Photochemistry and photoinduced chemical crosslinking activity of acrylated prepolymers by several commercial type I far UV photoinitiators

Some of the most important commercial type I far UV photoinitiators have be en analysed by various spectroscopic techniques to evaluate the relationshi p between their photophysical properties and photoinitiation activity for t he photocrosslinking of commercial acrylated monomers. The type of electron ic transitions occurring upon absorption of light are established via UV sp ectroscopy. Phosphorescence spectroscopy has also been used to study their triplet energies and lifetimes and phosphorescence quantum yields. Microsec ond flash photolysis has been undertaken to determine the nature of the fre e radical (benzoyl radical) intermediates. The foe radical intermediate dec ay kinetics of the different photoinitiators have also been determined, as well as the first- and second-order rate constants, the transient radical a bsorptions and radical lifetimes. Photocrosslinking studies have been under taken by real time infrared spectroscopy (RTIR) at different photoinitiator concentrations in the presence and absence of oxygen using a reactive dilu ent monomer. Pendulum hardness measurements were also carried out to correl ate the film hardness properties with the kinetic results obtained with RTI R. Analysis of the data shows that whilst type I initiator photoactivity is predominantly dependent upon the absorption maxima of the chromophore, str uctural influences play an important role in controlling the formation of b enzoyl radical intermediates. Triplet lifetimes are very short-lived (< 10 ms), indicative of a predominantly it n-pi* low lying triplet state. Photoc hemical crosslinking rates by RTIR appear to be closely linked to benzoyl r adical formation while in the presence of oxygen variable quenching effects are observed on the crosslinking reactivity. (C) 1999 Elsevier Science Ltd . All rights reserved.