New Mechanistic And Practical Aspects In Photoinitiators And Photosensitizers For Conventional And Laser Curing Of Multifunctional Organic Materials

INTRODUCTION
As known, the fast development during the last decade of high performance
UV-cürable systems hasinduced a growing number of applications in various
industrial areas [1]. The basic idea in the particular photopolymerization process
is to subject a monomer to a UV irradiation in order to generate an active species,

which in turn leads to a polymer chain by addition of other monomer units. Since
most of the monomers commonly employed are not producing initiating species
with a sufficient high yield upon light exposure, it is necessary to introduce a
photoinitiator that will start the polymerization.

The choice of photoinitiators is partly governed by the requirement of a high
curing speed. Many reactive monomers and efficient photoinitiators have been
synthesized by now and are commercially available. They make it possible to
formulate a broad range of appealing photocurable coatings. 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. 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.

Very attractive fields of research and development are found in the role
played by decisive parameters affecting the reactivity of a given structure, such as
chemical substitution in well chosen positions, the character of water solubility,
acid releasing ability and copolymerization availability. Although the reactivity of
photoinitiators can be successfully investigated through steady-state experiments,
laser spectroscopy proved a most convenient tool for the real time investigation of
the excited state dynamics and the processes involved [2].