Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1994-03-08
1995-09-12
Valentine, Donald R.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
20412943, 204DIG9, 20412975, C25F 304
Patent
active
054494419
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of electrochemically roughening an Al sheet, mainly for use as a lithographic plate support or a capacitor foil.
To make an aluminium sheet suitable for use as a lithographic plate support, the surface needs to be roughened or grained. Standard techniques for this include: mechanical graining by the use of balls or abrasives or wire brushes; electrochemical graining, by the application of an AC current in an acidic electrolyte; and chemical graining, by simple immersion in an etch. Roughening is carried out in order to enhance the adhesion of a photosensitive coating on the support, and to improve the water retention properties of the uncoated support surface, Irradiation and development of a lithographic plate generally result in ink-receptive image areas and water-retaining non-image areas, the latter generally being the uncovered support surface. For this purpose the aluminium sheet needs to be roughened on a scale of approximately 1-15 .mu.m.
This invention is concerned with an improvement in the electrochemical graining or roughening technique. It is known to subject the Al sheet to AC, the current being biased in the anodic direction. It is also known to subject the Al sheet to AC with the current biased in the cathodic direction. Different advantages are claimed for the two alternatives. But neither alternative is effective to achieve uniform graining of the Al sheet on a desired scale, and in the shortest possible time. In the economics of this industry, time considerations are of paramount importance. In commercial electrochemical graining, sheet is passed continuously through a long bath of electrolyte; treatment time determines the speed of passage of the sheet, and the length of the bath and hence the amount of electrolyte used.
U.S. Pat. No. 4,468,295 (Hoechst) describes an electrochemical roughening technique in which Al sheet is subjected to two superimposed alternating currents having different frequencies, the ratio of the frequencies being from 3 to 100. This is claimed to improve the uniformity of the electrochemical graining, but it is not suggested that the treatment time is reduced. In continuous operation, different parts of the Al sheet would be subjected to different initial treatment conditions, which would be expected to produce a banded and therefore non-uniform result.
U.S. Pat. No. 4,561,944 (Fuji) describes a two-step or three-step graining treatment. Al sheet is first mechanically roughened, then optionally chemically etched, and finally electrochemically roughened by means of AC biased on the cathodic side. Because several roughening steps are involved, this treatment is relatively expensive.
U.S. Pat. No. 4,897,168 describes a process for the production of a roughened aluminium printing plate support by means of a pulsed direct current in an acidic electrolyte. During roughening the plate is mainly anodic. The roughened plate may thereafter be subjected to the cathodic half of the power supply which may exert a cleaning effect.
It is an object of this invention to provide a method for electrochemical roughening of Al sheet, in which the graining is uniform and on a desired scale, and which is effected as quickly or more quickly than conventional techniques.
The invention provides a method of electrochemically roughening a surface of an Al sheet, by subjecting the sheet in an aqueous electrolyte to the action of an alternating electric current, characterised in that the AC is initially biased in a first way for a time sufficient at least to initiate formation of a first pitting structure on the surface, and is then biased in a second way for a time sufficient at least to initiate formation of a second pitting structure on the surface, wherein the bias in the cathodic direction is of limited extent, such that the ratio of the areas, on opposite sides of a zero potential line under the curve of a voltage/time plot, is no greater than about 3:1.
The following observations led up to the present invention. Microscopic examination of conventiona
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Amor Martin P.
Andrews Peter R.
Ball Jonathan
Limbach Peter K. F.
Alcan International Limited
Valentine Donald R.
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