Diaryliodonium Salts Ascationic Initiators For Coatings Under EB Irradiation

1. Introduction
Recently there has been a growing interest in radiation curing of coatings
Most'progress has been made in the-field of radical polymerization and network'
formation of acrylic systems by UV-light Since Crivello and 1 reported the
possibility of cationic polymerization by UV-light in 1977, cationically
curablesystems have developed:th'éinsélvés from.'acuriosity:to' a serious
industrial activity. Normally,' diaryliodonium- and triarylsulfoniuxn-salts,
with non-nucleophilic, complex metal halides such as hexafluorophosphate as
the counterion are used as initiators Because of their rather low response
toUV-light, sensitizers such as benzoinmethylether are used. The initiation
mechanism of these sensitized reactions is schematically expressed as follows with a diaryliodonium salt as initiator.

Upon UV-radiation the radical initiator (the sensitizer) cleaves and radicals
are formed. The radical from reaction (1) reacts with the onium salt in the
so called electron transfer reaction (2). Through the dissociation of, the
iodonium salt, a cation (carbenium ion) is formed and the anion is released
from the onium salt. The cation is the true initiator for the polymerization
and is stabilized by the non-nucleophilic anion By cleavage of the iodoniuxn
salt, aryliodide and anaryl radical are formed. This aryl radical is not
active towards. the onium salt, but by abstracting a hydrogen from the
surrounding medium an active radical can be formed. This leads to a chain
reaction.