Photochemistry and photoinduced chemical crosslinking activity of several type II commercial photoinitiators in acrylated prepolymers

Some of the most important commercial benzophenone and thioxanthone photoinitiators have been analysed by various spectroscopic techniques to evaluate the relationship between their photophysical properties and photoinitiation activity for photocrosslinking of commercial acrylated monomers and prepolymers. The type of electronic transitions occurring upon absorption of light are established via UV spectroscopy. Phosphorescence spectroscopy has been used to study their triplet energies and lifetimes and phosphorescence quantum yields. Microsecond flash photolysis has also been undertaken to determine the nature of the free radical semiquinone (ketyl) 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 low viscosity reactive diluent and a commercial resin of higher viscosity. 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 II initiator photoactivity is predominantly dependent upon the absorption maxima of the chromophore, structural influences play a major role in controlling the nature of the lowest excited triplet state and the formation of radical intermediates.