الفهرس 
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Abstract
This study was conducted to evaluate the effect of radiation exposure and aging time on the degree of conversion (DC%), flexural strength (FS), micro-hardness (MH) and surface topography (ST) of three different resin composites. In total, 360 rod-shaped resin composite specimens were prepared to evaluate FS and 90 disc-shaped resin composite specimens were prepared for evaluation of each of DC% and MH. In addition, 36 disc-shaped resin composite specimens were prepared for ST examination. Specimens were divided into 18 groups (DC%: n=5, FS: n=20 and MH: n=5, ST: n=2) according to the three experimental factors of the study; 1- Radiation exposure (no-radiation exposure and radiation exposure), 2- Resin composite (Herculite XRV Ultra, Z250 XT and Grandio), and 3- Aging time (24-h in distilled water, 3-m and 6-m in 70% ethanol). Fourier-Transform Infrared Spectrometer (FT-IR) was used to determine the DC %. For FS, specimens were subjected to a three-point bending test at 1 mm/min crosshead speed. Vickers Micro-hardness tester was used for testing MH. The ST for each resin composite material was evaluated using the Scanning Electron Microscope (SEM). Data were analysed using 3-Way ANOVA/Tukey HSD test. Significant level was set at P=0.05. DC% results showed that, there was no significant difference between no-radiation exposure and radiation exposure. Herculite XRV Ultra revealed the highest statistically significant DC%. The 24-h aging time revealed the least significant DC% (P<0.05). For FS, radiation exposure showed a statistically significant higher value compared to no-radiation exposure. Herculite XRV Ultra showed the least FS value. The 24-h aging time showed the highest significant value. For MH, there was no significant difference between no-radiation exposure and radiation exposure. Grandio showed the highest
statistically significant MH value. There was no statistically significant difference between the three different aging periods. SEM micrographs showed that radiation exposure affected the surface of the tested resin composite specimens. The effect was presented as voids and cracks. Increasing the aging time in ethanol solution increased the deteriorations of the tested resin composites, with Herculite XRV Ultra exhibiting the most affected surface of the tested resin composite specimens.
Under the limitations of this study, the following conclusions could be suggested:
1- Radiation exposure positively affected the FS of the different composites, with higher impact on the materials, which have a higher filler loading. On the other hand, it had no impact on the DC% or MH of the investigated resin composites.
2- The DC%, FS and MH showed dependency on the filler concentration, as well as the matrix composition of the tested resin composites.
3- Aging in ethanol improved the DC% of the different composites, with no impact on MH. However, ethanol aging showed a considerable negative effect on the FS of the tested resin composites.
4- Surface topography yielded different surface morphological appearance. Aging in ethanol showed a common morphological
feature in all tested materials, which is in the form of multiple cracks starting from material’s surface and extending deeply through the entire thickness of the specimen.