Reactivity and Oxygen Inhibition Of Uv Cured Formulations Studied By Real-Time Infrared Spectroscopy

Oxygen inhibition of free radical polymerization is the most severe handicap for radiation
curing technology. Oxygen inhibition is slowing down the reaction rate, limiting
production speed, and is the reason for bad curing of the surface layer (tackiness). For most
applications a properly cured surface is the most important asset of the coating with respect
to the final properties of consumer goods.

Oxygen has two pathways to interact with the initiator: One is quenching, whereby
energy from the excited triplet state of the initiator is transferred to the oxygen molecule
and the initiator becomes deactivated. The amount of quenching depends on the triplet
energy and triplet lifetime of the initiator. For a-cleavage type a-hydroxyketones the triplet
lifetime is very short (1,2) and so quenching can be neglected. The second path is the
addition reaction of the oxygen molecule with the radicals that were formed by the initator
system. In this case peroxy radicals are formed which do not initiate polymerization and
hence initiation activity is lost. However the oxygen does not only react with the initiator
but also with the growing polymer chain radicals leading to termination of the polymerization
reaction.