Dual curing of cationic UV-curable clear and pigmented coating systems photosensitized by thioxanthone and anthracene

We investigated the effects of photosensitizers (2,4-diethylthioxanthone (DETX) and anthracene) and dual curing on the kinetic and physical properties of cationic photocurable clear and pigmented coatings based on cycloaliphatic diepoxide/epsilon-caprolactone polyol. The cationic photopolymerizations were investigated by Fourier-transform infrared spectroscopy with attenuated total internal reflection (FTIR-ATR), real-time infrared spectroscopy (RTIR), and differential scanning photocalorimetry (photo-DSC). The coatings were shown to polymerize in the dark due to the "living" character of cationic polymerization. Measurements of the yellowness index revealed that the yellowing of the coatings containing the blend of DETX with anthracene photosensitizer was less than that of the coatings containing DETX only. Depth-profile analyses of FTIR-ATR spectroscopy data showed that the dual curing significantly increased both surface and interior curing of the coating films. The RTIR and Photo-DSC analyses showed that the polymerization reactivity and the ultimate percentage conversion for the cationic clear and pigmented formulations containing photosensitizers were higher than the corresponding values for the formulation without photosensitizer. (C) 2003 Elsevier Science Ltd. All rights reserved.