Effects of critical thermal cycling on the flexural strength of resin composites

We compared flexural strength (FS) in four resin composites before and after three protocols for thermal cycling aging. Four resin composites were evaluated: Enamel Plus Hri, Gradia Direct Posterior, Grandioso, and Grandioso Flow. Sixty specimens (2 × 2 × 25 mm) were fabricated using a split metallic mold and light-cured for 30 s. The specimens were then randomly divided into four groups and tested using one of the following thermal cycling procedures: (1) storage in deionized water for 24 h (control group), (2) 15,000 cycles, 3) 30,000 cycles, and 4) 45,000 cycles. Each thermal cycling procedure was conducted between 5 °C and 55 °C, with a dwell time of 30 s. All specimens were subjected to a three-point bending test, to determine FS (0.5 mm/min). "Material" and "thermal aging" were significantly associated with FS (P < 0.001). A statistically significant interaction between the two factors was also detected (P < 0.001). In the non-aged groups, nanohybrid composites had the highest FS. FS significantly decreased after thermal cycling protocols in all composites tested. Gradia composite exhibited decrease in FS only after 45,000 cycles. In contrast, FS significantly decreased in the Grandioso Flow composite at 15,000 cycles. The trend in the decrease varied among composites, and the decrement in FS was not proportional to baseline values.