Advances in light-induced polymerizations: I. Shadow cure in free radical photopolymerizations II. Experimental and modeling studies of photoinitiator systems for effective polymerizations with LEDs

Photopolymerization has become the standard for many coating and printing applications that require rapid curing at room temperature due to its potential to reduce volatile organic compound (VOC) emissions while providing a means for efficient manufacturing processes. These advantages could be useful in a variety of emerging applications, such as anisotropic conductive films (ACF) if photopolymerization could extend into relatively narrow shadow regions which are not directly illuminated, and if visible wavelengths that are not absorbed by polyimide films could be used to trigger the reaction.;The broad objectives of this research are i) to examine the factors that determine the attainable extent of shadow cure in free radical polymerizations, and ii) to develop initiator systems effective for polymerization using visible light and light emitting diode (LED) lamps.;Project I: Shadow Cure in Free Radical Photopolymerizations. In this project, the extent of shadow cure in visible-light-induced free radical photopolymerization is investigated. A number of effective methods such as adding additives, utilizing a reflective stage, and increasing the light intensity are introduced. In addition, the use of fluorescent dyes in multi-component photoinitiator systems proved to be very effective for shadow cure because the fluorescent light emitted from the dye could irradiate the shadow region.;When considering practical resins, mixtures of oligomers and monomers, the viscosity is the major barrier that must be overcome in order to achieve high conversion in the shadow regions using visible-light-induced multi-component photoinitiator systems. Hence, instead of using multi-component systems, a commercial visible-light-induced single-component photoinitiator is investigated. As a result, a high conversion in shadow regions of the viscous oligomer containing resin is achieved.;Project II: Experimental and Modeling Studies of Photoinitiator Systems for Effective Polymerizations with LEDs. In this project, various LED photocuring systems are investigated and characterized. The light intensities of LEDs become weaker as their peak emission wavelengths decrease. Therefore, to design the practical process of LED curing, the effect of both the light intensity and the emission spectrum of the lamp must be considered. Photopolymerization for four representative UV photoinitiators with different LEDs are investigated experimentally and theoretically. The effective light source is dependent on the photoinitiators and several LEDs demonstrate high thin cure ability. The calculated results from a model display good qualitative correspondence with the experimental results. Various interesting suggestions are obtained using this model. For example, the commercialization of 355 nm LEDs might be able to superior photopolymerization compared to other currently available LED lamps.