Aminoketone sensitizers for aqueous soluble photopolymer compositions

DETAILED DESCRIPTION OF THE INVENTION Aqueous-soluble photopolymer compositions have been recognized in the industry as one means for reducing volatile organic emissions during the manufacture and subsequent development of lithographic plates.
The conversion of organic solvent-based photopolymer compositions to aqueous solvent-based photopolymer compositions has presented a variety of technical problems.
Among these problems has been the need for spectral sensitizers of high sensitivity which are soluble enough in water-based solvents for coating and which provide good developability in aqueous developers.
Aminoketone spectral sensitizers have been proven to be useful sensitizers for iodonium salt containing photopolymerizable compositions (for example see U.
S.
Pat.
No.
3,729,313 (Smith); Apr.
24, 1973), but these materials have little to no solubility in water-based solvent systems of moderate pH (i.
e.
, pH 5-11 units).
The instant invention overcomes this problem by incorporating a quaternary ammonium substituent into the aminoketone sensitizer, thereby enhancing aqueous solubility.
The term "aminoketone" as used herein means a chemical substance having within its structural formula the sub-structure: ##STR3## The choice of quaternary ammonium as opposed to the many other available solubilizing groups, as a solubilizing substituent, imparts very high solubility in aqueous-based solvents.
Additionally, the sensitizers of the instant invention were found to have higher photosensitivity in aqueous-based printing plate applications than sulfonated aminoketones of the prior art, and surprisingly even higher photospeed than the parent unsubstituted aminoketones of the prior art.
The aminoketone sensitizers of the instant invention are preferably described by the following formula: ##STR4## wherein R.
sub.
1 and R.
sub.
2 each represent an alkyl group having 1 to 6 carbon atoms, and R.
sub.
3 and R.
sub.
4 each represent hydrogen; or at least one of R.
sub.
1 and R.
sub.
3 or R.
sub.
2 and R.
sub.
4 together represent an alkylene group having 2 to 4 carbon atoms, R