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Kinetics of photopolymerization of acrylics and epoxies

Involved People: Alfonso Maffezzoli, Antonio Greco, Carola Esposito Corcione
Curing reactions of photoactivated systems are assuming an increasing relevance in many industrial processes, such as coatings, printing, adhesives. Besides these processes, stereolithography makes use of photoactivated resins in a laser induced polymerization for 3D building. The kinetic behavior of photocuring is a key point for a full comprehension of the cure conditions and to determine the time needed for curing the polymer. In turn, the kinetic behavior of photocuring influences the temperature rise and the shrinkage associated with part building. A similar process occurs in dental composites. Cure kinetics are studied in isothermal mode using a modified Perkin Elmer DSC 7 differential scanning calorimeter (DSC). The standard lids furnace is replaced by two quartz plates, in order to irradiate the reactive mixture in open sample pans. Experiments can be run in static air or under nitrogen. Also, different types of lamps can be used to reproduce different processes. The research activity is focused on different aspects of photopolymerization:
  • Characterization of the reaction kinetic of particulate filled acrylate systems for dental composites. Acrylate based resin cure through a radical mechanism, in which a steady state assumption is usually assumed to be valid. Accordingly, the reaction kinetic of these systems can be modeled through a pseudo-autocatalytic equation. The temperature dependence of reaction kinetics is attributed to a temperature dependent maximum degree of reaction and to a temperature dependent kinetic constant. The temperature dependency of the maximum degree of reaction is attributed to vitrification of the polymer matrix as its Tg reaches the temperature at which it is cured. Results reported in Figure 1show the temperature dependent maximum degree of reaction.
  • Characterization of phtoactivated epoxy systems. The mechanisms involved in a cationic photopolymerization are complex when compared with radical photopolymerization. In facts, the steady state assumption, which is valid for radical polymerization, cannot be applied to cationic polymerization, which is the basic reaction mechanism of epoxy resins. As a consequence, cationic photopolymerization show a continuous reaction even after removal of the light source, which is usually referred to as dark reaction. A suitable mathematical model able to represent the kinetic behavior was developed. The model prediction show a very good agreement with experimental results, as shown in Figure 2. The developed mathematical model can also be used to represent the reaction kinetics of multi phase systems, where different reacting species are present, and reaction of the different species proceed independently, and no copolymerization is observed. This is the case of epoxy systems for stereolitography, which show the presence of multiple peaks. For the same epoxy systems, it was shown that the maximum degree of reaction is also dependent on the irradiation intensity, which is in contrast with a kinetically controlled reaction hypothesis. Indeed , it can be shown that the reaction becomes diffusion controlled at high degree of conversion. As a consequence, vitrification of the systems is delayed at higher degree of reaction as the rate of reaction is increased. At low conversion, it was possible to build master curves, reported in Figure 3, indicating that the mechanisms involved in network formation in the kinetic controlled regime are the same regardless of temperature and irradiation intensity.At high conversion, master curves were built in the diffusion controlled regime, using different shift factors, as shown in Figure 4.
  • Prediction of temperature evolution of during photopolymerization. Heat transfer model coupled with reaction kinetic have been applied to predict the temperature evolution during phtopolymerization (refs 2-7). To this aim, the kinetic expressions for free radical polymerization, coupled with an energy balance, represent a system of differential equation solved using a numerical method. This approach was applied to predict the temperature evolution during curing of dental composite in a tooth cavity A similar approach is also use to model the temperature evolution during the laser photopolimerization in a stereolitography equipment in presence of a cation reaction mechanism.
Figure 1: maximum degree of reaction for particulate filled acrylate systems as a function of temperature (ref 1)
Figure 2: experimental and model prediction rate of photopolymerization for epoxy systems at different temperatures (ref 10)

Figure 3: master curves for photopolymerization of epoxy system at different irradiation intensities in the kinetic controlled regime (ref 11)

Figure 4: master curves for photopolymerization of epoxy system at different irradiation intensities in the diffusion controlled regime (ref 11)


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University of Salento   FacoltÓ di Ingengeria    Department of Engineering for Innovation