Most visible-light photoinitiators are based on electron transfer processes and are comprised of two or more components. These initiators can lose effectiveness in viscous systems because the underlying reactions are diffusion controlled. In this contribution, the visible-light photoinitiator bis(cyclopentadienyl) bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium is characterized for polymerization of viscous systems and low light intensities. This compound absorbs visible light at wavelengths up to 550 nm, and does not rely on diffusion-controlled electron transfer reactions because it undergoes unimolecular decomposition. In contrast to trends observed for other photoinitiators, the effectiveness of the compound is found to increase markedly with the addition of protonic acids and with increasing system viscosity. For a given concentration of initiator and acid, a remarkably low optimal light intensity for effective polymerization is observed. The origins of these surprising results are discussed in terms of the mechanism of decomposition of the photoinitiator.

In photopolymerization systems, “shadow cure” may be defined as polymerization which extends into regions which are not illuminated by the incident initiating light source. The objective of this study is to evaluate the use of fluorescent additives for polymerization in masked regions that are unilluminated by the incident initiating light. Two different fluorescent dyes are investigated: fluorescein (FL) and eosin Y spirit soluble (EYss). A systematic series of studies was performed to characterize the effects of fluorescence intensity, the incident light intensity, and the presence of a diphenyl iodonium salt on the observed degree of shadow cure. It was concluded that shadow cure may be enhanced if one or more fluorescent compounds emit fluorescent light at wavelengths absorbed by the dye in a two- or three-component photoinitiator system. The addition of DPI to the two-component systems containing MDEA and FL or EYss led to a significant enhancement in the observed shadow cure. This result was attributed to the fact that DPI will increase both the number of active centers and the mobility of the active centers as a result of the electron transfer reactions in which it participates.

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 PhotopolymerizationsIn 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. Part II: Experimental and Modeling Studies of Photoinitiator Systems for Effective Polymerizations with LEDsIn this second aspect of the research, various LED photocuring systems were investigated and characterized. LEDs are very energy efficient, however, the light intensities of LEDs are reduced as the peak emission wavelength is decreased. Therefore, to identify conditions for effective LED curing, the effect of both the light intensity and the emission spectrum of the lamp must be considered. Photopolymerization using 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 theoretical model display good qualitative correspondence with the experimental results, and provide insight into effective operating conditions. For example, the commercialization of 355 nm LEDs is predicted to achieve superior photopolymerization compared to other currently available LED lamps. Abstract Approved: ____________________________________ Thes...