KINETIC-STUDY OF ULTRAFAST PHOTOPOLYMERIZATION REACTIONS

The photopolymerization kinetics of UV-curable polyurethane-acrylate r esins has been studied by real-time infrared spectroscopy (RT-IR). Fro m the conversion versus time curves recorded during and after UV expos ure, the rate constants of propagation (k(p)) and termination (k(t)) r eactions were determined at different stages of the curing process. Th e k(p) value was shown to remain fairly constant at about 10(4) L . mo l(-1). s(-1), until 30% conversion, and to decrease later on due to ge lation and related molecular mobility restrictions. The increasing vis cosity of the medium undergoing polymerization was found to have a str onger effect on the k(t) value, which decreased rapidly from the very beginning of the irradiation. The proportionality of k(p) and k(t) obs erved above 10% conversion was taken as evidence of a termination proc ess occurring by a reaction diffusion mechanism. The rate of polymeriz ation was found to increase with the light intensity up to an upper li mit above which the reaction kinetics is not controlled by the initiat ion rate but only by the diffusion rate of the reactive species. The t emperature profile has also been recorded by RT-IR spectroscopy and sh own to correlate well with the cure profile. The increase of the tempe rature shows the same light intensity dependence as the polymerization rate and levels at a maximum value under very intense illumination.