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

Abstract

Some of the most important commercial type I far UV photoinitiators have been analysed by various spectroscopic techniques to evaluate the relationship between their photophysical properties and photoinitiation activity for the photocrosslinking of commercial acrylated monomers. The type of electronic transitions occurring upon absorption of light are established via UV spectroscopy. Phosphorescence spectroscopy has also been used to study their triplet energies and lifetimes and phosphorescence quantum yields. Microsecond flash photolysis has been undertaken to determine the nature of the free radical (benzoyl radical) intermediates. The free radical intermediate decay kinetics of the different photoinitiators have also been determined, as well as the first- and second-order rate constants, the transient radical absorptions and radical lifetimes. Photocrosslinking studies have been undertaken by real time infrared spectroscopy (RTIR) at different photoinitiator concentrations in the presence and absence of oxygen using a reactive diluent monomer. Pendulum hardness measurements were also carried out to correlate the film hardness properties with the kinetic results obtained with RTIR. Analysis of the data shows that whilst type I initiator photoactivity is predominantly dependent upon the absorption maxima of the chromophore, structural influences play an important role in controlling the formation of benzoyl radical intermediates. Triplet lifetimes are very short-lived (<10 ms), indicative of a predominantly it n−π* low lying triplet state. Photochemical crosslinking rates by RTIR appear to be closely linked to benzoyl radical formation while in the presence of oxygen variable quenching effects are observed on the crosslinking reactivity. ©