Changes in mechanical properties of seven light-cured composite resins after thermal cycling

Objective To examine the changes of the mechanical properties of 7 different light-cured composite resins after thermal cycling and the correlations between these properties. Methods Seven different light-cured composite resins, including 2 microfilled composites (A110:AH and ESTELITE ∑:ET), 3 microhybrid composites (AELITE:AT, Z250:ZS, and CharmFil plus:CP), and 2 nanohybrid composites (Z350:ZH and Grandio:GD), were prepared into test specimens with a diameter of 12 mm and a thickness of 1.0 mm. The specimens were stored in distilled water at 37 ℃ for 24 h prior to 1 000 thermal cycles of 5 ℃ for 15 s and 55 ℃ for 15 s. The biaxial flexural strength (σf) was tested using the ball-on-three-ball method at a crosshead speed of 0.5 mm/min (ISO4049). The fracture surface was observed under scanning electron microscope (SEM), and the remaining specimens underwent Knoop hardness test with a 50-g loading for 10 s. Results The highest and lowest Weibull modulus was observed in AH (18.752) and AT (5.290) group, respectively. The highest and lowest biaxial flexural strength was observed in ZS (158.2 MPa) and ET (54.0 MPa) groups, respectively. The σf of the tested materials decreased in the order of microhybrid composite, nanohybrid composite, and microfiller composite, and the σf showed no significant difference between the composites with a similar filler (P>0.05). The fracture number was positively correlated to the strength of the material. The Knoop hardness numbers (H) was the highest in GD group (110.81±14.77 kg/mm 2 ) and the lowest in AH group (42.81±1.91 kg/mm 2 ). SEM showed that the interface region of the matrix and the filler was vulnerable to crack formation. Conclusion The nanohybrid composite resins better suit clinical applications than microhybrid composites. The applicability of Knoop hardness test in hardness measurement of the composite resins needs to be further demonstrated.