Effects of Exposure Duration on Composite Flexural Strength

Objective: To measure and compare the flexural strength (FS) at different layer depths (LD) within composite as a function of exposure duration (ED). Methods: Composite cylinders were simulated by stacking 0.5-mm thick Teflon molds (having center holes 6.5-mm diameter) between Mylar sheets. Molds were positioned so that the holes were aligned. A “stack” was assembled by placing uncured composite paste (A3 Premise, Kerr Sybron) in each mold, covering with Mylar, and pressing under pressure, after which another mold was added and filled in a similar manner. Ten molds were assembled to form a “stack” representing 3.0-mm overall composite thickness with layer depths of 0-0.5; 0.5-1.0, 1.0-1.5, 1.5-2.0, 2.0-2.5, 2.5-3.0 mm. Ten stacks were made for each ED. The stack was exposed for 5, 10, 15, or 30 s using a high-powered LED light (DEMI, Kerr Sybron) positioned 2 mm from the top surface. Stacks were dark-stored for 24 h and disassembled. Cured composite discs were recovered at each LD and tested in biaxial flexure using a universal testing machine. FS was calculated and compared statistically between LD and ED (two-way ANOVA, Tukey and Dunnett's post-hoc tests). Results: Exposure duration (p=0.0004), layer depth (p=0.0001), and their interaction (p=0.0008) significantly affected composite flexural strength. For the 5-s ED, FS was significantly lower only at 2.0-2.5 and 2.5-3.0 mm. For the 10-s and 15-s ED, FS was significantly lower only at 2.5-3.0 mm. There was no difference in FS among LD for the 20-s ED. Conclusion: With flexural strength as a test parameter, a 5-s LED exposure duration resulted in similar properties as those using longer exposures up to a depth of 2.0 mm, indicating the potential adequacy of a short, 5-s exposure for a 2-mm composite depth. Longer exposure durations provide increased flexural strength at greater depths.