The repair microshear bond strengths (µSBSs) to resin matrices in 4 different cured-composites after water storage (0, 60 s, 1 week, 1 month) were evaluated. Three different adhesive application methods to the cured-composites were performed; (1) none, (2) onestep self-etch adhesive application, and (3) one-step self-etch adhesive application with a silane coupling agent. Degree of conversion (DC) of the composite discs was determined using ATR/FT-IR with a time-based spectrum analysis. Initially, the amount of unreacted resin monomers in the repaired cured-composite contributed to the bonding performance of newly-filled uncured-composite to resin matrix of the cured-composite. Adhesive application could not improve their repair µSBS. After 1-month of water-storage, the repair µSBS was dependent on material, which either reduced or did not and was not influenced by their amount of un-reacted resin monomers. When repairing aged composite resin, the appropriate adhesive application procedures were different among resin composites.

The aim of this study was to investigate volumetric polymerization shrinkage (VS), using swept-source optical coherence tomography (SS-OCT), of bulk-fill composites with different light-curing strategies; immediately after light-irradiation and after 24 h, and to evaluate their regional ultimate tensile strength (UTS) at different curing depths. The immediate VS after photo-polymerization decreased when the curing light-curing time was reduced from 20 to 10 to 5 s. On the other hand, their VS values after 24 h significantly increased due to the post-cure polymerization, resulting in similar VS values at all the light-curing times. Five seconds light-curing decreased the regional UTS of resin composites after the 24 h period compared with the 10 and 20 s light-curing regimes; therefore, the remarkable progress of post-cure polymerization after light-curing for a short time would not lead to an improvement in the mechanical properties of resin composites. The influence of the light-curing time on the curing depth was dependent upon the bulk-fill composite material.

The effect of long-term water storage on the regional ultimate tensile strength (UTS), water sorption (Wsp) and water solubility (Wsl) of conventional and bulk-fill resin composites at various depths was investigated. Composite specimens light-cured from one side were sectioned into beams corresponding to different depths (1-5 mm) and stored in water for 24 h, 1 week, 1 month, 6 months or 1 year. UTS increased during the first week and then gradually decreased over time, especially in deeper regions. Bulk-fill composites initially exhibited similar UTS at all depths, whereas the UTS of conventional composites at 1 mm and 5 mm differed significantly at all time points. Wsp and Wsl increased with depth and storage time, markedly at 3-5 mm after 1 month for conventional composites and after 6 months for bulk-fill composites. The signs of degradation at depths beyond 3 mm suggested that even bulk-fill composites have suboptimal properties in layers more than 3 mm in thickness.