Kinetic Study of Ultrafast Photopolymerization Reactions

Abstract

The photopolymerization kinetics of UV-curable polyurethane-acrylate resins has been studied by real-time infrared spectroscopy (RT-IR). From the conversion versus time curves recorded during and after UV exposure, the rate constants of propagation (k_p ) and termination (k_t ) reactions were determined at different stages of the curing process. The k_p value was shown to remain fairly constant at about 10⁴ L·mol⁻¹·s⁻¹, until 30% conversion, and to decrease later on due to gelation and related molecular mobility restrictions. The increasing viscosity of the medium undergoing polymerization was found to have a stronger effect on the k_t value, which decreased rapidly from the very beginning of the irradiation. The proportionality of k_p and k_t , observed above 10% conversion was taken as evidence of a termination process occurring by a reaction diffusion mechanism. The rate of polymerization was found to increase with the light intensity up to an upper limit above which the reaction kinetics is not controlled by the initiation rate but only by the diffusion rate of the reactive species. The temperature profile has also been recorded by RT-IR spectroscopy and shown to correlate well with the cure profile. The increase of the temperature shows the same light intensity dependence as the polymerization rate and levels at a maximum value under very intense illumination.