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Spatial and Temporal Evolution of the Photoinitiation Rate for Thick Polymer Systems Illuminated by Polychromatic Light: Selection of Efficient Photoinitiators for L.E.D. or Mercury Lamps

Cindy C. Hoppe, Nicole Stephenson Kenning, Beth A. Ficek, and Alec B. Scranton. Chemical and Biochemical Engineering, University of Iowa, 4133 SC, Iowa City, IA 52242

Photopolymerizations offer a unique set of advantages, such as spatial and temporal control of initiation, cost efficiency, and solvent-free systems, over traditional thermal polymerization. They are now well established in thin film and coating applications. However, photopolymerizations of thick systems (greater than 1 mm) are relatively uncommon due to light attenuation with increasing sample depth. Light-emitting diode (LED) light sources offer a number of advantages over mercury lamps traditionally used in photopolymerizations, including high energy efficiency, long lamp lifetimes, low heat generation, and absence of hazardous vapors. In this contribution, four common free-radical photoinitiators were evaluated for use in thick photopolymerizations illuminated with a medium pressure 200 W mercury/xenon arc lamp and a high intensity 400 nm LED lamp. For each photoinitiator/lamp combination, the spatial and temporal evolution of the photoinitiation rate profile was analyzed by solving a set of differential equations which govern the light intensity gradient and initiator concentration gradient for polychromatic illumination. This mathematical solution provides a complete and accurate description of polychromatic photoinitiation in thick systems in which the incident light intensity and molar absorptivities of all components may be independently specified at each incident wavelength. A simulation developed from this mathematical description helps to gain a fundamental understanding how a host of variables affect photoinitiation behavior, which is needed for effective design of photopolymerization reaction systems. The simulation results from this study revealed that two of the four photoinitiators evaluated were ineffective for photoinitiating thick polymer systems. The photoinitiator bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (BAPO), in combination with the 400 nm LED lamp, was shown to be the most efficient photoinitiator/light source combination for photoinitiation of thick systems. These results show that some photoinitiators commonly used for photopolymerization of thin coatings are ineffective for curing thick systems. These results also illustrate that, for some photoinitiators, LED light sources may lead to more effective photoinitiation of thick polymer systems.