Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1994-01-21
1995-03-07
Valentine, Donald R.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
20412975, C25F 304
Patent
active
053954890
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of electrochemically roughening an Al sheet for use as a lithographic plate support.
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 results 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, and to have an average roughness R.sub.a (measured by an optical technique) in the range 0.5-2.5 .mu.m.
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. Chemical etching is necessarily a relatively slow process, and is not envisaged here. This invention is an improvement in the electrochemical roughening process, based on the inventors discovery that added components, preferably in the Al sheet or alternatively in the electrolyte, permit treatment time to be reduced.
European patent application 96347A (Fuji) describes aluminium alloy sheets for use as lithographic printing plate supports, which sheets contain 0.005-0.01% of Sn, In, Ga or Zn, together with Fe and optionally Cu or Mg. These sheets are subjected to a chemical etch, optionally followed by an electrochemical treatment, and the added components improve the rate of chemical etching and produce a desired pitting structure on the metal sheet surface. There is no teaching to treat electrochemically a sheet which has not previously been subjected to a chemical etch.
British Patent Specification 1 374 787 (Kalle) mentions that the hydrochloric acid electrolyte, used to electrochemically roughen metal foil for lithographic printing plates, may contain mercury ions.
U.S. Pat. No. 4,272,342 (Fuji) states that lithographic plates to be electrochemically roughened may be of an alloy of aluminium with zinc, lead or bismuth; and that a nitric acid electrolyte for electrochemical roughening may contain zinc nitrate as a corrosion inhibitor.
U.S. Pat. Nos. 4,566,959 and 4,566,960 (Hoechst) describe electrolytes for electrochemical roughening, comprising hydrochloric acid or nitric acid together with an inorganic fluorine compound such as SbF.sub.6. The aluminium metal sheet to be roughened may contain small amounts of zinc.
Where no function of an alloying component is stated, the function may be inferred as being to improve the mechanical properties of the lithographic plate. None of the references discussed in the four preceding paragraphs teaches or suggest that the added component can accelerate the electrograining (electrochemical roughening).
The present invention provides a method of electrochemically roughening an aluminium metal sheet for use as a lithographic plate support, which method comprises subjecting the sheet in an electrolyte to an alternating current treatment to roughen the surface of the sheet, the treatment being performed in the presence of an added component selected from at least one of Hg, Ga, In, Sn, Bi, Tl, Cd, Pb, Zn and Sb, said component being present, in the sheet at a concentration of 0.01-0.5% by weight and/or as a coating on the sheet and/or in combined form in solution in the electrolyte at a concentration of 0.001M to 0.1M, the concentration being effective to increase the rate of electrochemical roughening.
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Ball Jonathan
Hunter John A.
Limbach Peter K. F.
Ward John A.
Alcan International Limited
Valentine Donald R.
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