Influence of water aging on surface hardness of low-shrinkage light-cured composite resins

: This study evaluated the microhardness of four types of low-shrinkage composite resins and two types of universal composite resins with either 12 or 16 J/cm2 light power energy. Three disks were made for each group (n = 3) for a total of 36 specimens. The specimens were prepared by condensing the composite resin into a circular copper mold (diameter: 6 mm; height: 2 mm) and polymerizing with 700 mW/cm2 light power density. The microhardness values of the resin specimens were measured using a Vickers hardness tester after different storage durations. Z250 and Clearfil Majesty Posterior composites showed softer subsurfaces when comparing the 24 h samples for all six types. Conversely, Kalore GC and Admira (AD) composites showed harder subsurfaces during the 24 h samples. All the composite resins showed significant differences (P < 0.05) in microhardness values at one of the aging times when they were polymerized with either 12 or 16 J/cm2 light power energy. Composite resin AD had a higher microhardness value after polymerization with 12 J/cm2 than with 16 J/cm2. The results indicated that low-shrinkage composite resins have better subsurface characteristics, and the light power energy of 16 J/cm2 is better for the polymerization of most composites.