Determination of curing time in visible-light-cured composite resins of different thickness by electron paramagnetic resonance.

The irradiation time of a visible-light-activated composite necessary to achieve full polymerization throughout the material was studied. Curing-time dependence on the thickness of the material was also investigated. To monitor the visible light-activation effect, the free radical concentration was measured as a function of irradiation time. If the composite sample is less than 0.5 mm thick and exposed to light for a time interval recommended by the manufacturer, full radical concentration is indeed created uniformly. This is not the case in thicker samples. Electron paramagnetic resonance (EPR) was used to monitor the concentration of free radicals in the samples. The number of radicals was monitored as a function of irradiation time during which the radicals were generated in samples 0.5, 0.8, 2.0, 3.0 and 5.0 mm thick. An EPR X-band spectro-meter was used to detect the free radical spectra. The number of free radicals per unit mass as a function of irradiation time shows that 60% of the maximum concentration of radicals in a 1 mm sample is reached in 24 s curing time, while in thicker samples it takes hundreds of seconds. On the basis of the experiments, a depth and irradiation time-dependent radical concentration model was developed. This model shows that a 2.0 mm thick sample is cured at the bottom side if irradiated for 60 s. It is proposed that the measure of the degree of polymerization in composite materials should be the polymerization of the bottom layer of the sample which is modelled from the number of free radicals generated in the sample.