Photochemistry and photoinduced chemical crosslinking activity of type I &II co-reactive photoinitiators in acrylated prepolymers

The photoreactivity of Type I and II photointiators, namely, 4-(2-hydroxyet hoxy)-phenyl-(2-hydroxy2-methylpropyl) ketone (Irgacure 2959, Ciba-Geigy) a nd 4-hydroxybenzophenone, respectively, have been examined in the free and bound form in melamine and urethane acrylate prepolymers. Go-reaction in th e prepolymers was undertaken by partial replacement of the hydroxyacrylate components with the photoinitiators in the reaction with the isocyanate com ponent. Both the photoinitiators were also converted into their correspondi ng acrylate derivatives for study. The free, bound and acrylated derivative s were then analysed by various spectroscopic techniques to evaluate the re lationship between their photophysical properties and photoinitiation activ ity for photocrosslinking of commercial acrylated monomers and prepolymers. Photocuring studies using Hg sources (conveyor and RTIR) indicated that wh ilst binding the Type I photointiator reduced its efficiency in cure, the T ype II photoinitiator was enhanced. The presence of an amine co-synergist, however, generally eliminated these differentials. Acrylation of the Irgacu re 2959 also reduced photoactivity. Absorption spectroscopy indicated the p resence of a strongly absorbing n pi* transition in the far UV region for b oth chromophore types which undergoes a blue shift on binding to the prepol ymer. Phosphorescence spectra, lifetimes and quantum yields also indicate t he presence of low lying triplet n pi* states for both types of chromophore . Binding appears to enhance the triplet Lifetime and reduce the quantum yi eld of emission due to increased electron donation into the chromophore by the resin component and may in part account for their lower photoactivity. Microsecond flash photolysis identified the formation of benzoyl radicals i n the case of the Type I system with a small enhancement in radical formati on on binding. The Type IT system gave ketyl radicals formed by hydrogen at om abstraction which were enhanced in the bound resin system. The latter wo uld account for increased photoactivity for the bound Type II system. Nanos econd laser flash photolysis experiments identified the triplet-triplet abs orption in the case of the Type II initiator. In the presence of a tertiary amine (triethylamine) the transient absorption and its Lifetime were signi ficantly enhanced due to exciplex formation. However, binding the benzophen one initiator to the resin significantly quenched the transient absorption and significantly reduced its lifetime. In the case of the Type I initiator the benzoyl radical was observed directly, confirming the microsecond flas h photolysis data. Binding of the initiator to the melamine acrylate resin also reduced benzoyl radical formation and reduced the radical lifetime. Th is may account for the reduced photoactivity of the Type I bound initiator. Radical formation was also reduced when bound to the urethane acrylate but the lifetime was enhanced. For the initiator bound resin systems the prese nce of an amine co-synergist enhanced benzoyl radical formation. The implic ations of initiator co-reactions are discussed in terms of both the photoch emical and commercial benefits. (C) 1999 Elsevier Science S.A. All rights r eserved.