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.

Effect of thermocycling on the bond strength between dual-cured resin cements and zirconium-oxide ceramics.

The present study evaluated the durability of bond strength between zirconia and 3 different resin cements. Thirty stabilized tetragonal zirconium-dioxide blocks were duplicated in dual-curing resin core build-up material specimens. Resin blocks were randomly luted to zirconium surfaces using 1) Clearfil Esthetic Cement (CLF), 2) RelyX Unicem Aplicap (RELX), or 3) Multilink Automix (MLA). After 24 h, half of the specimens from each of the 3 groups were loaded in tension until fracture (0.5 mm/min). The remaining half were tested after 6,000 thermal cycles (5 to 55°C). Data were analyzed using 2-way ANOVA and the Tukey test (α = 0.05). Fractographic analysis was performed using a stereomicroscope. Tensile bond strength values were significantly affected by the luting agent system employed and by thermal aging (P < 0.001). The highest tensile bond strength values in non-thermal-aged groups were observed for specimens from the RELX and CLF groups. In contrast, in the thermal-aged groups, the highest tensile bond strength values were for the MLA and RELX groups. Moreover, while thermocycling significantly affected bond strengths in the RELX and CLF groups, the mean strength of the MLA group did not significantly change after aging. There was little difference in the distribution of failure modes in any group.