Lithographic printing plate

DETAILED DESCRIPTION OF THE INVENTION The suitable positively charged colloidal silica and insolubilized hydrophilic polymers containing face coatings are discussed in Ser.
No.
493,528.
The introduction into such face coatings of pigment and resinous binder must be controlled carefully in terms of proportions to maintain the lithographic effectiveness of the plate while at the same time securing the advantages of water resistance and surface smoothness conventionally obtained by the use of a barrier coat.
Thus, the resinous binder proportion in such single coating or face coating is restricted to 30% at most, but preferably is at least about 5% of such face coating to be practical.
In the present invention, in addition to the colloidal silica and insolubilized hydrophilic polymer, is the pigment, which must be restricted to 60% at most of the face coat formulation, but should be at least about 10%.
The proportion of positively charged silica should be at least about 10% and can go as high as 70%.
The insolubilized hydrophilic polymer (including insolubilizing crosslinking agent if that is used) should be at least about 5%, and not more than 25%, and the ratio of the colloidal silica to such insolubilized hydrophilic polymer should be broadly between about 2:1 and 6:1.
Preferably, these ranges are from about 10 to 20% for the resinous binder, about 30 to 50% for the pigment, about 30 to 50% for the silica, and about 10 to 20% for the insolubilized hydrophilic polymer including crosslinkers therefor, and the ratio of silica to such insolubilized hydrophilic polymer advantageously is about 3:1 to 5:1.
The foregoing weight percentages are based on the dried lithographic printing surface coating; volatiles such as water and fugitive solvents are not considered.
The resinous binder should be cationic or nonionic or a mixture, but it should not be anionic to avoid coagulation.
Thus, as a practical matter, resins exhibiting carboxyl functionality should be avoided as, of course, are even stronger acidic functionalities as sulfonic, phosphonic, etc