Time-temperature and time-irradiation intensity superposition for photopolymerization of an epoxy based resin

In this paper the photoinitiated polymerization of a commercially available epoxy based resin for stereolithography (SL5170) was studied by means of photo differential scanning calorimetry (pDSC) at different temperature and irradiation intensities. Experimental results showed that temperature and light intensity have the same qualitative effects on the reaction kinetics. Time-temperature and time-intensity superposition principle was applied to the experimental curves. In both cases, it was shown that curves shifting on the time axis determines a single master curve. Individual curves significantly diverge from the master curve for high conversions, indicating that different regimes are encountered during photocuring of commercial stereolithography cationic resin. For low conversions, the reaction is kinetically controlled, whereas it becomes mainly diffusion controlled when the glass transition temperature approaches the isothermal cure temperature. At high conversion, master curves were also built in the diffusion-controlled regime, using different shift factors. The applicability of master curves to photopolymerization of epoxy based resin for stereolithography indicates that the mechanisms involved in network formation in the kinetic controlled regime are the same regardless of temperature and irradiation intensity. Consequently, time temperature transformation (TTT) and time intensity transformation (TIT) curves were built, as the time needed to reach polymer vitrification at each temperature and irradiation intensity.