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

Some of the most important commercial benzophenone and thioxanthone photoin itiators have been analysed by various spectroscopic techniques to evaluate the relationship between their photophysical properties and photoinitiatio n activity for photocrosslinking of commercial acrylated monomers and prepo lymers. The type of electronic transitions occurring upon absorption of lig ht are established via UV spectroscopy. Phosphorescence spectroscopy has be en used to study their triplet energies and lifetimes and phosphorescence q uantum yields. Microsecond flash photolysis has also been undertaken to det ermine the nature of the free radical semiquinone (ketyl) intermediates. Th e free radical intermediate decay kinetics of the different photoinitiators have also been determined, as well as the first and second order rate cons tants, the transient radical absorptions and radical lifetimes. Photocrossl inking studies have been undertaken by Real Time Infrared Spectroscopy (RTI R) at different photoinitiator concentrations in the presence and absence o f oxygen using a low viscosity reactive diluent and a commercial resin of h igher viscosity. Pendulum hardness measurements were also carried out to co rrelate the film hardness properties with the kinetic results obtained with RTIR. Analysis of the data shows that whilst type II initiator photoactivi ty 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. (C ) 1999 Elsevier Science S.A. All rights reserved.