A method of treating aluminium foil or a lithographic printing plate support and products so obtained

Abstract

 Aluminium (1) is immersed in a bath (2) containing phosphate ions before anodising in a bath (3) containing sulphate ions. When the substrate is aluminium foil in web form, preferably the phosphate bath serves as a liquid junction cell by which the anodising current is introduced into the web. Aluminium foil or sheet so processed is especially suitable as a base for a presensitised lithographic printing plate.

Description

[0001] This invention relates to the treatment of aluminium (including its alloys); especially aluminium foil in continuous web form. Primarily, but not exclusively, it is concerned with the pre-treatment of aluminium that is subsequently to be coated with photosensitive material to make a ^presensitised lithographic printing plate.

[0002] Aluminium for this purpose is almost always anodised, generally using a bath based on sulphuric acid, though phosphoric acid is used by at least one manufacturer. Sulphuric acid gives a hard, adherent and abrasion-resistant coating, but there are problems in getting satisfactory adhesion of some attractive sensitive coatings (for example polyvinyl cinnamate among negative-working coatings and orthoquinone diazide sulphonate-based resists amongst positive-working coatings). Phosphoric acid anodising gives much better adhesion with these and sane other coatings, but the anodic coating is much softer and tends to abrade away during printing to an extent that may limit the number of copies that can be taken off the plate.

[0003] In some cases-aluminium that has been anodised in sulphuric acid has been treated subsequently with phosphoric acid, and this has been found to give a useful improvement in adhesion while retaining part of the advantages of the use of sulphuric acid for anodising, but this presents obvious difficulties in control of the operation.

[0004] The invention is based in part on the realisation that anodic oxidation takes place at the surface of the metal, not of the coating, and that it will not of itself destroy the existing exposed surface of the coating.

[0005] In accordance with the invention, a process of treating aluminium comprising anodising it in a bath containing sulphate ions is characterised by first immersing it in a bath containing phosphate ions.

[0006] Immersion may be by dipping or, with some advantages, by spraying or flooding the surface of the aluminium.

[0007] The phosphate ions are preferably orthophosphate ions, but more complex phosphates can be used.

[0008] The phosphate and sulphate ions may be the only anions present to any substantial extent in the respective baths, but the presence of other anions that are inert or that modify, rather than radically alter., the effect of the treatment may be present. In particular the presence of minor amounts of sulphate in the phosphate bath and/or of phosphate in the sulphate .bath is not excluded. We prefer that the associated cations should be wholly or mainly hydrogen ions, but neutral or alkaline salt solutions may be suitable in some cases.

[0009] Subject to suitable choice of its composition (its pH especially) the phosphate treatment may be by a simple immersion process, or the aluminium surface may be subjected to anodising treatment in the phosphate bath asrwell as in the sulphate bath. When the aluminium is a foil in continuous web form, however, we prefer that the phosphate bath should serve also as a "liquid junction" cell for introducing anodising current into the web without the use of contact rolls or the like; in this case the aluminium will be electrolytically etched in the phosphate bath.

Example

[0010] The accompanying drawing shows the distinctive part of apparatus in which a web of aluminium foil 1 is passed, on the run, through a first bath 2 containing 45% aqueous orthophosphoric acid and, with or without intermediate washing (not shown), through a second bath 3 containing 15% sulphuric acid. Current is supplied from a conventional power pack and flows between an inert anode 4 immersed in the first bath and an inert cathode 5 immersed in the second bath via both the electrolytes and the part 6 of the aluminium web extending between the two baths. Under typical conditions the bath temperatures are both around 20°_C, the applied potential is about 16.5 V and the mean current density at each of the immersed length of the foil is about 200 Am-². The line speed is such that the dwell time in each bath is approximately four minutes.

[0011] After washing and drying the anodised web is coated with a proprietary positive-working lithographic resist comprising a resin sensitised with the 2, 4 dihydroxybenzophenone ester of 1, 2, 5 diazo naphthol sulphonic acid and coloured with a blue dye to yield presensitised plates with excellent adhesion and press life compared with the best current commercial anodised- aluminium presensitised lighographic printing plates. The anodic coating is thin compared with conventional coatings, and penetration of dye into the coating is not significant.

Claims

1. A method of treating aluminium comprising anodising it in a bath containing sulphate ions characterised by first immersing it in a bath containing phosphate ions.

2. A method in accordance with Claim 1 of treating aluminium foil in web form comprising passing the web through a first bath containing phosphate ions and afterwards through an anodising bath containing sulphate ions characterised by passing current between an anode immersed in the first bath and a cathode immersed in the anodising bath.

3. A method of making a presensitised lithographic printing plate comprising anodising aluminium sheet or foil subsequently applying a lithographic photosensitive coating to it, characterised in that the sheet or foil is anodised by the method claimed in Claim 1 or Claim 2..

4. Aluminium treated by the method claimed in Claim 1.

5. Aluminium foil treated by the method claimed in Claim 2.

6. A presensitised lithographic printing plate made by the method claimed in Claim 3.

Drawing