Fluorescent Probes for Monitoring the UV Curing of Acrylic Adhesives, 1. FTIR and Fluorescence in Real Time

Abstract

The use of the fluorescence technique for monitoring the UV-curing of two polyurethane-based adhesives containing acrylic monomers is described in this paper. The increase of the fluorescence intensity as well as the maximum wavelength shift on emission was measured in real time during the photopolymerisation process. The established relationship between fluorescence and conversion allowed monitoring of the course of the process beyond vitrification. In that sense, all the fluorescent probes studied were sensitive to the formulation curing and in some cases, the range of conversions in which they were useful was limited by their size and probably, by the free volume fraction in the polymeric matrix, Moreover, polymerisation rates were obtained from fluorescent kinetic profiles and used to carry out a comparative study of the different photosensitive formulations. The real time FTIR (RTIR) results show the highest photoinitiation efficiency of the photoinitiator 2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone (Irgacure 369) compared to that of 2,2-dimethoxy-2-phenylacetophenone (Irgacure 651). The limitation of RTIR method for monitoring slow post-reactions was overcome by the use of the fluorescence-based method. Thus, a fluorescence intensity increase and a blue-shift of the emission band were measured after the limiting conversion was attained. Dynamic mechanical analysis showed that the glass transition temperature of the polymer increases during prolonged exposure to irradiation confirming the increase of the system rigidity.