Lithographic plate of tin-plated steel and method of manufacture

Abstract

A method of making a presensitized lithographic plate is disclosed wherein a steel support having a tin-plated layer is depassivated and coated with a photosensitive product. Depassivation is accomplished by immersion of the support in an electrolytic bath of sodium carbonate. The electrolysis is performed with the support as a cathode and reversed so that the support is an anode of the bath. Preferably, the bath is an aqueous solution of 50 grams per liter of sodium carbonate at a temperature of 65.degree. C. with a current density of the bath being approximately 2.5 amperes per dm.sup.2. It is also contemplated that the depassivated tin-plated layer may be coated with copper before application of the photosensitive product. This can be accomplished by immersion in an electrolytic bath of copper cyanide. Preferably, the copper cyanide bath is at a temperature of 65.degree. C. with immersion occurring for approximately 3 minutes with the support in the cathode position and having a current density of 2.5 amperes per dm.sup.2. This results in a copper coating of approximately 3-4 microns in thickness. The copper coating may be passivated before the photosensitive product is added by a coating of aqueous benzotriazole solution. The photosensitive product may be a diazo resin of the orthoquinone diazide type condensed with a formophenolic resin. Over the diazo resin, after development a firing solution which is subsequently annealed may be added. Alternatively, the depassivation of the tin-plated layer may be accomplished by wet brushing in the presence of an aqueous pounce solution followed by impregnation with a hydrochloric acid and stannous chloride solution.