Free Radical Shadow Cure Initiated Using Two-Component and Three-Component Initiator Systems

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2012, Article ID 213846, 8 pages doi:10.1155/2012/213846 Research Article Free Radical Shadow Cure Initiated Using Two-Component and Three-Component Initiator Systems Hajime Kitano,1, 2 Karthik Ramachandran,2 and Alec B. Scranton2 1 Bridgestone Corporation, 3-1-1 Ogawahigashi-cho, Kodaira-shi, Tokyo 187-8531, Japan 2 Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, IA 52242-1219, USA Correspondence should be addressed to Alec B. Scranton, Received 29 November 2011; Accepted 3 February 2012 Academic Editor: L. Maria Gómez Copyright © 2012 Hajime Kitano et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 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. 1. Introduction Photopolymerization has received considerable attention due to its environmental and processing advantages [1, 2]. Compared to traditional thermal polymerization processes, light-induced polymerization offers reduced volatile organic compound emissions, lower energy requirements, and shorter cure times. Furthermore, light sources are generally much more compact than ovens and autoclaves used for thermal cure. For these reasons photopolymerization can be used not only for the replacement of conventional thermal processes but also for new systems and applications. In addition, the recent development of initiators for visible-light-induced photopolymerization enables processes based upon inexpensive light sources and lower energy photons which are not damaging to biological tissues and cells. Due to these advantages, visible-light-induced photopolymerization has been employed for various applications including coatings, adhesives, and printing. Despite these advantages, the inability to cure shaded regions which are inaccessible to the initiating light is a limitation for many applications. If this disadvantage is overcome a number of new applications of photopolymerization could emerge, such as three-dimensional adhesives [3], optical impact films [4], solar-light-induced coatings, and pigmented systems. In photopolymerization systems, “shadow cure” may be defined as polymerization which extends into regions which are not illuminated by the incident initiating light source. Dual-cure systems containing both photoinitiators and thermal initiators for thick shadow cure are well known [5, 6]; however, shadow cure in systems containing only photoinitiators has only recently been reported by Ficek et al. [7]. These authors demonstrated cationic photopolymerizations of thick systems in which polymerization could occur in shadow regions due to the mobility of the long-lived cationic active centers. Although this method could be attractive for some systems and applications, it has a number of limitations. It may only be applied to cationically polymerizable monomers, exhibits relatively slow cure rates compared to free radical systems, and the photo-generated protons may be corrosive to electronic devices. Therefore, in this contribution, shadow cure in free radical photopolymerizations 2 International Journal of Photoenergy O O C O OH OH Br Br OH O OH C O O Br Br (b) (a) CH2 CH2 OH O H3 C N Cl− I+ CH2 CH2 OH (c) (d) (e) Figure 1: Chemical structures of (a) fluorescein (FL), (b) eosin Y spirit soluble (EYss), (c) benzophenone (BP), (d) N-methyldiethanolamine (MDEA), (e) diphenyl iodonium chloride (DPI). initiated using two-component and three-component photoinitiators is investigated. Multicomponent photoinitiator systems are commonly employed for visible-light-induced photopolymerization. The energy of a visible photon is generally lower than the bond dissociation energy of most organic molecules; therefore visible-light-induced photoinitiator systems are primarily two-component photoinitiator systems in which the active centers are produced via an electron transfer followed by a proton transfer from the electron donor (typically an amine) to the excited photosensitizer [1]. Many dyes and other compounds that absorb in the visible range have been used as the photosensitizer in this type of system, including camphorquinone, (thio)xanthone derivatives, (thio)xanthene derivatives, and ketocoumarin derivatives [8–13]. In order to enhance the electron transfer system by adding other reaction schemes including diphenyl iodonium chloride (DPI) into these two-component photoinitiator systems, three-component photoinitiator systems have been developed and investigated as well [14–22]. 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. In this method, the absorption of light by the dye molecules leads to emission of fluorescent light at a longer wavelength in all directions. Therefore, careful selection of the combination of the incident wavelength, the fluorescent additive, and the photosensitizer can lead to effective illumination and polymerization in shaded regions that are inaccessible to the incident light source. In this study, two different fluorescent dyes are investigated: fluorescein (FL) and eosin Y spirit soluble (EYss). Each of these dyes may form free radical active centers in two-component initiator systems containing Nmethyldiethanolamine (MDEA) or three-component initiator systems containing MDEA and DPI. 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. 2. Experimental 2.1. Materials. A mo (...truncated)