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Cationic UV curing systems used in current
industrial applications contain iylsulfonium salts as
cationic photoinitiator, with the exception of the epoxysilicone
release coating systems which use diaryliodonium
salts. Upon irradiation by UV light, these onium salts
generate a stable, long-lived, strong Bronsted acid which
can induce a variety of cationic polymerization processes.
One serious deficiency of these onium salts is
their limited absorption capacity above 300 nm, in
particular their inability of absorbing the high energy band
at 366 urn in the case of a mercury lamp or the high
energy provided at longer wavelength by the so-called
doped lamps. It is described in the literature and proven
that photosensitizers and free radical photoinitiators
may be used to broaden the spectral response of the onium
saltS. In the case of a sensitizer, this is achieved via
electron transfer from the excited state photosensitizer to
the onium salt: This is obviously useful. when the
absorption capacity of the onium salt is dramatically
affected, like:
