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

Abstract

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.

Phototriggered Base Amplification for Thiol-Michael Addition Reactions in Cross-linked Photopolymerizations with Efficient Dark Cure

“…38 Herein, the time extended reaction or persistent reaction character of the phototriggered base-amplified thiol-Michael polymerization reaction motivated exploration of its feasibility toward dark or shadow curing where the persistent catalyst is generated in the irradiated area and left to diffuse, amplify, and catalyze the reaction throughout the unexposed remainder of the sample. In order to precisely characterize the extent of dark cure, the conversion profile was measured using confocal Raman spectroscopy 39 for TMPTA/PETMP in the presence of 20 mol % NPPOC-DEA and 1 mol % Fmoc-TMG. As shown in Figure 5, the acrylate conversion in the illuminated region was found to be ∼99% and relatively constant in the x-direction (negative values of the abscissa correspond to illuminated region, SI Figure S9).…”

A new safranin based three-component photoinitiating system for high resolution and low shrinkage printed parts via digital light processing

“…It seems that the lower number of created radicals in the two-component PIS in comparison to that of three-component PIS is much lower, thereby leading to slower conversion rates. 36,53 …”

“…It seems that the lower number of created radicals in the two-component PIS in comparison to that of three-component PIS is much lower, thereby leading to slower conversion rates. 36,53 By looking more closely to the rst seconds of polymerization in Fig. 9, it can be seen that there was an inhibition time (about 3.5 s) in both 2KSD samples.…”

“…In these systems, the active species are usually generated via an electron/energy transfer between a photosensitizer and coinitiator. 36 The photosensitizer is typically an organic dye such as safranin, 37 methylene blue, 38 rose bengal 39 or ketone sensitizer like isopropyl thioxanthone (ITX). 40 The second 3 component is an electron or/and a hydrogen donor that is usually an amine or thiol, and the third component is an electron acceptor such as onium salt.…”

Photopolymer material sensitized by xanthene dyes for holographic recording using forbidden singlet–triplet electronic transitions

“…As a result the excited dye reactions with other components of photoinitiating system free radicals are produced. Further, these radicals initiate polymerization of the monomeric components [15,[17][18][19]. Methods of photoinitiation for three-component systems of this type are shown in figure 4.…”