As set forth in U.S. Pat. No. 4,477,317, many methods have been proposed in the treatment of aluminum surfaces to render them suitable for use in the preparation of lithographic printing plates. A commercially utilized method involves subjecting aluminum metal plates or sheets to electrolytic graining or etching in an aqueous acidic electrolyte. U.S. Pat. Nos. 3,072,546 and 3,073,765 disclose the treatment of aluminum surfaces in a hydrochloric acid electrolyte while applying an alternating current. Aluminum plates treated by the so-called alternating current acid electrolyte procedure renders the plates suitable for the production of lithographic printing plates. More specifically, this procedure results in an aluminum surface which is uniformly roughened, hydrophilic surfaces which are more receptive and adherent to light-sensitive coatings. The average roughness values of such treated surfaces are designated as Ra micrometers.
In recent years there has been a need to increase the production of lithographic printing plates in order to meet growing commercial demands. An attempt was made to speed up the line of aluminum sheet passing through the alternating current tank containing the aqueous acidic electrolyte and thereby produce additional substrate material for lithographic printing plates. It was found, however, that the increase in line speed has a serious disadvantage in that resulting aluminum surface does not have adequate average roughness values required to produce quality aluminum metal supports for lithographic printing plates. Upon investigation it was found that with conventional alternating current etching or graining it takes a relatively long etching time to achieve the desired metal surface roughness. More particularly, the alternating etching of an aluminum sheet in hydrochloric acid takes at least 30 seconds at 75 amps per square decimeter to reach an average roughness value of 0.7 micrometer starting from smooth aluminum. It follows that the replacement of present equipment, such etching tanks and electric power supplies, as well as increased amount of electrolyte solution that would be required to maintain product quality at higher aluminum sheet line speeds would be extremely costly.
As is also known, the use of direct current rather than alternating current results in aluminum sheet surfaces with a non-uniform roughened pattern characterized by undesirable deep pitting.
It would be advantageous therefore to have a method for increasing the line speed of aluminum sheet passing through an electrolyte solution in an etching or graining treatment without loss of the product quality required for suitable supports in lithographic printing plates.