Influence of Delayed Placement and Resin Matrix on Composite-Composite Interface Strength

Objectives: The aim was to investigate the effect of delay time in composite placement and resin matrix type on the shear bond strength (SBS) between two successive composite layers made from the same material.
Methods: Three experimental composites were prepared by mixing 79 wt% of particulate filler with 21 wt% of different combinations of dimethacrylate-based resin matrices (UDMA/TEGDMA; BisEMA/UDMA/TEGDMA; BisGMA/TEGDMA). Twelve groups of specimens were prepared (n=6/group) having a cured composite as substructure layer and then the successive surface composite layer was applied at various time intervals following light irradiation of the substructure layer (0 minute, 10 minutes, 1 hour, 24 hours, and 72 hours). After two-day of dry storage at 37°C, SBS between surface and substructure composite layers was measured in a universal testing machine, and failure modes were visually analyzed. SEM and 3D optical profilometer were used for surfaces analysis of the bonding interface. SBS values were statistically analyzed with a Two-way analysis of variance (ANOVA) followed by Tukey HSD test (α=.05).
Results: SBS values ranged from 24.5 ±3 to 10.3 ±2.4 MPa for all specimens. The mean SBS decreased (p<0.05) when the composite–composite interface was time delayed more than 1 hour. However, the effect was material dependent. Resin matrix type has a significant (p<0.05) effect on SBS values. Cohesive failure in substructure layer was found to be predominant in all specimens.
Conclusions: The data suggests that the shear bond strength between two successive composite layers could deteriorate after a one-hour placement delay.