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By S. Peter Pappas, Polymers and Coatings Department North Dakota State University - RadTech Europe Conference 1989
Curing and imaging processes, based on photogeneration of acid, have not as yet achieved the commercial significance of corresponding free radical processes. Several reasons for this may be advanced including (1) the development of photoinitiators for cationic polymerization was preceded by substantial advances in curing and polymer imaging technology based on photoinitiated radical polymerization; (2) the early developmental work on curing and imaging by cationic polymerization utilized aryldiazonium salts as photoinitiators which, while highly photoactive, are thermally unstable, thereby preventing long-term storage of fully-formulated compositions; and (3) discoveries of thermally stable photoinitiators for cationic polymerization, initially diaryliodoniuin and triarylsulfonium salts, and, subsequently, mixed ligand arene, cyclopentadienyl ferrocenium salts, were made almost simultaneously by several industrial groups, which has resulted in unclear patent situations--a situation which appears to be resolved, at least with regard to the oniurn salts.
Nevertheless, during the past decade, significant advances have been made in the technology of UV curing based on cationic polymerization of epoxy-functional resins with onium salts 1-3], including the beneficial effects of added polyols 4,5], and more recently with mixed ligand arene cyclopentadienyl Fe(II) salts 6,7]. Enhanced rates of cationic polymerization of epoxides in the presence of alcohols has been attributed to a new mechanism, involving activated monomer propagation 8].
Although commercially less-readily available at present, vinyl-ether functional resins, which tend to exhibit higher reactivity than epoxies, are gaining considerable interest for radiation (both UV and EB) curing by cationic polymerization 9,10].
Recent advances have also been made in exploiting photogeneration of acid in chemically-amplified imaging processes, including polymer systems which can function as both positive- and negative-tone photoresists, depending upon the developing solvent 11-13].
Following general considerations of photoinitiated catonic polymerization, emphasis will be placed, herein, on the direct and sensitized photochemistry of triarylsulfonium and diaryliodoniuin salts, together with selected applications. Recent studies on the synthesis and photochemisty of photosensitizer-bound sulfonium salts will also be presented.
