Kinetic study of free radicals trapped in dental resins stored in different environments

In this work, we used electron paramagnetic resonance to follow the decrease kinetics of free radicals trapped in an experimental resin (ER) and in a commercial composite (Charisma® (Ch)) stored under different conditions (in air at 25 and 37 °C; in argon, oxygen and water at 25 °C). During the first day, the decay was fast (0–24 h—rate of decay of allylic radical: 1700–1000 a.u. for Ch, 1700–1500 a.u. for ER) and the storage conditions had no influence on the kinetics. This phase was ascribed to a post-polymerization phenomenon. From 1 day to 1 month, the rate of decay depended on the storage environment. In argon, free radicals were quite stable (1 day to 1 month—rate of decay of allylic radical: 1200–1000 a.u. for Ch, 1400–1200 a.u. for ER). For the other storage environments, in ER, the rate of decay was higher in water than in oxygen and in air (1 day to 1 month—rate of decay of allyl radical: 1400 a.u. to 100, 500 and 800 a.u., respectively). In Ch, free radicals faded quicker than in ER, as undetectable levels were reached before 1 month, which attests to the influence of fillers on radical decrease kinetics. Heating experiments were also performed, and free radical concentrations decreased faster at higher temperatures, especially above the glass transition temperature. In conclusion, ambient oxygen is mainly involved in the termination process of free radicals. Therefore, conditions influencing oxygen diffusion have an impact on radical kinetics as well.