Effect of repeated heating and cooling cycles on the degree of conversion and microhardness of four resin composites

Objective This study evaluated the effect of repeatedly heating and cooling four resin-based composites (RBCs) for up to six cycles. Materials and Methods Four commercial RBCs were heated to 68 degrees C and cooled to room temperature for up to six cycles before photocuring at 30 degrees C. Specimens spent a total of 0, 30, 60, 90, 120, 150 min, or 7 days at 68 degrees C. The degree of conversion (DC) was measured at the bottom of the specimens immediately after photocuring. The Vickers microhardness was measured at the top and bottom of the RBC surfaces 24 h after photocuring. The data were analyzed using one-way analysis of variance, Dunnett's or Bonferroni post-hoc tests, and Spearman correlation analysis (alpha = 0.05). Results For two brands of RBC, the DC decreased at various time points; however, these decreases were small, and there was no correlation (negative or positive) between the number of heating cycles and the DC for any of the RBCs. Repeated heated and cooling resulted in small changes in the hardness (compared to the control) in both directions (Dunnett; p < 0.05). Two of the RBCs showed a significant, positive correlation between the number of heating cycles and their hardness at the bottom surface. Conclusion Repeated heating, cooling, and then reheating the RBCs for up to 1 week had little overall effect on their DC and microhardness values. The 2 mm thick specimens of all four RBCs achieved a bottom: top hardness ratio exceeding 0.8 after a 20 s exposure to light from a commercial LED curing light Clinical Significance Six repeated dry heating and cooling cycles of up to 1 week in duration had little effect on the DC and the microhardness of four commercial resin-based composites.