted on the A.B. Dick (model 350) press with a control as described in Example 1.

The results, summarised in Table 2, again show that as the immersion time is increased, the press run achieved increases, but the ink-up rate deteriorates. However, a condition again exists where a long press run and fast ink-up are compatible (Sample F).

Table 2 Time of immersion in No. of copies Press run trisodium phosphate to fully ink length (No.

Sample (10%) (seconds) plate of copies) Control 0 10 7,000 E 20 10 15,000 F 40 10 45,000 G 60 20 55,000 Example 3 Three sheets of 'Instafax' Offset Foil sold by Kodak Limited which comprise nuclei of colloidal silver on an anodised aluminium sheet were immersed in a solution of phosphoric acid (10% W/V) at a pH of 1.6 for times of two, five and seven minutes, washed immediately in demineralised water, and then dried. Without further treatment each sample was imaged using the 'Instafax' Offset Contact Negative and Developer, and 'Instafax' Offset Fix was applied. All samples were mounted on an A.B. Dick (model 350) press alongside an untreated sample of 'Instafax' Offset Foil (control sample) prepared in the usual manner. The control sample showed excessive image wear by 15,000 impressions while the remaining plates showed no wear by 20,000 impressions.

Exam 4 A further three sheets of 'Instafax' Offset Foil were treated as in Example 3 except that after treating with phosphoric acid (10% W/V) the foil was re-nucleated by immersing in a solution of the following composition: Colloidal Silver Solution (Prepared by the Carey Lea Method) 654 ml Mild Silver Protein (B.P.C.) 8.45 grams Distilled Water 4320 ml Lissapol - N ml The excess solution was removed using an air knife and the samples imaged and treated as in Example 3. The control plate showed image wear by 12,000 impressions, but of the samples treated in phosphoric acid, none was showing wear by 30,000 impressions when the press run was stopped.

WHAT WE CLAIM IS: 1. A method of making an anodised aluminium sheet capable of forming a lithographic printing plate which comprises the steps of treating an anodised aluminium sheet containing an aluminium compound more water-soluble than aluminium phosphate in the anodic layer, with an aqueous solution containing phosphate ions and having a pH less than 4 or greater than 10.5 in such a manner that a lithographic printing plate prepared therefrom has an increased press run, then washing the anodised aluminium with an aqueous solution having a pH between 4 and 10.5, preferably between 5 and 9, and a degree of total hardness of not more than 70 ppm and, before or after the above treatment and wash steps, uniformly depositing image-forming particles on the anodic surface.

2. A method as claimed in claim 1 in which the image-forming particles are silverprecipitating nuclei.

3. A method as claimed in claim 2 in which the silver precipitating nuclei are deposited from a solution comprising colloidal silver particles and, optionally, silver protein.

4. A method as claimed in any of claims 1-3 in which the aluminium has been anodised using sulphuric acid as the electrolyte.

5. A method as claimed in any of claims 1-4 in which the anodised aluminium is sealed by washing with water at a temperature of from 70 to 900C prior to the phosphate treatment.

6. A method as claimed in any of claims 1-5 in which the phosphate ion solution has a pH between 0.5 and 3.5 or 10.5 and 13.5.

7. A method as claimed in claim 6 in which the phosphate ion solution has a pH of from 11 to 13.

8. A method as claimed in any of claims 1-7 in which the image-forming particles are deposited after the phosphate treatment and washing steps are completed.

9. A method as claimed in claim 1 substantially as described herein and with reference to the Examples.

10. Anodised aluminium sheets capable of forming a lithographic printing plate when made by the method of any of claims 1-9.

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. treated in phosphoric acid, none was showing wear by 30,000 impressions when the press run was stopped. WHAT WE CLAIM IS:

1. A method of making an anodised aluminium sheet capable of forming a lithographic printing plate which comprises the steps of treating an anodised aluminium sheet containing an aluminium compound more water-soluble than aluminium phosphate in the anodic layer, with an aqueous solution containing phosphate ions and having a pH less than 4 or greater than 10.5 in such a manner that a lithographic printing plate prepared therefrom has an increased press run, then washing the anodised aluminium with an aqueous solution having a pH between 4 and 10.5, preferably between 5 and 9, and a degree of total hardness of not more than 70 ppm and, before or after the above treatment and wash steps, uniformly depositing image-forming particles on the anodic surface.

2. A method as claimed in claim 1 in which the image-forming particles are silverprecipitating nuclei.

3. A method as claimed in claim 2 in which the silver precipitating nuclei are deposited from a solution comprising colloidal silver particles and, optionally, silver protein.

4. A method as claimed in any of claims 1-3 in which the aluminium has been anodised using sulphuric acid as the electrolyte.

5. A method as claimed in any of claims 1-4 in which the anodised aluminium is sealed by washing with water at a temperature of from 70 to 900C prior to the phosphate treatment.

6. A method as claimed in any of claims 1-5 in which the phosphate ion solution has a pH between 0.5 and 3.5 or 10.5 and 13.5.

7. A method as claimed in claim 6 in which the phosphate ion solution has a pH of from 11 to 13.

8. A method as claimed in any of claims 1-7 in which the image-forming particles are deposited after the phosphate treatment and washing steps are completed.

9. A method as claimed in claim 1 substantially as described herein and with reference to the Examples.

10. Anodised aluminium sheets capable of forming a lithographic printing plate when made by the method of any of claims 1-9.