The Kinetics of Hybrid Photopolymerizations for Urethane Acrylate/epoxide Hybrid Mixture Systems as a Function of Water Concentration and Temperature
Ho Seop Eom and Julie L.P. Jessop. Chemical and Biochemical Engineering, The University of Iowa, 4133 SC, Iowa City, IA 52242
Radical and cationic photopolymerizations provide very fast curing rate with a less cost, compared to thermal polymerizations, due to the high rate of photoinitiation. However, their kinetics is often interrupted by atmospheric factors namely, oxygen and water molecules. In many cases, the kinetic failure is closely related to the end-product performance. For instance, radical species are easily scavenged by oxygen dissolved in the reaction systems and diffused from the atmosphere during the free-radical photopolymerization. Eventually, it produces a tacky surface in coating applications. In the cationic photopolymerization, cationic species are very sensitive to water molecules (or humidity), because water molecules plays a role as an inhibitor or chain transfer, resultig in poor performance products. In order to address the problems with the atmospheric factors for the entire spectrum including kinetics and end-product performance, a hybrid photopolymerization system was introduced where radical and cationic photopolymerizations simultaneously or sequentially take place in one-pot system. A synergetic effect of two complementary reactions on the kinetics of each type polymerization is expected, since the nature of radical species is not sensitive to water molecules and cationic species are not scavenged by oxygen. It is also expected that hybrid photopolymerizations give tunable physical property of the resulting polymers via the use of two resins with complementary property. In this study, the combination of elastic urethane acrylate oligomers with brittle cycloaliphatic epoxy resins was introduced to control physical/mechanical property of the resulting polymers by creating various interpenetrating network structures as a function of the functionality of the epoxides. In our previous study, it turned out the rate and final conversion of urethane acrylate free-radical photopolymerizations significantly increased due to the dilution effect of low viscous and small molecular weight epoxides, compared to relatively high viscous urethane acrylate only systems. In addition, the induction time introduced by oxygen inhibition in the initial stage of the reaction significantly decreased via a dual initiator system by increasing the consumption of dissolved oxygen in the hybrid mixture. However, the effect of water content on the cationic photopolymerization kinetics in the hybrid mixture systems is still ambiguous in terms of the role of water molecules. In addition, the effect of temperature on hybrid photopolymerizations will be addressed by elevating the temperature of the reaction system in various time stage, because the cycloaliphatic epoxides in the hybrid mixture systems tends to react very slow, relative to urethane acrylates. Finally, the kinetic information of two complementary polymerization systems needs to be correlated with physical/mechanical property of the resulting hybrid polymers. For those purposes, FT-Raman/IR spectroscopic technique was used to obtain simultaneous kinetic information, such as the rate of polymerization and final conversion, of free-radical and cationic photopolymerizations in the hybrid mixture systems. Photo-differential scanning calorimetry studies were performed to evaluate the elevated temperature effect in various time stage on the kinetics of hybrid photopolymerizations. Dynamic mechanical analysis (DMA) was performed to determine physical property including glass transition temperature and modulus of the resulting hybrid polymers. Therefore, the interplay between water concentration and temperature on the hybrid photopolymerization kinetics will be intensively discussed in correlation with physical and mechanical property.