Effect of stepped light intensity on polymerization force and conversion in a photoactivated composite

This study evaluated the effect of stepped light intensity on the polymerization shrinkage forces and degrees of conversion of a hybrid composite.

Composite specimens were bonded between two steel rods (5.00 mm diameter, 1.25 mm apart, configuration factor = 2) mounted in a universal testing machine using a constant displacement mode. Polymerization contraction force was recorded for 300 seconds under four light exposure conditions: group 1: 40 s x 800 mW/cm2; group 2: 10 s x 100 mW/cm2 + 30 s x 800 mW/cm2; group 3: 60 s x 800 mW/cm2; group 4: 10 s x 100 mW/cm2 + 50 s x 800 mW/cm2. Maximum curing force (N300 s) and maximum force rate of the four groups were compared using one-way analysis of variance (ANOVA) (alpha = 0.05) and the Tukey test. Degree of conversion in all groups was evaluated at two depths (top surface and 2 mm) using Fourier transform infrared spectroscopy (FTIR).

Mean maximum shrinkage forces and standard deviations (SD) were: group 1, 177 N (SD = 23); group 2, 172 N (SD = 11); group 3, 213 N (SD = 15); group 4, 197 N (SD = 17). Mean maximum forces for stepped and standard groups with the same duration (1 and 2; 3 and 4) were not statistically different; means for groups 2 and 3 were statistically different. Maximum force rates were not significantly different (p = .1548). Force:time curves were S-shaped. Specimens exposed to stepped curing exhibited longer delays before force was recorded. Mode of curing was shown not to contribute to overall cure, but both duration of cure and the depth (top surface vs. 2.00 mm) were significant with an interaction effect.