Recent Progress In The Photoinitiated Cationic Polymerization Of Cycloaliphatic Epoxy Systems

The photoinduced cationic polymerization of cycloaliphatic epoxide based
systems continues to succeed in an increasing number of applications. The growing
experience in the formulation of these systems have greatly improved their versatility and
potential.

The cure rate of cationic photopolymerization can be largely influenced by the
nature of the components of the formulation: cycloaliphatic epoxies react faster than
glycidylether type of epoxy; vinylethers are among the fastest cationically curing
monomers; hydroxyl-containing compounds lead to chain transfer reactions which may
slow down the curing process, depending on the nature and concentration of these
compounds.

Beside the formulation, the characteristics of the source of UV light can also
influence significantly the curing performance of a given UV curing system. Over the
recent years, a general trend in the development of new UV curing equipment has been
an increase in the power at which the lamps are operated, and, in addition, the access to
spectral distributions different from the standard mercury spectrum, by means of socalled
doped lamps. The first key element to the success of any UV curing process is the
highest possible adaptation of the emission spectrum of the lamp and the absorption
spectrum of the photoinitiator. In this respect, the recent development of the XeCl
excimer as source of UV light is of very high interest for the cationic UV curing process.
This type of UV source is characterized by a very narrow bandwidth emission at 308 nm
and a peak irradiance of 3 to 4 times higher than a typical mercury emission in this
region of the UV spectrum, i.e. exactly where the standard cationic photoinitiator, the
triarylsulfonium salts, show a maximum in the absorption spectrum with an interesting
extinction coefficient.

This paper presents comparative results in terms of cure rate and degree of
conversion of epoxy groups achieved when using the XeCI excimer source of UV light or
the standard mercury lamp, both microwave powered. The curing performance is
investigated for various cationic curing systems, applied at different coating film weights.