Kinetics study of the UV-initiated cationic polymerization of cycloaliphatic diepoxide resins

Photo-differential scanning calorimetry (photo-DSC) was used to study the cure kinetics of UV-initiated cationic photo-polymerization of epoxy resin monomers in presence of different photoinitiators and to investigate the photo-reactivity with respect to their chemical structure. Two cycloaliphatic diepoxy monomers - 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate and bis-(3,4-epoxycyclohexyl) adipate - were studied. Both monomers have two terminal epoxy functional groups and differ only in the number of, so-called, “space” carboxylate group and thus the overall length of the molecule. Different kinetics analysis results, including enthalpy of the reaction, induction time, peak maximum, percentage conversion were obtained for both epoxy systems for different amounts of added photoinitiator and at different isothermal temperatures (30-70 °C). Two kinetic parameters - rate coefficient (k) and the order of the initiating reaction (m) - were determined, using an autocatalytic kinetics model. Activation energy for the two epoxies curing process using the Arrhenius equation was calculated as well. Results show that photochemical reactivity is influenced by the chemical structures of monomers.