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. Part I: Shadow Cure in Free Radical Photopolymerizations In this aspect of the research, the extent of shadow cure in visible-light-induced free radical photopolymerization was investigated. A number of methods including specialized additives, reflective stages, and increased light intensity are considered. In addition, the use of fluorescent dyes in multi-component photoinitiator systems is proved to be very effective for shadow cure since the fluorescent light emitted from the dye could illuminate the shadow region. The high viscosities associated with industrially relevant reaction systems (mixtures of oligomers and monomers) reduce the effectiveness of multi-component photoinitiator systems since a diffusion-controlled molecular encounter is required during a short excited state lifetime. Therefore, a new single-component organo-metallic visiblelight-induced photoinitiator system was characterized. This innovative photoinitiator system resulted in high conversions in the shadow regions of the viscous oligomercontaining mixtures.