Fountain compositions for use in lithographic printing comprising aqueous solutions of polyacrylamide based polymers and blends of polyacrylamide and polyacrylic acid with an organic chelating agent

摘要

Precipitation at relatively high pH levels, e.g. above about 4 or more, in an acidic water/alcohol type fountain composition containing polyvalent metal cations and, as an active polymer component for rendering the non-image areas of a lithographic plate hydrophilic, a polyacrylamide-based polymer having a weight average molecular weight (M. sub.w) of from about 5,000 to about 1,000,000 and having from about 3 to about 70 percent of the carbonyl sites as carboxyl groups, or a blend of polyacrylamide and polyacrylic acid, or the like, is controlled by providing in the fountain composition an effective amount of an organic chelating agent for polyvalent metal cations which is substantially inert with respect to the active polymer component. The suitable chelating agents are further characterized in that at 25° C, each (A) has a 1:1 stability constant with zinc of at least about 10.sup.7.5 ; (B) has a solubility of at least about 0.001 moles per liter in the water/alcohol solvent system over the pH range of from about 3 to about 7; and (C) when in a stable complexed form with polyvalent metal cation(s), has a solubility of at least about 0.001 moles per liter in the water/alcohol solvent system over the pH range of from about 3 to about 7. A preferred chelating agent is the pentasodium salt of diethylenetriaminepentaacetic acid.