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There has been a growing interest in recent years in light-induced cationic
polymerization, due to the development of very efficient photoinitiators and highly,
reactive new monomers. A distinct feature of cationic polymerization is that a large
variety of monomers which are inactive towards radicals, like epoxides, cyclic ethers,
vinyl ethers or siloxanes, can be polyrnerized readily by UV irradiation in the presence
of onium salts. In addition, cationic-induced polymerization is not sensitive to
oxygen, by contrast to radical-induced polymerization so that this technology is
specially well suited to cure rapidly thin coatings in the presence of air. Morever, since
the propagating polymer cations are not reacting with each other, the termination step
plays a minor role and the chain reaction will continue to develop in the dark, well
after the end of the UV exposure. The manifold, advantages of radiation, curing are
retained, in particular a rapid cure, low energy comsumption, solvent-free
formulation and ambient temperature operations. As a result of these attractive
benefits, cationic UV-curable systems are used in a growing number of industrial
applications, in particular as coatings, adhesives, printing inks and photoresists.
The basic principle of photoinitiated cationic polymerization is represented
schematically on Fig. 1. Like in radical-induced UV-curing, light serves only to
decompose the photoinitiator (P1), thus generating the protonic acid which is needed
to initiate cationic polymerization. With multifunctional monomers, the reaction will
develop in the three dimensions to yield highly crosslinked polymers. The different
types 'of cationic photoinitiators and monomers currently used in UV curing
applications have been covered recently in a comprehensive review, and will only be
described briefly here, together with the curing kinetics. The mechanism of cationic
polymerization of epoxides and vinyl ethers, as well as the various factors influencing
the cure speed will be outlined. The advantages and disadvantages of photoinitiated
cationic polymerization will finally be discussed, in consideration of the present and
future' applications of this advanced technology.
