Some Studies on Wiped Lithographic Plate Coatings.

The diazo resin employed in lithographic wipe-on plate coatings is the condensation product of 4-diazo diphenylamine with formaldehyde and is usually furnished in the form of the zinc chloride salt. This condensation reaction, carried out in an acid medium, is not a straight-forward-reaction and the resulting resin can vary considerably from batch to batch even under controlled conditions. In solution it begins to deteriorate rapidly and even in the original powder form its stability depends to a great extent upon its past history and ambient conditions. Although for lithographic purposes a fair degree of variation can be tolerated, there obviously must be some limits. This paper reviews the history and literature on this particular resin, surface treatments of the aluminum support, and developing systems. It reports studies made in an attempt to understand chemical and physical properties of the resin which affects its suitability for lithographic platemaking.