Photo-curing kinetics for the UV-initiated cationic polymerization of a cycloaliphatic diepoxide system photosensitized by thioxanthone

Photo-differential scanning calorimetry (photo-DSC) was used to investigate the cure kinetics for the UV-initiated cationic photo-polymerization of cycloaliphatic diepoxide (CADE) systems with and without a photo-sensitizer, 2,4-diethylthioxanthone (DETX), in the presence of a diaryliodonium-salt photo-initiator. Two kinetics parameters-the rate constant (k) and the order of the initiation reaction (m)-were determined for the CADE epoxide system for different amounts of added DETX photo-sensitizer (0-1 wt%) and at different isothermal temperatures (25-100 °C) using an autocatalytic kinetics model. The photo-sensitized CADE epoxide system gave much higher k and m values than the nonphoto-sensitized one (due to the photo-sensitization effect). Moreover, k and m for both CADE epoxide systems with and without DETX increased significantly with increasing isothermal temperature due to a thermal contribution towards increasing the mobility of active species. We also observed that the addition of DETX lowered the activation energy for the UV-curable epoxide system; the collision factor for the system with DETX was higher than that obtained for the system without DETX, indicating that the reactivity of the former was greater than that of the latter due to the photo-sensitization effect.