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As known, the fast development during the last decade of high performance
UV-curable systems has induced a growing number of applications in various
industrial areas 1]. The choice of photoinitiators is partly governed by the
requirement of a high curing speed. Improved photoinitiator systems exhibiting
increasingly fast curing speeds or / and increased photosensitivity, however, are
required. The intrinsic reactivity in the excited states determines the interest of a
given photoinitiator with reference to its efficiency 2-12]. Other factors, however, e.g.
high molecular absorption coefficients and a broad spectral absorption range
(limiting the inner filter reabsorption in pigmented media), synergistic effects,
yellowing, extractability and the effect on the long-term stability of the material, must
be considered as important items. These different topics have been recently
discussed. For example, onium salts are known as a novel class of stable, nonhygroscopic,
highly efficient photoinitiators ; they are latent sources of cation radicals
and Bronsted acids, which can be generated on demand by irradiation. Recent
papers reviewed the behaviour of onium salts as cationic polymerization initiators.
The ability of 'diphenyliodonium salts to generate both cation radicals and free
radicals on photolysis, permits also their use as convenient amphifunctional
photoinitiators i.e. to initiate both cationic and free radical polymerization. Timeresolved
laser-spectroscopy gives the best opportunity to identify the excited states,
involved in the fundamental key steps of photoinitiation processes, through the
determination of the transient absorption spectra, the triplet state lifetimes and the
rate constants of the reactions 13,14]. Another example concerns the interactions
between photoinitiators. Combining photoinitiators of polymerization to enhance the
overall efficiency of the curing process is still a challenge. It has been shown 15-191
that the excitation transfer occurs in a mixture of a morpholino ketone MK and a
thioxanthone derivative TX, where MK and TX play the role of a photoinitiator I, and a
photosensitizer S, respectively. While the absorption spectra exhibit an intense
absorption in the UV, only the thioxanthone absorbs in the near UV. The excitation
transfer is described by a competition between electron transfer and energy transfer.
The relative efficiencies of these processes are dependent on two factors: first, the
energy position of the lowest lying triplet state. Second, the change of the
spectroscopic character of the triplet state that affects the rate constant of the electron
transfer. In the same way, the combined use of UV photoinitiators and additives can
enhance the overall activity in photopolymerizable formulations, e.g. : ketones -
amines -onium salt 20,21] or ketones - amines bromo compounds 22] (such
4 or bromo ketones or ketosulfones). Excited state processes have been carried
out 23].
